Multiwavelength videomicrofluorometric study of cytotoxic effects of a marine peptide, didemnin B, on normal and MDR resistant CCRF-CEM cell lines.

The development of multidrug resistance (MDR) in heterogeneous cell sensitive and resistant populations to a variety of clinically important cytotoxic drugs poses a major obstacle to cancer chemotherapy. Didemnin B, a marine cyclic depsipeptide, displays interesting biological properties: antiviral activity, inhibition of DNA, RNA and protein synthesis, initiation of apoptosis and ability to block the cell cycle. As very little is known about its mode of action, we studied the effect of increasing doses of Didemnin B on sensitive and resistant human leukemic lymphoblast cell lines. The fluorescence of living cells simultaneously stained with Hoechst 33,342, Rhodamine 123 and Nile Red, were analyzed in a multiparametric approach involving multiwavelength microfluorometry. High concentrations of Didemnin B induced, in the sensitive cell line, a very early decrease in the energetic state of the mitochondria that occurs before a significant decrease of nuclear DNA content, observed simultaneously on sensitive and resistant cells, that could be related to an apoptosis process. Furthermore low Didemnin doses (50 nM) affected CEM-WT and CEM VLB differently, while higher doses (200 nM-250 nM and over) affected the two cell lines in the same way. This indicated that, at these doses, the membranar Pgp has no effect on the mode of action of Didemnin, suggesting that Didemnin does not need to be internalized to be active.