AUGMENTATION OF 1-BETA-D-ARABINOFURANOSYLCYTOSINE (ARA-C) CYTOTOXICITY IN LEUKEMIA-CELLS BY COADMINISTRATION WITH ANTISIGNALLING DRUGS

The ribonucleotide reductase inhibitors hydroxyurea (HU), arabinosyl-2 -fluoroadenine (F-Ara-A) and 2-chlorodeoxyadenosine (2-CdA) and the an tisignalling drugs all-trans retinoic acid (ATRA), staurosporine and q uercetin have been reported to enhance the cytotoxicity of l-P-D-arabi nofuranosylcytosine (ara-C). We tested the hypothesis that the ara-C-s ensitising potency of the antisignalling agents is equipotent with tha t of the ribonucleotide inhibitors. The cytotoxicity, determined by th e 3-(4,5 dimethylthiazol-2-yl-)5 diphenyltetrazolium bromide (MTT) ass ay, of combinations of ara-C with the agents named above was compared in the leukaemia cell lines HL-60, ara-C-resistant HL-60 (HL-60/ara-C) and U937. Furthermore, a range of protein tyrosine kinase inhibitors, genistein, CGP 52411, tyrphostin A48 and nordihydroguaiaretic acid (N DGA), for which ara-C-sensitisation has hitherto not been described, w ere included in the study. All three cell types acquired increased sen sitivity to ara-C when co-incubated with HU or ATRA, but their ara-C s ensitivity was not affected by quercetin or genistein. 2-CdA, CGP 5241 1, tyrphostin A48, staurosporine and NDGA were active as sensitisers a gainst ara-C in HL-60 cells, CGP 52411 and tyrphostin A48 also in HL-6 0/ara-C cells, and 2-CdA, staurosporine and NDGA also in U937 cells. F -Ara-A increased ara-C toxicity in HL-60/ara-C and U937 cells. To addr ess the mechanism of the observed sensitisation, the influence of agen ts with ara-C-sensitising properties on ara-C-induced apoptosis was in vestigated in HL-60 cells as measured by cell shrinkage, DNA loss and DNA fragmentation. HU, ATRA, tyrphostin A48 and NDGA augmented apoptos is induced by ara-C as assessed by all three indicators. CGP 52411 dec reased the effect of ara-C on apoptotic indicators after incubation fo r 4h, but not after 12h. The results suggest that ATRA, CGP 52411, tyr phostin A48, staurosporine and NDGA may be suitable alternatives to th e clinically applied ribonucleotide reductase inhibitors as modifiers of ara-C cytotoxicity in the treatment of acute myeloid leukaemia. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.