Similar changes were induced by Cladribine and by gemcitabine, in the deoxypyrimidine salvage, during short-term treatments

Deoxycytidine kinase augments ATM-Mediated DNA repair and contributes to radiation resistance

Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

Pharmacokinetic study of gemcitabine, given as prolonged infusion at fixed dose rate, in combination with cisplatin in patients with advanced non-small-cell lung cancer

INTRODUCTION

Although some studies have suggested that gemcitabine delivered as a fixed dose rate (FDR) infusion of 10 mg/m(2)/min could be more effective than when administered as the standard 30-min infusion, the available pharmacokinetic data are still too limited to draw definitive conclusions. This study is aimed to investigate the plasmatic and intracellular pharmacokinetics of gemcitabine given as FDR at doses of 600 and 1,200 mg/m(2) in combination with 75 mg/m(2) of cisplatin in advanced non-small-cell lung cancer (NSCLC) patients.

PATIENTS AND METHOD

The patients were divided into two groups receiving different initial doses of the drug: 4 patients received 600 mg/m(2) gemcitabine 60-min i.v. infusion and 4 patients 1,200 mg/m(2) gemcitabine 120-min i.v. infusion both as a FDR of 10 mg/m(2)/min on days 1 and 8 of a 21-day cycle (at first cycle). At the second cycle, all patients were treated with gemcitabine at 1,200 mg/m(2) 120-min i.v. infusion (FDR of 10 mg/m(2)/min) on days 1 and 8 of a 21-day cycle. At each cycle, gemcitabine was administered alone on day one, and in combination with 75 mg/m(2) of cisplatin on day 8. Plasmatic and intracellular pharmacokinetic analyses were performed on blood samples collected at defined time points before, during and after gemcitabine infusion.

RESULTS

The plasmatic pharmacokinetic parameters were clearly different when the patients received a higher gemcitabine dose in the second cycle compared to the lower dose of the first course; in the same time, the intracellular drug levels were not modified. Comparing the pharmacokinetic parameters of different patients treated at different dose levels, the results appeared to be quite similar.

CONCLUSIONS

A substantially higher accumulation of metabolites in peripheral blood mononuclear cells was observed when the longer infusion time was employed, suggesting a pharmacological advantage for this treatment schedule.

No evidence of gemcitabine accumulation during weekly administration

Some anticancer agents tend to accumulate during repeated administration. We determined whether gemcitabine or its metabolites would accumulate during repeated administration. Gemcitabine was administered over two courses with each course consisting of a 30-min infusion at 1000 mg/m(2) weekly for 3 weeks followed by 1 week of rest. In 14 patients we evaluated eventual accumulation by comparing the concentrations in blood samples taken before, and at 30 and 60 min after the start of infusion on days 1, 8 and 15, in both cycles. At the end of the infusion gemcitabine concentrations at day 1 of both courses varied between 18 and 77 microM and at day 15 between 13 and 90 microM. The mean ratios day 8/day 1 and day 15/day 1 varied from 0.94 to 1.18. For the inactive metabolite 2',2'-difluoro-2'-deoxyuridine (dFdU) these values varied between 54 and 152 microM and 55 and 157, respectively, and the ratios from 0.96 to 1.08. The concentration of the active metabolite of gemcitabine, gemcitabine triphosphate (dFdCTP) in peripheral white blood cells, ranged between 37 and 283 pmol/10(6) cells at the end of infusion on day 1 and 35 and 115 pmol/10(6) cells on day 15. Potential accumulation was evaluated using a mixed effects model and no evidence was observed of accumulation for either gemcitabine or its metabolites. Gemcitabine can be administered safely without the risk that the drug will accumulate.

Basis for effective combination cancer chemotherapy with antimetabolites

Most current chemotherapy regimens for cancer consist of empirically designed combinations, based on efficacy and lack of overlapping toxicity. In the development of combinations, several aspects are often overlooked: (1) possible metabolic and biological interactions between drugs, (2) scheduling, and (3) different pharmacokinetic profiles. Antimetabolites are used widely in chemotherapy combinations for treatment of various leukemias and solid tumors. Ideally, the combination of two or more agents should be more effective than each agent separately (synergism), although additive and even antagonistic combinations may result in a higher therapeutic efficacy in the clinic. The median-drug effect analysis method is one of the most widely used methods for in vitro evaluation of combinations. Several examples of classical effective antimetabolite-(anti)metabolite combinations are discussed, such as that of methotrexate with 6-mercaptopurine or leucovorin in (childhood) leukemia and 5-fluorouracil (5FU) with leucovorin in colon cancer. More recent combinations include treatment of acute-myeloid leukemia with fludarabine and arabinosylcytosine. Other combinations, currently frequently used in the treatment of solid malignancies, include an antimetabolite with a DNA-damaging agent, such as gemcitabine with cisplatin and 5FU with the cisplatin analog oxaliplatin. The combination of 5FU and the topoisomerase inhibitor irinotecan is based on decreased repair of irinotecan-induced DNA damage. These combinations may increase induction of apoptosis. The latter combinations have dramatically changed the treatment of incurable cancers, such as lung and colon cancer, and have demonstrated that rationally designed drug combinations offer new possibilities to treat solid malignancies.