[Frontiers in Bioscience 2, d232-241, June 1, 1997]

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Steven Grant.

Division of hermatology , Department of Pharmacology and Microbiology , Medical College of Virginia

Received 5/23/97; Accepted 5/28/97

3. ARA-C

The deoxycytidine analog ara-C is among the most effective agents used in the treatment of acute leukemia in man (10). It is transported across the cell membrane by facilitated nucleoside diffusion (11), and converted to its nucleoside monophosphate derivative, ara-CMP, by the pyrimidine salvage pathway enzyme, deoxycytidine kinase. This process represents the rate limiting step in ara-C metabolism (12), and has been reported to involve, at least in in vitro systems, participation of the PKC isoform (13). The metabolism of ara-C and its derivatives are opposed by degradative enzymes such as cytidine deaminase and deoxycytidylate deaminase, which convert them to inactive ara-U catabolites (14). ara-C is ultimately converted to ara-CTP, which is an inhibitor of DNA polymerase and (15), and is also incorporated into elongating strands, thereby interfering with chain elongation and inducing premature chain termination (16). The lethal actions of ara-C correlate closely with its incorporation into DNA (17), and the bulk of evidence suggests that it is this phenomenon, rather than inhibition of DNA polymerase, that leads to cell death.

The metabolism of ara-C is also linked to perturbations in levels of intracellular lipid messengers that regulate apoptosis. For example, conversion of ara-CTP to ara-CDP involves a reversal of the cholinephosphotransferase reaction, which leads to the generation of diacylglyceride (DAG) (18). DAG is a potent activator of PKC, which may explain reported increases in activity of this enzyme in ara-C-treated leukemic cells (19). Since PKC serves to oppose apoptosis in hematopoietic cells (20), elevations in DAG activity could theoretically antagonize ara-C-mediated cell killing. Treatment of leukemic cells also leads to the generation of ceramide (21), which is known to be a strong inducer of apoptosis in multiple cell lines (22,23). Thus, the lethal actions of ara-C may be regulated by its net effect on pro- versus anti-apoptotic lipid messengers.

Resistance of leukemic cells to ara-C, as in the case of virtually all antineoplastic drugs, may occur at two distinct levels. The first level is drug-specific, and involves alterations in drug metabolism or interaction with cellular targets. In the case of ara-C, resistance has been correlated with decreased transport (24), diminished activity of deoxycytidine kinase (25), increased activity of cytidine deaminase (26), increased rates of ara-CTP dephosphorylation (27), increased intracellular levels of dCTP (28), and reduced incorporation of ara-C into DNA (29). However, resistance to ara-C can also result from a generalized reduction in the susceptibility of cells to an apoptotic cell death. For example, increased leukemic cell expression of the proto-oncogene bcl-2 confers resistance to a broad range of cytotoxic drugs, including ara-C (30). Cells expressing increased levels of the Bcl-2 protein are less susceptible to ara-C-related activation of proteases involved in the degradation phase of apoptosis, particularly the Yama protease (CPP32) (31,32). Diminished sensitivity of Bcl-2-overexpressing cells to ara-C-mediated lethality occurs in the absence of alterations in ara-C metabolism or the degree of DNA damage (33), supporting the notion that resistance stems from a distal defect in the cell death pathway.