Drug resistance in multiple myeloma: cyclosporin A analogues and their metabolites as potential chemosensitizers

Abstract

The malignant clone in myeloma is not eradicated by chemotherapy. Cyclosporins inhibit drug transport mechanisms, particularly the multidrug transporter p-glycoprotein 170, leading to their use as chemosensitizers. In myeloma, clonotypic blood B cells represent the major drug-resistant subset. This study compares the ability of cyclosporin A analogues and metabolites to inhibit cellular transporter(s) in myeloma and normal B cells in vitro, and evaluates their potential role in vivo. Cyclosporin A (CsA), CsG, PSC 833 or SDZ 280-446, and primary CsA and CsG metabolites, were tested for their ability to inhibit drug transport mechanisms of ex vivo malignant B cells from 81 patients with multiple myeloma as compared to B cells from normal donors, as measured by the export of the dye rhodamine 123 (Rh123) using multiparameter flow cytometry. The majority of myeloma B and normal B cells had efficient transporter function as measured by their CsA-sensitive export of Rh123. CsA and CsA analogues mediated efficient inhibition of this transport. Inhibition of dye transport by normal B cells required an approximately six-fold greater concentration of the synthetic peptolide SDZ 280-446 than was needed to optimally inhibit transport by myeloma B cells. PSC 833 and CsG were inhibitory at concentrations approximately five-fold lower than were required for CsA. Assessment of inhibitory potency in vivo indicated that the in vivo chemosensitizer levels of CsA and PSC 833 exceeded the transporter inhibitory concentration by four- and 20-fold respectively. In vivo, cyclosporins are rapidly and almost completely converted to metabolites. AM1 and AM4N, primary metabolites of CsA, mediated inhibition of transport, as did CsG metabolites GM1, GM4N and GM9. AM1 and GM9 are known to reach steady-state in vivo levels that exceed the inhibitory concentration identified here by 1.1- to 1.9-fold. Thus, cyclosporin metabolites, which accumulate in the blood during infusion of CsA and other cyclosporins, are shown here to be effective chemosensitizers for normally drug-resistant myeloma cells in vitro. Cyclosporin metabolites are considered to be less toxic than the parent drugs, suggesting that novel chemosensitization strategies designed to minimize concentrations of parent drug and maximize accumulation of primary metabolites in vivo may optimize cytotoxicity to the malignant clone in myeloma.