MODULATION OF RESISTANCE TO ARA-C BY BRYOSTATIN IN FRESH BLAST CELLS FROM PATIENTS WITH AML

Bryostatin has shown promise both as a cytotoxic agent and more recent ly as a modulator of 1-beta-D-arabinofuranosylcytosine (ara-C) resista nce. This compound is currently in phase I and II trials as a single a gent. We have used the 3-4,5-dimethylthiazol, 2,5-diphenyltetrazolium bromide (MTT) assay as a means of investigating the direct effects of bryostatin and the effects of co-incubating this agent with ara-C on f resh blast cells from 53 patients with acute myeloid leukaemia (AML) a nd myelodysplastic syndrome (MDS). Additional studies evaluated the le vels of accumulation and retention of 1-beta-D-arabinofuranosylcytosin e 5'-triphosphate (ara-CTP) in cells exposed to ara-C with and without bryostatin. Cells were exposed to bryostatin at a range of concentrat ions (0.1-100 nM) for 48 h and at 1 nM for both modulation studies and assessment of ara-CTP production. We found bryostatin to be cytotoxic in 18/58 (31%) tests whilst potentiation of formazan production in th e MTT assay was seen in 21/58 (36%) patients. On co-incubation with br yostatin, 16/58 (27%) tests showed increased cytotoxicity to ara-C. Fu rthermore, there was a significant increase in the accumulation of ara -CTP on co-incubation with bryostatin (p = 0.0401). We found patients with in vitro resistance were more likely to become sensitised followi ng exposure to bryostatin (p< 0.01). This study has emphasised the nee d to optimise treatment regimens for individual patients using this ap proach. (C) 1998 Elsevier Science Ltd. All rights reserved.