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 sensiti ve 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 apopto sis and ability to block the cell cycle. As very little is known about its mode of action, we studied the effect of inn-easing closes of Didemnin B on sensitive and resistant human leukemic lymphoblast cell lines. The fluores cence of living cells simultaneously stained with Hoechst 33342, Rhodamine 123 and Nile Red were analyzed in a multi-parametric approach involving mul tiwavelength micro-fluorometry. High concentrations of Didemnin B induced i n the sensitive cell line, a very early decrease in the energetic state of the mitochondria that occurs before a significant decrease of nuclear DNA c ontent, observed simultaneously on sensitive and resistant cells, that coul d be related to an apoptosis process. Furthermore low Didemnin doses (50 nM ) affected GEM-WT and CEM VLB differently, while higher doses (200nM-250 nM and over) affected the two cell lines in the same way. This indicated that , at these noses, the membranar Pgp has no effect on the mode of action of Didemnin, suggesting that Didemnin does not need to be internalized to be a ctive.