P-glycoprotein (P-gp) is a membranous ATPase responsible for the multidrug resistance phenotype. The effect on P-gp-mediated transport of anthracyclines of cell irradiation in the presence of 2,2-diphenyl-5-[N-1-(o-azidophenyl)ethylamino]valeronitrile (VP*), a photoactivable analogue of verapamil was studied in viable K562/ADR cells. The derivatives were daunorubicin (DNR), idarubicin (IDA), 8-(S)-fluoro-idarubicin (F-IDA), 2'-bromo-4'-epidaunorubicin (Br-DNR) and pirarubicin (PIRA). It was observed that the irradiation in the presence of the verapamil analogue was unable to completely inhibit the P-gp-mediated efflux of anthracyclines and we estimated that P-gp retained 10-20% of its ability to pump these toxins. The ability of verapamil, DNR, IDA, F-IDA, Br-DNR and PIRA to inhibit the effect of VP* was studied. For this purpose, cells were irradiated in the presence of VP* and various concentrations of either verapamil or of one of the anthracyclines and then the P-gp functionality was checked by its ability to pump pirarubicin. It was observed that (i) the effect observed, when cells were irradiated in the presence of VP*, was completely blocked by the presence of verapamil; (ii) that anthracyclines are able to partially inhibit the VP* effect. This inhibition occurs at low concentration of anthracycline and depends on the nature of the derivative used. With those used in that study, after the photoirradiation of K562 ADR cells in the presence of VP* and anthracycline, P-gp has retained 50 +/- 5% of its functionality. The anthracycline concentration required for this inhibition is rather low, the total drug concentration yielding 50% of the effect ranged from 0.5 (Br-DNR) to 4 microM (F-IDA). The corresponding cytosolic concentrations are highly correlated with the values of Km determined previously.