Overcoming Multidrug Resistance with Inhibitor of ABC Transporters Bound to HPMA Copolymer Carrier as a Potential Therapeutic Approach in Cancer Treatment

A significant problem in cancer chemotherapy is that even originally sensitive tumors become resistant to the effects of cytostatic drugs. This phenomenon has been described as multidrug resistance (MDR). MDR can develop in several ways, with the predominant mechanism being the overexpression of ATP-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp).

The aim of the study was to examine the potential of novel polymeric therapeutics based on N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers bearing either anticancer drug, inhibitor of ABC transporters or both, for overcoming MDR mediated by P-gp in doxorubicin (Dox) resistant cell line P388/MDR.

Series of low-molecular weight derivatives of inhibitor reversin 121 (R121) were tested for their ability to block the P-gp efflux pump. 5-methyl-4-oxohexanoyl R121 (MeOHe-R121) showed the highest inhibitory activity out of all the tested derivatives. This derivative was consequently conjugated to the HPMA copolymer carrier. Conjugate P-Ahx-NH-N=MeOHe-R121 showed superior ability to sensitize P388/MDR cells in a dose-dependent manner and induced an almost 50-fold increase of cytostatic activity of Dox at 24 µM. Moreover, P-Ahx-NH-N=MeOHe-R121 exerted gradually increasing intracellular accumulation of Dox in P388/MDR cells and thus increased cytotoxic activity of Dox in concentration dependent manner. Additionally, the cytostatic activity of HPMA copolymer conjugate P-Ahx-NH-N=MeOHe-R121(Dox) bearing both Dox and P-gp inhibitor MeOHe-R121 bound via pH-sensitive hydrazone bond was almost 45 times higher than that of the conjugate bearing only Dox in P388/MDR cells.

Finally, the in vivo evaluation of anti-tumor activity of conjugate P-Ahx-NH-N=MeOHe-R121(Dox) or mixture of the conjugates P-Ahx-NH-N=MeOHe-R121 and P-Ahx-NH-N=Dox in mouse P388/MDR model showed significant inhibition of tumor growth with the best effects of P-Ahx-NH-N=MeOHe-R121(Dox). The treatment with conjugate P-Ahx-NH-N=Dox alone exerted similar growth as obtained in the control group.