Pharmacological basis for cladribine resistance in a human acute T lymphoblastic leukaemia cell line selected for resistance to etoposide

Cross-resistance between different classes of antineoplastic agents can jeo pardize successful combination cancer chemotherapy. In this study, we obser ved an unexpected cross-resistance between the podophyllotoxine derivative etoposide (VP) and the nucleoside analogue cladribine (CdA) in CCRF-CEM cel ls developed for resistance to VP. The resistant cells also displayed 14- a nd twofold resistance to cytarabine (ara-C) and gemcitabine respectively, C loser analysis of these cells showed that they contained lower amounts of t opoisomerase (topo) II alpha (P < 0.001) and P protein (P < 0.026), formed substantially lower amounts of the topo II-DNA complex, and had a markedly decreased level of Fas (CD95/APO-1)-ligand mRNA expression. Interestingly, Fas expression in the resistant cells did not differ from that in the paren tal cell line. No differences were observed in the accumulation/efflux of d aunorubicin or in the gene expressions of P-glycoprotein, multidrug resista nce-associated protein and the lung resistance-related protein. The activit y of deoxycytidine kinase (dCK), responsible for activation of CdA and ara- C, was the same for resistant and wild-type cells. However, there was an in crease in the activity of the cytosolic 5'-nucleotidases (5'-NT), responsib le for deactivation of nucleotides, amounting to 206% (P < 0.001) for the h igh K-m and 134% (P < 0.331) for the low K-m 5'-NT in resistant cells. The high K-m 5'-NT is probably responsible for the decreased amount of the acti ve metabolite CdA 5'-triphosphate [40% decreased (P < 0.045)], as well as f or other purine ribonucleosides and deoxyribonucleosides triphosphates in t he resistant cells. In contrast, a significantly higher deoxycytidine triph osphate (dCTP) level (167%, P < 0.001) was observed in the resistant cells, Thus, this study suggests that the major cause of resistance to the nucleo side analogues CdA and ara-C in cells selected for resistance to VP is a re sult of metabolic alterations producing increased activity of 5'-NT and hig her dCTP levels. Furthermore, these results indicate that there is a common factor in the regulation of nucleotide-degrading enzymes and DNA topoisome rases, which may be altered in cross-resistant cells.