In vivo cellular kinetic and pharmacological studies1–β–d– arabinofuranosylcytosine and 3–deazauridine chemotherapy for relapsing acute leukemia

In 1 –β–D–arabinofuranosylcytosine (ara–C)–resistant acute leukemia, a treatment program with ara–C and 3–deazauridine (DAU) was designed to take advantage of cellular kinetic and pharmacological effects using DNA flow cytometry and high–pressure liquid chromatography techniques. There were two treatment arms, one using the sequential administration of ara–C and DAU in order to maximize DAU–induced cell kill after ara–C–induced S–phase synchronization. In the absence of hematological improvement, the two drugs were administered in reverse sequence. This treatment schedule aimed at restoration of ara–C sensitivity by pretreatment with DAU, thus reducing intracellular deoxycytidine 5^‘–triphosphate levels and hence decreasing both the competitor of 1 –β–D–arabinofuranosylcy–tosine 5’–triphosphate (ara–CTP) and a feedback inhibitor of deoxycytidine kinase as the rate–limiting enzyme in the formation of ara–CTP. Following a pilot trial in two patients, all six subsequent patients were initially treated with ara–C, and DAU was added at the time of maximum accumulation of bone marrow cells in S phase. In cases of less than 50% reduction in leukemic infiltrate, the reverse drug sequence was given. ara–CTP accumulated in the leukemic bone marrow cells of all seven evaluable patients and continued to increase during subsequent DAU treatment in four patients. This pharmacological response was associated with an increase in mean S–phase percentage by a factor of 2.1 (ρ < 0.006), occurring between 12 and 48 hr after treatment started. 3–Deazauridine 5’–tri–phosphate (deaza–UTP) levels during the ara–C→DAU sequence were markedly lower than during the alternate treatment arm administered to three patients failing the previous schedule. ara–C was then administered at the time of minimum intracellular cytidine 5’–triphosphate (CTP) concentration or of maximum S–phase accumulation during DAU infusion, whichever occurred first. When given as initial therapy, DAU increased the proportion of marrow cells in S phase in two of three patients. Subsequent ara–C treatment reduced deaza–UTP to levels noted previously during the opposite drug sequence. Cellular CTP decreased as a result of DAU therapy in four of seven instances, including repeat observations on both treatment arms in two patients; DAU–induced reduction in CTP was not consistently associated with an increase in ara–CTP. A comparison between the two treatment arms suggests that the principal effects of initial S–phase delay, CTP reduction by DAU and deaza–UTP reduction by ara–C, occurred regardless of the drug sequence. While there was no significant correlation between peak levels of ara–CTP and pretreatment S–phase proportion, an inverse relationship was noted between peak levels of deaza–UTP and the minimum CTP concentration attained during the treatment course. A significant antileukemic effect was observed in five of eight patients, including one complete 3–month remission and one early death with a marrow in complete remission. This result is quite remarkable in view of the established ara–C resistance in all patients and DAU resistance in three of eight patients. Our data indicate that, in the absence of lethal cell sensitivity, ara–C can effectively promote S–phase synchronization for more effective cell kill by a second S–phase–specific agent, such as DAU. On the other hand, DAU reduced intracellular CTP levels (as an indicator of deoxycytidine 5’–triphosphate concentration) which, however, were not uniformly associated with an increase in ara–CTP levels. The observation of deaza–UTP reduction by ara–C may suggest a potentially negative interaction between the two agents, resulting in a decrease in bioavailability of active metabolites. We therefore propose a treatment schedule using concomitant intermittent administration of both agents in order to achieve maximum intracellular levels of both ara–CTP and deaza–UTP for optimum drug syn–ergism.