DECREASED POTENCY OF MDR-MODULATORS UNDER SERUM CONDITIONS DETERMINED BY A FUNCTIONAL ASSAY

A variety of agents are capable of overcoming P-glycoprotein-mediated multidrug resistance (MDR) in vitro, However, the clinical potential of these compounds is often limited due to high plasma protein binding. We compared the efficacy of several MDR-reversing compounds in serum-free culture medium and under serum conditions by means of a functional assay, Using flow cytometry the efflux of the fluorescent dye rhodamine 123 (Rh123) was measured from normal peripheral blood CD8(+) T-lymphocytes which express low levels of P-glycoprotein. Inhibition of Rh123 efflux by R-verapamil, dexnigludipine-HCl, cyclosporin A, SDZ PSC833 and the protein kinase C (PKC) inhibitor CGP 41251 was determined in serum-free medium and in serum at concentrations from 0.1 to 50 mu mol/l. With the exception of SDZ PSC833 all MDR modulators showed an insufficient or suboptimal modulation of P-glycoprotein under serum conditions at concentrations achievable in vivo. The highest potency under serum conditions demonstrated SDZ PSC833: even at a concentration of 0.5 mu mol/l a sufficient inhibitory effect was observed. Subsequently this approach was applied to patients suffering from B-cell chronic lymphocytic leukaemia (B-CLL; n=3) and acute myeloid leukaemia (AML; n=2) which were positive in the Rh123 efflux assay. As for normal CD8(+) T-lymphocytes, much higher drug concentrations were required under serum conditions to effectively inhibit: Rh123 efflux from the leukaemic cells. Thus the interpretation of results of clinical 'modulator' trials should consider the decreased bioavailability of MDR-reversing agents.