Decreased biotolerability for ivermectin and cyclosporin A in mice exposed to potent P-glycoprotein inhibitors.

SDZ PSC 833 or SDZ 280-446 are strong blockers of the function of class I mdr gene-encoded P-glycoprotein molecules, which were developed for the reversal of multi-drug-resistance of tumor cells. When treated with such drugs, normal mice may display hypersensitivity to cyclosporin A and ivermectin. The recorded signs of acute toxicity are compatible with alterations of the murine central nervous system functions and with earlier data suggesting that P-glycoprotein expressed at the murine blood-brain barrier might be involved in the exclusion of cyclosporin A or ivermectin from brain tissue.

SDZ PSC 833 and SDZ 280-446 are the most active of various resistance-modifying agents in restoring rhodamine-123 retention within multidrug resistant P388 cells.

Multidrug resistance (MDR) of tumor cells may result from overexpression of P-glycoprotein (Pgp) but may be down-modulated by resistance-modifying agents (RMAs). The cyclosporin SDZ PSC 833 and the cyclopeptolide SDZ 280-446 were found to be the strongest RMAs known to date for restoring the sensitivity of MDR cells to anticancer drugs, as well as for restoring their retention of daunomycin, a fluorescent anthracycline. Using rhodamine-123 (Rhod-123), another fluorescent probe of Pgp function which also differentiates sensitive and MDR cells, several RMAs were compared for their capacity to inhibit Pgp function. At variance with the data obtained with the daunomycin probe, a series of RMAs did not detectably restore Rhod-123 retention by the MDR cells. With the remaining RMAs, achieving the same levels of Rhod-123 retention required 3 times lower RMA concentrations when the RMA was added to the MDR cells for both the initial uptake and the efflux of Rhod-123 rather than for its uptake only. Nevertheless, the data emphasized the large superiority of SDZ PSC 833 and SDZ 280-446 over all other RMAs.