Comprehensive association analysis of speech recognition thresholds after cisplatin-based chemotherapy in survivors of adult-onset cancer

PURPOSE

Deficits in speech understanding constitute one of the most severe consequences of hearing loss. Here we investigate the clinical and genetic risk factors for symmetric deterioration of speech recognition thresholds (SRT) among cancer survivors treated with cisplatin.

METHODS

SRT was measured using spondaic words and calculating the mean of measurements for both ears with symmetric SRT values. For clinical associations, SRT-based hearing disability (SHD) was defined as SRT≥15 dB hearing loss and clinical variables were derived from the study dataset. Genotyped blood samples were used for GWAS with rank-based inverse normal transformed SRT values as the response variable. Age was used as a covariate in association analyses.

RESULTS

SHD was inversely associated with self-reported health (p = 0.004). Current smoking (p = 0.002), years of smoking (p = 0.02), BMI (p < 0.001), and peripheral motor neuropathy (p = 0.003) were positively associated with SHD, while physical activity was inversely associated with SHD (p = 0.005). In contrast, cumulative cisplatin dose, peripheral sensory neuropathy, hypertension, and hypercholesterolemia were not associated with SHD. Although no genetic variants had an association p value < 5 × 10-8 , 22 genetic variants were suggestively associated (p < 10-5 ) with SRT deterioration. Three of the top variants in 10 respective linkage disequilibrium regions were either positioned within the coding sequence or were eQTLs for genes involved in neuronal development (ATE1, ENAH, and ZFHX3).

CONCLUSION

Current results improve our understanding of risk factors for SRT deterioration in cancer survivors. Higher BMI, lower physical activity, and smoking are associated with SHD. Larger samples would allow for expansion of the current findings on the genetic architecture of SRT.

Impact of cisplatin-induced hearing loss (CIHL) on patient-reported social and emotional functioning

12120

Background: Cisplatin is one of the most commonly used ototoxic drugs, but no study has quantified the handicap imposed by CIHL in U.S. adult-onset cancer survivors. Identification of survivors with high degrees of handicap and related risk factors is vital, as hearing loss (HL) in the general population is strongly related to adverse health outcomes, including cognitive decline, dementia, and poor mental health and social well-being. Methods: Eligible testicular cancer survivors (TCS) (age < 60 y at diagnosis, given first line cisplatin) completed comprehensive health surveys, including the validated 25-item Hearing Handicap Inventory for Adults (HHIA). HHIA quantifies emotional (13 items) and social difficulties (12 items) related to HL; for each scale (0-100%), based on their responses, TCS were grouped into 3 handicap levels: 0-16% (none/minimal), 17-42% (mild/moderate), and 43-100% (significant) following HHIA recommendations. A Spearman correlation evaluated the associations between increasing HL severity and HHIA group. The association between HL and reported cognitive dysfunction was evaluated by a logistic regression analysis adjusted for age, income, education, yrs since therapy, cisplatin dose, BMI, smoking, and hypertension, with results presented as OR[CI], p-value. Results: Among 213 TCS [median age at evaluation, 46 y (IQR: 38-52 y); median time since cisplatin completion, 10.6 y (IQR: 6.8-16.6 y)], CIHL was reported by 127 TCS (60%). Of TCS with CIHL, 31% reported some degree of related handicap (for total HHIA scale: 13% TCS and 18% TCS reporting mild/moderate and significant handicap, respectively). HL severity was significantly correlated with handicap level in all domains (social, ρ = 0.85, p <.001; emotional, ρ = 0.72, p <.001; and total, ρ = 0.81, p <.001). Cognitive dysfunction was more commonly reported by TCS with CIHL than TCS without (35% and 22%, p =.049). HL was a significant independent predictor of cognitive dysfunction, 2.20 [1.09-4.47], p =.028. Since many patients with HL in the general population report tinnitus (TINN), and TINN may suggest a worse HL phenotype, we additionally adjusted for TINN severity and found a significant independent association for TINN and cognitive dysfunction, 1.49 [1.05-2.11], p = 0.027. Despite these outcomes, only 10% of TCS with HL used hearing aids (n = 13). Conclusions: After cisplatin chemotherapy, 60% TCS report CIHL, and TCS with CIHL report poorer social and emotional function. One in 5 TCS with CIHL reported significant overall handicap. Despite these outcomes, the low prevalence of hearing aid use suggests a potential clinical intervention that could improve social and emotional well-being. If confirmed, the possible association between CIHL with TINN and cognitive dysfunction may be of particular interest, as the Lancet Commission on Dementia Prevention (2020) identified untreated HL as a key modifiable risk factor.

Building a model to predict the risk of multiple severe neurotoxicities in cancer survivors after cisplatin treatment

e24066

Background: Cisplatin treatment is used for many cancers, including testicular, ovarian, and head and neck malignancies. Cancer survivors with multiple cisplatin-related toxicities can have poor health-related quality of life (HRQOL). Identification of clinical and genetic factors that predict the risk of these neurotoxicities is critical. Methods: Testicular cancer survivors (TCS) enrolled in the Platinum Study completed surveys, underwent physical examination, extensive audiometric testing, and phlebotomy for genotyping and serum platinum analysis. Cases included TCS with two or more severe toxicities (hearing loss [HL], tinnitus, and peripheral sensory neuropathy [PSN]), defined as follows: hearing threshold > 40dB based on geometric mean of 4-12kHz, responding yes to “Do you have ringing or buzzing in the ears?” and/or EORTC-CIPN20 scores in the severe range for items related to sensory neuropathy. Controls were restricted to TCS without any toxicities. TCS with a single toxicity were excluded from analyses. Penalized logistic regression lasso method was used to create the model to predict the binary outcome. Creatinine clearance and residual serum platinum levels were calculated. Polygenic risk scores (PRS) for traits commonly associated with pharmacokinetics and HL, tinnitus, and PSN were calculated for TCS in the training (n = 284) and validation (n = 157) data sets using PRS publicly available in The Polygenic Score Catalog using PRSice 2.3.3. Models were trained and tested in R 4.1.2. Results: A model to assess the risk of developing multiple severe neurotoxicities that could be used without blood work and additional analysis was developed. Clinical predictors incorporated into the model were age at testicular cancer diagnosis, age at phlebotomy, weight and height. PRS incorporated were age-related sensorineural hearing loss (PGS000762), body fat percentage (PGS002133), creatinine in urine (PGS001944), and peripheral nervous system disease (PGS002039). The accuracy of this model was 77.71%, which was significantly greater than the no information rate (NIR) of 65.61% (p = .00067). The positive and negative predictive values (PPV and NPV) were 72.09% and 79.82%, respectively. The AUC-ROC was 0.804. Adding residual platinum levels and creatinine clearance increased the accuracy of the model to 78.34%, which was significantly greater than the NIR (p = .00035). The PPV was 75.00% and the NPV was 79.49%. The area under the receiver operating characteristic curve (AUC-ROC) was 0.832. Conclusions: TCS are often faced with multiple severe neurotoxicities such as HL, tinnitus, and PSN, which impact HRQOL for many decades. If confirmed, a penalized regression model using clinical and genetic characteristics can predict the risk of developing these phenotypes to guide clinicians in treatment and post-treatment management plans.

Cisplatin-induced tinnitus (CIS-TINN) and patient-reported outcomes in adult-onset cancer survivors

e24089

Background: Cisplatin is one of the most used cytotoxic drugs worldwide, but few studies have comprehensively evaluated CIS-TINN in adult-onset cancer survivors. It is critical to identify survivors with high degrees of handicap and related risk factors, as tinnitus (TINN) is strongly related to adverse health outcomes (AHO) including reduced quality of life, and poorer physical and mental health. Methods: Eligible cisplatin-treated testicular cancer survivors (TCS) (aged < 60 y at diagnosis) completed comprehensive, validated health surveys, including the 20-item Tinnitus Primary Function Questionnaire (TPFQ). TPFQ assesses the 4 main TINN-impaired domains (see Table), each with a response metric of 0% (no interference) to 100% interference by TINN, with final impairment groupings of 0-16% (none/minimal), 17-42% (mild/moderate), and 43-100% (significant). TPFQ measures were compared by 2-sided signed-rank tests. Multinomial logistic regression models (adjusted for age, income, education, yrs since therapy, cisplatin dose, BMI, smoking, hypertension, and hearing loss (HL)) tested for associations with TINN severity. Results are shown as OR [95% CI], p-value. Results: Among 213 TCS [median age 46 y (IQR: 38-52 y); median time since therapy: 10.6 y (IQR: 6.8-16.6 y)], the most common AHOs were CIS-TINN (60%), and HL (60%). For TCS with CIS-TINN, the mean degree of reported interference across all TPFQ domains was 21% (median: 15, IQR: 3-33). TCS with CIS-TINN reported significantly greater interference with emotion (median: 22, range 5-40) than with concentration (median 13, IQR: 1.0-37, p = .004), hearing (median: 10, IQR: 0-30, p < .001), and sleep (median: 2.4, IQR: 0-21, p < .001). Conclusions: Following cisplatin chemotherapy, CIS-TINN is a leading AHO and often reported with HL and reduced quality of life. Severe CIS-TINN was associated with greater handicap on the TPFQ as well as with depression, cognitive dysfunction, and hearing aid use. Overall, however, TCS with CIS-TINN and HL reported underutilization of hearing aids.[Table: see text]

Abstract P3-15-10: Human neuronal model to study chemotherapeutic-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy is the major dose-limiting toxicity for several anti-cancer drugs including taxanes, platinating agents and microtubule inhibitors. In general, animal models have been used to study neuropathy. Our goals are twofold: 1) to create a human model for study of chemotherapeutic induced neuropathy using iCell Neurons derived from human induced pluripotent stem cells (iPSCs) and; 2) to elucidate the molecular genetic mechanisms underlying chemotherapeutic induced peripheral neuropathy by identifying genetic variants and genes that increase the likelihood of this devastating adverse event. We have used this model system to perform functional studies on candidate targets of interest from clinical studies of chemotherapeutic induced neuropathy. Upon treatment of iCell Neurons with increasing concentrations of paclitaxel (0.001-100 μM) for 72 hours, we identified a reproducible 3-5 μm (12-14%) decrease in cell median neurite process length and a 13-19 μm (10-13%) decrease in cell total neurite outgrowth per order of magnitude increase in drug. The same concentrations of vincristine and cisplatin for 72 hours result in a decrease in cell median neurite process length of 6-18% and 2-4% per order of magnitude increase in drug, respectively. No decrease in neurite length is observed with hydroxyurea, a drug that does not result in neuropathy. Paclitaxel binds to beta-tubulin to exert its cytotoxic effect and genetic variants within the promoter of TUBB2A were shown to be associated with paclitaxel induced neuropathy (Clin Cancer Res, 18(16):4441-8, 2012). As a proof of concept, we have shown that decreased expression of the beta-tubulin isotype TUBB2A by siRNA transfection causes decreased median neurite process length (interaction P = 2.0 × 10-4) and decreased total neurite outgrowth (interaction P = 6.2 × 10-10) of iCell Neurons 48 hours post-paclitaxel treatment. To determine clinical relevance, we have developed a protocol to collect blood of breast and ovarian cancer patients experiencing severe paclitaxel-induced peripheral neuropathy and matched patients without neuropathy following similar paclitaxel regimens to create iPSCs and eventually neurons. We hope to create a resource of these cells for the scientific community.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-15-10.

Integration of cell line and clinical trial genome-wide analyses supports a polygenic architecture of Paclitaxel-induced sensory peripheral neuropathy

PURPOSE

We sought to show the relevance of a lymphoblastoid cell line (LCL) model in the discovery of clinically relevant genetic variants affecting chemotherapeutic response by comparing LCL genome-wide association study (GWAS) results to clinical GWAS results.

EXPERIMENTAL DESIGN

A GWAS of paclitaxel-induced cytotoxicity was conducted in 247 LCLs from the HapMap Project and compared with a GWAS of sensory peripheral neuropathy in patients with breast cancer (n = 855) treated with paclitaxel in the Cancer and Leukemia Group B (CALGB) 40101 trial. Significant enrichment was assessed by permutation resampling analysis.

RESULTS

We observed an enrichment of LCL cytotoxicity-associated single-nucleotide polymorphisms (SNP) in the sensory peripheral neuropathy-associated SNPs from the clinical trial with concordant allelic directions of effect (empirical P = 0.007). Of the 24 SNPs that overlap between the clinical trial (P < 0.05) and the preclinical cytotoxicity study (P < 0.001), 19 of them are expression quantitative trait loci (eQTL), which is a significant enrichment of this functional class (empirical P = 0.0447). One of these eQTLs is located in RFX2, which encodes a member of the DNA-binding regulatory factor X family. Decreased expression of this gene by siRNA resulted in increased sensitivity of Neuroscreen-1(NS-1; rat pheochromocytoma) cells to paclitaxel as measured by reduced neurite outgrowth and increased cytotoxicity, functionally validating the involvement of RFX2 in nerve cell response to paclitaxel.

CONCLUSIONS

The enrichment results and functional example imply that cellular models of chemotherapeutic toxicity may capture components of the underlying polygenic architecture of related traits in patients.

Clinical translation of cell-based pharmacogenomic discovery

The use of cell-based models has emerged as a promising means to discover and validate pharmacologic phenotype-genotype relationships. The availability of large-scale genome studies in both human and model systems is now allowing us an unprecedented opportunity to understand how well cell-based models identify clinically relevant genetic variants associated with drug response and toxicity. Here we review these studies and the emerging translational information.

Abstract 2486: African ancestry is associated with high-risk disease and event-free survival in children with neuroblastoma

Background In a cohort of 3539 children with neuroblastoma, we have previously reported that self-reported black race is significantly associated with high-risk disease (p &lt; 0.001) and late relapse or death (p = 0.04). To investigate if the component of genomic variation that co-segregates with African ancestry is associated with risk group and survival in neuroblastoma, we interrogated genome-wide single nucleotide polymorphism (SNP) genotypes against those phenotypes in 2790 neuroblastoma patients. Methods Genome-wide SNP genotypes from 3404 children with neuroblastoma were obtained from 3 Illumina platforms: HumanHap550 v1, HumanHap550 v3 and HumanQuad 610. Quality filtering removed 17 patients who were genotyped twice, 15 patients with ambiguous sex, 72 patients with some degree of relatedness, 120 patients with poor genotype call rates (defined as &gt;5% total SNPs interrogated without a genotype call) and 390 patients with missing clinical data. Genotypes from the remaining 2790 patients were included in the analysis. Using SNPs common to each of the genotyping platforms, ancestral reference populations from the International HapMap Project (Caucasian - CEU, African - YRI and Asian - CHB/JPT) and the EIGENSTRAT method, a principal components analysis was performed and an ancestry map of all patients was created. Principal Component 1 (PC1), which separated black neuroblastoma patients and the YRI HapMap samples from all other ethnic groups, was tested as a continuous variable in a Cox Proportional Hazard Regression model of event-free survival (EFS) and in an ordinal logistic regression of risk group. Using a k-means clustering algorithm, patients were grouped into 1 of 5 ethnic clusters;EFS of clusters was compared using a log-rank test, and Kaplan-Meier curves were plotted. Results PC1 was correlated with neuroblastoma risk group classification (p=0.0063). PC1 was also correlated with EFS (p=0.044). Compared to Caucasian patients, patients with African ancestry had the poorest EFS (log rank p = 0.0257). Discussion Here we show that the component of genomic variation that co-segregates with African ancestry is associated with high-risk disease and event-free survival. Patients with African ancestry had inferior outcomes to patients of Caucasian ancestry. We hypothesize that germline genetic variants associated with both the development of high-risk neuroblastoma and event-free survival after chemotherapy will have different allelic frequencies in Caucasian and African populations and that patients of African ancestry may also harbor unique genetic variants associated with high-risk disease and event-free survival not found in Caucasian patients. Efforts to identify these genetic variants are underway. These results underscore the need to perform association studies from a variety of ancestries, as each group may yield novel associations.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2486. doi:1538-7445.AM2012-2486

Genome-wide meta-analysis identifies variants associated with platinating agent susceptibility across populations

Platinating agents are used in the treatment of many cancers, yet they can induce toxicities and resistance that limit their utility. Using previously published and additional world population panels of diverse ancestry totaling 608 lymphoblastoid cell lines (LCLs), we performed meta-analyses of over 3 million SNPs for both carboplatin- and cisplatin-induced cytotoxicity. The most significant SNP in the carboplatin meta-analysis is located in an intron of NBAS (p = 5.1 × 10⁻⁷). The most significant SNP in the cisplatin meta-analysis is upstream of KRT16P2 (p = 5.8 × 10⁻⁷). We also show that cisplatin-susceptibility SNPs are enriched for carboplatin-susceptibility SNPs. Most of the variants that associate with platinum-induced cytotoxicity are polymorphic across multiple world populations; therefore, they could be tested in follow-up studies in diverse clinical populations. Seven genes previously implicated in platinating agent response, including BCL2, GSTM1, GSTT1, ERCC2, and ERCC6 were also implicated in our meta-analyses.

Heritable and non-genetic factors as variables of pharmacologic phenotypes in lymphoblastoid cell lines

Publicly available genetic and expression data on lymphoblastoid cell lines (LCLs) make them a unique resource for understanding the genetic underpinnings of pharmacological outcomes and disease. LCLs have been used for pharmacogenomic discovery and validation of clinical findings associated with drug response. However, variation in cellular growth rate, baseline Epstein-Barr virus (EBV) copy number and ATP levels can all be confounders in such studies. Our objective is to better define confounding variables that affect pharmacological end points in LCLs. To this end, we evaluated the effect of these three variables on drug-induced cytotoxicity in LCLs. The drugs evaluated included daunorubicin, etoposide, carboplatin, cisplatin, cytarabine, pemetrexed, 5'-deoxyfluorouridine, vorinostat, methotrexate, 6-mercaptopurine, and 5-fluorouracil. Baseline ATP or EBV copy number were not significantly correlated with cellular growth rate or drug-induced cytotoxicity. In contrast, cellular growth rate and drug-induced cytotoxicity were significantly, directly related for all drugs except vorinostat. Importantly, cellular growth rate is under appreciable genetic influence (h²=0.30-0.39) with five suggestive linkage regions across the genome. Not surprisingly, a percentage of SNPs that significantly associate with drug-induced cytotoxicity also associate with cellular growth rate (P ≤ 0.0001). Studies using LCLs for pharmacologic outcomes should therefore consider that a portion of the genetic variation explaining drug-induced cytotoxicity is mediated via heritable effects on growth rate.

FstSNP-HapMap3: a database of SNPs with high population differentiation for HapMap3

The International HapMap Project has recently made available genotypes and frequency data for phase 3 (NCBI build 36, dbSNPb129) of the HapMap providing an enriched genotype dataset for approximately 1.6 million single nucleotide polymorphisms (SNPs) from 1,115 individuals with ancestry from parts of Africa, Asia, Europe, North America and Mexico. In the present study, we aim to facilitate pharmacogenetics studies by providing a database of SNPs with high population differentiation through a genomewide test on allele frequency variation among 11 HapMap3 samples. Common SNPs with minor allele frequency greater than 5¢ from each of 11 HapMap3 samples were included in the present analysis. The population differentiation is measured in terms of fixation index (Fst), and the SNPs with Fst values over 0.5 were defined as highly differentiated SNPs. Our tests were carried out between all pairs of the 11 HapMap3 samples or among subgroups with the same continental ancestries. Altogether we carried out 64 genomewide Fst tests and identified 28,215 highly differentiated SNPs for 49 different combinations of HapMap3 samples in the current database.

Etoposide sensitivity does not predict MLL rearrangements or risk of therapy-related acute myeloid leukemia

Therapy-related acute myeloid leukemia (t-AML) caused by MLL rearrangements (rMLL) can arise from topoisomerase II agents. However, whether rMLL-related leukemogenesis is inextricably linked to drug cytotoxicity remains controversial. We therefore compared (i) rMLL in children with acute lymphoblastic leukemia (ALL) who developed t-AML and those who did not, (ii) epipodophyllotoxin toxicity in patients with t-AML and in controls, and (iii) rMLL in cells sensitive to etoposide and in those resistant to etoposide. In children with ALL, rMLL appeared to be more frequent in children who developed t-AML than in those who did not (seven pairs, P = 0.04), although independent of the cumulative etoposide dose (P = 0.5). Similarly, the frequency of epipodophyllotoxin-related toxicities did not differ between patients with t-AML and controls (26 pairs, P > 0.17). Moreover, in 25 cell lines, etoposide-induced MLL fusions did not differ in sensitive vs. resistant lines at equitoxic concentrations (P = 0.65). Together, these results indicate that epipodophyllotoxin-mediated leukemogenesis is not directly linked to drug cytotoxicity.

Genome scan implicates adhesion biological pathways in secondary leukemia

The genetic risk factors for etoposide-induced leukemia with MLL translocations remain largely unknown. To identify genetic risk factors for and novel characteristics of secondary leukemia, we profiled 116,204 single nucleotide polymorphisms (SNPs) in germline and paired leukemic cell DNA from 13 secondary leukemia/myelodysplasia cases and germline DNA from 13 matched and 156 unmatched controls, all with acute lymphoblastic leukemia treated with etoposide. We analyzed global gene expression from a partially overlapping cohort. No single locus was altered in most cases. We discovered 81 regions of loss of heterozygosity (LOH) in leukemic blasts and 309 SNPs whose allele frequencies differed in cases vs controls. Candidate genes were prioritized on the basis of genes whose SNPs or expression differentiated cases from controls or showed LOH or copy number change in germline vs paired blast DNA from the 13 cases. Three biological pathways were altered: adhesion, Wnt signaling and regulation of actin. Validation experiments using a genome scan for etoposide-induced leukemogenic MLL chimeric fusions in 15 HapMap cell lines also implicated genes involved in adhesion, a process linked to de novo leukemogenesis. Independent clinical epidemiologic and in vitro genome-wide approaches converged to identify novel pathways that may contribute to therapy-induced leukemia.

The pharmacogenetics research network: from SNP discovery to clinical drug response

The NIH Pharmacogenetics Research Network (PGRN) is a collaborative group of investigators with a wide range of research interests, but all attempting to correlate drug response with genetic variation. Several research groups concentrate on drugs used to treat specific medical disorders (asthma, depression, cardiovascular disease, addiction of nicotine, and cancer), whereas others are focused on specific groups of proteins that interact with drugs (membrane transporters and phase II drug-metabolizing enzymes). The diverse scientific information is stored and annotated in a publicly accessible knowledge base, the Pharmacogenetics and Pharmacogenomics Knowledge base (PharmGKB). This report highlights selected achievements and scientific approaches as well as hypotheses about future directions of each of the groups within the PGRN. Seven major topics are included: informatics (PharmGKB), cardiovascular, pulmonary, addiction, cancer, transport, and metabolism.

A phase II trial of oral temozolomide in patients with metastatic renal cell cancer

PURPOSE

To determine the activity of temozolomide, an oral imidazotetrazine alkylating agent that has exhibited broad antitumor activity in preclinical studies, in renal cell cancer (RCC) patients. METHODS. Metastatic RCC patients were treated with temozolomide, 200 mg/m(2) per day orally, and traditional radiologic response endpoints were assessed. O(6)-Alkylguanine-DNA alkyltransferase (AGT) activity was measured in four pretreatment biopsies.

RESULTS

Among 12 patients, there were no responses. High AGT activity was observed in all four biopsies analyzed.

CONCLUSIONS

Temozolomide is not active against RCC and this clinical observation may be due to high levels of AGT in this tumor.

High efficiency electroporation of human umbilical cord blood CD34+ hematopoietic precursor cells

Human umbilical cord blood provides an alternative source of hematopoietic cells for purposes of transplantation or ex vivo genetic modification. The objective of this study was to evaluate electroporation as a means to introduce foreign genes into human cord blood CD34+ cells and evaluate gene expression in CD34+/CD38(dim) and committed myeloid progenitors (CD33+, CD11b+). CD34+ cells were cultured in X-VIVO 10 supplemented with thrombopoietin, stem cell factor, and Flt-3 ligand. Electroporation efficiency and cell viability measured by flow cytometry using enhanced green fluorescent protein (EGFP) as a reporter indicated 31% +/- 2% EGFP+ /CD34+ efficiency and 77% +/- 3% viability as determined 48 hours post-electroporation. The addition of allogeneic cord blood plasma increased the efficiency to 44% +/- 5% with no effect on viability. Of the total CD34+ cells 48 hours post-electroporation, 20% were CD38(dim)/EGFP+. CD34+ cells exposed to interleukin-3, GM-CSF and G-CSF for an additional 11 days differentiated into CD33+ and CD11b+ cells, and 9% +/- 3% and 8% +/- 7% were expressing the reporter gene, respectively. We show that electroporation can be used to introduce foreign genes into early hematopoietic stem cells (CD34+/CD38(dim)), and that the introduced gene is functionally expressed following expansion into committed myeloid progenitors (CD33+, CD11b+) in response to corresponding cytokines. Further investigation is needed to determine the transgene expression in functional terminal cells derived from the genetically modified CD34+ cells, such as T cells and dendritic cells.

Role of cytochrome P450 isoenzymes in metabolism of O(6)-benzylguanine: implications for dacarbazine activation

O(6)-Benzylguanine (BG) effectively inactivates the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase and enhances the effectiveness of alkylating agents, such as 1,3-bis(2-chloroethyl)-1-nitrosourea and temozolomide, in vitro and in vivo. BG is presently in clinical trials with 1,3-bis(2-chloroethyl)-1-nitrosourea and temozolomide. Preclinical data demonstrate that BG enhances the sensitivity of cells to 5-[3-methyl-triazen-1-yl]-imidazole-4-carboxamide, the active intermediate of dacarbazine (DTIC), making the combination BG plus DTIC attractive for additional clinical development. DTIC requires metabolism by cytochrome P450 (CYP450) isoforms, 1A1, 1A2, and 2E1 to form two reactive N-demethylated metabolites, 5-[3-hydroxy-methyl-3-methyl-triazen-1-yl]-imidazole-4-carboxamide and 5-[3-methyl-triazen-1-yl]-imidazole-4-carboxamide, ultimately forming a methylating species responsible for its cytotoxicity. The objective of this study was to examine the role of CYP450 1A1 and 1A2 in the metabolism of BG and identify possible drug-drug interactions with DTIC. Our data show that CYP450 isoforms 1A1 and 1A2 are primarily responsible for both BG oxidation to O(6)-benzyl-8-oxoguanine (8-oxoBG) and additional debenzylation to 8-oxoguanine. The catalytic efficiency of BG oxidation is 16 times lower for CYP1A1 than CYP1A2; however, the catalytic efficiency of 8-oxoBG debenzylation is 11 times greater for CYP1A1 than CYP1A2. Furthermore, BG inhibits CYP1A1 and 1A2 catalyzed conversion of DTIC to active methylating species. 8-OxoBG also inhibited conversion of DTIC to active methylating species but to a much lesser extent. The concentrations of BG required to inhibit 50% of DTIC N-demethylation were 2.8, 0.13, and 3.8 microM in human liver microsomes, baculovirus overexpressed CYP1A1, and CYP1A2, respectively. Our results suggest that treating patients with BG as a means to enhance the effectiveness of DTIC is unlikely to result in a therapeutic benefit as a result of inhibition of the enzymes responsible for conversion of DTIC to its active methylating species.

Effect of O6-benzylguanine on nitrogen mustard-induced toxicity, apoptosis, and mutagenicity in Chinese hamster ovary cells

O6-Benzylguanine (BG) inactivates O6-alkylguanine-DNA alkyltransferase (AGT), resulting in an increase in the sensitivity of cells to the toxic effects of O6-alkylating agents. BG significantly enhances the cytotoxicity and decreases the mutagenicity of nitrogen mustards [i.e., phosphoramide mustard (PM), melphalan, and chlorambucil], a group of alkylating agents not known to produce O6-adducts in DNA. The enhancement is observed in cells irrespective of AGT activity. Exposure of Chinese hamster ovary cells to 100 microM BG results in enhancement in the cytotoxicity of PM (300 microM), chlorambucil (40 microM), and melphalan (10 microM) by 9-, 7-, and 18-fold, respectively. In contrast, mutation frequency after treatment with 300 microM PM is decreased from 259 mutants/10(6) cells to 22 mutants/10(6) cells when cells are pretreated with BG. The enhancement of toxicity of these bis-alkylating agents appears to involve cross-link formation, because neither cytotoxicity nor mutagenicity of a monoalkylating PM analogue is significantly altered when combined with BG. Enhanced cytotoxicity and decreased mutagenicity is concomitant with a dramatic increase in the number of cells undergoing apoptosis when BG is combined with PM, melphalan, or chlorambucil at 72-94 h after treatment. Cell cycle analysis demonstrates that BG alone or combined with nitrogen mustards arrests cells in G1 phase of the cell cycle. At 16 h after treatment, 11 and 57% of cells treated with PM alone or with BG plus PM are in G1 phase, respectively. Our data suggest that treatment with BG causes G1 arrest and drives noncycling cells treated with nitrogen mustards into apoptosis, thus protecting against mutagenic DNA damage introduced by nitrogen mustards.

Optimization of culture conditions to enhance transfection of human CD34+ cells by electroporation

The ability to culture CD34+ stem cells, while maintaining their pluripotency, is essential for manipulations such as gene transfection for therapeutic trials. Human peripheral blood (PB) CD34+ cells (> or = 90% purity) were cultured for up to 4 days in serum-free culture medium supplemented with thrombopoietin (TPO), stem cell factor (SCF), Flt-3 ligand (Flt-3L), with or without PIXY321 (IL-3/GM-CSF fusion protein) and human serum. The CD34 mean fluorescence intensity (MFI) and cell cycle status were evaluated daily using flow cytometry and hypotonic propidium iodide. Prior to culture (day 0), 97.0 +/- 0.9%, 1.9 +/- 0.3% and 1.0 +/- 0.6% of the selected CD34+ cells were in G0-G1, S-phase, or G2-M, respectively. After 2-4 days in culture with TPO/SCF/Flt-3L, there was an increase in the percent of cells in S-phase to 26.4 +/- 0.1% without significant loss of CD34 MFI. The addition of PIXY321 increased.the percentage of CD34+ cells in S-phase to 36.3 +/- 4.0%, but the CD34 MFI and numbers of CFU (colony-forming units) were significantly decreased at day 3 when cultured with PIXY321 or various recombinant cytokine combinations that included IL-3 and IL-6. There is an increase from day 0 to day 4 in the percentages of CD34+ with CD38-, HLA-DR-, and c-kit(low), but not Thy-1+ cells. Electroporation with EGFP reporter gene showed that 1-2 days of pre-stimulation in X-VIVO 10 supplemented with TPO/SCF/Flt-3L was necessary and sufficient for efficient transfection. Flow cytometry analysis demonstrated that 22% of the viable cells are CD34+/EGFP+ 48 h post electroporation. The introduced reporter gene appears to be stable as determined by EGFP+/LTC-IC (long-term colony-initiating cells), at 30-40 positive colonies (16 +/- 7%) per 1 x 10(5) electroporated CD34+ cells.

Debenzylation of O(6)-benzyl-8-oxoguanine in human liver: implications for O(6)-benzylguanine metabolism

O(6)-Benzylguanine (BG) effectively inactivates the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase, and enhances the effectiveness of 1,3-bis(2-chloroethyl)-1-nitrosourea in cells in culture and tumor-bearing animals. BG is presently in phase II clinical trials. In humans, BG is converted to O(6)-benzyl-8-oxoguanine (8-oxoBG), a longer-lived, yet equally potent inactivator. We have isolated and identified the debenzylated product, 8-oxoguanine, in plasma and urine of patients following administration of BG. The purpose of this work was to determine the human liver enzymes responsible for the debenzylation of 8-oxoBG. Therefore, 8-oxoBG was incubated with human liver microsomes and cytosol, and the concentration of 8-oxoguanine was determined. No appreciable product was formed in the cytosol; however, increasing amounts of 8-oxoguanine were formed with increasing concentrations of pooled human liver microsomes. The amount of 8-oxoguanine formed increased with time and substrate concentration. Co-incubation of human liver microsomes with 8-oxoBG and various cytochrome P450 isoform-selective inhibitors suggested the possible involvement of CYP1A2, 2E1, and/or 2A6 in this reaction. Incubation of 8-oxoBG with baculovirus cDNA-overexpressed CYP1A2, 2E1, 2A6, and 3A4 demonstrated that formation of 8-oxoguanine was due mainly to CYP1A2. Debenzylation of 8-oxoBG complied with Michaelis-Menten kinetics with K(m) and V(max) values of 35.9 microM and 0.59 pmol/min/pmol of CYP1A2, respectively. CYP1A2 appears to be mainly responsible for the debenzylation of 8-oxoBG in human liver.

Inactivation of O6-alkylguanine-DNA alkyltransferase by 8-substituted O6-benzylguanine analogs in mice

PURPOSE

The purpose of this study was to determine the usefulness of various 8-substituted O6-benzylguanine (BG) analogs as modulators of the DNA repair protein. O6-alkylguanine-DNA alkyltransferase (AGT). More specifically, the degree of inactivation of AGT in mouse brain, liver, kidney and tumor by O6-benzyl-8-oxoguanine (8-oxoBG), 8-aza-O6-benzylguanine (8-azaBG), O6-benzyl-8-bromoguanine (8-bromoBG) and O6-benzyl-8-trifluoromethylguanine (8-tfmBG) was compared to inactivation by BG, a modulator in phase II clinical trials. BG is converted rapidly to 8-oxoBG in rodents, monkeys and humans. It was reasoned that 8-substituted analogs of BG would exhibit different pharmacological properties compared to BG which could influence tissue bioavailability and, thus, the extent of AGT inactivation in vivo. We compared the tissue distribution of these agents and AGT activity following administration of the 8-substituted analogs.

MATERIALS AND METHODS

At various time points up to 24 h after i.p. administration of the BG analogs, tissues (i.e. brain, liver, kidney), A549 lung tumor xenografts (i.p.) or D456 brain tumor xenografts (i.c.) were harvested from athymic nude mice for AGT analysis. AGT activity was quantified in tissue extracts using a biochemical assay with [3H]methylated DNA as a substrate. In addition, concentrations of BG and 8-oxoBG were determined by HPLC with fluorescence detection in mouse tissues following administration of drug.

RESULTS

Each of the 8-substituted analogs of BG demonstrated variable AGT inactivation capabilities that were comparable to or better than those of BG especially in kidney and brain tissues. There was a more pronounced depletion of AGT inactivation in brain and D456 brain tumor xenografts following administration of BG compared to 8-oxoBG that could be explained by a much greater concentration of AGT-inactivating drug (BG plus the metabolite 8-oxoBG for mice treated with BG versus 8-oxoBG for mice treated with 8-oxoBG) present in these tissues. The AUCs for brain, kidney and liver were 3.2, 6.9 and 1 1.8 times greater for BG than for 8-oxoBG.

CONCLUSIONS

8-substituted analogs of BG possess unique AGT-inactivation profiles in vivo that are different from that of BG. The AGT-inhibitory activities of BG and its major metabolite, 8-oxoBG, are related to tissue disposition of both drugs.

Distribution and functional consequences of nucleotide polymorphisms in the 3'-untranslated region of the human Sep15 gene

Selenium has been shown to prevent cancer in a variety of animal model systems. Both epidemiological studies and supplementation trials have supported its efficacy in humans. However, the mechanism by which selenium suppresses tumor development remains unknown. Selenium is present in known human selenoproteins as the amino acid selenocysteine (Sec). Sec is inserted cotranslationally in response to UGA codons within selenoprotein mRNAs in a process requiring a sequence within the 3'-untranslated region (UTR), referred to as a Sec insertion sequence (SECIS) element. Recently, a human Mr 15,000 selenoprotein (Sep15) was identified that contains an in-frame UGA codon and a SECIS element in the 3'-UTR. Examination of the available cDNA sequences for this protein revealed two polymorphisms located at position 811 (C/T) and at position 1125 (G/A) located within the 3'-UTR. Here, we demonstrate significant differences in Sep15 allele frequencies by ethnicity and that the identity of the nucleotides at the polymorphic sites influences SECIS function in a selenium-dependent manner. This, together with genetic data indicating loss of heterozygosity at the Sep15 locus in certain human tumor types, suggests that Sep15 may be involved in cancer development, risk, or both.

Efficient expression of foreign genes in human CD34(+) hematopoietic precursor cells using electroporation

Introduction of foreign genes into human CD34(+) hematopoietic precursor cells offers a means to correct inborn errors or to protect human stem cells from chemotherapeutic damage. Electroporation is a non-chemical, nonviral, highly reproducible means to introduce foreign genes into mammalian cells that has been used primarily for rapidly dividing cells. CD34(+) cells isolated from mobilized peripheral blood of patients were cultured for 48 h in serum-free culture medium supplemented with Flt-3 ligand, stem cell factor and thrombopoietin. Cell cycle analysis showed an increase in % S-phase from 2% on day 0 to 28% on day 2 without significant loss of mean fluorescence intensity (MFI). Optimal electroporation conditions for CD34(+) cells were 550 V/cm, 38 ms, 30 microg DNA/500 microl at cell densities between 0.2 x 10(6) and 10 x 10(6) cells/ml resulting in transient EGFP gene expression in 21% (+/- 1%) of CD34(+) precursor cells, as determined by flow cytometry 48 h after electroporation. The more primitive cells were also found to be EGFP(+) as determined by subset analysis using Thy1, CD38, AC133 and c-kit conjugated monoclonal antibodies. Methylcellulose assays on electroporated CD34(+) cells yielded 20% (+/- 7%) EGFP(+) colonies (CFU-GM, BFU-E and CFU-mix) and 22% (+/- 5%) EGFP(+) long-term colony-initiating cells (LTC-IC). The reporter gene was found to be integrated into the LTC-IC genomic DNA as determined by inverse PCR and DNA sequencing. These results suggest that electroporation has the potential to effectively and stably deliver exogenous genes into human hematopoietic precursor cells.

O6-benzylguanine potentiates the antitumor effect of locally delivered carmustine against an intracranial rat glioma

Local delivery of carmustine (BCNU) via biodegradable polymers prolongs survival against experimental brain tumors and in human clinical trials. O6-benzylguanine (O6-BG), a potent inhibitor of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), has been shown to reduce nitrosourea resistance and, thus, enhance the efficacy of systemic BCNU therapy in a variety of tumor models. In this report, we demonstrate that O6-BG can potentiate the activity of BCNU delivered intracranially via polymers in rats challenged with a lethal brain tumor. Fischer 344 rats received a lethal intracranial challenge of 100,000 F98 glioma cells (F98 cells have significant AGT activity, 328 fmol/mg protein). Five days later, animals receiving an i.p. injection of O6-BG (50 mg/kg) 2 h prior to BCNU polymer (3.8% BCNU by weight) implantation had significantly improved survival (n = 7; median survival, 34 days) over animals receiving either O6-BG alone (n = 7; median survival, 22 days; P = 0.0002) or BCNU polymer alone (n = 8; median survival, 25 days; P = 0.0001). Median survival for the control group (n = 8) was 23.5 days. Moreover, there was no physical, behavioral, or pathological evidence of treatment-related toxicity. These findings suggest that O6-BG can potentiate the effects of interstitially delivered BCNU and, for tumors expressing significant AGT, may be necessary for the BCNU to provide a meaningful therapeutic benefit. Given the clinical use of BCNU polymers against malignant gliomas, concurrent treatment with O6-BG may provide an important addition to our therapeutic armamentarium.

Phase I trial of carmustine plus O6-benzylguanine for patients with recurrent or progressive malignant glioma

PURPOSE

The major mechanism of resistance to alkylnitrosourea therapy involves the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT), which removes chloroethylation or methylation damage from the O(6) position of guanine. O(6)-benzylguanine (O(6)-BG) is an AGT substrate that inhibits AGT by suicide inactivation. We conducted a phase I trial of carmustine (BCNU) plus O(6)-BG to define the toxicity and maximum-tolerated dose (MTD) of BCNU in conjunction with the preadministration of O(6)-BG with recurrent or progressive malignant glioma.

PATIENTS AND METHODS

Patients were treated with O(6)-BG at a dose of 100 mg/m(2) followed 1 hour later by BCNU. Cohorts of three to six patients were treated with escalating doses of BCNU, and patients were observed for at least 6 weeks before being considered assessable for toxicity. Plasma samples were collected and analyzed for O(6)-BG, 8-oxo-O(6)-BG, and 8-oxoguanine concentration.

RESULTS

Twenty-three patients were treated (22 with glioblastoma multiforme and one with anaplastic astrocytoma). Four dose levels of BCNU (13.5, 27, 40, and 55 mg/m(2)) were evaluated, with the highest dose level being complicated by grade 3 or 4 thrombocytopenia and neutropenia. O(6)-BG rapidly disappeared from plasma (elimination half-life = 0. 54 +/- 0.14 hours) and was converted to a longer-lived metabolite, 8-oxo-O(6)-BG (elimination half-life = 5.6 +/- 2.7 hours) and further to 8-oxoguanine. There was no detectable O(6)-BG 5 hours after the start of the O(6)-BG infusion; however, 8-oxo-O(6)-BG and 8-oxoguanine concentrations were detected 25 hours after O(6)-BG infusion. The mean area under the concentration-time curve (AUC) of 8-oxo-O(6)-BG was 17.5 times greater than the mean AUC for O(6)-BG.

CONCLUSION

These results indicate that the MTD of BCNU when given in combination with O(6)-BG at a dose of 100 mg/m(2) is 40 mg/m(2) administered at 6-week intervals. This study provides the foundation for a phase II trial of O(6)-BG plus BCNU in nitrosourea-resistant malignant glioma.

Schedule-dependent activity of temozolomide plus CPT-11 against a human central nervous system tumor-derived xenograft

Temozolomide, an imidazole tetrazinone, and CPT-11, a camptothecin derivative, have previously been shown to have anti-central nervous system tumor activity in laboratory and clinical studies. The current experiments were designed to evaluate the activity of temozolomide plus CPT-11 against a malignant glioma-derived xenograft, D-54 MG, growing s.c. in athymic nude mice. The initial schedule of i.p. drug administration was temozolomide at 0.1 LD10 on day 1 and CPT-11 at 0.1 LD10 on days 1-5 and 8-14. The combination of these two agents produced greater than additive activity against D-54 MG. This enhanced activity was maintained when the initial administration of CPT-11 was delayed to day 3 or day 5. However, when CPT-11 was administered first on day 1 using 0.5 LD10 (for the single dose schedule) followed by temozolomide (0.1 LD10) 5 h, 3 days, or 5 days later, the enhancement of activity was substantially reduced. These results demonstrate that the combination of temozolomide plus CPT-11 displays a schedule-dependent enhancement of antitumor activity, suggest a mechanistic explanation for the enhanced activity, and provide the rationale for a Phase I trial of this regimen.

Effect of O6-benzylguanine on alkylating agent-induced toxicity and mutagenicity. In Chinese hamster ovary cells expressing wild-type and mutant O6-alkylguanine-DNA alkyltransferases

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) has been shown to protect cells from the toxic and mutagenic effect of alkylating agents by removing lesions from the O6 position of guanine. O6-Benzylguanine (BG) is a potent inactivator of AGT, resulting in an increase in the sensitivity of cells to the toxic effects of chemotherapeutic alkylating agents. Chinese hamster ovary (CHO) cells and CHO cells transfected with wild-type AGT (CHOWTAGT) and a mutant AGT [P138 M/V139I/P140K (CHOMIK)] known to be resistant to BG were treated with BG and various alkylating agents. BG treatment alone dramatically decreased AGT activity in CHOWTAGT cells but resulted in no depletion in AGT activity in CHOMIK cells. In the absence of AGT, these cells are highly sensitive to the toxic and mutagenic effects of temozolomide and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and no further sensitization occurs in the presence of BG. In contrast, CHOWTAGT cells are resistant to temozolomide and BCNU, and treatment with BG resulted in a significantly higher cell killing and mutation frequency. CHOMIK cells were completely resistant to temozolomide or BCNU in the presence and absence of BG. Both cell killing and mutation frequency of 4-hydroperoxycyclophosphamide (4-HC) in CHO, CHOWTAGT, and CHOMIK cells were increased in the presence of BG. 4-HC generates two active metabolites, phosphoramide mustard (PM) and acrolein. BG had no effect on 4hydroperoxydidechlorocyclophosphamide (which generates acrolein and a nonalkylating form of PM) in CHO cells and CHOMIK cells, but enhancement of toxicity was observed with PM in both these cell lines. Therefore, we attribute the enhancement to the PM metabolite of 4-HC. Our results demonstrate that wild-type AGT plays an important role in protecting against the toxic and mutagenic effect of O6 alkylating agents and that a mutant AGT resistant to inactivation by BG effectively prevents BG-enhanced toxicity and mutagenicity induced by these agents. Expression of the AGT protein contributes to resistance of 4-HC. BG also enhances the toxicity of 4-HC and PM by a mechanism that may not involve the AGT repair protein.

Chronic daily low dose of 4-methyl-5-(2-pyrazinyl)-1,2-dithiole-3-thione (Oltipraz) in patients with previously resected colon polyps and first degree female relatives of breast cancer patients

The chemoprevention agent oltipraz, one of the most active chemopreventive compounds in preclinical studies, has been shown to induce glutathione-S-transferase (GST) activity in animals. Oltipraz was evaluated in a Phase I trial at daily oral doses of 20 mg (L1), 50 mg (L2), and 100 mg (L3) and twice weekly doses of 125 mg (L4) taken for 6 months with 6 patients entered at L1 and L2 and 7 patients entered at L3 and L4 (26 subjects: 19 females and 7 males). The subject population included patients with previously resected colon polyps and first-degree female relatives of breast cancer patients. Patients with resected colon polyps underwent rectal biopsy for GST and glutathione (GSH) analyses. Of the 26 subjects, the following completed 6 months of therapy: 4 of 6 patients (L1), 4 of 6 patients (L2), 5 of 7 patients (L3), and 4 of 7 patients (L4). Toxicities were mild to severe and included: gastrointestinal symptoms, photosensitivity/heat intolerance, and neurological symptoms. Monthly plasma samples were obtained 2-3 h after oltipraz ingestion with minimally detectable plasma concentrations at L1. There was a significant difference in mean oltipraz concentration across the four doses, with no significant differences in mean oltipraz concentration over time. Rectal tissue and lymphocyte GSH and GST were variable, with no significant difference in mean levels across doses. At the 100-mg/day dose (L3), 1 patient experienced significant increase in rectal tissue GSH and GST activity, whereas 3 additional patients (L1 and L4) had >50% increase in tissue GSH. Lymphocyte GSH level was significantly related to plasma oltipraz concentration. There were no significant correlations between plasma oltipraz concentration and lymphocyte GST level nor any significant correlation between plasma concentration and percentage of change in tissue GSH or GST. Further investigation of dose/schedule and biological end points is ongoing.

Pharmacokinetics of oral O6-benzylguanine and evidence of interaction with oral ketoconazole in the rat

PURPOSE

O6-Benzylguanine (BG) is a modulator of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT). BG is converted in mice, rats and humans to an equally active, yet longer-lived metabolite, O6-benzyl-8-oxoguanine (8-oxo-BG) by CYP1A2, CYP3A4 and aldehyde oxidase. Since intravenous BG is expected to enter phase I development with orally administered anticancer agents such as temozolomide, procarbazine or SarCNU, we determined the bioavailability of orally administered BG, as well as the effect of ketoconazole, a potent intestinal and hepatic CYP3A4 inhibitor, on the disposition of BG.

METHODS

Following intravenous or oral administration of BG in PEG-400/saline (40:60) to Sprague-Dawley rats, the pharmacokinetics of BG and 8-oxo-BG were determined. To determine the effect of CYP3A inhibition on disposition, oral BG was coadministered with ketoconazole.

RESULTS

The peak plasma concentration (Cmax), time to Cmax (tmax), and bioavailability (F) of oral BG were: 2.3 +/- 0.9 microg/ml, 2.3 +/- 0.6 h, and 65.5% respectively. The AUCs of BG and 8-oxo-BG were 13.1 +/- 4.6 microg x h/ml and 1.7 +/- 0.4 microg x h/ml after oral administration of BG. Coadministration with ketoconazole resulted in an increase in mean absorption time from 2.0 +/- 0.3 h to 6.0 +/- 0.9 h, a shift in tmax to 5 +/- 3.3 h, a decrease in Cmax to 0.96 +/- 0.8 microg/ml, and a decrease in AUC0-inf ratio of 8-oxo-BG:BG from about 0.12 to 0.04 (P < 0.05). The bioavailability of BG was not changed (65.5% vs 56.9%, P= 0.78).

CONCLUSIONS

The oral bioavailability of BG is high, warranting consideration of an oral formulation for clinical development. Coadministration of ketoconazole and BG resulted in delayed oral absorption and inhibition of conversion of BG to 8-oxo-BG in the rat model.

Plasma and cerebrospinal fluid pharmacokinetics of O6-benzylguanine and analogues in nonhuman primates

O6-Benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase, that enhances the sensitivity of tumor cell lines and tumor xenografts to chloroethylnitrosoureas. To search for BG analogues with greater penetration into the cerebrospinal fluid (CSF), we evaluated plasma and CSF pharmacokinetics of BG, 8-aza-O6-benzylguanine (8-azaBG), O6-benzyl-8-bromoguanine (8-BrBG), O6-benzyl-8-oxoguanine (8-oxoBG), O6-benzyl-8-trifluoromethylguanine (8-tfmBG), and O6-benzyl-2'-deoxyguanosine (B2dG) after i.v. administration of 200 mg/m2 of drug through an indwelling Ommaya reservoir in a nonhuman primate model. BG and its analogues were quantified in plasma and CSF using reverse-phase high-performance liquid chromatography assays. The plasma clearances of the four 8-substituted BG analogues were similar (0.04-0.06 l/h/kg), but half-lives ranged from <2 to >24 h. BG was converted to 8-oxoBG, an equally potent O6-alkylguanine-DNA alkyltransferase inactivator, and the elimination of 8-oxoBG was much slower than that of BG. As a result, the plasma area under the curve of 8-oxoBG was 3.5-fold greater than that of BG. B2dG was metabolized to BG and 8-oxoBG, but this pathway accounted for only 20% of B2dG elimination. The CSF penetration percentages (based on the ratio of AUC(CSF): AUCplasma) for BG, 8-azaBG, 8-oxoBG, 8-tfmBG, 8-BrBG, and B2dG were 3.2, 0.18, 4.1, 1.4, <0.3, and 2.0%, respectively. The CSF penetration of BG and its active metabolite 8-oxoBG is greater than the penetration of 8-azaBG, 8-BrBG, 8-tfmBG, and B2dG.

Phase I clinical and pharmacological study of O6-benzylguanine followed by carmustine in patients with advanced cancer

O6-benzylguanine (BG) is a potent inactivator of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) that enhances sensitivity to nitrosoureas in tumor cell lines and tumor-bearing animals. The major objectives of this study were to define the optimal modulatory dose and associated toxicities of benzylguanine administered alone and in combination with carmustine; to define the maximally tolerated dose and associated toxicities of carmustine administered with benzylguanine and to describe the pharmacokinetics of BG in humans and its effects on AGT depletion and recovery in peripheral blood mononuclear cells. Patients with histologically confirmed advanced solid tumors or lymphoma that had failed to respond to standard therapy or for which no standard therapy was available were eligible to participate in this study. Patients initially received BG as a 1-h i.v. infusion without carmustine. After a 14-day washout (ie., without therapy) period, patients received BG as a 1-h i.v. infusion followed, 1 h later, by a 15-min i.v. infusion of carmustine. Cycles of chemotherapy were repeated every 6 weeks. Cohorts of patients received BG doses ranging from 10 to 120 mg/m2 and carmustine doses ranging from 13 to 50 mg/m2. Plasma and urine samples were collected and analyzed for BG, and O6-benzyl-8-oxoguanine concentrations and AGT activity was determined in peripheral blood mononuclear cells. There was no toxicity attributable to BG alone at any dose tested. Bone marrow suppression was the primary and dose-limiting toxicity of BG combined with carmustine and was cumulative in some patients. The neutrophil nadir occurred at a median of day 27, with complete recovery in most patients by day 43. Nonhematological toxicity included fatigue, anorexia, increased bilirubin, and transaminase elevation. Recommended doses for Phase II testing are 120 mg/m2 BG given with carmustine at 40 mg/m2. BG rapidly disappeared from plasma and was converted to a major metabolite, O6-benzyl-8-oxoguanine, which has a 2.4-fold higher maximal concentration and 20-fold higher area under the concentration versus time curve than BG. AGT activity in peripheral blood mononuclear cells was rapidly and completely suppressed at all of the BG doses. The rate of AGT regeneration was more rapid for patients treated with the lowest dose of BG but was similar for BG doses ranging from 20-120 mg/m2. In conclusion, coadministration of BG and carmustine is feasible in cancer patients, but the maximal dose of carmustine that can be safely administered with BG is approximately one-third of the standard clinical dose. Bone marrow suppression, which may be cumulative, is the dose-limiting toxicity of the combination. Prolonged AGT suppression is likely attributable primarily to the effect of O6-benzyl-8-oxoguanine.

O6-benzylguanine-mediated enhancement of nitrosourea activity in Mer- central nervous system tumor xenografts--implications for clinical trials

PURPOSE

To evaluate the role of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) plus O6-benzylguanine (O6-BG) in the treatment of both Mer+ and Mer- tumors.

METHODS

The effect of pretreatment with O6-BG on the activity of BCNU against Mer- human central nervous tumor xenografts D-54 MG and D-245 MG was evaluated in athymic nude mice.

RESULTS

BCNU (1.0 LD10; dose lethal to 10% of treated animals) produced growth delays of 8.9 days and 7.5 days and tumor regressions in six of ten and one of nine animals against D-54 MG, which was derived from a human malignant glioma xenograft. Dose reduction of BCNU to 0.38 LD10 eliminated antitumor activity. The combination of BCNU (0.38 LD10) plus O6-BG produced growth delays of 8.8 days and 7.9 days, with tumor regressions in four of ten and two of nine animals, respectively. BCNU (1.0 LD10) produced a growth delay of 49.8 days and ten of ten tumor regressions against D-245 MG, which was derived from a glioblastoma multiforme. BCNU (0.38 LD10) produced a growth delay of 19.4 days, with nine of ten tumor regressions. The combination of BCNU (0.38 LD10) plus O6-BG produced a growth delay of 65.7 days and seven of eight tumor regressions.

CONCLUSION

These results suggest that the combination of BCNU plus O6-BG may be a rational intervention for both Mer+ as well as Mer- tumors.

Concomitant chemoradiotherapy as primary therapy for locoregionally advanced head and neck cancer

PURPOSE

To achieve locoregional control of head and neck cancer, survival, and organ preservation using intensive concomitant chemoradiotherapy.

PATIENTS AND METHODS

This study was a phase II trial of chemoradiotherapy with cisplatin 100 mg/m(2) every 28 days, infusional fluorouracil 800 mg/m(2)/d for 5 days, hydroxyurea 1 g orally every 12 hours for 11 doses, and radiotherapy twice daily at 1.5 Gy/fraction on days 1 through 5 (total dose, 15 Gy). Five days of treatment were followed by 9 days of rest, during which time patients received granulocyte colony-stimulating factor. Five cycles (three with cisplatin) were administered over 10 weeks (total radiotherapy dose, </= 75 Gy). Adjuvant chemoprevention with retinoic acid and interferon alfa-2A was offered.

RESULTS

Seventy-six patients were treated (stage IV, 93%; N2, 54%; N3, 21%). At a median follow-up of 38 months, the 3-year progression-free survival is 72%, locoregional control 92%, systemic control 83%, and overall survival 55%. Toxicities included mucositis (grade 3, 45%; grade 4, 12%), neutropenia (grade 4, 39%), and thrombocytopenia (grade 4, 53%). Surgery at the primary site was performed in 13 patients, and 39 had neck dissection. A majority of patients declined adjuvant chemoprevention. Pharmacokinetic parameters were not prognostic of tumor control. Quality of life declined during treatment but returned from good to excellent by 12 months after treatment.

CONCLUSION

Intensive concomitant chemoradiotherapy leads to high locoregional control and survival rates with organ preservation and a reversal of the historical pattern of failure (distant > locoregional). Surgery after concomitant chemoradiotherapy is feasible. Compliance with adjuvant chemoprevention is poor. Identification of less toxic regimens and improved distant disease control emerge as important future research goals.

Characterization of CPT-11 hydrolysis by human liver carboxylesterase isoforms hCE-1 and hCE-2

7-Ethyl-10-[4-(1-piperidino)-1-piperidino] carbonyloxy-camptothecin (irinotecan; CPT-11) is a prodrug activated by carboxylesterase enzymes. We characterized the hydrolysis of CPT-11 by two recently identified human carboxylesterase (hCE) enzymes, hCE-1 and hCE-2. Km and Vmax for hCE-1 and hCE-2 are 43 microM and 0.53 nmol/min/mg protein and 3.4 microM and 2.5 nmol/min/mg protein, respectively. hCE-2 has a 12.5-fold higher affinity for CPT-11 and a 5-fold higher maximal rate of CPT-11 hydrolysis when compared with hCE-1. In cytotoxicity assays, incubation of 1 microM CPT-11 with hCE-2 (3.6 microg/ml) resulted in a 60% reduction in survival of SQ20b cells. No significant reduction in cell survival was observed after incubation of CPT-11 with hCE-1. These data indicate that hCE-2 is a high-affinity, high-velocity enzyme with respect to CPT-11. hCE-2 likely plays a substantial role in CPT-11 activation in human liver at relevant pharmacological concentrations.

In vitro and in vivo activity of protein kinase C inhibitor chelerythrine chloride induces tumor cell toxicity and growth delay in vivo

Although clonogenic or divisional death is the main mechanism by which DNA-damaging agents demonstrate antitumor activity, recent data indicate that strategies specifically designed to trigger apoptosis may also prove to be useful antitumor agents. Protein kinase C (PKC) isoenzymes are involved in the regulation of cell proliferation, differentiation, and survival. Whereas pharmacological inhibition of PKC activity triggers apoptosis in most mammalian cells, cell line and tissue differences in sensitivities to these inhibitors remain. Whereas PKC inhibitors have potential as antitumor agents, issue of kinase specificity and solubility have remained obstacles to their clinical use. In this report, we investigated the antitumor activity of the PKC inhibitor chelerythrine chloride (chelerythrine), a selective inhibitor of group A and B PKC isoforms. Chelerythrine exhibited cytotoxic activity against nine human tumor cell lines tested in vitro. On the basis of the finding that radioresistant and chemoresistant squamous cell carcinoma lines (HNSCC) undergo apoptosis rapidly after treatment with chelerythrine in vitro, we assessed the effects of this agent on p53-deficient SQ-20B HNSCC cells in vivo. The results demonstrate that chelerythrine treatment of nude mice bearing SQ-20B is associated with significant tumor growth delay. Significantly, treatment with chelerythrine resulted in minimal toxicity. These findings demonstrate a potential for chelerythrine as an antitumor drug against squamous cell carcinoma.

Inhibition of O(6)-methylguanine-DNA methyltransferase increases azoxymethane-induced colonic tumors in rats

Azoxymethane (AOM) causes O(6)-methylguanine adduct formation which leads to G-->A transitions. Their repair is carried out by O(6)-methylguanine-DNA methyltransferase (MGMT). To evaluate the importance of this repair event in AOM-induced carcinogenesis, we examined the effect of O(6)-benzylguanine (BG), a potent inhibitor of MGMT, on colonic tumor development. Rats were treated weekly for 2 weeks at 0 and 24 h with BG (60 mg/kg body wt i.p.) or vehicle (40% polyethylene glycol, PEG-400), followed 2 h after the first dose of BG with AOM (15 mg/kg body wt) or vehicle (saline) i.p. Rats were killed 35 weeks later and tumors harvested and DNA extracted. In the AOM-treated groups, BG caused a significant increase in tumor incidence with tumors in 65.9%, versus 30.8% in the AOM/PEG-treated group (P < 0.05). In the BG/AOM group there was also a significant increase in tumor multiplicity, with 2.3 tumors/tumor-bearing rat, versus 1.6 tumors/tumor- bearing rat in the AOM/PEG group (P < 0.05). Since O(6)-methylguanine adducts can cause activating mutations in the K-ras and beta-catenin genes, we examined the effects of BG on these mutations. In the BG group there were seven mutations in codon 12 or 13 of exon 1 of the K-ras gene in 51 tumors examined, compared with no K-ras mutations in 17 tumors analyzed in the AOM/PEG group (P = 0.12). In the BG/AOM group there were 10 mutations in exon 3 of the beta-catenin gene among 48 tumors evaluated, compared with six mutations in 16 tumors analyzed in the PEG/AOM group (P = 0.16). In summary, MGMT inhibition increases AOM-induced colonic tumor incidence and multiplicity in rats.

Determination of 8-oxoguanine in human plasma and urine by high-performance liquid chromatography with electrochemical detection

A highly sensitive and selective method for determining 8-oxoguanine in plasma and urine was developed by high-performance liquid chromatography with electrochemical detection. The compound was separated by gradient elution on a C18 reversed-phase column with a mobile phase of acetonitrile and 0.1 M sodium acetate, pH 5.2. 8-Hydroxy-2'-deoxyguanosine was used as internal standard. 8-Oxoguanine was detected electrochemically by setting the potential to +300 mV vs. Pd reference. The sensitivity of the assay was 22 ng/ml with a signal-to-noise ratio of 7:1. The within-day relative standard deviations for 8-oxoguanine quality control samples with concentrations of 3340, 1340 and 84 ng/ml were 3.6, 4.3 and 5.7% for plasma, and 4.1, 4.6 and 6.2% for urine, respectively. The day-to-day relative standard deviations for the same samples were 3.8, 6.8 and 7.1% for plasma, and 3.9, 7.0 and 7.9% for urine, respectively. The method is designed to study the pharmacokinetics and metabolic fate of O6-benzylguanine in a phase I clinical trial. Previously, O6-benzyl-8-oxoguanine was identified as the primary metabolite of O6-benzylguanine in humans. We now demonstrate that 8-oxoguanine is a further metabolite of O6-benzylguanine.

Mechanisms of resistance to the toxicity of cyclophosphamide

Resistance to cyclophosphamide therapy continues to be a major reason for treatment failure. This chapter covers some of the mechanisms implicated in resistance to the toxic and mutagenic effects of cyclophosphamide therapy in the laboratory and clinic. Since resistance is likely to be the result of a number of interrelating factors, this chapter evaluates the contribution of both glutathione and DNA repair processes to cyclophosphamide resistance. Glutathione appears to be involved directly in the detoxification of cyclophosphamide and metabolites and may play a more indirect role in other processes. The ability of the cell to repair cyclophosphamide-induced DNA lesions, possibly through nucleotide excision repair or other processes, may be a key contributor to drug resistance. Interestingly, the presence of the repair enzyme, O6-alkylguanine-DNA alkyltransferase, long thought to be involved with resistance to methylating and chloroethylating agents, may also contribute to resistance to the cytotoxic and mutagenic effects of cyclophosphamide.

O6-alkylguanine-DNA alkyltransferase in cutaneous T-cell lymphoma: implications for treatment with alkylating agents

Mycosis fungoides is a low-grade cutaneous T-cell lymphoma. Early treatment often involves the use of topical chemotherapy such as mechlorethamine or carmustine although single-agent oral chemotherapy with alkylators is common for advanced disease. Recently, in a Phase I study of the new alkylating agent temozolomide, two mycosis fungoides patients experienced a complete response. The mechanism of resistance to alkylating drugs such as temozolomide is thought to be due to the presence in tumor cells of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT). The protein mediates a reaction with the O6-position of guanine in DNA, removing the lesion and leaving guanine intact. We, therefore, examined the levels of AGT in CD4+ T lymphocytes obtained by negative antibody selection from the blood of noncancerous individuals and mycosis fungoides patients, and in paraffin-embedded sections from mycosis fungoides patch, plaque, or tumor lesions and cells from involved lymph nodes. AGT protein levels were measured by quantitative immunofluorescence microscopy using a monoclonal antibody against human AGT. Using this approach, the mean level of our positive control (AGT-expressing cells) was 84,807 molecules/nucleus; values below 5,000 molecules/nucleus are considered very low. The mean AGT level in CD4+ T lymphocytes from noncancerous and cancerous individuals was 18,618 (n = 12) and 8,593 (n = 5), respectively. The mean fraction of outliers in circulating CD4+ T lymphocytes from mycosis fungoides patients was statistically significantly lower than T cells in lymph nodes. AGT molecules/nucleus from lymph node biopsies from 8 of 10 patients showed low (< 10,000 molecules/nucleus) or undetectable levels (n = 5) of AGT. The mean AGT level from samples of mycosis fungoides patch/plaque and tumor was also low at 221 (n = 4) and 2,363 (n = 6), respectively. Surprisingly, Hut78, a mycosis fungoides T-cell lymphoma cell line, was positive for AGT activity (median: 77,700 molecules/nucleus), and Hut102--another mycosis fungoides cell line--was low (median: 5,990 molecules/nucleus). Because AGT is a primary means of cell resistance to alkylating agents, the low level of AGT in neoplastic T lymphocytes from patients with mycosis fungoides suggests that treatment with alkylating agents producing O6-alkylguanine adducts, such as carmustine or temozolomide, may produce improved clinical outcomes.

Role of O6-alkylguanine-DNA alkyltransferase in protecting against cyclophosphamide-induced toxicity and mutagenicity

Cyclophosphamide is used to treat a wide range of human malignancies. However, it is also a known carcinogen associated with induction of therapy-related leukemia and bladder cancer. The DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), protects cells from the toxic and mutagenic effects of O6-alkylating agents. We report here the contribution of AGT in protecting against the toxic and mutagenic effects of cyclophosphamide. CHO cells transduced with wild-type human AGT (CHO(AGT)) and pcDNA3 (CHOpcDNA3) were treated with activated cyclophosphamide derivatives, 4-hydroperoxycyclophosphamide (4-HC), 4-hydroperoxydidechlorocyclophosphamide (4-HDC), a progenitor of acrolein, and phosphoramide mustard (PM). The results show that CHO(AGT) is 7- or 20-fold less sensitive to the toxic effects of 30 microM 4-HC or 300 microM 4-HDC, respectively, than CHOpcDNA3 cells as measured by cell survival using a colony-forming assay. CHO(AGT) cells treated with 20 microM 4-HC or 200 microM 4-HDC produced 4- or 7-fold lower mutation frequency as measured at the HPRT locus than CHOpcDNA3 cells treated with the same dose of drugs. At 30 microM acrolein, the cell survival for CHO(AGT) was 30% compared with 18.7% for CHOpcDNA3. The mutation frequency of acrolein at the same dose was 57 mutants/10(6) cells in CHOpcDNA3 compared with no mutants in CHO(AGT). In contrast, CHO(AGT) and CHOpcDNA3 cells treated with PM had similar survival curves and exhibited no difference in mutation frequency. The present study demonstrates that AGT plays an important role in protecting against the toxic and mutagenic effect of cyclophosphamide and suggests that acrolein, not PM, is responsible for generating the toxic and mutagenic lesion(s) protected by the AGT protein.

Phase I study of eniluracil, a dihydropyrimidine dehydrogenase inactivator, and oral 5-fluorouracil with radiation therapy in patients with recurrent or advanced head and neck cancer

5-Fluorouracil (5-FU) is an effective enhancer of radiation therapy (RT) in head and neck cancers. Due to rapid, predominantly hepatic metabolism by dihydropyrimidine dehydrogenase (DPD) and suggested clinical benefit from prolonged drug exposure, 5-FU is commonly given by continuous infusion. Eniluracil is a novel DPD-inactivator designed to prolong the half-life of 5-FU and provide sustained plasma concentrations of 5-FU with oral dosing. We conducted a Phase I study of the safety and efficacy of eniluracil given with oral 5-FU in patients receiving concurrent RT for recurrent or advanced squamous cell carcinomas of the head and neck. Thirteen patients with recurrent, metastatic, or high-risk (defined as an expected 2-year survival rate of <10%) head and neck cancer were enrolled and treated with concomitant chemoradiotherapy on an every-other-week schedule. Eniluracil at a fixed dose [20 mg twice a day (BID)] was given for 7 consecutive days (days 1-7). 5-FU and RT were given on 5 consecutive days (days 2-6). One patient was treated with once-daily RT (2.0 Gy fractions). The remaining patients received hyperfractionated RT (1.5-Gy fractions BID). The initial dose of 5-FU was 2.5 mg/m2 given BID. Dose escalation in patient cohorts was scheduled at 2.5-mg/m2 increments, with intrapatient dose escalation permitted. Lymphocyte DPD activity and serum 5-FU and uracil concentrations were monitored during two cycles. DPD activity was completely or nearly completely inactivated in all patients. Sustained, presumed therapeutic concentrations of 5-FU were observed at a dose of 5.0 mg/m2 given BID. Cumulative dose-limiting myelosuppression (both neutropenia and thrombocytopenia) was observed during the fourth and fifth cycles following administration of 5.0 mg/m2 5-FU BID. One patient died of neutropenic sepsis during cycle 4. Other late cycle toxicities included diarrhea, fatigue, and mucositis. Grade 3 mucositis was observed in 4 patients, but no grade 4 mucositis or grade 3 or 4 dermatitis was observed. A second patient death occurred during cycle 1 of treatment. No specific cause of death was identified. The study was subsequently discontinued. Cumulative myelosupression was the significant dose-limiting toxicity of oral 5-FU given with the DPD-inactivator eniluracil on an every-other-week schedule. Clinical radiation sensitization was not observed, based on the absence of dose-limiting mucositis and dermatitis. Alternative dosing schedules need to be examined to determine the most appropriate use of eniluracil and 5-FU as radiation enhancers.

Modulation of cyclophosphamide activity by O6-alkylguanine-DNA alkyltransferase

PURPOSE

The human medulloblastoma cell line D283 Med (4-HCR), a line resistant to 4-hydroperoxycyclophosphamide (4-HC), displays enhanced repair of DNA interstrand crosslinks induced by phosphoramide mustard. D283 Med (4-HCR) cells are cross-resistant to 1,3-bis(2-chloroethyl)- -nitrosourea, but partial sensitivity is restored after elevated levels of O6-alkylguanine-DNA alkyltransferase (AGT) are depleted by O6-benzylguanine (O6-BG). Studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) after AGT is depleted by O6-BG.

METHODS

Limiting dilution and xenograft studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide with or without O6-BG.

RESULTS

The activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) was increased after AGT depletion by O6-BG preincubation. Similar studies with Chinese hamster ovary cells, with or without stable transfection with a plasmid expressing the human AGT protein, revealed that the AGT-expressing cells were significantly less sensitive to 4-HC and 4-hydroperoxydidechlorocyclophosphamide. Reaction of DNA with 4-HC, phosphoramide mustard, or acrolein revealed that only 4-HC and acrolein caused a decrease in AGT levels.

CONCLUSIONS

We propose that a small but potentially significant part of the cellular toxicity of cyclophosphamide in these cells is due to acrolein, and that this toxicity is abrogated by removal of the acrolein adduct from DNA by AGT.

Lack of evidence for a polymorphism at codon 160 of human O6-alkylguanine-DNA alkyltransferase gene in normal tissue and cancer

O6-benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT), which enhances sensitivity to nitrosoureas in cells and tumor-bearing animals. BG is presently undergoing clinical trials for development as an agent to enhance the therapeutic index of alkylating agent chemotherapy. It has been reported that a polymorphism exists in the human agt gene, with about 15% of the Japanese population having arginine at codon 160 instead of glycine on the polypeptide (Y. Imai et al., Carcinogenesis, 16: 2441-2445, 1995). The resultant mutant AGT protein is equally effective against both methylated DNA as compared with wild type protein. However, this mutant AGT protein was less sensitive to inactivation by BG with a 20-fold increase in the ED50 value. This observation raised the possibility that a subpopulation of patients may be resistant to BG due to a single base change. We have demonstrated that this alteration also reduces the sensitivity to O6-benzyl-8-oxoguanine, an equally potent, yet much longer-lived human metabolite of BG. To test the possibility that this germ-line mutation of the agt gene might explain resistance to BG and O6-benzyl-8-oxoguanine of patients on our Phase I clinical trials, we evaluated the DNA from lymphocytes of 18 patients. The G160R mutation was not found in any of the 18 patients. To determine the frequency of this mutation in the United States population, DNA from 181 healthy individuals were investigated and, again, the mutation was not observed in this cohort. Therefore, if the mutation exists, it is in statistically <1.6% of the United States noncancerous population. To investigate the possibility that this mutation might be somatic, we evaluated genomic DNA samples from 94 human primary cancers of four different histological subtypes (brain, colon, esophageal, and head and neck). Again, none were found to have the G160R mutation.

Phase I trial of O6-benzylguanine for patients undergoing surgery for malignant glioma

PURPOSE

The major mechanism of resistance to alkylnitrosourea therapy is the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT), which removes chlorethylation or methylation damage from the O6-position of guanine. O6-benzylguanine (O6-BG) is an AGT substrate that inhibits AGT by suicide inactivation. We conducted a phase I trial to define the presurgical dose required for depletion of tumor AGT activity in patients with malignant glioma.

MATERIALS AND METHODS

Patients were to be treated 18 hours before craniotomy with intravenous doses that ranged between 40 and 100 mg/m2 given over 1 hour. Resected tumor was snap-frozen in liquid nitrogen and AGT activity analyzed by high-pressure liquid chromatography (HPLC). Up to 13 patients were treated at a specific dose of O6-BG, with a target end point of > or = 11 of 13 patients with undetectable tumor AGT levels (< 10 fmol/mg protein).

RESULTS

Thirty patients with malignant gliomas were enrolled, with 11 of 11 patients treated at 100 mg/m2 O6-BG demonstrating tumor AGT levels less than 10 fmol/mg protein. No toxicity was noted in any patient treated.

CONCLUSION

These results indicate that 100 mg/m2 of O6-BG can maintain tumor AGT levels less than 10 fmol/mg protein for at least 18 hours after treatment, a time interval in which bis(2-chloroethyl)nitrosourea (BCNU)-induced chloroethyl adducts are fully converted into interstrand cross-links. A 100-mg/m2 dose of O6-BG will be used in combination with BCNU in another phase I trial designed to determine the maximal-tolerated dose of BCNU.

Effects of 1,2-naphthoquinones on human tumor cell growth and lack of cross-resistance with other anticancer agents

The sensitivity of human tumor and rat prostate tumor cells to a series of naphthoquinones, including tricyclic compounds of the beta-lapachone and dunnione families as well as 4-alkoxy-1,2-naphthoquinones, was evaluated. To better understand the mechanism of cytotoxicity of 1,2-naphthoquinones, the roles of various resistance mechanisms including P-glycoprotein, multidrug resistant associated protein, glutathione (GSH) and related enzymes, altered topoisomerase activity, and overexpression of genes that control apoptosis (bcl-2 and bc-xL) were studied. MCF7 cells were most sensitive to the naphthoquinones with IC50 values ranging from 1.1 to 10.8 microM, as compared to 2.5 to >32 microM for HT29 human colon, A549 human lung, CEM leukemia and AT3.1 rat prostate cancer cells. MCF7 ADR cells, selected for resistance to adriamycin (ADR), displayed cross-resistance to the tricyclic 1,2-naphthoquinones. Drug efflux via a P-glycoprotein mechanism was ruled out as a mechanism of resistance to 1,2-naphthoquinones, since KB-V1 cells expressing high levels of P-glycoprotein and the KB-3.1 parent line were equally sensitive to these compounds. Any resistance of the tricyclic naphthoquinones noted in ADR-resistant cells appeared to relate to the GSH redox cycle and could be circumvented by exposure to buthionine sulfoximine or by changing the structure from a tricyclic derivative to a 4-alkoxy-1,2-naphthoquinone. The 1,2-naphthoquinones were found to be cytotoxic against CEM/VM-1 and CEM/M70-B1 cells that were selected for resistance to teniposide or merbarone, respectively. In addition, cells overexpressing bcl-2 or bcl-xL proteins were as sensitive to 1,2-naphthoquinones as were control cells. Because of their effectiveness in drug-resistant cells, these agents appear to hold promise as effective chemotherapeutic agents.

Ventilator-related Acinetobacter outbreak in an intensive care unit

An outbreak of 16 cases of ciprofloxacin-resistant Acinetobacter baumannii (calcoaceticus subspecies anitratus) infections occurred during a 7-month period in a medical intensive care unit. Fifteen of the patients developed pneumonia associated with ventilator support. Possible sources considered in the outbreak investigation were sinks, ice, personnel, patients on multiple antibiotic therapy, reusable ventilator circuits, and hemodialysis. The equipment and environment associated with the outbreak were cultured. Patients on ventilators were significantly more susceptible to Acinetobacter nosocomial infection compared with the rest of the patients in the medical intensive care unit (p < 0.05). Sputum cultures were only 5% sensitive to ciprofloxacin and gentamicin, but they were 100% sensitive to imipenem (p < 0.0001). Uncloaking imipenem was a significant contributing factor in controlling this outbreak. Once outbreak control measures were instituted, Acinetobacter isolates dropped from 77 (during the outbreak year) to 9 (during the subsequent year) and no new pneumonia cases occurred.

O6-alkylguanine-DNA alkyltransferase inactivation by ester prodrugs of O6-benzylguanine derivatives and their rate of hydrolysis by cellular esterases

To modulate the bioavailability and perhaps improve the tumor cell selectivity of O6-alkylguanine-DNA alkyltransferase (AGT) inactivators, pivaloyloxymethyl ester derivatives of O6-benzylguanine (BG) were synthesized and tested as AGT inactivators and as substrates for cellular esterases. The potential prodrugs examined were the 7- and 9-pivaloyloxymethyl derivatives of O6-benzylguanine (7- and 9-esterBG), and of 8-aza-O6-benzylguanine (8-aza-7-esterBG and 8-aza-9-esterBG) and the 9-pivaloyloxymethyl derivative of 8-bromo-O6-benzylguanine (8-bromo-9-esterBG). The benzylated purines were all potent inactivators of the pure AGT and of the AGT activity in HT29 cells and cell extracts. Each ester was at least 75 times less potent than the corresponding benzylated purine against the pure human AGT. In contrast, the activities of esters and their respective benzylated purine were similar in crude cell extracts and in intact cells. The increase in potency of esters in cellular extracts could be explained by a conversion of the respective prodrug to the more potent benzylated purine in the presence of cellular esterases. The apparent catalytic activity (Vmax/Km) of liver microsomal esterase for 8-azaBG ester prodrugs was 70-130 times greater than for BG prodrugs and 10-20 times greater than for 8-bromo-9-esterBG. Tumor cell hydrolysis of the esters varied considerably as a function of cell type and prodrug structure. These data suggest that these or related prodrugs may be advantageous for selective AGT inactivation in certain tumor types.

O6-benzylguanine in humans: metabolic, pharmacokinetic, and pharmacodynamic findings

PURPOSE

O6-Benzylguanine is a potent inactivator of the DNA-repair protein, O6-alkylguanine-DNA alkyl-transferase (AGT), that enhances sensitivity to nitrosoureas in tumor-cell lines and tumor-bearing animals. The objective of this study was to determine the pharmacokinetics and metabolic fate of O6-Benzylguanine in humans and its effect on AGT activity in peripheral-blood mononuclear cells (PBMCs).

PATIENTS AND METHODS

Twenty-five cancer patients were treated with O6-Benzylguanine at a dose level of 10, 20, 40, and 80 mg/m2 intravenously (IV) over 1 hour. Plasma and urine samples were collected and analyzed for O6-Benzylguanine and O6-Benzyl-8-oxoguanine concentrations. AGT activity in PBMCs was determined up to 2 weeks postinfusion.

RESULTS

There was no toxicity attributable to O6-Benzylguanine alone at all doses tested. O6-Benzylguanine rapidly disappeared from plasma and was converted to a major metabolite, O6-Benzyl-8-oxoguanine. The half-life of O6-Benzyl-8-oxoguanine increased with dose from 2.8 to 9.2 hours at doses of 10 and 80 mg/m2, respectively. The maximum concentration Cmax and area under the concentration-time curve (AUC) for O6-Benzyl-8-oxoguanine were, respectively, 2.2- and 12- to 29-fold greater than those of O6-Benzylguanine. At all doses, depletion of AGT activity was observed in lymphocytes with a return to baseline by 1 week posttreatment.

CONCLUSION

This study demonstrates that administration of O6-Benzylguanine to humans results in a rapid conversion to O6-Benzyl-8-oxoguanine, which follows nonlinear kinetics. Both compounds contribute to an effective depletion of AGT activity in lymphocytes; however, prolonged depletion of AGT activity is likely due primarily to the effect of O6-Benzyl-8-oxoguanine.