Mitochondrial Inhibitor Atovaquone Increases Tumor Oxygenation and Inhibits Hypoxic Gene Expression in Patients with Non-Small Cell Lung Cancer

PURPOSE

Tumor hypoxia fuels an aggressive tumor phenotype and confers resistance to anticancer treatments. We conducted a clinical trial to determine whether the antimalarial drug atovaquone, a known mitochondrial inhibitor, reduces hypoxia in non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

Patients with NSCLC scheduled for surgery were recruited sequentially into two cohorts: cohort 1 received oral atovaquone at the standard clinical dose of 750 mg twice daily, while cohort 2 did not. Primary imaging endpoint was change in tumor hypoxic volume (HV) measured by hypoxia PET-CT. Intercohort comparison of hypoxia gene expression signatures using RNA sequencing from resected tumors was performed.

RESULTS

Thirty patients were evaluable for hypoxia PET-CT analysis, 15 per cohort. Median treatment duration was 12 days. Eleven (73.3%) atovaquone-treated patients had meaningful HV reduction, with median change -28% [95% confidence interval (CI), -58.2 to -4.4]. In contrast, median change in untreated patients was +15.5% (95% CI, -6.5 to 35.5). Linear regression estimated the expected mean HV was 55% (95% CI, 24%-74%) lower in cohort 1 compared with cohort 2 (P = 0.004), adjusting for cohort, tumor volume, and baseline HV. A key pharmacodynamics endpoint was reduction in hypoxia-regulated genes, which were significantly downregulated in atovaquone-treated tumors. Data from multiple additional measures of tumor hypoxia and perfusion are presented. No atovaquone-related adverse events were reported.

CONCLUSIONS

This is the first clinical evidence that targeting tumor mitochondrial metabolism can reduce hypoxia and produce relevant antitumor effects at the mRNA level. Repurposing atovaquone for this purpose may improve treatment outcomes for NSCLC.

Development and pre-clinical testing of a novel hypoxia-activated KDAC inhibitor

Tumor hypoxia is associated with therapy resistance and poor patient prognosis. Hypoxia-activated prodrugs, designed to selectively target hypoxic cells while sparing normal tissue, represent a promising treatment strategy. We report the pre-clinical efficacy of 1-methyl-2-nitroimidazole panobinostat (NI-Pano, CH-03), a novel bioreductive version of the clinically used lysine deacetylase inhibitor, panobinostat. NI-Pano was stable in normoxic (21% O₂) conditions and underwent NADPH-CYP-mediated enzymatic bioreduction to release panobinostat in hypoxia (<0.1% O₂). Treatment of cells grown in both 2D and 3D with NI-Pano increased acetylation of histone H3 at lysine 9, induced apoptosis, and decreased clonogenic survival. Importantly, NI-Pano exhibited growth delay effects as a single agent in tumor xenografts. Pharmacokinetic analysis confirmed the presence of sub-micromolar concentrations of panobinostat in hypoxic mouse xenografts, but not in circulating plasma or kidneys. Together, our pre-clinical results provide a strong mechanistic rationale for the clinical development of NI-Pano for selective targeting of hypoxic tumors.

The effects of nitric oxide or oxygen on the stable products formed from the tyrosine phenoxyl radical

Tyrosine is a critical component of many proteins and can be the subject of oxidative posttranslational modifications. Furthermore, the oxidation of tyrosine residues to phenoxyl radicals, sometimes quite stable, is essential for some enzymatic functions. The lifetime and fate of tyrosine phenoxyl radicals in biological systems are largely driven by the availability and proximity of oxidants and reductants. Tyrosine phenoxyl radicals have extremely low reactivity with molecular oxygen whereas reactions with nitric oxide are diffusion controlled. This is in contrast to equivalent reactions with tryptophanyl and cysteinyl radicals where reactions with oxygen are much faster. Despite, the quite disparate apparent reactivity of tyrosine phenoxyl radicals with oxygen and nitric oxide being known, the products of the reactions are not well established. Changes in the levels from expected basal concentrations of stable products resulting from tyrosine phenoxyl radicals, for example naturally occurring 3,3'-dityrosine, 3-nitrotyrosine, and 3-hydroxytyrosine, can be indicative of oxidative and/or nitrosative stress. Using the radiolytic generation of specific oxidizing radicals to form tyrosine phenoxyl radicals in an aqueous solution at a known rate, we have compared the products in the absence and presence of nitric oxide or oxygen. Possible reactions of the phenoxyl radicals with oxygen remain unclear although we show evidence for a small decrease in the yield of dityrosine and loss of tyrosine in the presence of 20% oxygen. Low concentrations of nitric oxide in anoxic conditions react with tyrosine phenoxyl radicals, by what is most probably through the formation of an unstable intermediate, regenerating tyrosine and forming nitrite.

Targeting IGF Perturbs Global Replication through Ribonucleotide Reductase Dysfunction

Inhibition of IGF receptor (IGF1R) delays repair of radiation-induced DNA double-strand breaks (DSB), prompting us to investigate whether IGF1R influences endogenous DNA damage. Here we demonstrate that IGF1R inhibition generates endogenous DNA lesions protected by 53BP1 bodies, indicating under-replicated DNA. In cancer cells, inhibition or depletion of IGF1R delayed replication fork progression accompanied by activation of ATR-CHK1 signaling and the intra-S-phase checkpoint. This phenotype reflected unanticipated regulation of global replication by IGF1 mediated via AKT, MEK/ERK, and JUN to influence expression of ribonucleotide reductase (RNR) subunit RRM2. Consequently, inhibition or depletion of IGF1R downregulated RRM2, compromising RNR function and perturbing dNTP supply. The resulting delay in fork progression and hallmarks of replication stress were rescued by RRM2 overexpression, confirming RRM2 as the critical factor through which IGF1 regulates replication. Suspecting existence of a backup pathway protecting from toxic sequelae of replication stress, targeted compound screens in breast cancer cells identified synergy between IGF inhibition and ATM loss. Reciprocal screens of ATM-proficient/deficient fibroblasts identified an IGF1R inhibitor as the top hit. IGF inhibition selectively compromised growth of ATM-null cells and spheroids and caused regression of ATM-null xenografts. This synthetic-lethal effect reflected conversion of single-stranded lesions in IGF-inhibited cells into toxic DSBs upon ATM inhibition. Overall, these data implicate IGF1R in alleviating replication stress, and the reciprocal IGF:ATM codependence we identify provides an approach to exploit this effect in ATM-deficient cancers. SIGNIFICANCE: This study identifies regulation of ribonucleotide reductase function and dNTP supply by IGFs and demonstrates that IGF axis blockade induces replication stress and reciprocal codependence on ATM. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/2128/F1.large.jpg.

Results of a phase II clinical trial of 6-mercaptopurine (6MP) and methotrexate in patients with BRCA-defective tumours

BACKGROUND

Tumour cells with BRCA1/2 gene mutations demonstrate increased sensitivity to platinum and poly (ADP-ribose) polymerase (PARP) inhibitors. 6-mercaptopurine (6MP) was found to selectively kill BRCA-defective cells in a xenograft model as effectively as the PARP inhibitor AG014699, even after these cells acquired resistance to a PARP inhibitor or cisplatin.

METHODS

This phase II single-arm trial investigated the activity of 6MP 55-75 mg/m2 per day, and methotrexate 15-20 mg/m2 per week in advanced breast or platinum-resistant ovarian cancer patients with a BRCA1/2 germline mutation, who had progressed after ≥1 previous line of chemotherapy. The primary outcome was objective response including stable disease (SD) as an assessment of clinical benefit rate (CBR), at 8 weeks, by RECIST v1.1. Secondary outcomes included overall survival (OS) and progression-free survival (PFS).

RESULTS

In total, 67 evaluable patients were recruited; 55 ovarian and 11 breast cancer patients. In total, 21 patients had SD (31%), one had a partial response (1.5%); CBR was 33% at 8 weeks. In total, 12/67 patients (18%) had SD at 16 weeks. In total, five ovarian cancer patients had SD for over 200 days. Median OS was 10.3 months (95% CI 6.9-14.5), median PFS 1.9 months (1.7-2.8).

CONCLUSIONS

The overall activity of 6MP and methotrexate in these patients was low; however, there was a small group of patients who appeared to derive longer-term clinical benefit.

TRIAL REGISTRATION

NCT01432145 http://www.ClinicalTrials.gov.

Probing the limits of Q-tag bioconjugation of antibodies

Site-selective labelling of antibodies (Abs) can circumvent problems from heterogeneity of conventional conjugation. Here, we evaluate the industrially-applied chemoenzymatic 'Q-tag' strategy based on transglutaminase-mediated (TGase) amide-bond formation in the generation of 89Zr-radiolabelled antibody conjugates. We show that, despite previously suggested high regioselectivity of TGases, in the anti-Her2 Ab Herceptin™ more precise native MS indicates only 70-80% functionalization at the target site (Q298H), in competition with modification at other sites, such as Q3H critically close to the CDR1 region.

An essential role for dNTP homeostasis following CDK-induced replication stress

Replication stress is a common feature of cancer cells, and thus a potentially important therapeutic target. Here, we show that cyclin-dependent kinase (CDK)-induced replication stress, resulting from Wee1 inactivation, is synthetic lethal with mutations disrupting dNTP homeostasis in fission yeast. Wee1 inactivation leads to increased dNTP demand and replication stress through CDK-induced firing of dormant replication origins. Subsequent dNTP depletion leads to inefficient DNA replication, DNA damage and to genome instability. Cells respond to this replication stress by increasing dNTP supply through histone methyltransferase Set2-dependent MBF-induced expression of Cdc22, the catalytic subunit of ribonucleotide reductase (RNR). Disrupting dNTP synthesis following Wee1 inactivation, through abrogating Set2-dependent H3K36 tri-methylation or DNA integrity checkpoint inactivation results in critically low dNTP levels, replication collapse and cell death, which can be rescued by increasing dNTP levels. These findings support a 'dNTP supply and demand' model in which maintaining dNTP homeostasis is essential to prevent replication catastrophe in response to CDK-induced replication stress.

Focused Ultrasound Hyperthermia for Targeted Drug Release from Thermosensitive Liposomes: Results from a Phase I Trial

Purpose To demonstrate the feasibility and safety of using focused ultrasound planning models to determine the treatment parameters needed to deliver volumetric mild hyperthermia for targeted drug delivery without real-time thermometry. Materials and Methods This study was part of the Targeted Doxorubicin, or TARDOX, phase I prospective trial of focused ultrasound-mediated, hyperthermia-triggered drug delivery to solid liver tumors ( ClinicalTrials.gov identifier NCT02181075). Ten participants (age range, 49-68 years; average age, 60 years; four women) were treated from March 2015 to March 2017 by using a clinically approved focused ultrasound system to release doxorubicin from lyso-thermosensitive liposomes. Ultrasonic heating of target tumors (treated volume: 11-73 cm³ [mean ± standard deviation, 50 cm³ ± 26]) was monitored in six participants by using a minimally invasive temperature sensor; four participants were treated without real-time thermometry. For all participants, CT images were used with a patient-specific hyperthermia model to define focused ultrasound treatment plans. Feasibility was assessed by comparing model-prescribed focused ultrasound powers to those implemented for treatment. Safety was assessed by evaluating MR images and biopsy specimens for evidence of thermal ablation and monitoring adverse events. Results The mean difference between predicted and implemented treatment powers was -0.1 W ± 17.7 (n = 10). No evidence of focused ultrasound-related adverse effects, including thermal ablation, was found. Conclusion In this 10-participant study, the authors confirmed the feasibility of using focused ultrasound-mediated hyperthermia planning models to define treatment parameters that safely enabled targeted, noninvasive drug delivery to liver tumors while monitored with B-mode guidance and without real-time thermometry. Published under a CC BY 4.0 license. Online supplemental material is available for this article. See also the editorial by Dickey and Levi-Polyachenko in this issue.

A phase 1 study to assess the safety, tolerability, and pharmacokinetics of CXD101 in patients with advanced cancer

BACKGROUND

In the current study, the authors sought to determine the maximum tolerated dose (MTD) of the novel class 1 selective histone deacetylase inhibitor CXD101 in a dose escalation study in patients with advanced solid tumors or recurrent/refractory lymphoma.

METHODS

The authors escalated the dose of CXD101 from 1 mg twice daily orally for 5 days in a 21-day cycle (3+3 design).

RESULTS

A total of 39 patients were enrolled, 36 of whom received CXD101. Of the 30 patients in the escalation cohort, 29 were evaluable for determination of the dose-limiting toxicity (DLT). DLTs were noted at doses of 16 mg twice daily (1 of 6 patients), 20 mg twice daily (1 of 6 patients), and 24/25 mg twice daily (2 of 5 patients, both of whom developed neutropenic fever). The MTD was 20 mg twice daily, which achieved maximal plasma concentrations (±standard deviation) of 231±76 nM to 342±126 nM, which was within the biologically active range. Six patients received 20 mg twice daily in an expansion cohort. The most frequent adverse events were fatigue, nausea, and reversible cytopenia. Key grade 3 to 4 adverse events (according to Common Terminology Criteria for Adverse Events criteria [version 4.03]) included thrombocytopenia (11%), neutropenia (17%), and neutropenic fever (2%) across the 133 CXD101 cycles given. The toxicity profile was similar to that of licensing studies with other histone deacetylase inhibitors. In 22 evaluable patients receiving a dose of ≥16 mg twice daily (17 of whom had lymphoma and 5 of whom had solid tumors), 3 partial responses (2 in patients with classic Hodgkin lymphoma after allogenic stem cell transplantation and 1 in a patient with angioimmunoblastic T-cell lymphoma) and 1 complete response (in a patient with follicular lymphoma) were noted (overall response rate of 18%) in addition to 9 patients who achieved durable stable disease. Responses were noted predominantly among patients with lymphoma (tumor reduction noted in 63% of patients on standard computed tomography).

CONCLUSIONS

The MTD in the current study was found to be 20 mg twice daily. Encouraging and durable activity was observed in patients with Hodgkin lymphoma, T-cell lymphoma, and follicular lymphoma.

Safety and feasibility of ultrasound-triggered targeted drug delivery of doxorubicin from thermosensitive liposomes in liver tumours (TARDOX): a single-centre, open-label, phase 1 trial

BACKGROUND

Previous preclinical research has shown that extracorporeal devices can be used to enhance the delivery and distribution of systemically administered anticancer drugs, resulting in increased intratumoural concentrations. We aimed to assess the safety and feasibility of targeted release and enhanced delivery of doxorubicin to solid tumours from thermosensitive liposomes triggered by mild hyperthermia, induced non-invasively by focused ultrasound.

METHODS

We did an open-label, single-centre, phase 1 trial in a single UK hospital. Adult patients (aged ≥18 years) with unresectable and non-ablatable primary or secondary liver tumours of any histological subtype were considered for the study. Patients received a single intravenous infusion (50 mg/m²) of lyso-thermosensitive liposomal doxorubicin (LTLD), followed by extracorporeal focused ultrasound exposure of a single target liver tumour. The trial had two parts: in part I, patients had a real-time thermometry device implanted intratumourally, whereas patients in part II proceeded without thermometry and we used a patient-specific model to predict optimal exposure parameters. We assessed tumour biopsies obtained before and after focused ultrasound exposure for doxorubicin concentration and distribution. The primary endpoint was at least a doubling of total intratumoural doxorubicin concentration in at least half of the patients treated, on an intention-to-treat basis. This study is registered with ClinicalTrials.gov, number NCT02181075, and is now closed to recruitment.

FINDINGS

Between March 13, 2015, and March 27, 2017, ten patients were enrolled in the study (six patients in part I and four in part II), and received a dose of LTLD followed by focused ultrasound exposure. The treatment resulted in an average increase of 3·7 times in intratumoural biopsy doxorubicin concentrations, from an estimate of 2·34 μg/g (SD 0·93) immediately after drug infusion to 8·56 μg/g (5·69) after focused ultrasound. Increases of two to ten times were observed in seven (70%) of ten patients, satisfying the primary endpoint. Serious adverse events registered were expected grade 4 transient neutropenia in five patients and prolonged hospital stay due to unexpected grade 1 confusion in one patient. Grade 3-4 adverse events recorded were neutropenia (grade 3 in one patient and grade 4 in five patients), and grade 3 anaemia in one patient. No treatment-related deaths occurred.

INTERPRETATION

The combined treatment of LTLD and non-invasive focused ultrasound hyperthermia in this study seemed to be clinically feasible, safe, and able to enhance intratumoural drug delivery, providing targeted chemo-ablative response in human liver tumours that were refractory to standard chemotherapy.

FUNDING

Oxford Biomedical Research Centre, National Institute for Health Research.

A first-in-human phase I study to determine the maximum tolerated dose of the oral Src/ABL inhibitor AZD0424

BACKGROUND

Src is involved in cancer invasion and metastasis. AZD0424, an oral inhibitor of Src and ABL1, has shown evidence of anti-tumour activity in pre-clinical studies.

METHODS

A phase Ia, dose escalation study was performed to assess the safety of continuous oral dosing with AZD0424 in advanced solid tumours. Secondary objectives included investigation of AZD0424 pharmacokinetics, effect on Src activity using markers of bone turnover, and anti-tumour activity.

RESULTS

41 patients were treated; 34 received AZD0424 once-daily at doses ranging from 5 mg to 150 mg, and 7 received 40 mg bi-daily 41.5% of patients experienced at least one AZD0424-related adverse event that was Grade 3-5 in severity, with patients treated at doses above 60 mg per day experiencing multiple treatment-related toxicities. The most commonly observed AZD0424-related adverse events were nausea, fatigue, anorexia and alopecia. Cmax and AUC increased linearly with dose and the mean±standard deviation t1/2 was 8.4±2.8 h. Clear evidence of Src target inhibition was seen at doses ⩾20 mg per day. No responses were observed and 7 patients (17.1%) achieved stable disease lasting 6 weeks or more.

CONCLUSIONS

AZD0424 displayed no evidence of efficacy as monotherapy despite a clear pharmacodynamic effect. Further evaluation of AZD0424 monotherapy in patients with solid tumours is not recommended.

CYP450 Enzymes Effect Oxygen-Dependent Reduction of Azide-Based Fluorogenic Dyes

Azide-containing compounds have broad utility in organic synthesis and chemical biology. Their use as powerful tools for the labeling of biological systems in vitro has enabled insights into complex cellular functions. To date, fluorogenic azide-containing compounds have primarily been employed in the context of click chemistry and as sensitive functionalities for hydrogen sulfide detection. Here, we report an alternative use of this functionality: as fluorogenic probes for the detection of depleted oxygen levels (hypoxia). Oxygen is imperative to all life forms, and probes that enable quantification of oxygen tension are of high utility in many areas of biology. Here we demonstrate the ability of an azide-based dye to image hypoxia in a range of human cancer cell lines. We have found that cytochrome P450 enzymes are able to reduce these probes in an oxygen-dependent manner, while hydrogen sulfide does not play an important role in their reduction. These data indicate that the azide group is a new bioreductive functionality that can be employed in prodrugs and dyes. We have uncovered a novel mechanism for the cellular reduction of azides, which has implications for the use of click chemistry in hypoxia.

The anti-malarial atovaquone increases radiosensitivity by alleviating tumour hypoxia

Tumour hypoxia renders cancer cells resistant to cancer therapy, resulting in markedly worse clinical outcomes. To find clinical candidate compounds that reduce hypoxia in tumours, we conduct a high-throughput screen for oxygen consumption rate (OCR) reduction and identify a number of drugs with this property. For this study we focus on the anti-malarial, atovaquone. Atovaquone rapidly decreases the OCR by more than 80% in a wide range of cancer cell lines at pharmacological concentrations. In addition, atovaquone eradicates hypoxia in FaDu, HCT116 and H1299 spheroids. Similarly, it reduces hypoxia in FaDu and HCT116 xenografts in nude mice, and causes a significant tumour growth delay when combined with radiation. Atovaquone is a ubiquinone analogue, and decreases the OCR by inhibiting mitochondrial complex III. We are now undertaking clinical studies to assess whether atovaquone reduces tumour hypoxia in patients, thereby increasing the efficacy of radiotherapy.

Identification of vitamin B1 metabolism as a tumor-specific radiosensitizing pathway using a high-throughput colony formation screen

Colony formation is the gold standard assay for determining reproductive cell death after radiation treatment, since effects on proliferation often do not reflect survival. We have developed a high-throughput radiosensitivity screening method based on clonogenicity and screened a siRNA library against kinases. Thiamine pyrophosphokinase-1 (TPK1), a key component of Vitamin B1/thiamine metabolism, was identified as a target for radiosensitization. TPK1 knockdown caused significant radiosensitization in cancer but not normal tissue cell lines. Other means of blocking this pathway, knockdown of thiamine transporter-1 (THTR1) or treatment with the thiamine analogue pyrithiamine hydrobromide (PyrH) caused significant tumor specific radiosensitization. There was persistent DNA damage in cells irradiated after TPK1 and THTR1 knockdown or PyrH treatment. Thus this screen allowed the identification of thiamine metabolism as a novel radiosensitization target that affects DNA repair. Short-term modulation of thiamine metabolism could be a clinically exploitable strategy to achieve tumor specific radiosensitization.

Modification of DNA damage mechanisms by nitric oxide during ionizing radiation

Nitric oxide ((•)NO) is a very effective radiosensitizer of hypoxic mammalian cells. In vivo (•)NO may have effects on tumor vasculature and hence on tumor oxygenation and it may also interact with radiation-produced radicals to modify DNA lesions. Few studies have addressed this last aspect, and we report here specific base modifications that result from reaction of (•)NO with radicals in DNA bases and in plasmid DNA after irradiation. 2'-Deoxyxanthosine monophosphate and 2'-deoxy-8-azaguanosine monophosphate (8azadGMP) are formed upon γ-irradiation of 2'-deoxyguanosine monophosphate (dGMP) in the presence of micromolar levels of (•)NO in anoxia. In addition, the presence of (•)NO at physiological pH inhibits the formation of the well-described (•)OH-induced oxidation product of dGMP, 8-oxo-2'-deoxyguanosine monophosphate. Single-strand breaks are induced in plasmid DNA when γ-irradiated in anoxia, whereas in the presence of (•)NO the number of breaks is reduced by approximately threefold, and evidence is shown for the formation of 8azadGMP in these plasmids. The consequence of the base modifications by (•)NO are as yet unknown although additional breaks are revealed in irradiated plasmid DNA after treatment with glycosylases involved in base excision repair. V79-4 cells irradiated in anoxia show an enhancement in the number of γH2AX foci when (•)NO is present, particularly evident a few hours postirradiation, indicative of the formation of replication-induced DNA damage. We propose that the consequence of (•)NO-induced base modifications in anoxia contributes to its radiosensitization of cells.

DNA damage induced by nitric oxide during ionizing radiation is enhanced at replication

Nitric oxide (NO) is a very effective radiosensitizer of hypoxic mammalian cells, at least as efficient as oxygen in enhancing cell death in vitro. NO may induce cell death through the formation of base lesions which are difficult to repair, and if they occur within complex clustered damage common to ionizing radiation, they may lead to replication-induced DNA strand breaks. It has previously been shown that 8-azaguanine and xanthine result from the reaction of guanine radicals with nitric oxide. We have now shown that adenine radicals also react with NO to form hypoxanthine and 8-azaadenine. Cells irradiated in exponential growth in the presence of NO are twice as radiosensitive compared to those irradiated in anoxia alone, whereas confluent cells are less radiosensitive to (•)NO. In addition, the numbers of DNA double strand breaks observed as γH2AX staining following radiosensitization by NO, are higher in exponential cells than in confluent cells. DNA damage, detected as 53BP1 foci, is also higher in HF-19 cells expressing Cyclin A, a marker for cells in S and G2 phases of the cell cycle, following radiosensitization by NO. RAD51 foci are highest in V79-4 cells irradiated in the presence of NO compared to in anoxia, 24h after radiolysis. This work presents evidence that radiosensitization of cells by NO is in part through the formation of specific DNA damage, difficult to repair, which in dividing cells may induce the formation of stalled replication forks and as a consequence replication-induced DNA strand breaks which may lead to cell death.

Reactivity of hydrogen sulfide with peroxynitrite and other oxidants of biological interest

Hydrogen sulfide (H(2)S) is an endogenously generated gas that can also be administered exogenously. It modulates physiological functions and has reported cytoprotective effects. To evaluate a possible antioxidant role, we investigated the reactivity of hydrogen sulfide with several one- and two-electron oxidants. The rate constant of the direct reaction with peroxynitrite was (4.8±1.4)×10(3)M(-1) s(-1) (pH 7.4, 37°C). At low hydrogen sulfide concentrations, oxidation by peroxynitrite led to oxygen consumption, consistent with a one-electron oxidation that initiated a radical chain reaction. Accordingly, pulse radiolysis studies indicated that hydrogen sulfide reacted with nitrogen dioxide at (3.0±0.3)×10(6)M(-1) s(-1) at pH 6 and (1.2±0.1)×10(7)M(-1) s(-1) at pH 7.5 (25°C). The reactions of hydrogen sulfide with hydrogen peroxide, hypochlorite, and taurine chloramine had rate constants of 0.73±0.03, (8±3)×10(7), and 303±27M(-1) s(-1), respectively (pH 7.4, 37°C). The reactivity of hydrogen sulfide was compared to that of low-molecular-weight thiols such as cysteine and glutathione. Considering the low tissue concentrations of endogenous hydrogen sulfide, direct reactions with oxidants probably cannot completely account for its protective effects.

Break-induced ATR and Ddb1-Cul4(Cdt)² ubiquitin ligase-dependent nucleotide synthesis promotes homologous recombination repair in fission yeast

Nucleotide synthesis is a universal response to DNA damage, but how this response facilitates DNA repair and cell survival is unclear. Here we establish a role for DNA damage-induced nucleotide synthesis in homologous recombination (HR) repair in fission yeast. Using a genetic screen, we found the Ddb1-Cul4(Cdt)² ubiquitin ligase complex and ribonucleotide reductase (RNR) to be required for HR repair of a DNA double-strand break (DSB). The Ddb1-Cul4(Cdt)² ubiquitin ligase complex is required for degradation of Spd1, an inhibitor of RNR in fission yeast. Accordingly, deleting spd1(+) suppressed the DNA damage sensitivity and the reduced HR efficiency associated with loss of ddb1(+) or cdt2(+). Furthermore, we demonstrate a role for nucleotide synthesis in postsynaptic gap filling of resected ssDNA ends during HR repair. Finally, we define a role for Rad3 (ATR) in nucleotide synthesis and HR through increasing Cdt2 nuclear levels in response to DNA damage. Our findings support a model in which break-induced Rad3 and Ddb1-Cul4(Cdt)² ubiquitin ligase-dependent Spd1 degradation and RNR activation promotes postsynaptic ssDNA gap filling during HR repair.

Effects of acute versus chronic hypoxia on DNA damage responses and genomic instability

Questions exist concerning the effects of acute versus chronic hypoxic conditions on DNA replication and genomic stability that may influence tumorigenesis. Severe hypoxia causes replication arrest independent of S-phase checkpoint, DNA damage response, or transformation status. Arrests occur during both the initiation and elongation phases of DNA replication, correlated with a rapid decrease in available deoxynucleotide triphosphates. With fluctuating oxygen tensions in tumors, arrested hypoxic cells may undergo rapid reperfusion and reoxygenation that leads to reoxygenation-induced DNA damage. In cells subjected to chronic hypoxia, we found that replicative restart was inhibited along with numerous replication factors, including MCM6 and RPA, the latter of which limits the hypoxia-induced DNA damage response. In contrast, in cells where replicative restart occurred, it was accompanied by extensive reoxygenation-induced DNA damage and compromised DNA repair. We found that cells reoxygenated after acute hypoxia underwent rapid p53-dependent apoptosis. Our findings suggest that cells lacking functional p53 are more susceptible to genomic instability and potentially tumorigenesis if they experience reoxygenation after acute exposure to hypoxia.

A phase I trial of radioimmunotherapy with 131I-A5B7 anti-CEA antibody in combination with combretastatin-A4-phosphate in advanced gastrointestinal carcinomas

PURPOSE

In preclinical models, radioimmunotherapy with (131)I-A5B7 anti-carcinoembryonic antigen (CEA) antibody ((131)I-A5B7) combined with the vascular disruptive agent combretastatin-A4-phosphate (CA4P) produced cures unlike either agent alone. We conducted a phase I trial determining the dose-limiting toxicity (DLT), maximum tolerated dose, efficacy, and mechanism of this combination in patients with gastrointestinal adenocarcinomas.

EXPERIMENTAL DESIGN

Patients had CEA of 10 to 1,000 microg/L, QTc < or =450 ms, no cardiac arrhythmia/ischaemia, and adequate hematology/biochemistry. Tumor was suitable for blood flow analysis by dynamic contrast enhanced-magnetic resonance imaging (MRI). The starting dose was 1,800 MBq/m(2) of (131)I-A5B7 on day 1 and 45 mg/m(2) CA4P given 48 and 72 hours post-(131)I-A5B7, then weekly for up to seven weeks.

RESULTS

Twelve patients were treated, with mean age of 63 years (range, 32-77). Two of six patients at the first dose level had DLTs (grade 4 neutropenia). The dose was reduced to 1,600 MBq/m(2), and CA4P escalated to 54 mg/m(2). Again, two of six patients had DLTs (neutropenia). Of ten assessable patients, three had stable disease and seven had progressive disease. Single-photon emission computed tomography confirmed tumor antibody uptake in all 10 patients. DCE-MRI confirmed falls in kinetic parameters (K(trans)/IAUGC(60)) in 9 of 12 patients. The change of both pharmacokinetic parameters reached a level expected to produce efficacy in one patient who had a minor response on computed tomography and a reduced serum tumor marker level.

CONCLUSIONS

This is believed to be the first trial reporting the combination of radioimmunotherapy and vascular disruptive agent; each component was shown to function, and myelosuppression was dose-limiting. Optimal dose and timing of CA4P, and moderate improvements in the performance of radioimmunotherapy seem necessary for efficacy.

Kinetics of reaction of nitrogen dioxide with dihydrorhodamine and the reaction of the dihydrorhodamine radical with oxygen: implications for quantifying peroxynitrite formation in cells

Dihydrorhodamine 123 (RhH2) has been used to detect 'reactive nitrogen species', including peroxynitrite and its radical decomposition products, peroxynitrite probably oxidizing RhH2 to rhodamine (Rh) via radical products rather than directly. In this study, the radical intermediate (RhH(.)) was generated by pulse radiolysis, and shown to react with oxygen with a rate constant k approximately 7 x 10(8) M(-1) s(-1). This fast reaction was exploited in experiments observing Rh being formed slowly (k approximately 4-7 x 10(5) M(-1) s(-1)) from oxidation of RhH2 by nitrogen dioxide in a rate-limiting step, >1000-fold slower than the corresponding oxidation by carbonate radicals. The time-dependent uptake of RhH2 into mammalian cells was measured, with average intracellular levels reaching only approximately 10 microM with the protocol used. The combination of low loading and relatively low reactivity of oxidants towards RhH2 compared to competing cellular nucleophiles suggests rather a small fraction of peroxynitrite-derived radicals (mainly CO3(.-)) may be scavenged intracellularly by RhH2.

Thiyl radicals react with nitric oxide to form S-nitrosothiols with rate constants near the diffusion-controlled limit

A possible route to S-nitrosothiols in biology is the reaction between thiyl radicals and nitric oxide. D. Hofstetter et al. (Biochem. Biophys. Res. Commun.360:146-148; 2007) claimed an upper limit of (2.8+/-0.6)x10(7) M(-1)s(-1) for the rate constant between thiyl radicals derived from glutathione and nitric oxide, and it was suggested that under physiological conditions S-nitrosation via this route is negligible. In the present study, thiyl radicals were generated by pulse radiolysis, and the rate constants of their reactions with nitric oxide were determined by kinetic competition with the oxidizable dyes 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) and a phenothiazine. The rate constants for the reaction of nitric oxide with thiyl radicals derived from glutathione, cysteine, and penicillamine were all in the range (2-3) x10(9) M(-1)s(-1), two orders of magnitude higher than the previously reported estimate in the case of glutathione. Absorbance changes on reaction of thiyl radicals with nitric oxide were consistent with such high reactivity and showed the formation of S-nitrosothiols, which was also confirmed in the case of glutathione by HPLC/MS. These rate constants imply that formation of S-nitrosothiols in biological systems from the combination of thiyl radicals with nitric oxide is much more likely than claimed by Hofstetter et al.

Radiosensitization by nitric oxide at low radiation doses

Nitric oxide was shown to radiosensitize anoxic V79 and CHO hamster cells and MCF7 and UT-SCC-14 human cells, measuring clonogenic survival and/or DNA damage in vitro at low radiation doses (0.1-5 Gy). Radiosensitization was easily detected after 2 Gy in anoxic V79 cells exposed to 40 ppm ( approximately 70 nM) nitric oxide, indicating that nitric oxide is a significantly more efficient radiosensitizer than oxygen. The yield of double-strand breaks (as gamma-H2AX foci) in V79 and MCF7 cells was doubled by irradiation in 1% v/v nitric oxide/N(2), and there was a longer repair time in cells irradiated in nitric oxide than in air or anoxia; single-strand breaks ("comet" assay) also appeared to be enhanced. Potent radiosensitization by nitric oxide is consistent with near diffusion-controlled reaction of nitric oxide with purine and pyrimidine radicals observed by pulse radiolysis, with nitric oxide reacting two to three times faster than oxygen with the 5-hydroxy-uracil-6-yl radical. Stable NO/base adducts were formed with uracil radicals. Effects on the radiosensitivity of cells exposed to as low as 40 ppm v/v nitric oxide after doses of 1-2 Gy suggest that variations in radiosensitivity in individual patients after radiotherapy might include a component reflecting differing levels of nitric oxide in tumors.

Effect of nitric-oxide synthesis on tumour blood volume and vascular activity: a phase I study

BACKGROUND

Nitric oxide has been implicated in tumour angiogenesis and in the maintaining of vasodilator tone of tumour blood vessels. The tumour vascular effects of inhibition of nitric-oxide synthesis have not been investigated in patients with cancer.

METHODS

Seven women and 11 men (12 with non-small-cell lung cancer, five prostate cancer, and one cervical cancer) were recruited onto a phase I dose-escalation study and received a single dose of the nitric oxide synthase inhibitor, N-nitro-L-arginine (L-NNA). Dose escalation was done by a modified Fibonacci scale with three patients at each dose level, starting with 0.1 mg/kg. Changes in dynamic contrast-enhanced CT measures of tumour relative blood volume and transfer constant (K) were measured at 1 h and 24 h after L-NNA administration.

FINDINGS

In the 18 patients, toxic effects were self-limiting cardiovascular changes: three patients had Common Toxicity Criteria version 2.0 grade 1 hypertension; two had grade 1 sinus bradycardia; and one had grade 1 palpitation. L-NNA area under the curve (AUC) increased linearly with dose from 163 micromol min(-1) L(-1) at 0.1 mg/kg L-NNA to 2150 micromol min(-1) L(-1) at 0.9 mg/kg L-NNA. In eight patients that underwent dynamic CT scanning, tumour blood volume decreased 1 h after L-NNA treatment (mean 42.9% [range 12.0-62.1]; paired t test p=0.0070), which was sustained for up to 24 h (mean 33.9% [range 6.5-64.9]; p=0.035). This decrease in blood volume was associated with an increase in the number of non-perfused pixels from 7.3% (SD 5.5) at baseline to 25.1% (15.3; p=0.0089) at 1 h, and 18.2% (12.9; p=0.050) at 24 h. There was a significant correlation between L-NNA plasma AUC and the reduction in tumour blood volume at 24 h after L-NNA (r=0.83; p=0.010).

INTERPRETATION

We have shown in vivo in patients with cancer that nitric oxide has a role in maintaining tumour blood supply, and we provide early clinical evidence that inhibition of nitric-oxide synthesis has tumour antivascular activity.

Kinetics of the reaction between nitric oxide and glutathione: implications for thiol depletion in cells

Nitric oxide in the absence of oxygen was suggested to react with 5-50 mM glutathione (GSH) over many minutes when [NO*] << [GSH] (N. Hogg et al., FEBS Lett. 382:223-228; 1996). However, Aravindakumar et al. (J. Chem. Soc. Perkin Trans. 2:663-669; 2002) provided data suggesting approximately 200-fold higher reactivity under conditions of [NO*] >> [GSH]. To help resolve these differences, the rate of loss of NO* ( approximately 9 microM) in aqueous solutions of GSH (2.5-20 mM) was measured by chemiluminescence. An apparent second-order rate constant of 0.080 +/- 0.008 M(-1) s(-1) at pH 7.4, 37 degrees C, was calculated based on the total [GSH] and "pseudo-first-order" kinetics; thiolate anion was much more reactive than undissociated thiol. These data imply a half-life of approximately 30 min for low concentrations of NO* with 5 mM GSH, 37 degrees C, pH 7.4, in the absence of oxygen. Possible kinetic schemes that can partially explain the divergent literature reports are discussed, notably an equilibrium in the reaction between NO* and GSH. Human breast carcinoma MCF-7 cells were exposed to NO* (initially approximately 18 microM) in alidded six well plate in an anaerobic chamber in vitro; intracellular GSH levels decreased by half in approximately 60 min. Aerobic exposure depletes GSH in cells in vitro much faster because of autoxidation of NO* to NO2*, >10(8) times more reactive toward GSH.

Phase I clinical trial of weekly combretastatin A4 phosphate: clinical and pharmacokinetic results

PURPOSE

A phase I trial was performed with combretastatin A4 phosphate (CA4P), a novel tubulin-binding agent that has been shown to rapidly reduce blood flow in animal tumors.

PATIENTS AND METHODS

The drug was delivered by a 10-minute weekly infusion for 3 weeks followed by a week gap, with intrapatient dose escalation. Dose escalation was accomplished by doubling until grade 2 toxicity was seen. The starting dose was 5 mg/m2.

RESULTS

Thirty-four patients received 167 infusions. CA4P was rapidly converted to the active combretastatin A4 (CA4), which was further metabolized to the glucuronide. CA4 area under the curve (AUC) increased from 0.169 at 5 mg/m2 to 3.29 micromol * h/L at 114 mg/m2. The mean CA4 AUC in eight patients at 68 mg/m2 was 2.33 micromol * h/L compared with 5.8 micromol * h/L at 25 mg/kg (the lowest effective dose) in the mouse. The only toxicity that possibly was related to the drug dose up to 40 mg/m2 was tumor pain. Dose-limiting toxicity was reversible ataxia at 114 mg/m2, vasovagal syncope and motor neuropathy at 88 mg/m2, and fatal ischemia in previously irradiated bowel at 52 mg/m2. Other drug-related grade 2 or higher toxicities seen in more than one patient were pain, lymphopenia, fatigue, anemia, diarrhea, hypertension, hypotension, vomiting, visual disturbance, and dyspnea. One patient at 68 mg/m2 had improvement in liver metastases of adrenocortical carcinoma.

CONCLUSION

CA4P was well tolerated in 14 of 16 patients at 52 or 68 mg/m2; these are doses at which tumor blood flow reduction has been recorded.

Enhancing the efficacy of photodynamic cancer therapy by radicals from plant auxin (indole-3-acetic acid)

Indole-3-acetic acid (plant auxin) has low toxicity but dramatically enhances the killing of mammalian cells on illuminating phenothiazinium dyes with red light. Suitable dyes include toluidine blue, used in cancer diagnosis because of localization in tumors, and methylene blue, used in experimental photodynamic therapy of cancer. The photosensitized oxidation of indole acetic acid forms a free radical that fragments in microseconds, forming reactive cytotoxins. Unlike conventional photodynamic therapy, requiring excitation of oxygen to the reactive singlet state, the treatment is effective even at the low oxygen levels common in tumors and with much lower light doses than normally used.

Halogenated indole-3-acetic acids as oxidatively activated prodrugs with potential for targeted cancer therapy

Substituted indole-3-acetic acid (IAA) derivatives, plant auxins with potential for use as prodrugs in enzyme-prodrug directed cancer therapies, were oxidised with horseradish peroxidase (HRP) and toxicity against V79 Chinese hamster lung fibroblasts was determined. Rate constants for oxidation by HRP compound I were also measured. Halogenated IAAs were found to be the most cytotoxic, with typical surviving fractions of <10(-3) after incubation for 2h with 100 microM prodrug and HRP.

Reactivity toward thiols and cytotoxicity of 3-methylene-2-oxindoles, cytotoxins from indole-3-acetic acids, on activation by peroxidases

Oxidation of indole-3-acetic acid and its derivatives by peroxidases such as that from horseradish produces many products, including 3-methylene-2-oxindoles. These have long been associated with biological activity, but their reactivity has not been characterized. We have previously demonstrated the potential value of substituted indole acetic acids and horseradish peroxidase as the basis for targeted cancer therapy, since the compounds are of low cytotoxicity until oxidized, when high cytotoxicity is observed; the combination of prodrug and enzyme depletes intracellular thiols. In this study, 3-methylene-2-oxindole and derivatives substituted in the 4-, 5-, or 6-position with methyl, F, or Cl have been synthesized and their reactivity toward representative thiol nucleophiles (glutathione, cysteine, and a cysteinyl peptide) measured using stopped-flow kinetic spectrophotometry. Rate constants were in the range approximately 2 x 10(3) to 2 x 10(4) M(-)(1) s(-)(1) at pH 7.4, 25 degrees C, implying a lifetime of a few tens of milliseconds for these methylene oxindoles in the cellular environment and diffusion distances of a few micrometers. As expected, halogen substitution decreased the rate of production of the methylene oxindoles on treatment of horseradish peroxidase. The cytotoxicities of the compounds were measured using Chinese hamster V79 fibroblast-like cells in vitro. The halogen-substituted derivatives were much more cytotoxic than the 5-methyl analogue or the parent (unsubstituted) compound, consistent with the trends in rate constant for reaction with the thiols. The results show that the cytotoxic response in the prodrug (indole acetic acid) and enzyme (horseradish peroxidase) system reflects the reactivity of methylene oxindoles toward nucleophiles much more than the rate of generation of the oxindoles, and helps explain the possible advantages of 5-fluoroindole-3-acetic acid compared to IAA as a lead compound for investigation in targeted cancer therapy.

Pharmacology and toxicity of nicotinamide combined with domperidone during fractionated radiotherapy

BACKGROUND AND PURPOSE

Treatment of head and neck tumors by the ARCON regimen has yielded high local control rates. As a result of this treatment intensification there was some increase in mainly acute toxicity of radiotherapy, but nicotinamide by itself has specific side effects such as nausea and vomiting. Due to these side effects and with the initial dose of 80 mg/kg, 31% of the patients discontinued nicotinamide intake. The aim of the study was to investigate the effect of a dose reduction to 60 mg/kg, and the addition of domperidone on the side effects of nicotinamide and its pharmacokinetic profile.

PATIENTS AND METHODS

In 22 patients blood plasma nicotinamide levels were determined after intake of 60 mg/kg nicotinamide. A next group of 87 patients received 60 mg/kg nicotinamide in combination with domperidone. In ten of these patients blood plasma nicotinamide levels were also determined. A full pharmacokinetic profile was constructed over the first 24 h after intake of the first drug dose. Furthermore, daily plasma levels at 1 h after nicotinamide intake was determined in the first and last weeks of radiotherapy. All patients were treated according to the ARCON schedule.

RESULTS AND DISCUSSION

The mean maximum plasma nicotinamide concentration was 793 nmol/ml without domperidone and 776 nmol/ml with domperidone. The median time at which the maximum concentration occurred was not significantly different for 60 mg/kg nicotinamide without or with domperidone (0.46 versus 0.54 h). The side effects were drastically reduced if nicotinamide was accompanied by domperidone. The percentage of patients that stopped nicotinamide intake was reduced from 32% without domperidone to 14% with domperidone. No correlation was found between the plasma peak concentrations of nicotinamide and the severity of side effects.

CONCLUSION

The currently used dose of 60 mg/kg nicotinamide results in a 30% reduction in peak plasma concentrations compared with 80 mg/kg nicotinamide. If nicotinamide was given in combination with domperidone, 86% of the patients continued the nicotinamide medication until the end of the treatment period.

5-Fluoroindole-3-acetic acid: a prodrug activated by a peroxidase with potential for use in targeted cancer therapy

Indole-3-acetic acid and some derivatives are oxidized by horseradish peroxidase, forming a radical-cation that rapidly fragments (eliminating CO(2)) to form cytotoxic products. No toxicity is seen when either indole-3-acetic acid or horseradish peroxidase is incubated alone at concentrations that together form potent cytotoxins. Unexpectedly, 5-fluoroindole-3-acetic acid, which is oxidized by horseradish peroxidase compound I 10-fold more slowly than indole-3-acetic acid, is much more cytotoxic towards V79 hamster fibroblasts in the presence of peroxidase than the unsubstituted indole. The fluorinated prodrug/peroxidase combination also shows potent cytotoxic activity in human and rodent tumor cell lines. Cytotoxicity is thought to arise in part from the formation of 3-methylene-2-oxindole (or analogues) that can conjugate with thiols and probably DNA or other biological nucleophiles. Levels of the fluorinated prodrug in the murine carcinoma NT after intraperitoneal administration of 50 mg/kg were about 200 microM. Although these were 4-5-fold lower than plasma levels (which reached 1mM), the integrated area under the concentration/time curve in tumors over 2 hr was approximately 20 mM min, almost double the exposure needed to achieve approximately 90-99% cell kill in human MCF7 breast or HT29 colon tumor cell lines and CaNT murine cells in vitro, although the human bladder T24 carcinoma cell line was more resistant. The high cytotoxicity of 5-fluoroindole-3-acetic acid after oxidative activation suggests its further evaluation as a prodrug for targeted cancer therapy involving antibody-, polymer-, or gene-directed delivery of horseradish peroxidase or similar activating enzymes.

Revisiting the interaction of the radical anion metabolite of nitrofurantoin with glutathione

There have been several conflicting reports as to the scavenging nature of glutathione toward the nitro radical anion of the drug nitrofurantoin. We produced the radical anion enzymatically using the xanthine oxidase/hypoxanthine system at pH 7.4 and pH 9.0 in the presence of various levels of glutathione from 10 to 100 mM and monitored any changes in the radical concentration via electron spin resonance spectroscopy. Independent of glutathione concentration, there was no decrease in the steady-state concentration of the radical. In fact, there was an average 30% increase in the concentration of the radical anion, which suggests enhanced enzyme activity in the presence of glutathione (GSH). These results, together with observations of the effects of glutathione on the stability of the radical anion generated by radiolysis or dithionite, rule out any detectable reaction between the nitrofurantoin radical anion and GSH under physiologically relevant conditions.

Measurement of plasma 5-hydroxyindoleacetic acid as a possible clinical surrogate marker for the action of antivascular agents

BACKGROUND

Serotonin (5HT), a naturally occurring vasoactive substance, is released from platelets into plasma under various pathological conditions. Recently, anticancer drugs that act by selectively disrupting tumour blood flow have been found to increase plasma 5HT concentrations in mice. Two such antivascular agents, flavone acetic acid (FAA) and 5,6-dimethylxanthenone-4-acetic acid (DMXAA), have completed Phase I clinical trial and raise the important question of whether suitable surrogate markers for antivascular effects can be identified.

METHODS

5HT is unstable to storage, precluding routine clinical assay, but the 5HT metabolite, 5-hydroxyindoleacetic acid (5HIAA) accumulates in plasma following 5HT release and is a more suitable marker because of its greater stability. We have developed an automated procedure for the assay of the low concentrations of 5HIAA found in humans by combining solid-phase extraction with high-performance liquid chromatography (HPLC).

RESULTS

Efficient separation of 5HIAA from possible interfering substances in human plasma, including a variety of pharmaceutical agents, was achieved on C18 columns using cetyltrimethylammonium bromide (CETAB) as an organic modifier. Adequate precision, accuracy and sensitivity were achieved by electrochemical detection (ECD) at +400 mV. Analysis of plasma from two patients treated with DMXAA in a Phase I trial demonstrated DMXAA-induced elevation of plasma 5HIAA with a time course similar to that previously described in mice.

CONCLUSIONS

Measurement of changes in plasma 5HIAA provides a new approach to the monitoring of therapies with an antivascular effect. The assay is sensitive to dietary sources of 5HT, which should be minimised.

Horseradish peroxidase-mediated gene therapy: choice of prodrugs in oxic and anoxic tumor conditions

We have previously proposed the plant enzyme horseradish peroxidase (HRP) and the plant hormone indole-3-acetic acid (IAA) as an enzyme/prodrug combination for cancer gene therapy. In the current study, we evaluated the potential of HRP/IAA for gene-directed enzyme/prodrug therapy in three human tumor cell lines (T24 bladder carcinoma, MCF-7 breast adenocarcinoma, and FaDu nasopharyngeal squamous carcinoma) and one endothelial cell line (HMEC-1). The action of 10 IAA analogues in combination with HRP was studied in vitro in normoxic conditions as well as in the extreme tumor conditions of anoxia. Compounds characterized by prompt normoxic or anoxic cytotoxic activation and high HRP transfectant killing or selectivity were identified. Some variations were observed in the response of cells of different origin, with IAA, 1-Me-IAA, and 5-Br-IAA representing the most promising candidates for HRP gene therapy. In particular, 5-Br-IAA showed a very prompt and selective activation in anoxia. A strong bystander effect was produced by activated IAA and analogues because 70-90% cell kill was obtained when only 5% of the cells expressed the HRP enzyme. These results indicate that HRP/IAA represents an effective system for enzyme/prodrug-based anticancer approaches, and further improvements could be achieved by the use of novel IAA derivatives.

Influence of plasma glutathione levels on radiation mucositis

PURPOSE

To test the hypothesis that there is a link between plasma glutathione (GSH) or other antioxidants (uric acid, ascorbate) and the severity of radiation mucositis following radiation treatment of tumors of the head and neck.

PATIENTS AND METHODS

Patients with carcinomas of the head-and-neck region were treated with the continuous hyperfractionated accelerated radiotherapy (CHART) regimen (54 Gy in 36 fractions over 12 days). Samples of blood plasma were analyzed for GSH, cysteine, urate, and ascorbate by high-pressure liquid chromatography. Patients were graded for dysphagia and requirement for analgesics. The areas under the curves of scores over 2-6 weeks following treatment were computed, and Spearman's rank-correlation coefficient was used to test for an association between plasma GSH levels (or those of other antioxidants) and mucositis.

RESULTS

The pretreatment plasma GSH level in 18 patients scored in the study was 1.0 +/- 0.7 M. Analysis of these and the dysphagia scores produced a correlation coefficient of 0.22 (confidence interval -0.28, 0.61; p = 0.39). No correlation was seen between mucositis severity and other measures of plasma antioxidants: cysteine (7.6 +/- 1.7 M), cysteine + GSH (8.6 +/- 1.9 M), uric acid (317 +/- 86 M), ascorbate (29 +/- 20 M), or whole-blood GSH concentrations (1,010 +/- 239 M).

CONCLUSION

The measurements of approximately micromolar levels of plasma GSH, or about 10 M cysteine + GSH (almost all of the total nonprotein thiols), are consistent with most other published data for either healthy adults or cancer patients; however, the values reported in an earlier study, suggesting a link between GSH and mucositis, are much higher. The hypothesis of a possible link between radiation mucositis and plasma-free (nonprotein) thiols was not supported.

Oxidative activation of indole-3-acetic acids to cytotoxic species- a potential new role for plant auxins in cancer therapy

Indole-3-acetic acid (IAA) and some derivatives can be oxidised by horseradish peroxidase (HRP) to cytotoxic species. Upon treatment with IAA/HRP, liposomes undergo lipid peroxidation, strand breaks and adducts are formed in supercoiled plasmid DNA, and mammalian cells in culture lose colony-forming ability. IAA is only toxic after oxidative decarboxylation; no effects are seen when IAA or HRP is incubated independently in these systems at equivalent concentrations. Toxicity is similar in both hamster fibroblasts and some human tumour cells. The effect of IAA/HRP is thought to be due in part to the formation of 3-methylene-2-oxindole, which may conjugate with DNA bases and protein thiols. Our hypothesis is that IAA/HRP could be used as the basis for targeted cancer therapy involving antibody-, polymer-, or gene-directed approaches. HRP can thus be targeted to a tumour allowing non-toxic IAA delivered systemically to be activated only in the tumour. Exposure to newly synthesised analogues of IAA shows a range of four orders of magnitude difference in cellular toxicity but no structure-activity relationships are apparent, in contrast to well-defined redox dependencies of oxidation by HRP intermediates or rates of decarboxylation of radical-cation intermediates.

Development of a novel enzyme/prodrug combination for gene therapy of cancer: horseradish peroxidase/indole-3-acetic acid

This paper demonstrates the potential for utilizing the plant enzyme, horseradish peroxidase (HRP), in a gene-directed enzyme prodrug therapy context. Human T24 bladder carcinoma cells transfected with a mammalian expression vector containing the HRP cDNA were selectively sensitized to the nontoxic plant hormone, indole-3-acetic acid (IAA). The HRP/IAA-induced cell kill was effective in normoxic and anoxic conditions. The activated drug is a long-lived species able to cross cell membranes, and cell contact appears not to be required for a bystander effect to take place. These preliminary results suggest that the delivery of the HRP gene to human tumors followed by IAA treatment may provide a novel cancer gene-directed enzyme prodrug therapy approach, with potential to target hypoxic cells.

Effect of local radiotherapy for bone pain on urinary markers of osteoclast activity

Urinary markers of bone resorption, pyridinoline and deoxypyridinoline were measured before and at 4 weeks after radiotherapy for metastatic bone pain. An association was shown between relief of metastatic skeletal pain by radiotherapy and low marker concentrations before and after treatment, lending support to the hypothesis that relief of metastatic bone pain by radiotherapy relates to an effect on bone, rather than tumour physiology.

Peroxidase-catalyzed effects of indole-3-acetic acid and analogues on lipid membranes, DNA, and mammalian cells in vitro

This study aimed to explore the mechanisms and molecular parameters which control the cytotoxicity of derivatives of indole-3-acetic acid (IAA) when oxidatively activated by horseradish peroxidase (HRP). Lipid peroxidation was measured in liposomes, damage to supercoiled plasmid DNA assessed by gel electrophoresis, free radical intermediates detected by EPR following spin trapping, binding of IAA-derived products demonstrated by 3H labelling, stable products measured by HPLC, and cytotoxicity in hamster fibroblasts measured by clonogenic survival. IAA, and nine analogues more easily oxidized by HRP, caused lipid peroxidation in liposomes, but not detectably in membranes of hamster fibroblasts, and were cytotoxic after HRP activation to varying degrees. Cytotoxicity was not correlated with activation rate. The hydrophilic vitamin E analogue, Trolox, inhibited cytotoxicity, whereas loading fibroblasts with vitamin E was ineffective, consistent with an oxidative mechanism in which radical precursors to damage are intercepted by Trolox in the aqueous phase. However, two known oxidation products were nontoxic (the 3-carbinol and 3-aldehyde, both probably produced from 3-CH2OO* peroxyl radicals via the 3-CH*2 [skatolyl] radical following decarboxylation of the radical cation). The skatolyl radical from IAA was shown by EPR with spin trapping to react with DNA; electrophoresis showed binding to occur. Treatment of hamster fibroblasts with 5-3H-IAA/HRP resulted in intracellular bound 3H. Together with earlier results, the new data point to unknown electrophilic oxidation products, reactive towards intracellular targets, being involved in cytotoxicity of the IAA/HRP combination, rather than direct attack of free radicals, excited states, or membrane lipid peroxidation.

Toward targeted "oxidation therapy" of cancer: peroxidase-catalysed cytotoxicity of indole-3-acetic acids

PURPOSE

The study aimed to identify suitable prodrugs that could be used to test the hypothesis that peroxidase activity in cells, either endogenous or enhanced by immunological targeting, can activate prodrugs to cytotoxins. We hypothesized that prototype prodrugs based on derivatives of indole-3-acetic acid (IAA), when activated by peroxidase enzymes (e.g., from horseradish, HRP) should produce peroxyl radicals, with deleterious biological consequences.

METHODS AND MATERIALS

V79 hamster cells were incubated with IAA or derivatives +/- HRP and cytotoxicity assessed by a clonogenic assay. To assess the toxicity of stable oxidation products, prodrugs were also oxidized by HRP without cells, and the products then added to cells.

RESULTS

The combination of prodrug and enzyme resulted in cytotoxicity, but neither indole nor enzyme in isolation was toxic under the conditions used. Although lipid peroxidation was stimulated in liposomes by the prodrug/enzyme treatment, it could not be measured in mammalian cells. Adding oxidized prodrugs to cells resulted in cytotoxicity.

CONCLUSIONS

Although the hypothesis that prodrugs of this type could enhance oxidative stress via lipid peroxidation was not established, the results nonetheless demonstrated oxidatively-activated cytotoxicity via indole acetic acid prodrugs, and suggested these as a new type of substrate for antibody-directed enzyme-prodrug therapy (ADEPT). The hypothesized free-radical fragmentation intermediates were demonstrated, but lipid peroxidation associated with peroxyl radical formation was unlikely to be the major route to cytotoxicity.

Interpretation of the reactivity of peroxidase compounds I and II with phenols by the Marcus equation

The catalytic cycle of heme peroxidases involves two reactive states, compound I and compound II. Although their reduction potentials at pH 7 are similar, compound I is in general more reactive towards organic substrates than compound II. The different reactivities have until now remained unexplained. In this study, the reactions of compounds I and II of peroxidase from horseradish with phenols were analyzed using the Marcus equation of electron-transfer. Both reactions exhibit similar reorganization energies, and the different reactivities of the two enzyme states can be ascribed to a higher apparent rate of activationless electron-transfer in the compound I reactions. This can be attributed to the shorter electron-tunneling distance on electron-transfer to the porphyrin radical cation in compound I, compared to electron-transfer to the iron ion in compound II.