Bioreductive activation of tumor targeted drug delivery systems containing melphalan

Solid tumors provide an environment conducive to bioreduction due to hypoxia and overexpression of bioreductive enzymes. Therefore, TDDS were designed with various substituents to modulate bioreductive activation. TDDS contain a quinone-based carrier coupled to the model drug, melphalan methyl ester (MME). Controlled bioreductive activation of TDDS can lead to specific drug release only at the tumor sites and hence reduce toxicity during systemic distribution. The rate and extent of bioreductive activation of TDDS was determined in presence of DT-diaphorase, xanthine oxidase, human breast tumor cells (MCF-7) and colon tumor cells (Caco-2). Stability of TDDS under aqueous buffer conditions and in the presence of glutathione was also evaluated. Anticancer activity of TDDS was determined based on alkylating activity, cytotoxicity and apoptotic induction in both the tumor cell lines. Results show that all TDDS especially CH3-TDDS improved stability of melphalan under pH 7.4. H-TDDS possessed significantly high bioreductive ability in both tumor cells, which was predominantly catalyzed by the reductive enzymes, DT-diaphorase. TDDS with the electron-withdrawing substituent (Br-TDDS) underwent rapid glutathione-catalyzed degradation in tumor cell homogenates. TDDS with electron-donating substituents exhibited either relatively low (CH₃-TDDS) or no (C₄H₄-TDDS or CH₃NH-TDDS) bioreduction in tumor cell homogenates. Thus, bioreductive activation of TDDS was influenced by the electronic properties of the substituents. Bioreduction of TDDS (in the presence of purified reductase enzymes and in the MCF-7 and Caco-2 cells) was consistent with their reductive ability (reduction potentials and sodiumborohydride reduction). Both H-TDDS and CH₃-TDDS exhibited alkylating activity similar to MME and melphalan. While H-TDDS demonstrated relatively high cytotoxicity and apoptosis induction in the MCF-7 cells, CH ₃-TDDS demonstrated relatively high cytotoxicity and apoptosis induction in the Caco-2 cells. MME and melphalan exhibited higher cytotoxicity and apoptosis induction than TDDS in the tumor cells. Based on stability under physiological pH and susceptibility to DT-diaphorase in human tumor cells, H-TDDS and CH₃-TDDS appear to be most promising candidates for in vivo antitumor efficacy studies. Such bioreductive TDDS could be useful in targeting a variety of anticancer agents to solid tumors.