The cytotoxicity of mafosfamide on G-CSF mobilized hematopoietic progenitors is reduced by SH groups of albumin - implications for further purging strategies

The efficacy of mafosfamide purging depends on factors like incubation time , drug and erythrocyte concentration. To determine the influence of protein -bound SH groups in the incubation medium, the cytotoxicity of mafosfamide on G-CSF mobilized CD34(+) cells was evaluated by short-term culture assays and drug concentration measurements. 100 mu mol/ml mafosfamide was incubat ed for 30 min in five buffers (PBS, PBS with 1%, 5% and 10% BSA and plasma) . The mean calculated areas under the concentration-time curves (AUC) were 2489 +/- 198, 1561 +/- 286, 976 +/- 201, 585 +/- 62 and 605 +/- 196 mu mol/ l/min. The mean reductions of CFU-GM growth were 79.4%, 73.0%, 62.5%, 30.3% , 6.2% respectively, Similar results were obtained for BFU-E. Regression an alysis showed a good correlation between cytotoxicity and AUCs (CFU-GM: r = 0.8195; BFU-E: r = 0.8207). This effect is well explained by the different concentrations of SH moieties in the incubation medium resulting in a high er drug binding capacity. The profound difference between AUCs and CFU-GMs in plasma and 10% BSA cannot be explained by the quantity of SH-groups. It is probably due to an additional enzymatic drug degeneration by the 3'-5'-e xonuclease subsite of plasma DNA polymerase. In conclusion, the concentrati on of albumin-associated SH groups strongly influences the cytotoxicity of mafosfamide, It has to be considered as a new and important aspect in ex vi vo bone marrow purging.