Heterogenous effects of bryostatin on human myeloid leukemia clonogenicity: dose and time scheduling dependency

The potent anti-neoplastic actions displayed in vitro by bryostatins have led to the introduction of short-term bryostatin-1 infusions in phase I clinical trials. Because bryostatin (bryo) undergoes a rapid clearance in vivo, we were interested in its scheduling effects on acute myeloid leukemia (AML) clonogenicity. Therefore, we assessed the primary plating efficiency (PE1) of AML samples in response to several bryo concentrations after various preincubation periods in a semi-solid colony forming assay. Whereas continuous exposure to 10 nM bryo during the assay period reduced the PE1 in nearly all samples tested, preincubation of eight AML patients' specimens for 1, 2, 3 or 4 days with bryo in a dose range of 0.1-10 nM showed a heterogenous PE1 response. Stimulatory as well as inhibitory effects on leukemic clonogenic growth were seen within individual specimens depending on dose and preincubation time with no single incubation time or concentration that caused unequivocal common overall inhibition of clonogenic growth in most or all of the samples. Higher doses of bryo also failed to result in specific inhibition of leukemic cells: 4/8 samples showed an increased clonogenic response to 250 nM bryo whereas normal bone marrow progenitor cells were consistently inhibited in their clonogenicity at this dose. A marked similarity, i.e. undulatory effects with increasing bryo concentrations, was found for HL60 leukemic cells. In conclusion, the effects of bryo on clonogenic leukemia cell growth are strongly dependent on scheduling and dose varying between and within individual AML samples. These results caution against in vivo bryo pulse therapy, as currently applied, for treatment of AML.