Synthesis of 4'-thio-beta-D-arabinofuranosylcytosine (4'-thio-ara-C) and comparison of its anticancer activity with that of ara-C

Discovery of a Novel Template, 7-Substituted 7-Deaza-4'-Thioadenosine Derivatives as Multi-Kinase Inhibitors

The development of anticancer drugs remains challenging owing to the potential for drug resistance. The simultaneous inhibition of multiple targets involved in cancer could overcome resistance, and these agents would exhibit higher potency than single-target inhibitors. Protein kinases represent a promising target for the development of anticancer agents. As most multi-kinase inhibitors are heterocycles occupying only the hinge and hydrophobic region in the ATP binding site, we aimed to design multi-kinase inhibitors that would occupy the ribose pocket, along with the hinge and hydrophobic region, based on ATP-kinase interactions. Herein, we report the discovery of a novel 4'-thionucleoside template as a multi-kinase inhibitor with potent anticancer activity. The in vitro evaluation revealed a lead 1g (7-acetylene-7-deaza-4'-thioadenosine) with potent anticancer activity, and marked inhibition of TRKA, CK1δ, and DYRK1A/1B kinases in the kinome scan assay. We believe that these findings will pave the way for developing anticancer drugs.

Safety Assessment of 4′-Thio-β-d-Arabinofuranosylcytosine in the Beagle Dog Suggests a Drug-Induced Centrally Mediated Effect on the Hypothalamic-Pituitary-Adrenal Axis

4′-Thio- β-d-arabinofuranosylcytosine (OSI-7836) is a nucleoside analogue with structural similarity to gemcitabine and cytarabine (ara-C). Myelosuppression, reversible transaminase elevations, and flu-like symptoms are common side effects associated with human use of gemcitabine and ara-C. Fatigue is also associated with the use of gemcitabine and OSI-7836 in humans. To better understand the toxicity of OSI-7836, subchronic studies were conducted in dogs. OSI-7836 was administered on days 1 and 8 or on days 1, 2, and 3 of a 21-day dose regimen. These schedules attempted to match clinical trial dosing regimens. Routine toxicity study end points demonstrated that OSI-7836 was primarily cytotoxic to the gastrointestinal tract, bone marrow, and testes; the myelotoxicity was mild and reversible. Plasma pharmacokinetics were dose-linear with an elimination half-life of 2.2 h. Follow-up single dose experiments in dogs assessed drug effects on lymphocyte subpopulations and on adrenal and thyroid function. Populations of T and B cells were equally reduced following OSI-7836 administration. There were no adverse effects on thyroid function, but there were marked reductions in circulating cortisol and adrenocorticotropic hormone concentrations suggesting a centrally mediated impairment of the hypothalamic-pituitary-adrenal axis. These findings show a toxicological profile with OSI-7836 similar to other nucleoside analogues and suggest that the beagle is a model for studying one possible cause of OSI-7836-related fatigue, impaired function of the hypothalamic-pituitary-adrenal axis.

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

Advances in the development of nucleoside and nucleotide analogues for cancer and viral diseases

Nucleoside analogues have been in clinical use for almost 50 years and have become cornerstones of treatment for patients with cancer or viral infections. The approval of several additional drugs over the past decade demonstrates that this family still possesses strong potential. Here, we review new nucleoside analogues and associated compounds that are currently in preclinical or clinical development for the treatment of cancer and viral infections, and that aim to provide increased response rates and reduced side effects. We also highlight the different approaches used in the development of these drugs and the potential of personalized therapy.

Advances in the development of nucleoside and nucleotide analogues for cancer and viral diseases

Nucleoside analogues have been in clinical use for almost 50 years and have become cornerstones of treatment for patients with cancer or viral infections. The approval of several additional drugs over the past decade demonstrates that this family still possesses strong potential. Here, we review new nucleoside analogues and associated compounds that are currently in preclinical or clinical development for the treatment of cancer and viral infections, and that aim to provide increased response rates and reduced side effects. We also highlight the different approaches used in the development of these drugs and the potential of personalized therapy.

Preclinical antitumor activity of thiarabine in human leukemia and lymphoma xenograft models

Thiarabine was evaluated for antitumor activity in seven human leukemia, lymphoma, and myeloma xenograft models to explore the activity in hematological malignancies. Thiarabine was active against all of the human leukemia and lymphoma lines tested, being curative against HL-60 leukemia and AS283 lymphoma and effecting tumor regressions in CCRF-CEM, MOLT-4, and K-562 leukemia and RL lymphoma models, but did not exhibit any appreciable activity against RPMI-8226 myeloma. For the leukemia/lymphoma models, thiarabine was more efficacious than ara-C/palmO-ara-C (four models), clofarabine (three models), fludarabine monophosphate (five models), cladribine (four models), and gemcitabine (six models). Thiarabine warrants future clinical trials with leukemias/lymphomas.

Preclinical combination therapy of thiarabine plus various clinical anticancer agents

Thiarabine is undergoing clinical trials. In support of that effort combination therapy of thiarabine plus six clinical anticancer agents was evaluated using various human tumor xenograft models. The antitumor activity of thiarabine in combination appeared to be greater than additive with irinotecan (DLD-1 colon), paclitaxel (PC-3 prostate), cisplatin (PC-3 prostate), or cyclophosphamide (RL lymphoma), additive with irinotecan (NCI-H460 NSCLC), cisplatin (NCI-H460 NSCLC) or methotrexate (CCRF-CEM leukemia), and less than additive with irinotecan (HT29 colon), paclitaxel (NCI-H460 NSCLC) or cisplatin (NCI-H23 NSCLC). Combining thiarabine with irinotecan, paclitaxel, cisplatin, or cyclophosphamide should receive consideration in the clinical treatment of cancer.

Isolation and characterization of a murine P388 leukemia line resistant to thiarabine

A murine P388 leukemia line fully resistant to thiarabine was obtained after five courses of intraperitoneal treatment (daily for nine consecutive days). The subline was sensitive as was the parental P388/0 line to 5-fluorouracil, gemcitabine, cyclophosphamide, cisplatin, melphalan, BCNU, mitomycin C, doxorubicin, mitoxantrone, etoposide, irinotecan, vincristine, and paclitaxel, but was cross resistant (at least marginally) to three antimetabolites: palmO-ara-C, fludarabine phosphate, and methotrexate. The deoxycytidine kinase activity in the subline was comparable to that for P388/0, whereas the dCMP deaminase activity was 43% of that for P388/0. No deoxycytidine deaminase activity was detected in either of the leukemias. There appeared to be little, if any, difference in the metabolism of deoxycytidine, cytidine, or thiarabine in the two leukemias.

Isolation and characterization of a murine P388 leukemia line resistant to clofarabine

A murine P388 leukemia line fully resistant to clofarabine was obtained after only two courses of intraperitoneal treatment (three times a day for nine consecutive days). The resistance was stable for at least 13 weeks without treatment. The subline was as sensitive to 5-fluorouracil, methotrexate, cyclophosphamide, cisplatin, melphalan, BCNU, doxorubicin, etoposide, irinotecan, vincristine, and docetaxel as was the parental P388/0 line but was cross-resistant to five antimetabolites [palmO-ara-C, 4'-thio-ara-C, fludarabine phosphate, cladribine, and gemcitabine-all of which require deoxycytidine kinase for activation] and paclitaxel. The subline had less than 1% of the deoxycytidine kinase activity in comparison to P388/0.

New nucleoside analogs for patients with hematological malignancies

INTRODUCTION

In the last few years, several new purine and pyrimidine nucleoside analogs have been synthesized and made available for both preclinical studies and clinical trials.

AREAS COVERED

This article summarizes recent achievements in the mechanism of action, pharmacological properties and clinical activity and toxicity as well as the emerging role of newer purine and pyrimidine nucleoside analogs potentially active in lymphoid and myeloid malignancies. A literature review was conducted from the MEDLINE database PubMed for articles in English. Publications from 2000 to October 2010 were scrutinized. The search terms used were clofarabine, nelarabine, forodesine, 8-chloroadenosine, LMP-420, azacitidine, decitabine, sapacitabine, troxacitabine, thiarabine and zebularine in conjunction with hematologic malignancies, leukemia and lymphoma. Conference proceedings from the previous 5 years of the American Society of Hematology, European Hematology Association, and American Society of Clinical Oncology were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

EXPERT OPINION

Several new nucleoside analogs are currently under investigation in preclinical and clinical studies concerning hematological malignancies. Clofarabine, nelarabine, azacitidine and decitabine have been recently approved for the treatment of leukemias and/or myelodysplastic syndromes. Other agents including forodesine, 8-chloroadenosine, LMP-420, sapacitabine, troxacitabine, thiarabine and zebularine seem to be promising for the treatment of lymphoid and myeloid malignancies. However, definitive data from ongoing and future clinical trials will aid in better defining their status in the treatment of hematological disorders.

Lack of in vivo cross-resistance with 4'-thio-ara-C against drug-resistant murine P388 and L1210 leukemias

PURPOSE

4'-Thio-β-D-arabinofuranosylcytosine (4'-thio-ara-C), which has shown a broad spectrum of antitumor activity against human tumor systems in mice and is undergoing clinical trials, was evaluated for cross-resistance to seven clinical agents in order to identify potentially useful guides for patient selection for further clinical trials of 4'-thio-ara-C and possible noncross-resistant drug combinations with 4'-thio-ara-C.

METHODS

A drug resistance profile for 4'-thio-ara-C, which was administered intraperitoneally daily for nine consecutive days, was obtained using seven drug-resistant P388 and L1210 leukemias that were implanted intraperitoneally in mice.

RESULTS

Multidrug-resistant P388 leukemias (leukemias resistant to doxorubicin, etoposide, or paclitaxel) exhibited no cross-resistance to 4'-thio-ara-C. Leukemias resistant to camptothecin, cisplatin, and 5-fluorouracil were also not cross-resistant to 4'-thio-ara-C. Only the leukemia resistant to 1-β-D-arabinofuranosylcytosine was cross-resistant to 4'-thio-ara-C.

CONCLUSIONS

The data suggest that (1) it may be important to exclude or to monitor with extra care patients who have previously been treated with 1-β-D-arabinofuranosylcytosine and (2) the lack of cross-resistance seen with 4'-thio-ara-C may contribute to therapeutic synergism when 4'-thio-ara-C is combined with other agents.

Inhibition of herpesvirus replication by 5-substituted 4'-thiopyrimidine nucleosides

A series of 4'-thionucleosides were synthesized and evaluated for activities against orthopoxviruses and herpesviruses. We reported previously that one analog, 5-iodo-4'-thio-2'-deoxyuridine (4'-thioIDU), exhibits good activity both in vitro and in vivo against two orthopoxviruses. This compound also has good activity in cell culture against many of the herpesviruses. It inhibited the replication of herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus with 50% effective concentrations (EC(50)s) of 0.1, 0.5, and 2 microM, respectively. It also inhibited the replication of human cytomegalovirus (HCMV) with an EC(50) of 5.9 microM but did not selectively inhibit Epstein-Barr virus, human herpesvirus 6, or human herpesvirus 8. While acyclovir-resistant strains of HSV-1 and HSV-2 were comparatively resistant to 4'-thioIDU, it retained modest activity (EC(50)s of 4 to 12 microM) against these strains. Some ganciclovir-resistant strains of HCMV also exhibited reduced susceptibilities to the compound, which appeared to be related to the specific mutations in the DNA polymerase, consistent with the observed incorporation of the compound into viral DNA. The activity of 4'-thioIDU was also evaluated using mice infected intranasally with the MS strain of HSV-2. Although there was no decrease in final mortality rates, the mean length of survival after inoculation increased significantly (P < 0.05) for all animals receiving 4'-thioIDU. The findings from the studies presented here suggest that 4'-thioIDU is a good inhibitor of some herpesviruses, as well as orthopoxviruses, and this class of compounds warrants further study as a therapy for infections with these viruses.

Activities of certain 5-substituted 4'-thiopyrimidine nucleosides against orthopoxvirus infections

As part of a program to identify new compounds that have activity against orthopoxviruses, a number of 4'-thionucleosides were synthesized and evaluated for their efficacies against vaccinia and cowpox viruses. Seven compounds that were active at about 1 microM against both viruses in human cells but that did not have significant toxicity were identified. The 5-iodo analog, 1-(2-deoxy-4-thio-beta-d-ribofuranosyl)-5-iodouracil (4'-thioIDU), was selected as a representative molecule; and this compound also inhibited viral DNA synthesis at less than 1 microM but only partially inhibited the replication of a recombinant vaccinia virus that lacked a thymidine kinase. This compound retained complete activity against cidofovir- and ST-246-resistant mutants. To determine if this analog had activity in an animal model, mice were infected intranasally with vaccinia or cowpox virus and treatment with 4'-thioIDU was given intraperitoneally or orally twice daily at 50, 15, 5, or 1.5 mg/kg of body weight beginning at 24 to 120 h postinfection and was continued for 5 days. Almost complete protection (87%) was observed when treatment with 1.5 mg/kg was begun at 72 h postinfection, and significant protection (73%) was still obtained when treatment with 5 mg/kg was initiated at 96 h. Virus titers in the liver, spleen, and kidney were reduced by about 4 log(10) units and about 2 log(10) units in mice infected with vaccinia virus and cowpox virus, respectively. These results indicate that 4'-thioIDU is a potent, nontoxic inhibitor of orthopoxvirus replication in cell culture and experimental animal infections and suggest that it may have potential for use in the treatment of orthopoxvirus infections in animals and humans.

Enhancement of the in vivo antitumor activity of clofarabine by 1-beta-D-[4-thio-arabinofuranosyl]-cytosine

PURPOSE

Clofarabine increases the activation of 1-beta-D-arabinofuranosyl cytosine (araC) in tumor cells, and combination of these two drugs has been shown to result in good clinical activity against various hematologic malignancies. 1-beta-D-[4-thio-arabinofuranosyl] cytosine (T-araC) is a new cytosine analog that has exhibited excellent activity against a broad spectrum of human solid tumors and leukemia/lymphoma xenografts in mice and is currently being evaluated in patients as a new drug for the treatment of cancer. Since T-araC has a vastly superior preclinical efficacy profile in comparison to araC, we have initiated studies to determine the potential value of clofarabine/T-araC combination therapy.

METHODS

In vitro studies have been conducted to determine the effect of clofarabine on the metabolism of T-araC, and in vivo studies have been conducted to determine the effect of the clofarabine/T-araC combination on five human tumor xenografts in mice.

RESULTS

Initial studies with various tumor cells in culture indicated that a 2-h incubation with clofarabine enhanced the metabolism of T-araC 24 h after its removal by threefold in three tumor cell types (HCT-116 colon, K562 leukemia, and RL lymphoma) and by 1.5-fold in two other tumor cell types (MDA-MB-435 breast (melanoma), and HL-60 leukemia). Pretreatment with clofarabine resulted in a slight decrease in metabolism of T-araC in RPMI-8226 myeloma cells (65% of control) and inhibited metabolism of T-araC in CCRF-CEM leukemia cells by 90%. In vivo combination studies were conducted with various human tumor xenografts to determine whether or not the modulations observed in vitro were reflective of the in vivo situation. Clofarabine and T-araC were administered on alternate days for five treatments each (q2dx5) with the administration of T-araC 24 h after each clofarabine treatment. Combination treatment of HCT-116, K562, HL-60, or RL tumors with clofarabine and T-araC resulted in dramatically superior anti-tumor activity than treatment with either agent alone, whereas this combination resulted in antagonism in CCRF-CEM tumors. The in vivo antitumor activity of clofarabine plus T-araC against HCT-116 tumors was much better than the activity seen with clofarabine plus araC.

CONCLUSIONS

These studies provide a rationale for clinical trials using this combination in the treatment of acute leukemias as well as solid tumors and suggest that this combination would exhibit greater antitumor activity than that of clofarabine plus araC.

Antiangiogenic activity of 4′-thio-β-d-arabinofuranosylcytosine

4'-Thio-beta-D-arabinofuranosylcytosine (T-araC), a new-generation deoxycytidine nucleoside analogue, showed significant efficacy against numerous solid tumors in preclinical studies and entered clinical development for cancer therapy. It is a structural analogue of cytarabine (araC), a clinically used drug in the treatment of acute myelogenous leukemia, which has no or very limited efficacy against solid tumors. In comparison with araC, the excellent in vivo activity of T-araC against solid tumors suggests that, in addition to inhibition of DNA synthesis, T-araC may target cellular signaling pathways, such as angiogenesis, in solid tumors. We studied T-araC and araC for their antiangiogenic activities in vitro and in vivo. Both compounds inhibited human endothelial cell proliferation with similar IC50s. However, only T-araC inhibited endothelial cell migration and differentiation into capillary tubules. T-araC also abrogated endothelial cell extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, a key signaling molecule involved in cellular processes of angiogenesis. Results from chick chorioallantoic membrane angiogenesis assays revealed that T-araC significantly inhibited the development of new blood vessels in vivo, whereas araC showed much less effect. The findings of this study show a role of T-araC in antiangiogenesis and suggest that T-araC combines antiproliferative and antiangiogenic activity in one molecule for a dual mechanism of drug action to achieve the excellent in vivo efficacy against several solid tumors. This study also provides important information for optimizing dosage and sequence of T-araC administration in clinical investigations by considering T-araC as both an antiproliferative and an antiangiogenic agent.

Synthesis and biological activity of 4'-thio-L-xylofuranosyl purine nucleosides

A series of some new 4'-thio-L-xylofuranosyl nucleosides were prepared and evaluated as potential anticancer agents. A versatile sugar intermediate for direct coupling with the purine moiety is also synthesized by an efficient and high-yielding route. Proof of structure and configuration at all chiral centers of the nucleosides was obtained by proton NMR. All target compounds were evaluated in a series of human cancer cell lines in vitro. The details of the synthesis of the carbohydrate precursor 1-O-acetyl-2,3,5-tri-O-benzyl-4-thio-L-xylofuranose (6) and corresponding purine nucleosides are presented in the manuscript.

A phase I, first in man study of OSI-7836 in patients with advanced refractory solid tumors: IND.147, a study of the Investigational New Drug Program of the National Cancer Institute of Canada Clinical Trials Group

PURPOSE

To determine the maximum tolerated dose (MTD), recommended phase II dose (RP2D), safety, tolerability, toxicity profile, dose-limiting toxicities (DLTs), anti-tumor activity and pharmacokinetics of OSI-7836 given IV on day 1 and day 8 every 3 weeks in patients with advanced incurable cancer.

METHODS

Twenty-seven previously treated patients with advanced or metastatic solid tumors were enrolled in this phase I study conducted by the National Cancer Institute of Canada Clinical Trial Group (NCIC CTG). OSI-7836 was administered IV on day 1 and day 8 every 3 weeks. The dose was initially escalated from 100 to 600 mg/m2 and finally de-escalated to 200 mg/m2 in seven cohorts of patients. Patients were evaluated every other cycle of treatment for radiological response. Pharmacokinetics were performed on day 1 and day 8 of cycle 1 for all patients.

RESULTS

Twenty-six patients were evaluable for toxicity. All patients experienced reversible Grade 3 lymphopenia beginning at cycle 1. The maximal delivered dose was 600 mg/m2. MTD was reached at 400 mg/m2. DLTs included fever, fatigue, rash, herpes simplex infection, nausea and vomiting. The RP2D was 200 mg/m2. No objective responses were seen in 21 evaluable patients. Pharmacokinetics were dose proportional, with a mean half-life of 46.0 min and a clearance of 34 l/(h.m2).

CONCLUSION

OSI-7836 given at 200 mg/m2 on day 1 and day 8 every 3 weekly is associated with manageable toxicity and is recommended for further study. While no objective responses were seen, the significant treatment related lymphopenia suggests that hematologic malignancies may warrant further investigation.

Chemical and enzymatic synthesis of 4'-thio-beta-D-arabinofuranosylcytosine monophosphate and triphosphate

N4-Acetyl-1-(2, 3-di-O-acetyl-4-thio-beta-D-arabinofuranosyl) cytosine (2) was synthesized in three steps from 1-(4-thio-beta-D-arabinofuranosyl) cytosine (1). The reaction of this partially blocked 4'-thio-ara-C derivative 2 with 2-chloro-4H-1,3,2-benzodioxaphosphorin-4-one gave the 5-phosphitylate derivative 3, which on reaction with pyrophosphate gave the 5'-nucleosidylcyclotriphosphite 4. Product 4 was then oxidized with iodine/pyridine/water and deblocked with concentrated ammonium hydroxide to provide the desired 4'-thio-ara-C-5'-triphosphate 5. This triphosphate 5 was converted to 4'-thio-ara-C -5'-monophosphate 6 by treatment with snake venom phosphodiesterase I. The details of the synthesis, purification, and characterization of both nucleotides are described.

Long intracellular retention of 4'-thio-arabinofuranosylcytosine 5'-triphosphate as a critical factor for the anti-solid tumor activity of 4'-thio-arabinofuranosylcytosine

4'-Thio-arabinofuranosylcytosine (T-araC) is a new cytosine analog, which exhibits excellent antitumor activity against various solid tumor xenografts in mice. T-araC is a structural analog of arabinofuranosylcytosine (araC), which is known to be marginally active against solid tumors. We have continued to study the biochemical pharmacology of T-araC in solid tumor cells to further characterize the mechanism of action of this new agent and to elucidate why these compounds show a profound difference in antitumor activity against solid tumors. AraC was a slightly more potent inhibitor of cell growth than T-araC when cells were continuously exposed to the drugs. However, T-araC was markedly more cytotoxic than araC when high concentrations of the compounds were given for short periods of time. Despite the fact that T-araC is a much poorer substrate, as compared to araC, for deoxycytidine kinase (the rate-limiting step in the formation of the triphosphates), similar intracellular concentrations of T-araC-5'-triphosphate (T-araCTP) and araCTP were formed in cells at these high, pharmacologically relevant concentrations due to similar Vmax's. The major difference in the metabolism of araC and T-araC was that the half-life of T-araCTP was tenfold longer than that of araCTP and much higher levels of T-araCTP were sustained in cells for long durations after exposure to T-araC. Inhibition of cytidine deaminase, deoxycytidylate deaminase, or DNA replication did not affect the half-life of either araCTP or T-araCTP. In addition, the rates of disappearance of the mono- and tri-phosphates of araC and T-araC in crude cell extracts were similar. These results indicated that these enzymes were not rate-limiting in the degradation of the respective triphosphates. However, the rate of phosphorylation of T-araC-5'-monophosphate (T-araCMP) in crude cell extracts was about tenfold greater than that of araCMP. The results of this work suggested that the longer intracellular retention of T-araCTP was responsible for the superior activity of T-araC against solid tumors in vivo, and that the greater activity of T-araCMP as a substrate of UMP/CMP kinase was responsible for the long intracellular half-life of T-araCTP.

Polymerization of the triphosphates of AraC, 2′,2′-difluorodeoxycytidine (dFdC) and OSI-7836 (T-araC) by human DNA polymerase α and DNA primase

OSI-7836 (4'-thio-araC, T-araC) is a nucleoside analogue that shows efficacy against solid tumor xenograft models. We examined how the triphosphates of OSI-7836 (T-araCTP), cytarabine (araCTP), and gemcitabine (dFdCTP) affected the initiation of new DNA strands by the pol alpha primase complex. Whereas dFdCTP very weakly inhibited primase, both T-araCTP and araCTP potently inhibited this enzyme. Primase polymerized T-araCTP and araCTP more readily than its natural substrate, CTP, and incorporation resulted in strong chain termination. dFdCTP, araCTP, and T-araCTP inhibited pol alpha competitively with respect to dCTP. When exogenously added primentemplates were used, pol alpha incorporated all three analogues into DNA, and incorporation caused either weak chain termination (dFdCTP), strong termination (araCTP), or extremely strong termination (T-araC). Furthermore, pol alpha polymerized T-araCTP only nine-fold less well than dCTP, whereas it polymerized araCTP and dFdCTP 24- and 83-fold less well, respectively. The presence of these three analogues in the template strand resulted in significant pausing by pol alpha, although the site and severity of pausing varied between the analogues. During the elongation of primase-synthesized primers, a reaction that is thought to mimic the normal sequence of events during the initiation of new DNA strands, pol alpha polymerized all three compounds. However, incorporation of araCTP and dFdCTP resulted in minimal chain termination, while incorporation of T-araCTP still caused extremely strong termination. The implications of these results with respect to how these compounds affect cells are discussed.

Incorporation of OSI-7836 into DNA of Calu-6 and H460 xenograft tumors

OSI-7836 (4'-thio-beta-D-arabinofuranosylcytosine) is a novel nucleoside analog in phase I clinical development for the treatment of cancer. As with other nucleoside analogs, the proposed mechanism of action involves phosphorylation to the triphosphate form followed by incorporation into cellular DNA, leading to cell death. This hypothesis has been examined by measuring and comparing the incorporation of ara-C, OSI-7836, and gemcitabine (dFdC) into DNA of cultured cells and by investigating the role of deoxycytidine kinase in OSI-7836 toxicity. We report here additional studies in which the role of cell cycling on OSI-7836 toxicity was investigated and incorporation of OSI-7836 into DNA of xenograft tumors measured. The role of the cell cycle was examined by comparing the toxicity of OSI-7836 in A549 NSCLC cells that were either in log phase growth or had reached confluence. A novel validated LC-MS/MS assay was developed to quantify the concentrations of OSI-7836 in DNA from Calu-6 and H460 human tumor xenografts in mice. Results showed that apoptosis induced by OSI-7836 was markedly greater in cycling cells than in confluent non-cycling cells despite only a modest increase in intracellular OSI-7836 triphosphate concentration. The LC-MS/MS assay developed to measure OSI-7836 incorporation into DNA had an on-column detection limit of 0.25 fmol, a quantification limit of 0.5 fmol, and a sensitivity of approximately 0.1 pmol OSI-7836/micromol dThy. Concentrations of OSI-7836 in splenic DNA (0.4 pmol OSI-7836/micromol dThy) averaged fivefold less than the average concentration in Calu-6 and H460 xenograft DNA (3.0 pmol OSI-7836/micromol dThy) following a 400 mg/kg dose of OSI-7836. Concentrations of OSI-7836 in Calu-6 tumor DNA isolated 24 h following a dose of 400, 1000, or 1600 mg OSI-7836/kg were approximately 1.3, 1 and 1.3 pmol OSI-7836/micromol dThy, respectively. Concentrations of OSI-7836 in DNA from H460 and Calu-6 xenografts did not appear to increase during repeated administration of 400 mg OSI-7836/kg on days 1, 4, 7, and 10. The majority of OSI-7836 in DNA from Calu-6 and H460 tumors of mice dosed with 1600 mg/kg was located at internal nucleotide linkages, similar to dFdC and ara-C. In conclusion, cell cycling studies supported the hypothesis that OSI-7836 cytotoxicity is dependent upon DNA synthesis. A validated LC-MS/MS assay was developed that could quantify OSI-7836 in DNA from tissues. The assay was used to show that OSI-7836 was incorporated in internal linkages in tumor DNA in a manner that was dose-independent at the doses tested and did not appear to accumulate during repeated dosing. The results suggest that if DNA incorporation is a toxic event, the relationships between administered dose, DNA incorporation, and toxicity are complex.

Synthesis and anti-cancer activity of some novel 5-azacytosine nucleosides

1-O-Acetyl-2-deoxy-3,5-di-O-toluoyl-4-thio-D-erythro-pentofuranose and 2-deoxy-1,3,5-tri-O-acetyl-4-thio-L-threo-pentofuranose were coupled with 5-azacytosine to obtain alpha and beta anomers of nucleosides. All four nucleosides were reduced to the corresponding dihydro derivatives and deblocked to give target compounds. All eight target compounds were evaluated in a series of human cancer cell lines in culture. Only 2'-deoxy-4'-thio-5-azacytidine (3beta) was found to be cytotoxic in all the cell lines and was further evaluated in vivo. Details of the synthesis and biological activity are reported.

Inhibitory Effects of 9-(4-Thio-.BETA.-D-ribo-pentofuranosyl)guanine on Tumor Growth and Angiogenesis

BACKGROUND

To find a nucleoside with anti-angiogenic activity, we tried to screen an active compound from our nucleoside library.

MATERIALS AND METHODS

The compound inhibiting the growth of human umbilical vein endothelial cell (HUVEC) induced by the conditioned medium of lung carcinoma cell line PC-9 was screened. The antitumor activity of the compound was evaluated against murine sarcoma S-180 implanted onto chick embryo chorioallantoic membrane (CAM).

RESULTS

9-(4-Thio-beta-D-ribo-pentofuranosyl)guanine (4'-thioguanosine) was found to be a potent inhibitor of the growth of HUVEC. The growth of S-180 implanted onto CAM was also inhibited by 4'-thioguanosine whereas the in vitro growth of S-180 was not inhibited. The administration of 4'-thioguanosine in mice caused unexpected side effect which suggested neurotoxicity.

CONCLUSIONS

Antitumor effect of 4'-thioguanosine on S-180 was suggested to be due to inhibition of tumor angiogenesis. Because of toxicity of 4'-thioguanosine in mice, further development of the derivatives which have lower toxicity is required.

Preclinical antitumor activity of 4'-thio-beta-D-arabinofuranosylcytosine (4'-thio-ara-C)

PURPOSE

4'-Thio-beta -d-arabinofuranosylcytosine (4'-thio-ara-C), which has shown significant cytotoxicity against a panel of human tumor lines, was evaluated for antitumor activity against a spectrum of human tumor systems in mice.

METHODS

Antitumor activity was evaluated in 15 subcutaneously implanted human tumor xenografts. 4'-Thio-ara-C was administered intraperitoneally using either q1dx9 (daily treatment for nine consecutive days) or q4hx3/q1dx9 (three treatments each day separated by 4-h intervals for nine consecutive days).

RESULTS

4'-Thio-ara-C exhibited an excellent spectrum of activity. Treatment with the compound was curative against HCT-116 colon, SW-620 colon, NCI-H23 NSCL, and CAKI-1 renal tumors and resulted in partial/complete regressions in the DLD-1 colon, NCI-H522 NSCL, DU-145 prostate, and PANC-1 pancreatic tumor models. Tumor stasis was noted for HT29 colon and NCI-H460 NSCL tumors. Tumor inhibition was observed for A549 NSCL, PC-3 prostate, LNCAP prostate, and MDA-MB-435 breast tumors. Of the 15 tumors examined, only CFPAC-1 pancreatic was unresponsive to the compound. In contrast, 1-beta -d-arabinofuranosylcytosine was minimally active at best against CAKI-1 renal, HCT-116 colon, NCI-H460 NSCL, and SW-620 colon tumors. Schedule- and route-dependency studies were conducted using the NCI-H460 NSCL tumor. The activity of 4'-thio-ara-C was independent of schedule when comparing q2dx5 (every other day for five treatments), q1dx9, and q4hx3/q1dx9 treatment schedules. 4'-Thio-ara-C was equally effective by the intravenous and intraperitoneal routes of administration, with the oral route being less efficacious.

CONCLUSIONS

On the basis of these results, 4'-thio-ara-C appears to have a profile distinct from other nucleoside antitumor agents and is being advanced to clinical trials.

Phosphorylation of 4'-thio-beta-D-arabinofuranosylcytosine and its analogs by human deoxycytidine kinase

4'-thio-beta-D-arabinofuranosylcytosine (T-araC) exhibits excellent in vivo antitumor activity against a variety of solid tumors despite its structural similarity to beta-D-arabinofuranosylcytosine (araC), an agent which is poorly active against solid tumors in vivo. It is of great interest to elucidate why these compounds show a profound difference in antitumor activity. Because deoxycytidine kinase (dCK) is the critical enzyme in the activation of both compounds, here we report the differences in the substrate characteristics with human dCK between these compounds. The catalytic efficiency (V(max)/K(m)) of araC was 100-fold higher than that of T-araC using either ATP or UTP as the phosphate donor. However, V(max) values of araC and T-araC were similar when UTP was the phosphate donor. Since UTP is believed to be the true phosphate donor for dCK in intact cells, these data indicated that the rates of phosphorylation of these two compounds at high pharmacologically relevant concentrations would be similar. This prediction was confirmed in intact cell experiments, which supported the hypothesis that UTP is the physiological phosphate donor for dCK phosphorylation in cells. The relative lack of importance of phosphate donor to the phosphorylation of T-araC by dCK revealed important insights into the activation of this compound in human cells at pharmacological doses. These studies indicated that replacement of the 4'-oxygen with sulfur significantly reduced the substrate activity of nucleoside analogs with dCK and that the superior activity of T-araC with respect to araC against solid tumors was not due to superior activity with dCK.

Synthesis and biological activity of 4'-thio-L-xylofuranosyl nucleosides

A series of 4'-thio-L-xylofuranosyl nucleosides were prepared and evaluated as potential anticancer and antiviral agents. The details of a convenient and high-yielding synthesis of the carbohydrate precursor 1-O-acetyl-2,3,5-tri-O-benzyl-4-thio-L-xylofuranose (6) are presented. Proof of structure and configuration at all chiral centers of the nucleosides was obtained by proton and carbon NMR. All target compounds were evaluated in a series of human cancer cell lines in culture and as antiviral agents.

Metabolism of 4'-thio-beta-D-arabinofuranosylcytosine in CEM cells

Because of the excellent in vivo activity of 4'-thio-beta-D-arabinofuranosylcytosine (T-araC) against a variety of human solid tumors, we have studied its metabolism in CEM cells to determine how the biochemical pharmacology of this compound differs from that of beta-D-arabinofuranosylcytosine (araC). Although there were many quantitative differences in the metabolism of T-araC and araC, the basic mechanism of action of T-araC was similar to that of araC: it was phosphorylated to T-araC-5'-triphosphate (T-araCTP) and inhibited DNA synthesis. The major differences between these two compounds were: (i) T-araC was phosphorylated to active metabolites at 1% the rate of araC; (ii) T-araCTP was 10- to 20-fold more potent as an inhibitor of DNA synthesis than was the 5'-triphosphate of araC (araCTP); (iii) the half-life of T-araCTP was twice that of araCTP; (iv) the catalytic efficiency of T-araC with cytidine deaminase was 10% that of araC; and (v) the 5'-monophosphate of araC was a better substrate for deoxycytidine 5'-monophosphate deaminase than was the 5'-monophosphate of T-araC. Of these differences in the metabolism of these two compounds, we propose that the prolonged retention of T-araCTP is a major factor contributing to the activity of T-araC against solid tumors. The data in this study represent another example of how relatively small structural changes in nucleoside analogs can profoundly affect the biochemical activity.

Synthesis and structure activity relationships of 5-substituted-4'-thio-beta-D-arabinofuranosylcytosines

Four 5-substituted (chloro, fluoro, bromo, methyl) 1-(4-thio-beta-D-arabinofuranosyl)cytosines and their alpha anomers were synthesized by a facile route in high yields. All of these nucleosides were evaluated for cytotoxicity against a panel of human tumor cell lines in vitro. Only 5-fluoro-1-(4-thio-beta-D-arabinofuranosyl)cytosine was found to be highly cytotoxic in all the cell lines and was further evaluated in vivo.