DRUG-BINDING TO P-GLYCOPROTEIN IS INHIBITED IN NORMAL-TISSUES FOLLOWING SDZ-PSC-833 TREATMENT

Rats were treated with daily injections of SDZ-PSC 833 (PSC) to study the interaction of this potent modulator of multidrug resistance (MDR) with P-glycoprotein (P-gp) expressed in normal tissues. After 2 days of treatment, the level of P-gp expression, detected by Western blot a nalysis, was not modified in renal brush border membranes (BBMs) and b rain capillaries. However, the amount of P-gp detected with the photoa ffinity probe [I-125]. arylazidoprazosin (IAAP) was decreased in both tissues, suggesting that the drug binding properties of P-gp were alte red by PSC treatment. This effect was further characterized by treatin g rats with PSC for 10 days. Following these treatments, the amount of immunodetected P-gp was increased in renal BBMs and brain capillaries . However, no increase in P-gp expression was observed in photolabelin g experiments, suggesting that induced P-gp was not functional. In vit ro experiments performed with renal BBMs showed that the inhibition of P-gp photolabeling by cyclosporin A (CsA), verapamil and vinblastine could be reversed by performing washing steps to remove these drugs be fore incubating the samples with IAAP. However, the inhibition mediate d by PSC was less reversible since photolabeling of P-gp remained inhi bited following the washing steps. Pre-incubation of intact CH(R)C5 ce lls with PSC, CsA and verapamil also inhibited P-gp photolabeling and increased rhodamine 123 accumulation. For PSC pretreated samples, thes e effects were not completely reversed following washing, but were abo lished for CsA and Ver pretreated samples. Our results suggest that PS C could block P-gp function by a different mechanism from that of CsA and verapamil, involving modification of the drug binding sites. (C) 1 998 Wiley-Liss, Inc.