A phase I-II study of combination therapy with thymidine and cytosine arabinoside

Increased ratio between deoxycytidine kinase and thymidine kinase 2 in CLL lymphocytes compared to normal lymphocytes

Deoxycytidine kinase (dCK) is important in the 5'-phosphorylation of deoxynucleoside analogs. Like dCK, thymidine kinase 2 (TK2) catalyzes the initial step of the phosphorylation of dcyd to dCTP. Thymidine is a strong inhibitor of the dCK activity of TK2. We examined the ratio of the dcyd phosphorylation carried out by dCK and by TK2 (dCK/TK2-dcyd) in lymphocytes from CLL patients and from donors. In the CLL lymphocytes we found a 3.5-fold average increase. Therefore, we conclude that addition of thymidine in the treatment of CLL with deoxynucleoside analogs will not be of any advantage. Furthermore, our results can explain earlier findings in CML and AML lymphocytes where the ara-C phosphorylation was twice the dcyd phosphorylation.

Biochemical modulation of arabinosylcytosine for therapy of leukemias

Analysis of different ribonucleotide reductase inhibitors to modulate arabinosylcytosine (ara-C) metabolism suggested that pretreatment with arabinosyl-2-fluoroadenine (F-ara-A) significantly potentiated the rate of ara-CTP (5'-triphosphate of ara-C) accumulation in both quiescent lymphocytes (p = 0.046) and in cycling blasts (p = 0.017). In vitro incubations of freshly isolated leukemia cells from patients with chronic (n = 7) or acute (n = 5) leukemias with F-ara-A, increased the rate of ara-CTP accumulation by a median of 1.5 or 1.7-fold, respectively, when subsequently incubated with ara-C. The objective of the present investigation was to test the hypothesis that ara-C can be biochemically modulated during therapy of leukemias. To test the biochemical modulation of ara-C in the clinical setting, we designed two protocols to administer fludarabine (clinical formulation of F-ara-A) and ara-C in a pharmacologically directed sequence for patients with chronic lymphocytic leukemia (CLL) refractory to conventional fludarabine therapy or for patients with acute myelogenous leukemia (AML) in relapse. Comparison of ara-CTP pharmacokinetics demonstrated a significant increase in the area under concentration curve (AUC) of ara-CTP both in CLL (median 1.5-fold) and AML cells (median 1.8-fold) after fludarabine infusion. Analyses of different processes involved in the metabolism of ara-CTP indicated that the increase in AUC was due to potentiation of the rate of ara-CTP accumulation. These studies demonstrate that protocols designed on biochemical and pharmacological rationales modulate ara-C metabolism during therapies.

Cytosine arabinoside in the treatment of acute myeloid leukemia: the role and place of high-dose regimens

Cytosine arabinoside (AraC) is one of the most active drugs in the treatment of acute leukemias and is widely applied at all phases of therapy. In spite of extensive clinical and experimental investigations, its intracellular metabolism and especially its mechanism of action are still not fully elucidated. Controversy also continues about the most appropriate way and dose of administration; which differs by more than 100 fold in clinical studies ranging from low-dose, over standard and intermediate dose regimens, to high-dose protocols. The present review is focused on the role and place of high-dose AraC treatment in acute myeloid leukemia (AML). Based on available clinical and experimental data, the following conclusions can be drawn: Not considering possible but not yet demonstrated beneficial long-term effects, the incorporation of high-dose AraC into induction therapy has not resulted in an improvement of overall remission rate, with the possible exception of patients with slow initial cytoreduction after a first course of conventional treatment. Promising results, however, emerge from high-dose AraC-based consolidation protocols, which need confirmation in prospectively randomized comparative trials. In relapsed and refractory AML, higher than conventional doses undoubtedly enhance the efficacy of AraC salvage therapy. The question of whether the antileukemic activity of intermediate-dose regimens with 500-1,000 mg/m2 AraC is equivalent to that of high-dose protocols applying 2,000-3,000 mg/m2 AraC, however, remains open but may soon be answered by ongoing controlled clinical and pharmacologic investigations.

Biochemical modulation of cytosine arabinoside

1-beta-D arabinofuranosylcytosine (ara-C) is an analog of the naturally occurring nucleoside 2'-deoxycytidine which is a potent antileukemic agent in man. Because the metabolism (and, ultimately, the effectiveness) of this agent is regulated by multiple processes involved in pyrimidine biosynthesis, attempts to improve its efficacy through biochemical modulation have been the focus of intense interest. These approaches have included combination of ara-C with inhibitors of de novo pyrimidine biosynthesis, deaminase inhibitors, nucleoside transport blockers, nucleosides, and more recently, hematopoietic growth factors. Although potentiation of ara-C metabolism and cytotoxicity has been documented in multiple experimental in vitro and in vivo experimental systems, clinical studies in humans have thus far failed to document definitive improvements in ara-C selectivity and efficacy through biochemical modulation. It is likely that such improvements will require the identification of more optimal schedules, sequences and dose relationships, and possibly combined modality approaches.

Potentiation of ara-C induced cytotoxicity by hydroxyurea in LoVo colon carcinoma cells

The present study was undertaken to determine whether cytotoxicity by 1-beta-D-arabinofuranosylcytosine (ara-C) in LoVo colon carcinoma cells, which are resistant to high concentrations of ara-C, would be enhanced by concurrent exposure to hydroxyurea (HU). Since mechanisms underlying the effects of HU on ara-C induced cytotoxicity are unclear, we also evaluated the effect of HU on the incorporation of ara-C into DNA, as well as potential consequences of misincorporation. Our results demonstrate that HU synergistically enhances cytotoxicity by ara-C in these cells. This effect was not present when HU was combined with aphidicolin, an agent that resembles ara-C in competing with dCTP for binding to polymerase alpha but that is not incorporated into DNA. Further, cells exposed to HU and ara-C incorporated up to 5-fold as much ara-C into DNA as cells solely treated with ara-C. While the extent of inhibition of DNA synthesis was comparable with cells exposed to HU and aphidicolin as those treated with HU and ara-C, recovery of DNA synthesis was delayed more significantly by the latter combination. These findings suggest that HU synergistically potentiates ara-C induced cytotoxicity by enhancing incorporation of ara-C in LoVo cell DNA.

Cancer chemotherapy. Progress and expectations, 1984

Progress in the treatment of cancer with drugs has radically altered the clinical approach to patients with malignancy. Not only have new drugs produced promising results in hitherto untreatable tumors, but they have extended and enhanced the effectiveness of other modalities, including surgery and radiotherapy. In this article, the authors consider avenues of research that likely to aid in the discovery of new anticancer drugs and improve the effectiveness of established agents. Promising new efforts in drug development include the use of new screening systems, particularly those employing human tumor material; the development of improved analogs of existing active agents particularly those of the anthracycline and platinum complex types; and the search for agents that which promote differentiation or prevent metastasis. In an effort to improve the effectiveness of established agents, the authors consider the application of pharmacokinetic principles in developing regional perfusion routes, intraperitoneal chemotherapy, and central nervous system penetration. Finally, the contribution of biochemical pharmacology to the current understanding of drug action, mechanisms of resistance, and drug interactions are considered, and the impact of this knowledge on clinical protocol design is assessed.