ANTHRACYCLINE SUBCELLULAR-DISTRIBUTION IN HUMAN LEUKEMIC-CELLS BY MICROSPECTROFLUOROMETRY - FACTORS CONTRIBUTING TO DRUG-INDUCED CELL-DEATHAND REVERSAL OF MULTIDRUG-RESISTANCE

There is a large discrepancy between the changes in drug accumulation and the changes in drug cytotoxicity that accompany development of ant hracycline in multidrug-resistant cells. Moreover, although different molecular targets for anthracyclines such as DNA, cell membranes, or e nzymes like topoisomerases could be involved, mechanisms by which thes e compounds exert their cytotoxic and differentiating effects remain u nclear. Studies of correlation between the biological effects of anthr acyclines and drug uptake have given conflicting conclusions. For exam ple, a decrease in drug cytotoxicity for different incubation temperat ures has been observed in spite of the same intracellular anthracyclin e amount, suggesting that temperature-dependent cytotoxic effects may be mediated by drug interaction with the cell membrane. What we propos e in this review are results of our laboratory which are in agreement with an action mechanism targeted to the nucleus. In fact, we have sho wn by using microspectrofluorometry, that identical nuclear anthracycl ine concentration induces the same degree of cytotoxicity, independent of cellular MDR phenotype and the anthracycline structure. Thus, we c ould acquire information on the mechanisms of drug resistance related to drug transport. We could also give evidence that this accumulation is increased when MDR modulators, such as verapamil and S9788 and cycl osporin A or anthracyclines are used. For clinical applications, our s tudies have already dealt with nuclear concentration measurements of d oxorubicin in leukocytes of treated patients, and in vitro measurement s of drug efflux from nuclei of acute leukemic cells and its correlati on with P-glycoprotein expression. However, in these studies, there wa s no correlation between anthracycline nuclear accumulation in vitro a nd P-glycoprotein expression. In addition, from preliminary results, w e have shown that some modulators such as quinine do not significantly increase nuclear accumulation of anthracyclines in MDR cells but are able to restore anthracycline sensitivity, Other authors have recently shown that quinine has a relatively weak effect on cellular doxorubic in accumulation in MDR cells but is able to completely restore doxorub icin sensitivity. They concluded that quinine has essentially intracel lular targets involved in drug distribution (cytoplasm nucleus) from s equestration compartments. Our data contradict this and we believe tha t such modulator modifies the molecular environment of anthracyclines and/or their binding to a possible cytoplasmic target leading to diffe rent cell death. Thus, we conclude that mechanisms by which anthracycl ines induce cell death, and ways by which chemotherapy fails in resist ant cells remain complex and are related to more than one target.