1,2-Bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (VNP40101M): I. Direct inhibition of O 6 -alkylguanine-DNA alkyltransferase (AGT) by electrophilic species generated by decomposition

Development and Validation of LC-MS-MS Assay for the Determination of the Emerging Alkylating Agent Laromustine and Its Active Metabolite in Human Plasma

The objective of this study was to validate a method for the determination of laromustine (VNP40101M) and short-lived its active metabolite (VNP4090CE) that has a half-life in human blood of <90 s in human plasma by liquid chromatography (LC) with tandem mass spectrometric (MS/MS) detection. We overcome the stability dilemma by acidified the human plasma with citric acid. Laromustine "breaks" down on the source of mass spectrometry to give m/z 249 which is the same m/z for VNP4090CE. Because VNP4090CE and laromustine elute at approximate retention time of 1.93 and 2.94 min, respectively, we were able to quantify both of them in one method. VNP40101M, VNP4090CE and the internal standards were extracted from human plasma by liquid-liquid extraction into ethyl ether. The ethyl ether layer was evaporated, reconstituted and analyzed using LC with MS/MS detection. Validation parameters such as selectivity, limit of quantitation, linearity, precision, accuracy, recovery, autosampler viability, freeze-thaw cycles and compounds stability are evaluated for this method. Results were calculated using peak area ratios, and calibration curves were generated using a weighted (1/x2) linear least-squares regression. Calibration curves for VNP40101M and VNP4090CE in human plasma ranged from 1.00 to 1,000 ng/mL. In this study, both intra- and inter-assay results demonstrated a relative standard deviation for calibration standards (inter-assay) and quality control samples (intra- and inter-assay) to be ≤15.0%. In this method, there is ~1.79% isotopic interference of VNP40101M to VNP40101M-IS, and ~3.76% isotopic interference of VNP4090CE to VNP4090CE-IS. It was concluded that there was no significant carryover.

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.

Clinical Advances of Hypoxia-Activated Prodrugs in Combination With Radiation Therapy

With the increasing incidence of cancer worldwide, the need for specific, effective therapies is ever more urgent. One example of targeted cancer therapeutics is hypoxia-activated prodrugs (HAPs), also known as bioreductive prodrugs. These prodrugs are inactive in cells with normal oxygen levels but in hypoxic cells (with low oxygen levels) undergo chemical reduction to the active compound. Hypoxia is a common feature of solid tumors and is associated with a more aggressive phenotype and resistance to all modes of therapy. Therefore, the combination of radiation therapy and bioreductive drugs presents an attractive opportunity for synergistic effects, because the HAP targets the radiation-resistant hypoxic cells. Hypoxia-activated prodrugs have typically been precursors of DNA-damaging agents, but a new generation of molecularly targeted HAPs is emerging. By targeting proteins associated with tumorigenesis and survival, these compounds may result in greater selectivity over healthy tissue. We review the clinical progress of HAPs as adjuncts to radiation therapy and conclude that the use of HAPs alongside radiation is vastly underexplored at the clinical level.

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.

Biotransformation and Rearrangement of Laromustine

This review highlights the recent research into the biotransformations and rearrangement of the sulfonylhydrazine-alkylating agent laromustine. Incubation of [(14)C]laromustine with rat, dog, monkey, and human liver microsomes produced eight radioactive components (C-1 to C-8). There was little difference in the metabolite profile among the species examined, partly because NADPH was not required for the formation of most components, which instead involved decomposition and/or hydrolysis. The exception was C-7, a hydroxylated metabolite, largely formed by CYP2B6 and CYP3A4/5. Liquid chromatography-multistage mass spectrometry (LC-MS(n)) studies determined that collision-induced dissociation, and not biotransformation or enzyme catalysis, produced the unique mass spectral rearrangement. Accurate mass measurements performed with a Fourier-transform ion cyclotron resonance mass spectrometer (FTICR-MS) significantly aided determination of the elemental compositions of the fragments and in the case of laromustine revealed the possibility of rearrangement. Further, collision-induced dissociation produced the loss of nitrogen (N2) and methylsulfonyl and methyl isocyanate moieties. The rearrangement, metabolite/decomposition products, and conjugation reactions were analyzed utilizing hydrogen-deuterium exchange, exact mass, (13)C-labeled laromustine, nuclear magnetic resonance spectroscopy (NMR), and LC-MS(n) experiments to assist with the assignments of these fragments and possible mechanistic rearrangement. Such techniques produced valuable insights into these functions: 1) Cytochrome P450 is involved in C-7 formation but plays little or no role in the conversion of [(14)C]laromustine to C-1 through C-6 and C-8; 2) the relative abundance of individual degradation/metabolite products was not species-dependent; and 3) laromustine produces several reactive intermediates that may produce the toxicities seen in the clinical trials.

Tertiary amine-catalyzed and direct synthesis of α-chloroalkanesulfonylhydrazines from azodicarboxylates and sulfonyl chlorides

α-Chloroalkanesulfonylhydrazines were synthesized directly and efficiently from alkanesulfonyl chlorides and dialkyl azodicarboxylates under the catalysis of tertiary amines.

Metabolic disposition of the anti-cancer agent [(14)C]laromustine in male rats

1. Laromustine (VNP40101M, also known as Cloretazine) is a novel sulfonylhydrazine alkylating (anticancer) agent. This article describes the use of quantitative whole-body autoradiography (QWBA) and mass balance to study the tissue distribution, the excretion mass balance and pharmacokinetics after intravenous administration of [(14)C]VNP40101M to rats. A single 10 mg/kg IV bolus dose of [(14)C]VNP40101M was given to rats. 2. The recovery of radioactivity from the Group 1 animals over a 7-day period was an average of 92.1% of the administered dose, which was accounted for in the excreta and carcass. Most of the radioactivity was eliminated within 48 h via urine (48%), with less excreted in feces (5%) and expired air accounted for (11%). The plasma half-life of [(14)C]laromustine was approximately 62 min and the peak plasma concentration (Cmax) averaged 8.3 μg/mL. 3. The QWBA study indicated that the drug-derived radioactivity was widely distributed to tissues through 7 days post-dose after a single 10 mg/kg IV bolus dose of [(14)C]VNP40101M to male pigmented Long-Evans rats. The maximum concentrations were observed at 0.5 or 1 h post-dose for majority tissues (28 of 42). The highest concentrations of radioactivity were found in the small intestine contents at 0.5 h (112.137 µg equiv/g), urinary bladder contents at 3 h (89.636 µg equiv/g) and probably reflect excretion of drug and metabolites. The highest concentrations in specific organs were found in the renal cortex at 1 h (28.582 µg equiv/g), small intestine at 3 h (16.946 µg equiv/g), Harderian gland at 3 h (12.332 µg equiv/g) and pancreas at 3 h (12.635 µg equiv/g). Concentrations in the cerebrum (1.978 µg equiv/g), cerebellum (2.109 µg equiv/g), medulla (1.797 µg equiv/g) and spinal cord (1.510 µg equiv/g) were maximal at 0.5 h post-dose and persisted for 7 days. 4. The predicted total body and target organ exposures for humans given a single 100 µCi IV dose of [(14)C]VNP40101M were well within the medical guidelines for maximum radioactivity exposures in human subjects.

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.

Chloroethylating and methylating dual function antineoplastic agents display superior cytotoxicity against repair proficient tumor cells

Two new agents based upon the structure of the clinically active prodrug laromustine were synthesized. These agents, 2-(2-chloroethyl)-N-methyl-1,2-bis(methylsulfonyl)-N-nitrosohydrazinecarboxamide (1) and N-(2-chloroethyl)-2-methyl-1,2-bis(methylsulfonyl)-N-nitrosohydrazinecarboxamide (2), were designed to retain the potent chloroethylating and DNA cross-linking functions of laromustine, and gain the ability to methylate DNA at the O-6 position of guanine, while lacking the carbamoylating activity of laromustine. The methylating arm was introduced with the intent of depleting the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Compound 1 is markedly more cytotoxic than laromustine in both AGT minus EMT6 mouse mammary carcinoma cells and high AGT expressing DU145 human prostate carcinoma cells. DNA cross-linking studies indicated that its cross-linking efficiency is nearly identical to its predicted active decomposition product, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE), which is also produced by laromustine. AGT ablation studies in DU145 cells demonstrated that 1 can efficiently deplete AGT. Studies assaying methanol and 2-chloroethanol production as a consequence of the methylation and chloroethylation of water by 1 and 2 confirmed their ability to function as methylating and chloroethylating agents and provided insights into the superior activity of 1.

Design of a hypoxia-activated prodrug inhibitor of O6-alkylguanine-DNA alkyltransferase

The efficacy of agents that alkylate the O-6 position of guanine is inhibited by O(6)-alkylguanine-DNA alkyltransferase (AGT) which removes these lesions from the tumor DNA. To increase differential toxicity, inhibitors must selectively deplete AGT in tumors, while sparing normal tissues where this protein serves a protective function. A newly synthesized prodrug of the AGT inhibitor O(6)-benzylguanine (O(6)-BG) with an α,α-dimethyl-4-nitrobenzyloxycarbonyl moiety masking the essential 2-amino group has demonstrated the feasibility of targeting hypoxic regions that are unique to solid tumors, for drug delivery. However, these modifications resulted in greatly decreased solubility. Recently, new potent global AGT inhibitors with improved formulatability such as O(6)-[(3-aminomethyl)benzylguanine (1) have been developed. However, acetylamino (N-(3-(((2-amino-9H-purin-6-yl)oxy)methyl)benzyl)acetamide) (2) exhibits a pronounced decrease in activity. Thus, 1 would be inactivated by N-acetylation and probably N-glucuronidation. To combat potential conjugational inactivation while retaining favorable solubility, we synthesized 6-((3-((dimethylamino)methyl)benzyl)oxy)-9H-purin-2-amine (3) in which the 3-aminomethyl moiety is protected by methylation; and to impart tumor selectivity we synthesized 2-(4-nitrophenyl)propan-2-yl(6-((3-((dimethylamino)methyl)benzyl)oxy)-9H-purin-2-yl)carbamate (7), a hypoxia targeted prodrug of 3 utilizing an α,α-dimethyl-4-nitrobenzyloxycarbonyl moiety. Consistent with this design, 7 demonstrates both hypoxia selective conversion by EMT6 cells of 7 to 3 and hypoxic sensitization of AGT containing DU145 cells to the cytotoxic actions of laromustine, while exhibiting improved solubility.

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.

A strategy for selective O(6)-alkylguanine-DNA alkyltransferase depletion under hypoxic conditions

Cellular resistance to chemotherapeutics that alkylate the O-6 position of guanine residues in DNA correlates with their O(6)-alkylguanine-DNA alkyltransferase activity. In normal cells high [O(6)-alkylguanine-DNA alkyltransferase] is beneficial, sparing the host from toxicity, whereas in tumor cells high [O(6)-alkylguanine-DNA alkyltransferase] prevents chemotherapeutic response. Therefore, it is necessary to selectively inactivate O(6)-alkylguanine-DNA alkyltransferase in tumors. The oxygen-deficient compartment unique to solid tumors is conducive to reduction, and could be utilized to provide this selectivity. Therefore, we synthesized 2-nitro-6-benzyloxypurine, an analog of O(6)-benzylguanine in which the essential 2-amino group is replaced by a nitro moiety, and 2-nitro-6-benzyloxypurine is >2000-fold weaker than O(6)-benzylguanine as an O(6)-alkylguanine-DNA alkyltransferase inhibitor. We demonstrate oxygen concentration sensitive net reduction of 2-nitro-6-benzyloxypurine by cytochrome P450 reductase, xanthine oxidase, and EMT6, DU145, and HL-60 cells to yield O(6)-benzylguanine. We show that 2-nitro-6-benzyloxypurine treatment depletes O(6)-alkylguanine-DNA alkyltransferase in intact cells under oxygen-deficient conditions and selectively sensitizes cells to laromustine (an agent that chloroethylates the O-6 position of guanine) under oxygen-deficient but not normoxic conditions. 2-Nitro-6-benzyloxypurine represents a proof of concept lead compound; however, its facile reduction (E(1/2) - 177 mV versus Ag/AgCl) may result in excessive oxidative stress and/or the generation of O(6)-alkylguanine-DNA alkyltransferase inhibitors in normoxic regions in vivo.

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.

AN ONIOM STUDY ON THE CROSSLINKED BASE PAIRS IN DNA REACTED WITH CHLOROETHYLNITROSOUREAS

Chloroethylnitrosoureas (CENUs) are clinically useful anticancer agents. Their cytotoxicity is associated with the generation of DNA interstrand crosslinks. QM/MM computations are carried out to investigate DNA crosslinks by CENUs with ONIOM hybrid method. The crosslinked DNA are subdivided into three layers, each of which are described at B3LYP/6-311+G(d,p), AM1, and UFF level of theory, respectively. The result shows that the deformation of DNA with dG ( N 1)– dC ( N 3) crosslink is much less than the other crosslinks, which indicate that the most favorable crosslink is between the N1 atom of guanine and the N3 atom of the complementary cytosine.

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.

Characterization of short-lived electrophilic metabolites of the anticancer agent laromustine (VNP40101M)

Laromustine (VNP40101M; 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-(methylamino) carbonylhydrazine) is a novel sulfonylhydrazine alkylating agent. Phase 1 metabolism of laromustine was reported recently and showed that laromustine undergoes rearrangement, dehalogenation, and hydrolysis at physiological pH to form active moieties. (1) A mechanism for the rearrangement was proposed on the basis of fragmentation ions. (1) (,) (2) In this article, we report the phase II conjugates of VNP40101M and VNP4090CE which were formed after incubation of VNP40101M or VNP4090CE with pooled human liver microsomes (HLM) and cofactors nicotinamide adenine dinucleotide phosphate (NADPH), glutathione (GSH), N-acetylecysteine (NAC), and cysteine (CYS). Eight novel phase II conjugates (M-1 to M-8) were identified and characterized by hydrogen-deuterium exchange (H-D), stable isotope ((13)C-labeled VNP40101M), and MS(n) experiments. M-4 and M-5 were further confirmed by nuclear magnetic resonance spectroscopy (NMR). The short-lived CH(3)SO(2)CH(2)CH(2)-, methylformamide and CH(3)SO(2)NHN═CHCH(2)- moieties were generated from VNP40101M. The reactive intermediates CH(3)SO(2)CH(2)CH(2)- and methylformamide formed conjugates with GSH, CYS, and NAC. The CH(3)SO(2)NHN═CHCH(2)- moiety formed conjugates with GSH and NAC. M-2, M-4, and M-6 were only detected from the incubation of VNP40101M because VNP4090CE does not contain a methylformamide group. All other conjugates were formed by both VNP40101M and VNP4090CE. The in vitro studies found that VNP40101M and VNP4090CE undergo activation in human liver microsomes. The results from this study showed that laromustine produces several reactive intermediates that may play a role in the toxicities seen in the clinical trials.

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.

An in vitro evaluation of the victim and perpetrator potential of the anticancer agent laromustine (VNP40101M), based on reaction phenotyping and inhibition and induction of cytochrome P450 enzymes

Laromustine (VNP40101M, also known as Cloretazine) is a novel sulfonylhydrazine alkylating (anticancer) agent. Laromustine generates two types of reactive intermediates: 90CE and methylisocyanate. When incubated with rat, dog, monkey, and human liver microsomes, [(14)C]laromustine was converted to 90CE (C-8) and seven other radioactive components (C-1-C-7). There was little difference in the metabolite profile among the species examined, in part because the formation of most components (C-1-C-6 and 90CE) did not require NADPH but involved decomposition and/or hydrolysis. The exception was C-7, a hydroxylated metabolite, largely formed by CYP2B6 and CYP3A4/5. Laromustine caused direct inhibition of CYP2B6 and CYP3A4/5 (the two enzymes involved in C-7 formation) as well as of CYP2C19. K(i) values were 125 microM for CYP2B6, 297 muM for CYP3A4/5, and 349 microM for CYP2C19 and were greater than the average clinical plasma C(max) of laromustine (25 microM). There was evidence of time-dependent inhibition of CYP1A2, CYP2B6, and CYP3A4/5. Treatment of primary cultures of human hepatocytes with up to 100 microM laromustine did not induce CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, or CYP3A4/5, but the highest concentration of laromustine decreased the activity and levels of immunoreactive CYP3A4. The results of this study suggest the laromustine has 1) negligible victim potential with respect to metabolism by cytochrome P450 enzymes, 2) negligible enzyme-inducing potential, and 3) the potential in some cases to cause inhibition of CYP2B6, CYP3A4, and possibly CYP2C19 during and shortly after the duration of intravenous administration of this anticancer drug, but the clinical effects of such interactions are likely to be insignificant.

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.

Anti proliferative activity of ELACY (CP-4055) in combination with cloretazine (VNP40101M), idarubicin, gemcitabine, irinotecan and topotecan in human leukemia and lymphoma cells

This study evaluated combination drug partners for CP-4055, the C18:1^Δ9,trans unsaturated fatty acid ester of cytarabine in HL-60 and U937 cells. Growth inhibition was assessed by ATP assay and drug interaction by the combination index and three dimensional methods. Synergy was observed in HL-60 cells for simultaneous combinations of CP-4055 with gemcitabine, irinotecan and topotecan, while combinations with cloretazine (VNP40101M) and idarubicin were additive. In U937 cells, synergy was observed with gemcitabine and additivity for the other drugs. In HL-60, the IC50 concentration of CP-4055 could be reduced 10-fold and that of gemcitabine 3-fold in combination versus the agents alone, an interaction that was independent of drug sequence, ratio and exposure time. In contrast, interactions of CP-4055 with the topoisomerase inhibitors became antagonistic when the drugs were administered 24 h prior to CP-4055 and at certain drug ratios, particularly in U937 cells. In summary, CP-4055 produced additive to synergistic anti proliferative activity when combined simultaneously with drugs from four mechanistic classes in cell culture models of human leukemia and lymphoma. The impact of drug sequence and ratio on the interactions argues for incorporation of these parameters into the design of combination chemotherapy regimens.

Phase I trial of VNP40101M (Cloretazine) in children with recurrent brain tumors: a pediatric brain tumor consortium study

PURPOSE

VNP40101M (Cloretazine), a novel DNA alkylating agent, was evaluated in a phase I study in children with recurrent brain tumors.

EXPERIMENTAL DESIGN

VNP40101M was given i.v. daily for 5 consecutive days every 6 weeks for up to eight cycles. Dose escalation was done independently in patients stratified based on intensity of prior therapy (moderately pretreated, stratum I; heavily pretreated, stratum II). Correlative studies included pharmacokinetics and measurement of O(6)-alkylguanine-DNA alkyl transferase levels in peripheral blood mononuclear cells before and after treatment.

RESULTS

Forty-one eligible patients (stratum I, 19; stratum II, 22) were enrolled on this study. The dose-limiting toxicity in 35 evaluable patients was myelosuppression, which occurred in 4 of 16 patients in stratum I and 3 of 19 patients in stratum II. Pharmacokinetic studies showed a median terminal half-life of 30 min (range, 14-39.5). The maximum tolerated dose in stratum I and II were 45 and 30 mg/m(2)/d daily for 5 days every 6 weeks, respectively. Peripheral blood mononuclear cells alkylguanine alkyl transferase levels did not decrease significantly after VNP40101M treatment. Central imaging review confirmed that three patients had stable disease for a median of 45 weeks (range, 37-61+) after therapy.

CONCLUSIONS

The recommended dose of VNP40101M for phase II studies in children with brain tumors is 45 mg/m(2)/d in moderately pretreated and 30 mg/m(2)/d in heavily pretreated patients when administered for 5 consecutive days every 6 weeks.

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.

Determination of Cloretazine (VNP40101M) and its active metabolite (VNP4090CE) in human plasma by liquid chromatography electrospray tandem mass spectrometry (LC-ESI-MS/MS)

A sensitive method for the determination of Cloretazine (VNP40101M) and its metabolite (VNP4090CE) with an internal standard (ISTD) in human plasma was developed using high-performance liquid chromatographic separation with tandem mass spectrometric detection. Acidified plasma samples (500 microL) were prepared using solid phase extraction (SPE) columns, and 25 microL of the reconstituted sample was injected onto an Ascentis C18 HPLC column (3 microm, 5 cmx2.1 mm) with an isocratic mobile phase. Analytes were detected with an API-3000 LC-MS/MS System at unit (Q1) and low (Q3) resolution in negative multiple reaction monitoring mode: m/z 249.0 (precursor ion) to m/z 114.9 (product ion) for both Cloretazine (at 3.64 min) and VNP4090CE (at 2.91 min), and m/z 253.0 (precursor ion) to m/z 116.9 (product ion) for the ISTD. The mean recovery for Cloretazine (VNP40101M) and its metabolite (VNP4090CE) was greater than 87% with a lower limit of quantification of 1.0 ng/mL for Cloretazine (S/N=9.7, CV<or=12%) and 0.5 ng/mL for VNP4090CE (S/N=11.3, CV<or=9.7%). This method was validated over a linear range of 1.0-1000 ng/mL for Cloretazine and 0.5-100 ng/mL for VNP4090CE, and results from a five day validation study demonstrated good within-day and between-day precision and accuracy. This method has been used to measure plasma Cloretazine and its metabolite concentrations in a Phase I study in children with recurrent progressive or refractory primary brain tumors.

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.

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.

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.