We have demonstrated that in vitro resistance of
tumor cells to
doxorubicin (Dox) can be fully circumvented by using
doxorubicin-loaded
nanospheres (Dox-NS), consisting of biodegradable
polyisohexylcyanoacrylate polymers of 300 nm diameter and containing 2.83 mg of Dox per 31.5 mg of
polymer. Five different multidrug-resistant cell lines, characterized by mdr1 amplification, were used in this study: Dox-R-MCF7, a human breast
adenocarcinoma; SKVBL1, a human ovarian
adenocarcinoma; K562-R, a human
erythroleukemia; and two murine lines: P388-Adr-R, a monocytic
leukemia of DBA2 mouse, and LR73MDR, a Chinese hamster ovarian cell line. These lines were 38.7, 210, 232, 143 and 20 times more resistant than their corresponding sensitive counterparts, respectively. Using Dox-NS, we obtained complete reversion of drug resistance in vitro, i.e. cell growth inhibition comparable with that obtained with sensitive cells exposed to free Dox. In vivo, we significantly prolonged the survival of DBA2 mice which had previously received P388-Adr-R cells by i.p.
injections of Dox-NS, while free Dox injection was ineffective toward this rapidly growing
tumor. (Prolongation of survival time: 115% vs 167% after Dox vs Dox-NS treatment, respectively.) Using the MCF7 cell line and its resistant variant, we studied the intracellular concentration and the cytoplasmic and nuclear distribution of Dox by
laser microspectrofluorometry (LMSF). In sensitive cells, we observed a similar accumulation and distribution of Dox whatever the form of Dox delivery, i.e. whether free or carried by
nanospheres. Analysis by LMSF showed that 99% of intranuclear Dox was bound to
DNA after treatment with both forms of Dox. Of Dox, 81 and 83% were found in the intranuclear compartment of sensitive cells incubated with free Dox and Dox-NS, respectively. Resistant cells incubated with Dox-NS accumulated the same amount of Dox as sensitive cells incubated with free Dox or with Dox-NS. Dox, when loaded in
nanospheres, bypasses the efflux mechanism responsible for multidrug resistance. LMSF analysis showed that Dox, transported and released by
nanospheres, interacts with
DNA identically in sensitive and resistant cells.