Estimation of the haematological toxicity of minor groove alkylators using tests on human cord blood cells

We evaluated the myelotoxicity and the anti-tumor potential of tallimustine, three of its analogues and carzelesin, with melphalan as reference substance. Tallimustine was tested by clonogenic assays on both human bone marrow (BM) and cord blood (hCB) cells, the other compounds on hCB only. The degree of inhibition of the haemopoietic progenitors GM-CFC, CFC-E and BFU-E was evaluated after exposure to different concentrations. The same schedules were tested on five tumour cell lines, We found that the dose-response curves for tallimustine on BM and hCB cells were similar. Carzelesin was shown to be the most potent of the substances tested and to be the one with the best in vitro therapeutic index; of the distamycin analogues, the one bearing an alpha-bromoacrylic group (FCE 25450) had the best index. For melphalan, tallimustine and carzelesin, the concentration inhibiting the growth of 70% of progenitor cells in vitro (ID70) was similar to the concentrations found in the serum of patients treated at the maximum tolerated dose (MTD). We conclude that hCB cells may be used instead of BM cells for in vitro myelotoxicity tests. Therapeutic indexes can be extrapolated from this model and could help in selecting the most promising analogue for further clinical development. The in vitro-active concentrations are similar to myelotoxic concentrations in patients, suggesting a predictive value for the assay.