Zosuquidar and an albumin-binding prodrug of zosuquidar reverse multidrug resistance in breast cancer cells of doxorubicin and an albumin-binding prodrug of doxorubicin

The P-glycoprotein (P-gp) is a 170-kDa protein that acts as an energy dependent, transmembrane efflux pump and is encoded by the MDR1 gene. It has been shown to be responsible for multidrug resistance (MDR) in a defined subpopulation of breast cancer patients and thus represents a molecular target for circumventing MDR in this tumor indication. MDR modulators have been developed and demonstrated high selectivity for P-gp with inhibitory activities in the low nanomolar range. Although some objective responses were achieved in clinical trials, combination therapy with these MDR modulators, such as Ca²⁺ antagonists caused unacceptable toxicity. Targeting P-gp inhibitors to the tumor site is a mean to increase their therapeutic index, and in this context binding of tailor-made prodrugs to circulating albumin is an established technology to reduce the toxicity and enhance the efficacy of anticancer drugs. In this study, we consequently developed an acid-sensitive albumin-binding prodrug of the P-gp inhibitor zosuquidar (LY335979) in a two-step synthesis using a maleimide hydrazone linker system established in our laboratory that first introduces acetylbenzoic acid at the HO-group of zosuquidar followed by derivatization with 6-maleimidocaproyl hydrazide to form the acid-sensitive hydrazone bond. The maleimide group enables the prodrug to bind rapidly and selectively to the cysteine-34 position of endogenous albumin after intravenous administration. HPLC analysis demonstrated rapid albumin binding of the zosuquidar prodrug as well as the quantitative release of the acetylbenzoic ester derivative of zosuquidar at pH 5.0. Subsequently, its ability to circumvent MDR was tested in two doxorubicin-resistant breast carcinoma cell lines (MCF-7/ADR and MT-3/ADR). The MDR status of these cell lines can be reversed by zosuquidar which was confirmed in a rhodamine 123 assay using fluorescence microscopy and FACS analysis. Furthermore, zosuquidar as well its acid-sensitive albumin conjugate re-sensitized cells to doxorubicin as well as to an albumin-binding prodrug of doxorubicin, i.e., the 6-maleimidocaproyl hydrazone derivative of doxorubicin, achieving IC₅₀ values in the same order of magnitude as the parental cell lines. Thus, a novel formulation of zosuquidar has been developed that could have the potential to improve the toxicity issues and tumor targeting properties of the original compound.