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 recently as a modulator of 1-beta-D-arabinofuranosylcytosine (ara-C) resistance. This compound is currently in phase I and II trials as a single agent. 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 fresh blast cells from 53 patients with acute myeloid leukaemia (AML) and myelodysplastic syndrome (MDS). Additional studies evaluated the levels of accumulation and retention of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (ara-CTP) in cells exposed to ara-C with and without bryostatin. Cells were exposed to bryostatin at a range of concentrations (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 the MTT assay was seen in 21/58 (36%) patients. On co-incubation with bryostatin, 16/58 (27%) tests showed increased cytotoxicity to ara-C. Furthermore, 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 following exposure to bryostatin (p < 0.01). This study has emphasised the need to optimise treatment regimens for individual patients using this approach.