A bioassay for the activity of PSC 833 in human serum for modulation of P-glycoprotein-mediated multidrug resistance

Abstract

We established a rapid and sensitive ex vivo bioassay to detect the multidrug resistance (MDR)-inhibitory activity of SDZ PSC 833 ([3'-keto-Bmt1]-[Val2]-cyclosporin (PSC 833)) in two RPMI 8226 human myeloma sublines (parent 8226 and doxorubicin-resistant subline Dox6) in 75% human serum. In vitro sensitivity of the tumor to doxorubicin was determined by 3-h drug exposure growth inhibition assay (MTT assay). PSC 833 in serum restored the IC50 of doxorubicin in the P-glycoprotein (P-gp)-positive resistant subline to the same level as in the sensitive cells at 1 microg/ml, which has been shown to be an achievable concentration in clinical trials. In addition, the cytotoxic effect of doxorubicin was enhanced by PSC 833 in the sera of the patient in whom the blood level was 705.7 ng/ml. However, 10 microg/ml PSC 833 in serum does not cause a complete recovery in the IC90 of doxorubicin in the resistant sublines. This MDR-inhibitory activity was supported by the finding that PSC 833 in serum does not increase accumulation of rhodamine 123 in doxorubicin-resistant cells in an in vitro functional assay. The present study provides evidence that PSC 833 in human serum is effective to modulate P-gp-mediated MDR but insufficient for the reversal of MDR from the clinicopharmacological point of view.

Overlapping Ligand Specificity of P-Glycoprotein and Serum α1-Acid Glycoprotein:Evidences and Potential Implications

“…In cancer cell culture, increased cytotoxicity of adriamycin (1 M) achieved by using the P-gp inhibitor toremifene (10 M) was reduced to the level of the control experiment (adriamycin only) in the presence of 2mg/ml AGP suggesting that elevated AGP levels in cancer patients may impair the ability of toremifene to modulate P-gp activity via the decrease of free fraction of the drug [26,27]. In relation to the serum-free medium, the second-generation non-immunosuppressive cyclosporin A analogue PSC833 (valspodar) failed to fully inhibit Pgp mediated drug transports when used in 100% fetal bovine serum at clinical drug level ( 2 M) [30,31]. In relation to the serum-free medium, the second-generation non-immunosuppressive cyclosporin A analogue PSC833 (valspodar) failed to fully inhibit Pgp mediated drug transports when used in 100% fetal bovine serum at clinical drug level ( 2 M) [30,31].…”