1,2-Bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (VNP40101M): II. Role of O 6 -alkylguanine-DNA alkyltransferase in cytotoxicity

Cell Models for Birth Defects Caused by Chloroethyl Nitrosourea-Induced DNA Lesions

ABSTRACT

Birth defects have been linked to administration of alkylating agents during pregnancy. The anti-tumor efficacy of alkylating agents correlate with their ability to induce DNA lesions, especially interstrand crosslinks (ICLs). Yet the role of DNA damages in birth defects remains to be clarified, owing, in part, to a lack of cell models. Here we generate DNA lesions in NIH/3T3 cells to mimic defects in fetus triggered by 3-Bis(2-chloroethyl)-1-nitrosourea (BCNU, carmustine). CCK-8 assay suggests that BCNU-induced cell death was dose-dependent. Alkaline comet tests and γ-H2AX staining confirm DNA ICLs and other forms of DNA damages caused by BCNUs. The cell cycle analysis shows cells arrest in G2/M phase until crosslinks repair is complete. Taken together, all these experiments demonstrate we have successfully established normal cell models for birth defects caused by BCNU-mediated DNA damages. The model can not only guide the development of effective and low-toxicity anticancer drugs, but also be of great significance for the study of neonatal malformation triggered by BCNUs.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

pH-dependent general base catalyzed activation rather than isocyanate liberation may explain the superior anticancer efficacy of laromustine compared to related 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine prodrugs

Laromustine (also known as cloretazine, onrigin, VNP40101M, 101M) is a prodrug of 90CE, a short-lived chloroethylating agent with anticancer activity. The short half-life of 90CE necessitates the use of latentiated prodrug forms for in vivo treatments. Alkylaminocarbonyl-based prodrugs such as laromustine exhibit significantly superior in vivo activity in several murine tumor models compared to analogs utilizing acyl, and alkoxycarbonyl latentiating groups. The alkylaminocarbonyl prodrugs possess two exclusive characteristics: (i) They are primarily unmasked by spontaneous base catalyzed elimination; and (ii) they liberate a reactive carbamoylating species. Previous speculations as to the therapeutic superiority of laromustine have focused upon the inhibition of enzymes by carbamoylation. We have investigated the therapeutic interactions of analogs with segregated chloroethylating and carbamoylating activities (singly and in combination) in the in vivo murine L1210 leukemia model. The combined treatment with chloroethylating and carbamoylating prodrugs failed to result in any synergism and produced a reduction in the therapeutic efficacy compared to the chloroethylating prodrug alone. Evidence supporting an alternative explanation for the superior tumor selectivity of laromustine is presented that is centered upon the high pH sensitivity of its base catalyzed activation, and the more alkaline intracellular pH values commonly found within tumor cells.

Influence of the Expression Level of O6-Alkylguanine-DNA Alkyltransferase on the Formation of DNA Interstrand Crosslinks Induced by Chloroethylnitrosoureas in Cells: A Quantitation Using High-Performance Liquid Chromatography-Mass Spectrometry

Chloroethylnitrosoureas (CENUs), which are bifunctional alkylating agents widely used in the clinical treatment of cancer, exert anticancer activity by inducing crosslink within a guanine-cytosine DNA base pair. However, the formation of dG-dC crosslinks can be prevented by O⁶-alkylguanine-DNA alkyltransferase (AGT), ultimately leading to drug resistance. Therefore, the level of AGT expression is related to the formation of dG-dC crosslinks and the sensitivity of cells to CENUs. In this work, we determined the CENU-induced dG-dC crosslink in mouse L1210 leukemia cells and in human glioblastoma cells (SF-763, SF-767 and SF-126) containing different levels of AGT using high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry. The results indicate that nimustine (ACNU) induced more dG-dC crosslinks in L1210 leukemia cells than those induced by carmustine (BCNU), lomustine (CCNU) and fotemustine (FTMS). This result was consistent with a previously reported cohort study, which demonstrated that ACNU had a better survival gain than BCNU, CCNU and FTMS for patients with high-grade glioma. Moreover, we compared the crosslinking levels and the cytotoxicity in SF-763, SF-767 and SF-126 cells with different AGT expression levels after exposure to ACNU. The levels of dG-dC crosslink in SF-126 cells (low AGT expression) were significantly higher than those in SF-767 (medium AGT expression) and SF-763 (high AGT expression) cells at each time point. Correspondingly, the cytotoxicity of SF-126 was the highest followed by SF-767 and SF-763. The results obtained in this work provided unequivocal evidence for drug resistance to CENUs induced by AGT-mediated repair of DNA ICLs. We postulate that the level of dG-dC crosslink has the potential to be employed as a biomarker for estimating drug resistance and anticancer efficiencies of novel CENU chemotherapies.

Antitumor sulfonylhydrazines: design, structure-activity relationships, resistance mechanisms, and strategies for improving therapeutic utility

1,2-Bis(sulfonyl)-1-alkylhydrazines (BSHs) were conceived as more specific DNA guanine O-6 methylating and chloroethylating agents lacking many of the undesirable toxicophores contained in antitumor nitrosoureas. O(6)-Alkylguanine-DNA alkyltransferase (MGMT) is the sole repair protein for O(6)-alkylguanine lesions in DNA and has been reported to be absent in 5-20% of most tumor types. Many BSHs exhibit highly selective cytotoxicity toward cells deficient in MGMT activity. The development of clinically useful MGMT assays should permit the identification of tumors with this vulnerability and allow for the preselection of patient subpopulations with a high probability of responding. The BSH system is highly versatile, permitting the synthesis of many prodrug types with the ability to incorporate an additional level of tumor-targeting due to preferential activation by tumor cells. Furthermore, it may be possible to expand the spectrum of activity of these agents to include tumors with MGMT activity by combining them with tumor-targeted MGMT inhibitors.

Carbamoylating activity associated with the activation of the antitumor agent laromustine inhibits angiogenesis by inducing ASK1-dependent endothelial cell death

The anticancer agent 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (laromustine), upon decomposition in situ, yields methyl isocyanate and the chloroethylating species 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE). 90CE has been shown to kill tumor cells via a proposed mechanism that involves interstrand DNA cross-linking. However, the role of methyl isocyanate in the antineoplastic function of laromustine has not been delineated. Herein, we show that 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine (101MDCE), an analog of laromustine that generates only methyl isocyanate, activates ASK1-JNK/p38 signaling in endothelial cells (EC). We have previously shown that ASK1 forms a complex with reduced thioredoxin (Trx1) in resting EC, and that the Cys residues in ASK1 and Trx1 are critical for their interaction. 101MDCE dissociated ASK1 from Trx1, but not from the phosphoserine-binding inhibitor 14-3-3, in whole cells and in cell lysates, consistent with the known ability of methyl isocyanate to carbamoylate free thiol groups of proteins. 101MDCE had no effect on the kinase activity of purified ASK1, JNK, or the catalytic activity of Trx1. However, 101MDCE, but not 90CE, significantly decreased the activity of Trx reductase-1 (TrxR1). We conclude that methyl isocyanate induces dissociation of ASK1 from Trx1 either directly by carbamoylating the critical Cys groups in the ASK1-Trx1 complex or indirectly by inhibiting TrxR1. Furthermore, 101MDCE (but not 90CE) induced EC death through a non-apoptotic (necroptotic) pathway leading to inhibition of angiogenesis in vitro. Our study has identified methyl isocyanates may contribute to the anticancer activity in part by interfering with tumor angiogenesis.

Hypoxia-selective O6-alkylguanine-DNA alkyltransferase inhibitors: design, synthesis, and evaluation of 6-(benzyloxy)-2-(aryldiazenyl)-9H-purines as prodrugs of O6-benzylguanine

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a DNA repair protein which removes alkyl groups from the O-6 position of guanine, thereby providing strong resistance to anticancer agents which alkylate this position. The clinical usefulness of these anticancer agents would be substantially augmented if AGT could be selectively inhibited in tumor tissue, without a corresponding depletion in normal tissue. We report the synthesis of a new AGT inhibitor (5c) which selectively depletes AGT in hypoxic tumor cells.

Thioredoxin reductase is inhibited by the carbamoylating activity of the anticancer sulfonylhydrazine drug laromustine

The thioredoxin system facilitates proliferative processes in cells and is upregulated in many cancers. The activities of both thioredoxin (Trx) and its reductase (TrxR) are mediated by oxidation/reduction reactions among cysteine residues. A common target in preclinical anticancer research, TrxR is reported here to be significantly inhibited by the anticancer agent laromustine. This agent, which has been in clinical trials for acute myelogenous leukemia and glioblastoma multiforme, is understood to be cytotoxic principally via interstrand DNA crosslinking that originates from a 2-chloroethylating species generated upon activation in situ. The spontaneous decomposition of laromustine also yields methyl isocyanate, which readily carbamoylates thiols and primary amines. Purified rat liver TrxR was inhibited by laromustine with a clinically relevant IC(50) value of 4.65 μM. A derivative of laromustine that lacks carbamoylating activity did not appreciably inhibit TrxR while another derivative, lacking only the 2-chloroethylating activity, retained its inhibitory potency. Furthermore, in assays measuring TrxR activity in murine cell lysates, a similar pattern of inhibition among these compounds was observed. These data contrast with previous studies demonstrating that glutathione reductase, another enzyme that relies on cysteine-mediated redox chemistry, was not inhibited by methylcarbamoylating agents when measured in cell lysates. Mass spectrometry of laromustine-treated enzyme revealed significant carbamoylation of TrxR, albeit not on known catalytically active residues. However, there was no evidence of 2-chloroethylation anywhere on the protein. The inhibition of TrxR is likely to contribute to the cytotoxic, anticancer mechanism of action for laromustine.

Preclinical evaluation of Laromustine for use in combination with radiation therapy in the treatment of solid tumors

PURPOSE

These studies explored questions related to the potential use of Laromustine in the treatment of solid tumors and in combination with radiotherapy.

MATERIALS AND METHODS

The studies used mouse EMT6 cells (both parental and transfected with genes for O(6)-alkylguanine-DNA transferase [AGT]), repair-deficient human Fanconi Anemia C and Chinese hamster VC8 (BRCA2(-/-)) cells and corresponding control cells, and EMT6 tumors in mice assayed using cell survival and tumor growth assays.

RESULTS

Hypoxia during Laromustine treatment did not protect EMT6 cells or human fibroblasts from this agent. Rapidly proliferating EMT6 cells were more sensitive than quiescent cultures. EMT6 cells expressing mouse or human AGT, which removes O(6)-alkyl groups from DNA guanine, thereby protecting against G-C crosslink formation, increased resistance to Laromustine. Crosslink-repair-deficient Fanconi Anemia C and VC8 cells were hypersensitive to Laromustine, confirming the importance of crosslinks as lethal lesions. In vitro, Laromustine and radiation produced additive toxicities to EMT6 cells. Studies using tumor cell survival and tumor growth assays showed effects of regimens combining Laromustine and radiation that were compatible with additive or subadditive interactions.

CONCLUSIONS

The effects of Laromustine on solid tumors and with radiation are complex and are influenced by microenvironmental and proliferative heterogeneity within these malignancies.

4-nitrobenzyloxycarbonyl derivatives of O(6)-benzylguanine as hypoxia-activated prodrug inhibitors of O(6)-alkylguanine-DNA alkyltransferase (AGT), which produces resistance to agents targeting the O-6 position of DNA guanine

A series of 4-nitrobenzyloxycarbonyl prodrug derivatives of O(6)-benzylguanine (O(6)-BG), conceived as prodrugs of O(6)-BG, an inhibitor of the resistance protein O(6)-alkylguanine-DNA alkyltransferase (AGT), were synthesized and evaluated for their ability to undergo bioreductive activation by reductase enzymes under oxygen deficiency. Three agents of this class, 4-nitrobenzyl (6-(benzyloxy)-9H-purin-2-yl)carbamate (1) and its monomethyl (2) and gem-dimethyl analogues (3), were tested for activation by reductase enzyme systems under oxygen deficient conditions. Compound 3, the most water-soluble of these agents, gave the highest yield of O(6)-BG following reduction of the nitro group trigger. Compound 3 was also evaluated for its ability to sensitize 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (laromustine)-resistant DU145 human prostate carcinoma cells, which express high levels of AGT, to the cytotoxic effects of this agent under normoxic and oxygen deficient conditions. While 3 had little or no effect on laromustine cytotoxicity under aerobic conditions, significant enhancement occurred under oxygen deficiency, providing evidence for the preferential release of the AGT inhibitor O(6)-BG under hypoxia.

Clinical activity of laromustine (Onrigin™) in hematologic malignancies

Laromustine (Onrigin™), formerly known as Cloretazine(®) (VNP40101M), belongs to a novel class of alkylating agents--the sulfonylhydrazines--and was selected for clinical development based on its broad anti-tumor activity in preclinical models. Laromustine is metabolized to yield 90CE and methylisocyanate, the former rapidly produces an alkylating, chloroethylating species, similar to the chloroethylating species generated by carmustine. However, several features distinguish laromustine from carmustine and possibly account for their biological differences in vitro and in vivo. The chloroethylating species responsible for laromustine's alkylator effect is relatively specific for guanine and forms a crosslink after incorporation into DNA. Laromustine has significant activity in both older patients with previously untreated acute myeloid leukemia or high-risk myelodysplastic syndrome, including those with very poor-risk disease, and in patients with relapsed disease. Further clinical studies are required with laromustine to evaluate its place as an anticancer agent in other hematological malignancies.

Quantitative relationship between guanine O(6)-alkyl lesions produced by Onrigin™ and tumor resistance by O(6)-alkylguanine-DNA alkyltransferase

O(6)-Alkylguanine-DNA alkyltransferase (AGT) mediates tumor resistance to alkylating agents that generate guanine O(6)-chloroethyl (Onrigin™ and carmustine) and O(6)-methyl (temozolomide) lesions; however, the relative efficiency of AGT protection against these lesions and the degree of resistance to these agents that a given number of AGT molecules produces are unclear. Measured from differential cytotoxicity in AGT-ablated and AGT-intact HL-60 cells containing 17,000 AGT molecules/cell, AGT produced 12- and 24-fold resistance to chloroethylating (90CE) and methylating (KS90) analogs of Onrigin™, respectively. For 50% growth inhibition, KS90 and 90CE generated 5,600 O(6)-methylguanines/cell and ∼300 O(6)-chloroethylguanines/cell, respectively. AGT repaired O(6)-methylguanines until the AGT pool was exhausted, while its repair of O(6)-chloroethylguanines was incomplete due to progression of the lesions to AGT-irreparable interstrand DNA cross-links. Thus, the smaller number of O(6)-chloroethylguanine lesions needed for cytotoxicity accounted for the marked degree of resistance (12-fold) to 90CE produced by AGT. Transfection of human or murine AGT into AGT deficient transplantable tumor cells (i.e., EMT6, M109 and U251) generated transfectants expressing AGT ranging from 4,000 to 700,000 molecules/cell. In vitro growth inhibition assays using these transfectants treated with 90CE revealed that AGT caused a concentration dependent resistance up to a level of ∼10,000 AGT molecules/cell. This finding was corroborated by in vivo studies where expression of 4,000 and 10,000 murine AGT molecules/cell rendered EMT6 tumors partially and completely resistant to Onrigin™, respectively. These studies imply that the antitumor activity of Onrigin™ stems from guanine O(6)-chloroethylation and define the threshold concentration of AGT that negates its antineoplastic activity.

Single-Agent Laromustine, A Novel Alkylating Agent, Has Significant Activity in Older Patients With Previously Untreated Poor-Risk Acute Myeloid Leukemia

Purpose

An international phase II study of laromustine (VNP40101M), a sulfonylhydrazine alkylating agent, was conducted in patients age 60 years or older with previously untreated poor-risk acute myeloid leukemia (AML).

Patients and Methods

Laromustine 600 mg/m2 was administered as a single 60-minute intravenous infusion. Patients were age 70 years or older or 60 years or older with at least one additional risk factor—unfavorable AML karyotype, Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 2, and/or cardiac, pulmonary, or hepatic comorbidities.

Results

Eighty-five patients (median age, 72 years; range, 60 to 87 years) were treated. Poor-risk features included age 70 years or older, 78%; adverse karyotype, 47%; PS of 2, 41%; pulmonary disease, 77%; cardiac disease, 73%; and hepatic disease, 3%. Ninety-six percent of patients had at least two risk factors, and 39% had at least four risk factors. The overall response rate (ORR) was 32%, with 20 patients (23%) achieving complete response (CR) and seven (8%) achieving CR with incomplete platelet recovery (CRp). ORR was 20% in patients with adverse cytogenetics; 32% in those age 70 years or older; 32% in those with PS of 2; 32% in patients with baseline pulmonary dysfunction; 34% in patients with baseline cardiac dysfunction; and 27% in 33 patients with at least four risk factors. Twelve (14%) patients died within 30 days of receiving laromustine therapy. Median overall survival was 3.2 months, with a 1-year survival of 21%; the median duration of survival for those who achieved CR/CRp was 12.4 months, with a 1-year survival of 52%.

Conclusion

Laromustine has significant single-agent activity in elderly patients with poor-risk AML. Adverse events are predominantly myelosuppressive or respiratory. Response rates are consistent across a spectrum of poor-risk features.

Phase 3 randomized, placebo-controlled, double-blind study of high-dose continuous infusion cytarabine alone or with laromustine (VNP40101M) in patients with acute myeloid leukemia in first relapse

Laromustine is a sulfonylhdrazine alkylator with significant antileukemia activity. An international, randomized (2:1), double-blind, placebo-controlled study was conducted to compare complete remission (CR) rates and overall survival (OS) in patients with first relapse acute myeloid leukemia (AML) treated with laromustine and high-dose cytarabine (HDAC) versus HDAC/placebo. Patients received 1.5 g/m(2) per day cytarabine continuous infusion for 3 days and laromustine 600 mg/m(2) (n = 177) or placebo (n = 86) on day 2. Patients in CR received consolidation with laromustine/HDAC or HDAC/placebo as per initial randomization. After interim analysis at 50% enrollment, the Data Safety Monitoring Board (DSMB) expressed concern that any advantage in CR would be compromised by the observed on-study mortality, and enrollment was held. The CR rate was significantly higher for the laromustine/HDAC group (35% vs 19%, P = .005). However, the 30-day mortality rate and median progression-free survival were significantly worse in this group compared with HDAC/placebo (11% vs 2%; P = .016; 54 days vs 34; P = .002). OS and median response durations were similar in both groups. Laromustine/HDAC induced significantly more CR than HDAC/placebo, but OS was not improved due to mortality associated with myelosuppression and its sequelae. The DSMB subsequently approved a revised protocol with laromustine dose reduction and recombinant growth factor support. The study was registered as NCT00112554 at http://www.clinicaltrials.gov.

Inhibition of human DNA polymerase beta activity by the anticancer prodrug Cloretazine

The antineoplastic prodrug Cloretazine exerts its cytotoxicity via a synergism between 2-chloroethylating and carbamoylating activities that are cogenerated upon activation in situ. Cloretazine is reported here to inhibit the nucleotidyl-transferase activity of purified human DNA polymerase beta (Pol beta), a principal enzyme of DNA base excision repair (BER). The 2-chloroethylating activity of Cloretazine alkylates DNA at the O(6) position of guanine bases resulting in 2-chloroethoxyguanine monoadducts, which further react to form cytotoxic interstrand DNA crosslinks. Alkylated DNA is often repaired via BER in vivo. Inhibition of the polymerase activity of Pol beta may account for some of the synergism between Cloretazine's two reactive subspecies in cytotoxicity assays. This inhibition was only observed using agents with carbamoylating activity. Furthermore, while therapeutically relevant concentrations of Cloretazine inhibited the polymerase activity of Pol beta, the enzyme's lyase activity, which may also participate in BER, was not significantly inhibited.

Initial testing of VNP40101M (Cloretazine) by the pediatric preclinical testing program

VNP40101M is a novel alkylating agent that yields two reactive compounds (a chloroethylating species and methylisocyanate) and has demonstrated activity against a wide spectrum of tumor xenografts. VNP40101M was tested against an in vivo panel of five pediatric brain tumor xenografts at a dose of 18 mg/kg/day administered for 5 days. O-6-methylguanine-DNA methyltransferase (MGMT) levels of xenografts were assessed by Western blot analysis. Only one xenograft (GBM2), which lacked detectable MGMT expression, demonstrated an objective response to VNP40101M. VNP4010M antitumor activity was observed only in the absence of MGMT expression, with resistance to VNP4010M seen even with low MGMT expression.

Lethality to leukemia cell lines of DNA interstrand cross-links generated by Cloretazine derived alkylating species

Cloretazine [1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine; VNP40101M; 101M] is a relatively new prodrug with activity in elderly acute myelogenous leukemia (AML) patients. Its therapeutic action is due largely to the production of 1-(3-cytosinyl),2-(1-guanyl)ethane cross-links (G-C ethane cross-links) in DNA. The numbers of cross-links produced in three experimental leukemia lines (L1210, U937 and HL-60) were fewer than 10 per genome at their respective LC50 concentrations. Only 1 in approximately 20,000 90CE molecules produces a cross-link in the AGT (O6-alkylguanine-DNA alkyltransferase) negative L1210 and U937 cell lines and 1 in 400,000 in the AGT positive HL-60 cell line.

Activity of VNP40101M (Cloretazine) in the treatment of CNS tumor xenografts in athymic mice

VNP40101M, or 1,2-bis(methylsulfonyl)-1-(2-choloroethyl)-2-(methylamino)carbonylhydrazine (Cloretazine), is a bifunctional prodrug that belongs to a class of DNA-modifying agents-the sulfonylhydrazines-that has been synthesized and been shown to have activity against a wide spectrum of xenografts. The current study was designed to assess the activity of VNP40101M administered at a dose of 18 mg/kg daily for five days against a panel of human adult and pediatric CNS tumors growing subcutaneously or intracranially in athymic nude mice. The results demonstrated statistically significant (p < 0.05) growth delays of 15.0, 8.3, 51.0, 60+, 60+, and 60+ days in subcutaneous xenografts derived from childhood glioblastoma multiforme (D-456 MG), childhood ependymoma (D-528 EP and D-612 EP), childhood medulloblastoma (D-425 MED), and adult malignant glioma (D-245 MG and D-54 MG), respectively, with corresponding tumor regressions in 10 of 10, 4 of 10, 8 of 10, 9 of 10, 9 of 10, and 10 of 10 treated mice, respectively. Delayed toxicity was seen more than 60 days after treatment, with 23 deaths in 100 treated animals, despite a median weight loss of only 0.06%. In mice bearing intracranial D-245 MG xenografts, treatment with VNP40101M at a dose of 18 mg/kg daily for five days produced a 50% increase in median survival compared with controls. Additional experiments conducted against subcutaneous D-245 MG xenografts by using reduced doses of 13.5 or 9.0 mg/kg daily for five days demonstrated tumor growth delays of 82.2 and 53.5 days, with corresponding tumor regressions in 8 of 9 and 9 of 10 treated mice, respectively (all values, p < 0.001), with one toxic death. These findings suggest that VNP40101M is active in the treatment of a wide range of human central nervous system tumors and warrants translation to the clinic.

Anti-tumor efficacy of Cloretazine (VNP40101M) alone and in combination with fludarabine in murine tumor and human xenograft tumor models

Cloretazine (VNP40101M), a new sulfonylhydrazine alkylating agent, has demonstrated broad-spectrum anti-tumor activity in preclinical studies. In this study, Cloretazine was evaluated both as a monotherapy and in combination with fludarabine in murine tumor and human tumor xenograft models. Cloretazine significantly inhibited the growth of subcutaneously implanted tumors, including B16F10 murine melanoma in C57BL/6 mice, and H460 human lung carcinoma and WiDr human colon carcinoma in athymic nude CD1 mice. The inhibition of tumor growth by Cloretazine was dose dependent, increasing from 42.2 to 87% as the dose escalated from 100 to 150 mg/kg. Cloretazine showed equivalent efficacy but lower toxicity compared to cyclophosphamide in these models. The combination therapy, consisting of a single dose of 10 mg/kg Cloretazine plus five doses of 70 mg/kg fludarabine, given every other day intraperitoneally, significantly increased the long-term survival of BDF1 mice bearing the L1210 murine leukemia. On Day 65 post-tumor implantation, the combination therapy yielded a 90% survival rate compared to 40% for Cloretazine alone and 0% for fludarabine alone.

Cloretazine (VNP40101M), a Novel Sulfonylhydrazine Alkylating Agent, in Patients Age 60 Years or Older With Previously Untreated Acute Myeloid Leukemia

Purpose

Cloretazine (VNP40101M) is a sulfonylhydrazine alkylating agent with significant antileukemia activity. A multicenter phase II study of cloretazine was conducted in patients 60 years of age or older with previously untreated acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS).

Patients and Methods

Cloretazine 600 mg/m2was administered as a single intravenous infusion. Patients were stratified by age, performance score, cytogenetic risk category, type of AML, and comorbidity.

Results

One hundred four patients, median age 72 years (range, 60 to 84 years), were treated on study. Performance status was 2 in 31 patients (30%) and no patient had a favorable karyotype. Forty-seven patients (45%) had cardiac disease, 25 patients (24%) had hepatic disease, and 19 patients (18%) had pulmonary disease, defined as per the Hematopoietic Cell Transplantation–Specific Comorbidity Index, at study entry. The overall response rate was 32%, with 29 patients (28%) achieving complete response (CR) and four patients (4%) achieving CR with incomplete platelet recovery. Response rates in 44 de novo AML patients, 45 secondary AML patients, and 15 high-risk MDS patients were 50%, 11%, and 40%, respectively. Response by cytogenetic risk category was 39% in 56 patients with intermediate cytogenetic risk and 24% in 46 patients with unfavorable cytogenetic risk. Nineteen (18%) patients died within 30 days of receiving cloretazine therapy. Median overall survival was 94 days, with a 1-year survival of 14%; the median duration of survival was 147 days, with a 1-year survival of 28% for those who achieved CR.

Conclusion

Cloretazine has significant activity and modest extramedullary toxicity in elderly patients with AML or high-risk MDS. Response rates remain consistent despite increasing age and comorbidity.

Mode of action of the chloroethylating and carbamoylating moieties of the prodrug cloretazine

Cloretazine is an antitumor sulfonylhydrazine prodrug that generates both chloroethylating and carbamoylating species. The cytotoxic potency of these species was analyzed in L1210 leukemia cells using analogues with chloroethylating or carbamoylating function only. Clonogenic assays showed that the chloroethylating-only agent 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE) produced marked differential cytotoxicity against wild-type and O6-alkylguanine-DNA alkyltransferase-transfected L1210 cells (LC10, 1.4 versus 31 micromol/L), indicating that a large portion of the cytotoxicity was due to alkylation of DNA at the O-6 position of guanine. Consistent with the concept that O-6 chloroethylation of DNA guanine progresses to interstrand cross-links, the comet assay, in which DNA cross-links were measured by a reduction in DNA migration induced by strand breaks, showed that cloretazine and 90CE, but not the carbamoylating-only agent 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine (101MDCE), produced DNA cross-links and that cloretazine caused more DNA cross-links than 90CE at equimolar concentrations. Cell cycle analyses showed that 90CE and 101MDCE at concentrations of 5 and 80 micromol/L, respectively, produced similar degrees of G2-M arrest. 90CE produced selective inhibition of DNA synthesis after overnight incubation, whereas 101MDCE caused rapid and nonselective inhibition of RNA, DNA, and protein syntheses. Both 90CE and 101MDCE induced phosphorylation of histone H2AX, albeit with distinct kinetics. These results indicate that (a) differential expression of O6-alkylguanine-DNA alkyltransferase in tumor and host cells seems to be responsible for tumor selectivity exerted by cloretazine; (b) 101MDCE enhances DNA cross-linking activity; and (c) 90CE induces cell death at concentrations lower than those causing alterations in the cell cycle and macromolecular syntheses.

Phase I study of cloretazine (VNP40101M), a novel sulfonylhydrazine alkylating agent, combined with cytarabine in patients with refractory leukemia

PURPOSE

Cloretazine (VNP40101M) is a novel sulfonylhydrazine alkylating agent with significant antileukemia activity. A phase I study of cloretazine combined with cytarabine (1-beta-d-arabinofuranosylcytosine, ara-C) was conducted in patients with refractory disease.

DESIGN

Ara-C was given i.v. at a fixed dose of 1.5 gm/m(2)/d by continuous infusion for 4 days (patients ages <65 years at time of diagnosis) or 3 days (patients ages > or =65 years). Cloretazine was given i.v. over 15 to 60 minutes on day 2 at a starting dose of 200 mg/m(2), with escalation in 100 mg/m(2) increments in cohorts of three to six patients until a maximum tolerated dose was established. The DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT) was measured at baseline.

RESULTS

Forty patients, including 32 with acute myeloid leukemia, received 47 courses of treatment. Complete responses were seen at cloretazine dose levels of > or =400 mg/m(2) in 10 of 37 (27%) evaluable patients, and in this patient subset, AGT activity was significantly lower in patients that responded to treatment than in patients who did not (P < or = 0.027). Dose-limiting toxicities (gastrointestinal and myelosuppression) were seen with 500 and 600 mg/m(2) of cloretazine combined with the 4-day ara-C schedule but not seen with the 3-day schedule.

CONCLUSION

The recommended cloretazine dose schedule for future studies is 600 mg/m(2) combined with 1.5 gm/m(2)/d continuous infusion of ara-C for 3 days. The cloretazine and ara-C regimen has significant antileukemic activity. AGT activity may be a predictor of response to cloretazine.

Role of O6-alkylguanine-DNA alkyltransferase in the cytotoxic activity of cloretazine

Cloretazine (VNP40101M; 101M; 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine) is a sulfonylhydrazine prodrug that generates both chloroethylating and carbamoylating species on activation. To explore the molecular mechanisms underlying the broad anticancer activity observed in preclinical studies, cloretazine and chloroethylating-only [i.e., 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine] and carbamoylating-only (i.e., 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine) analogues were evaluated in five murine hematopoietic cell lines. These cell lines were separable into two groups by virtue of their sensitivity to 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine; the sensitive group included L1210, P388, and F-MEL leukemias (IC50s, 6-8 micromol/L) and the resistant group consisted of Ba/F3 bone marrow and WEHI-3B leukemia cells (IC50s, 50-70 micromol/L). Resistant cells expressed O6-alkylguanine-DNA alkyltransferase (AGT), whereas sensitive cells did not. A correlation existed between AGT expression and the functional status of p53; AGT- cells possessed defective p53, whereas AGT+ cells contained wild-type p53. Based on recent findings on regulation of AGT gene expression by others, we suspect that silencing of the AGT gene by promoter hypermethylation frequently occurs during tumor progression involving p53 inactivation. O6-Chloroethylguanine is the initial DNA lesion that progresses to lethal interstrand DNA cross-links. Cloretazine exhibited a much higher preference toward the O6-chloroethylation of guanine, as measured by the difference in IC50s to wild-type and AGT-transfected L1210 cells, than 1,3-bis(2-chloroethyl)-1-nitrosourea, which targets the same site in DNA. Preferential toxicity of cloretazine against AGT- tumor cells coupled with decreased toxicity to AGT+ cells in host tissues constitute the therapeutic basis for cloretazine.

1,2-Bis(methylsulfonyl)-1-(2-chloroethyl)-2-[[1-(4-nitrophenyl)ethoxy]carbonyl]hydrazine: an anticancer agent targeting hypoxic cells

To target malignant cells residing in hypoxic regions of solid tumors, we have designed and synthesized prodrugs generating the cytotoxic alkylating species 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE) after bioreductive activation. We postulate that one of these agents, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[[1-(4-nitrophenyl)ethoxy]carbonyl]hydrazine (KS119), requires enzymatic nitro reduction to produce 90CE, whereas another agent, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(4-nitrobenzyloxy)carbonyl]hydrazine (PNBC), can also be activated by nucleophilic attack by thiols such as glutathione (GSH)/GST. We demonstrated that these agents selectively kill hypoxic EMT6 mouse mammary carcinoma and CHO cells. In hypoxia, 50 microM KS119 produced 5 logs of kill of EMT6 cells without discernable cytotoxicity in air; similar effects were observed with CHO cells. PNBC was less efficacious against hypoxic tumor cells and also had some toxicity to aerobic cells, presumably because of GST/thiol activation, making PNBC less interesting as a selective hypoxic-cell cytotoxin. BALB/c mice with established EMT6 solid tumors were used to demonstrate that KS119 could reach and kill hypoxic cells in solid tumors. To gain information on bioreductive enzymes involved in the activation of KS119, cytotoxicity was measured in CHO cell lines overexpressing NADH:cytochrome b5 reductase (NBR), NADPH:cytochrome P450 reductase (NPR), or NADPH: quinone oxidoreductase 1 (NQO1). Increased cytotoxicity occurred in cells overexpressing NBR and NPR, whereas overexpressed NQO1 had no effect. These findings were supported by enzymatic studies using purified NPR and xanthine oxidase to activate KS119. KS119 has significant potential as a hypoxia-selective tumor-cell cytotoxin and is unlikely to cause major toxicity to well oxygenated normal tissues.

Differential inhibition of cellular glutathione reductase activity by isocyanates generated from the antitumor prodrugs Cloretazine™ and BCNU

The antitumor, DNA-alkylating agent 1,3-bis[2-chloroethyl]-2-nitrosourea (BCNU; Carmustine), which generates 2-chloroethyl isocyanate upon decomposition in situ, inhibits cellular glutathione reductase (GR; EC 1.8.1.7) activity by up to 90% at pharmacological doses. GR is susceptible to attack from exogenous electrophiles, particularly carbamoylation from alkyl isocyanates, rendering the enzyme unable to catalyze the reduction of oxidized glutathione. Evidence implicates inhibition of GR as a cause of the pulmonary toxicity often seen in high-dose BCNU-treated animals and human cancer patients. Herein we demonstrate that the prodrug Cloretazine (1,2-bis[methylsulfonyl]-1-[2-chloroethyl]-2-[(methylamino)carbonyl]hydrazine; VNP40101M), which yields methyl isocyanate and chloroethylating species upon activation, did not produce similar inhibition of cellular GR activity, despite BCNU and Cloretazine being equally potent inhibitors of purified human GR (IC(50) values of 55.5 microM and 54.6 microM, respectively). Human erythrocytes, following exposure to 50 microM BCNU for 1h at 37 degrees C, had an 84% decrease in GR activity, whereas 50 microM Cloretazine caused less than 1% inhibition under the same conditions. Similar results were found using L1210 murine leukemia cells. The disparity between these compounds remained when cells were lysed prior to drug exposure and were partially recapitulated using purified enzyme when 1mM reduced glutathione was included during the drug exposure. The superior antineoplastic potential of Cloretazine compared to BCNU in animal models could be attributed in part to the contribution of the methyl isocyanate, which is synergistic with the co-generated cytotoxic alkylating species, while at the same time unable to significantly inhibit cellular GR.