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

Establishment and large-scale validation of a three-dimensional tumor model on an array chip for anticancer drug evaluation

Two-dimensional (2D) tumor model has always poorly predicted drug response of animal model due to the lack of recapitulation of tumor microenvironment. Establishing a biomimetic, controllable, and cost-effective three-dimensional (3D) model and large-scale validation of its in vivo predictivity has shown promise in bridging the gap between the 2D tumor model and animal model. Here, we established a matrigel-based 3D micro-tumor model on an array chip for large-scale anticancer drug evaluation. Compared with the 2D tumor model, the 3D tumor model on the chip showed spheroid morphology, slower proliferation kinetics, and comparable reproducibility. Next, the results of the chemotherapeutic evaluation from 18 drugs against 27 cancer cell lines showed 17.6% of drug resistance on the 3D tumor model. Moreover, the evaluation results of targeted drugs showed expected sensitivity and higher specificity on the 3D tumor model compared with the 2D model. Finally, the evaluation results on the 3D tumor model were more consistent with the in vivo cell-derived xenograft model, and excluded 95% false-positive results from the 2D model. Overall, the matrigel-based 3D micro-tumor model on the array chip provides a promising tool to accelerate anticancer drug discovery.

Diastereoselective and regioselective synthesis of adenosine thionucleoside analogues using an acyclic approach

An acyclic approach to synthesize thiofuranoside N-glycosides bearing an adenine nucleobase is presented herein. This approach provides a significant improvement in terms of regio- and diastereoselectivity compared with the current paradigms used for their formation. Activation of acyclic dithioacetal substrates bearing either a C2′-alkoxy or fluoro group and coupling with purine nucleobases selectively generates 1′,2′-syn thioaminals. The regiochemistry of the nucleobase coupling (N7 or N9) can also be controlled by using either silylated or unsilylated purines. A subsequent SN2-like cyclization of these 1′,2′-syn acyclic thioaminals results in the desired 1′,2′-cis thionucleoside analogues.

Design and synthesis of pyrimidine molecules endowed with thiazolidin-4-one as new anticancer agents

Design and synthesis of new pyrimidine derivatives clubbed with thiazolidin-4-one from 4-(2-chlorophenyl)-6-(2,4-dichlorophenyl)pyrimidin-2-amine and their in vitro anticancer activities were screened at National Cancer Institute (NCI), USA against full NCI 60 cell lines. Compound 2 (NSC: 765735) exhibited remarkable growth inhibition at single dose (10 μM) and encourage chosen for broadcast at 10-fold dilutions of five different concentrations (0.01, 0.1, 1, 10 and 100 μM). The compound 2 was found better quality for Lung cancer cell line (HOP-92) by viewing growth inhibition (GI50 0.52) and no cytotoxicity seen (LC50 > 100). Molecular docking study was performed using Maestro 9.0 (Schrodinger Inc. USA) to provide binding mode into binding sites of CDK2. Compound 2 could be used as a lead compound for developing new potential anticancer agents.

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.

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.

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.

Antitumor efficacy testing in rodents

The preclinical research and human clinical trials necessary for developing anticancer therapeutics are costly. One contributor to these costs is preclinical rodent efficacy studies, which, in addition to the costs associated with conducting them, often guide the selection of agents for clinical development. If inappropriate or inaccurate recommendations are made on the basis of these preclinical studies, then additional costs are incurred. In this commentary, I discuss the issues associated with preclinical rodent efficacy studies. These include the identification of proper preclinical efficacy models, the selection of appropriate experimental endpoints, and the correct statistical evaluation of the resulting data. I also describe important experimental design considerations, such as selecting the drug vehicle, optimizing the therapeutic treatment plan, properly powering the experiment by defining appropriate numbers of replicates in each treatment arm, and proper randomization. Improved preclinical selection criteria can aid in reducing unnecessary human studies, thus reducing the overall costs of anticancer drug development.

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.

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.

Proprotein convertases: "master switches" in the regulation of tumor growth and progression

Proprotein convertases (PCs) are a group of Ca2+-dependent serine proteases that have homology to the endoproteases subtilisin (bacteria) and kexin (yeast). This group is comprised of less than a dozen members, known as furin/PACE, PC1/PC3, PC2, PC4, PACE4, PC5/PC6, PC7/PC8/LPC, SKI/S1P, and NARC-1/PCSK9. Four PCs (Furin, PACE4, PC5, and PC7) have been localized to several different tissues and epithelial or nervous system tumors. PCs activate their cognate substrates by limited proteolysis at the consensus sequence RXR/KR downward arrow. Many PC substrates are well known cancer-associated proteins such as growth factors, growth factor receptors, integrins, and matrix metalloproteases (MMPs). For example, IGF-1 and its receptor, TGF-beta, VEGF-C, and MT-MMPs have direct roles in tumor progression and metastasis. Furin, a well-studied member of the PC family, has been associated with enhanced invasion and proliferation in head and neck, breast, and lung cancer. Conversely, inhibition of PC activity by PDX or several PC pro-segments, resulted in reduced processing of these key cancer-related substrates in human squamous cell carcinomas (SCC), colon adenocarcinoma, and astrocytoma cell lines. In parallel to these changes in cell proliferation and invasiveness as well as metastatic ability were markedly impaired. By controlling the maturation/activation of key cancer-associated proteins, PCs act as "master switches" at different levels during tumor development and progression. The manifold effects of PCs, influencing tumor cell proliferation, motility, adhesiveness, and invasiveness, should be exploited by further developing competitive/inhibitory therapeutic strategies that would be able to neutralize simultaneously the most salient cancer cell properties.

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.