From a number of studies it has been suggested that positive charge and degree of lipophilicity dictate, or at least influence, whether anthracyclines are recognized by the apparently clinical important mechanism of tumor cell resistance, i.e., P-gp-mediated multidrug resistance. Using a selected series of analogs in which lipophilicity and or positive charge are altered we find the following: (1) Positively-charged anthracyclines as compared to their neutral counterparts are better recognized by MDR+ cells. (2) With increasing lipophilicity charge becomes less important for MDR recognition. (3) In MDR+ cells with a resistance index to Adriamycin (ADR) of 4534, as compared to an MDR- parental line, almost all of the resistance is circumvented (resistance index = 3) with an anthracycline which does not contain a protonatable nitrogen and is highly lipophilic (partition coefficient, log p = > 1.99). (4) As lipophilicity is increased to log p > 1.99 and nuclear binding is decreased, anthracycline localization switches from nuclear to cytoplasmic which most likely indicates a different cytotoxic target and mechanism of action. (5) Cytoplasmically localized anthracyclines appear to distribute also in mitochondria which suggests these organelles as possible new anthracycline targets. In contrast, ADR shows no mitochondrial localization. (6) Photoaffinity analysis suggests that the highly lipophilic analogs, regardless of charge, interfere with NASV-Vp binding to P-gp. This is consistent with the idea that highly lipophilic anthracyclines act as modulators of MDR which may contribute to their mechanism of overcoming this form of resistance. The possible clinical significance of these data is that highly lipophilic anthracyclines are shown to circumvent MDR which most likely reflects their ability to localize in the cytoplasm and affect targets other than nuclear DNA, i.e., mitochondria, and to act as self modulators of MDR. Thus, a new approach to circumventing MDR and other mechanisms of resistance which involve nuclear targets is the use of active anthracyclines which are highly lipophilic and localize in the cytoplasm/mitochondria.
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