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

The chemistry and biology of natural ribomimetics and related compounds

Natural ribomimetics represent an important group of specialized metabolites with significant biological activities. Many of the activities, e.g., inhibition of seryl-tRNA synthetases, glycosidases, or ribosomes, are manifestations of their structural resemblance to ribose or related sugars, which play roles in the structural, physiological, and/or reproductive functions of living organisms. Recent studies on the biosynthesis and biological activities of some natural ribomimetics have expanded our understanding on how they are made in nature and why they have great potential as pharmaceutically relevant products. This review article highlights the discovery, biological activities, biosynthesis, and development of this intriguing class of natural products.

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.

HPLC method development, validation, and impurity characterization of a potent antitumor nucleoside, T-dCyd (NSC 764276)

An HPLC method for the assay of an anticancer nucleoside, 4'-thio-2'-deoxycytidine (T-dCyd, NSC 764276), has been developed and validated. The stress testing of T-dCyd was carried out in accordance with ICH guidelines Q1A (R2) under acidic, alkaline, oxidative, thermolytic, and photolytic conditions. The separation of T-dCyd from its impurities and degradation products was achieved in 40min on a Luna^® Phenyl-Hexyl column (150mm×4.6mm i.d., 3μm) with a gradient elution using ammonium phosphate buffer (pH 3.85) and methanol as the mobile phase. The gradient starts from 2% and ends at 80% of methanol. Detection is by UV at 282nm. LC-QTOF/MS was used to obtain mass data for characterization of impurities and degradation products. The proposed HPLC assay method was validated for specificity, linearity (concentration range 0.25-0.75mg/mL, r≥0.9998), accuracy (recovery 98.1-102.0%), precision (RSD≤1.5%), and sensitivity (LOD 0.1μg/mL). The developed method was suitable for the quality control and stability monitoring of the T-dCyd drug substance.

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.

Novel DNA methyltransferase-1 (DNMT1) depleting anticancer nucleosides, 4'-thio-2'-deoxycytidine and 5-aza-4'-thio-2'-deoxycytidine

PURPOSE

Currently approved DNA hypomethylating nucleosides elicit their effects in part by depleting DNA methyltransferase I (DNMT1). However, their low response rates and adverse effects continue to drive the discovery of newer DNMT1 depleting agents. Herein, we identified two novel 2'-deoxycytidine (dCyd) analogs, 4'-thio-2'-deoxycytidine (T-dCyd) and 5-aza-4'-thio-2'-deoxycytidine (aza-T-dCyd) that potently deplete DNMT1 in both in vitro and in vivo models of cancer and concomitantly inhibit tumor growth.

METHODS

DNMT1 protein levels in in vitro and in vivo cancer models were determined by Western blotting and antitumor efficacy was evaluated using xenografts. Effects on CpG methylation were evaluated using methylation-specific PCR. T-dCyd metabolism was evaluated using radiolabeled substrate.

RESULTS

T-dCyd markedly depleted DNMT1 in CCRF-CEM and KG1a leukemia and NCI-H23 lung carcinoma cell lines, while it was ineffective in the HCT-116 colon or IGROV-1 ovarian tumor lines. On the other hand, aza-T-dCyd potently depleted DNMT1 in all of these lines indicating that dCyd analogs with minor structural dissimilarities induce different DNMT1 turnover mechanisms. Although T-dCyd was deaminated to 4'-thio-2'-deoxyuridine, very little was converted to 4'-thio-thymidine nucleotides, suggesting that inhibition of thymidylate synthase would be minimal with 4'-thio dCyd analogs. Both T-dCyd and aza-T-dCyd also depleted DNMT1 in human tumor xenografts and markedly reduced in vivo tumor growth. Interestingly, the selectivity index of aza-T-dCyd was at least tenfold greater than that of decitabine.

CONCLUSIONS

Collectively, these data show that 4'-thio modified dCyd analogs, such as T-dCyd or aza-T-dCyd, could be a new source of clinically effective DNMT1 depleting anticancer compounds with less toxicity.

Metabolism, mechanism of action and sensitivity profile of fluorocyclopentenylcytosine (RX-3117; TV-1360)

A novel cytidine analog fluorocyclopentenylcytosine (RX-3117; TV-1360) was characterized for its cytotoxicity in a 59-cell line panel and further characterized for cytotoxicity, metabolism and mechanism of action in 15 additional cancer cell lines, including gemcitabine-resistant variants. In both panels sensitivity varied 75-fold (IC50: 0.4- > 30 μM RX-3117). RX-3117 showed a different sensitivity profile compared to cyclopentenyl-cytosine (CPEC) and azacytidine, substrates for uridine-cytidine-kinase (UCK). Dipyridamole, an inhibitor of the equilibrative-nucleoside-transporter protected against RX-3117. Uridine and cytidine protected against RX-3117, but deoxycytidine (substrate for deoxycytidine-kinase [dCK]) not, although it protected against gemcitabine, demonstrating that RX-3117 is a substrate for UCK and not for dCK. UCK activity was abundant in all cell lines, including the gemcitabine-resistant variants. RX-3117 was a very poor substrate for cytidine deaminase (66,000-fold less than gemcitabine). RX-3117 was rapidly metabolised to its nucleotides predominantly the triphosphate, which was highest in the most sensitive cells (U937, A2780) and lowest in the least sensitive (CCRF-CEM). RX-3117 did not significantly affect cytidine and uridine nucleotide pools. Incorporation of RX-3117 into RNA and DNA was higher in sensitive A2780 and low in insensitive SW1573 cells. In sensitive U937 cells 1 μM RX-3117 resulted in 90% inhibition of RNA synthesis but 100 μM RX-3117 was required in A2780 and CCRF-CEM cells. RX-3117 at IC50 values did not affect the integrity of RNA. DNA synthesis was completely inhibited in sensitive U937 cells at 1 μM, but in other cells even higher concentrations only resulted in a partial inhibition. At IC50 values RX-3117 downregulated the expression of DNA methyltransferase. In conclusion, RX-3117 showed a completely different sensitivity profile compared to gemcitabine and CPEC, its uptake is transporter dependent and is activated by UCK. RX-3117 is incorporated into RNA and DNA, did not affect RNA integrity, depleted DNA methyltransferase and inhibited RNA and DNA synthesis. Nucleotide formation is related with sensitivity.

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.

Synthesis, evaluation of anti-HIV-1 and anti-HCV activity of novel 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides

A series of 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides of both pyrimidine and purine nucleobases were synthesized in an efficient manner starting from commercially available L-pyroglutamic acid via glycosylation of difluorinated pyrrolidine derivative 15. Several 4'-azanucleosides were prepared as a separable mixture of α- and β-anomers. The 6-chloropurine analogue was obtained as a mixture of N(7) and N(9) regioisomers and their structures were identified based on NOESY and HMBC spectral data. Among the 4'-azanucleosides tested as HIV-1 inhibitors in primary human lymphocytes, four compounds showed modest activity and the 5-fluorouracil analogue (18d) was found to be the most active compound (EC(50)=36.9μM) in this series. None of the compounds synthesized in this study demonstrated anti-HCV activity.

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.

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.

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.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

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.

A Phase I Study of a New Nucleoside Analogue, OSI-7836, Using Two Administration Schedules in Patients with Advanced Solid Malignancies

PURPOSE

To investigate the safety, tolerability, and pharmacokinetic profile of the novel nucleoside analogue OSI-7836 in patients with advanced solid malignancies.

EXPERIMENTAL DESIGN

OSI-7836 was initially given as a 60-minute i.v. infusion on day 1 every 21 days. In view of its dose-limiting toxicities, the administration time was amended to a 5-minute bolus, and subsequently, the schedule was amended to weekly for 4 weeks followed by a 2-week rest. Blood and urine samples were collected for pharmacokinetic studies. Analyses of cytokines and lymphocyte subsets were added later in the study to elucidate a mechanism for the severe fatigue and lymphocyte depletion observed in earlier patients.

RESULTS

Thirty patients received a total of 61 treatment cycles. Fatigue was the main dose-limiting toxicity. Maximum-tolerated dose was defined as 300 mg/m2 in the 60-minute infusion, (three times per week) schedule; 400 mg/m2 in the 5-minute bolus infusion, (three times per week) schedule; and 100 mg/m2 in the weekly schedule. Other common toxicities were nausea, vomiting, rash, fever, and a flu-like syndrome. There were no clinically significant hematologic toxicities. Following the initial dose, OSI-7836 was eliminated from plasma with a median (range) elimination half-life of 48.3 minutes (22.6-64.8 minutes). Lymphocyte subset analysis showed a significant drop in B cell counts, which persisted to day 14 and beyond. Cytokine analysis showed significant elevations of interleukin-6 and interleukin-10 in all patients who received > or = 200 mg/m2 OSI-7836. Best response was disease stabilization in seven patients.

CONCLUSION

OSI-7836 was associated with excessive fatigue, and despite changes in its schedule and duration of administration, we did not observe an improvement in its tolerability. Its potentially selective effect on B lymphocytes could be exploited in further studies in specific hematologic malignancies.

c-Abl-independent p73 stabilization during gemcitabine- or 4'-thio-beta-D-arabinofuranosylcytosine-induced apoptosis in wild-type and p53-null colorectal cancer cells

Nucleoside anticancer drugs like gemcitabine (2'-deoxy-2',2'-difluorocytidine) are potent inducers of p53, and ectopic expression of wild-type p53 sensitizes cells to these agents. However, it is also known that nucleosides are efficient activators of apoptosis in tumor cells that do not express a functional p53. To clarify this issue, we examined the effects of gemcitabine and 4'-thio-beta-d-arabinofuranosylcytosine (T-ara-C) on p73, a structural and functional homologue of p53, whose activation could also account for nucleoside-induced apoptosis because no functionally significant mutations of p73 have been reported in cancers. Acute treatment of HCT 116 colon carcinoma cells with gemcitabine or T-ara-C induced marked cytotoxicity and cleavage of caspase-3 and poly(ADP-ribose) polymerase. T-ara-C and gemcitabine markedly induced p53 accumulation as well as increased levels of phospho-p53 (Ser15/Ser20/Ser46) and induced its binding to a consensus p53 response element. Despite robust activation of p53 by T-ara-C and gemcitabine, we found that wild-type and p53-/- HCT 116 cells exhibited almost equivalent sensitivity towards these nucleosides. Examination of p73 revealed that T-ara-C and gemcitabine markedly increased p73 protein levels and p73 DNA-binding activities in both p53-/- and wild-type cells. Furthermore, T-ara-C- and gemcitabine-induced increases in p73 levels occur due to a decrease in p73 protein turnover. RNA interference studies show that nucleoside-induced p73 increases are independent of c-Abl, a nucleoside-activated kinase recently implicated in p73 stabilization. HCT 116 lines, wherein the downstream p53/p73 targets Bax and PUMA (p53 up-regulated modulator of apoptosis) were deleted, were less sensitive to T-ara-C and gemcitabine. Together, these studies indicate that c-Abl-independent p73 stabilization pathways could account for the p53-independent mechanisms in nucleoside-induced apoptosis.

The Role of Human Nucleoside Transporters in Cellular Uptake of 4′-Thio-β-d-arabinofuranosylcytosine and β-d-Arabinosylcytosine

4'-Thio-beta-D-arabinofuranosyl cytosine (TaraC) is in phase I development for treatment of cancer. In human equilibrative nucleoside transporter (hENT) 1-containing CEM cells, initial rates of uptake (10 microM; picomoles per microliter of cell water per second) of [3H]TaraC and [3H]1-beta-D-arabinofuranosyl cytosine (araC) were low (0.007 +/- 003 and 0.034 +/- 0.003, respectively) compared with that of [3H]uridine (0.317 +/- 0.048), a highactivity hENT1 permeant. In hENT1- and hENT2-containing HeLa cells, initial rates of uptake (10 microM; picomoles per cell per second) of [3H]TaraC, [3H]araC, and [3H]deoxycytidine were low (0.30 +/- 0.003, 0.42 +/- 0.03, and 0.51 +/- 0.11, respectively) and mediated primarily by hENT1 (approximately 74, approximately 65, and approximately 61%, respectively). In HeLa cells with recombinant human concentrative nucleoside transporter (hCNT) 1 or hCNT3 and pharmacologically blocked hENT1 and hENT2, transport of 10 microM[3H]TaraC and [3H]araC was not detected. The apparent affinities of recombinant transporters (produced in yeast) for a panel of cytosine-containing nucleosides yielded results that were consistent with the observed low-permeant activities of TaraC and araC for hENT1/2 and negligible permeant activities for hCNT1/2/3. During prolonged drug exposures of CEM cells with hENT1 activity, araC was more cytotoxic than TaraC, whereas coexposures with nitrobenzylthioinosine (to pharmacologically block hENT1) yielded identical cytotoxicities for araC and TaraC. The introduction by gene transfer of hENT2 and hCNT1 activities, respectively, into nucleoside transport-defective CEM cells increased sensitivity to both drugs moderately and slightly. These results demonstrated that nucleoside transport capacity (primarily via hENT1, to a lesser extent by hENT2 and possibly by hCNT1) is a determinant of pharmacological activity of both drugs.

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.

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.

Stereoselective Synthesis of Conformationally Constrained 2‘-Deoxy-4‘-thia β-Anomeric Spirocyclic Nucleosides Featuring Either Hydroxyl Configuration at C5‘

An enantioselective approach to 2'-deoxy-4'-thia spirocyclic nucleosides featuring an alpha- or beta-hydroxyl substituent at C-5' of the carbocyclic ring is detailed. The starting point is the mandelate acetal 8. The overall strategy involves the stereocontrolled dihydroxylation of this dihydrothiophene, subsequent generation of the keto acetonide 12 followed by its Meerwein-Ponndorf-Verley reduction and beta-elimination, protection of the resulting dihydroxy thiaglycal, electrophilic glycosidation according to the Haraguchi protocol, reductive removal of the phenylseleno group, and end-game global deprotection. Acquisition of the alpha- and beta-5'-isomers is equally facile. Various 1D and 2D NMR techniques are used for assigning configuration.

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.

A novel method for the synthesis of 4′-thiopyrimidine nucleosides using hypervalent iodine compounds

The coupling reactions of cyclic sulfides with a silylated pyrimidine nucleobase using a hypervalent iodine reagent were investigated. The reaction of silylated uracil with cyclic sulfide 12 using PhI=O gave the desired beta-anomer 14 in moderate yield. 4'-Thiouridine (22) was obtained by deprotection of 14.