STUDY OF P-GLYCOPROTEIN FUNCTIONALITY IN LIVING RESISTANT K562 CELLS AFTER PHOTOLABELING WITH A VERAPAMIL ANALOG

To our knowledge, this is the first study to investigate the modificat ion of P-glycoprotein functionality in living resistant cells after ph otolabeling. For this purpose, four new photoactive verapamil analogue s were synthesized. Tnese compounds have the same efficacy as verapami l to increase pirarubicin (pira) incorporation into living multidrug r esistant (MDR) K562 cells and to sensitize them to the cytotoxic effec t of this anthracycline derivative, indicating that they act as typica l MDR modifiers in MDR cells. These compounds were used to photolabel P-glycoprotein (P-gp) in living resistant cells. Irradiation did not r esult in photodamage to cells, and P-gp functionality was verified by the ability of living cells to incorporate pira. The irradiation of re sistant cells, 10(6)/mL, in the presence of a verapamil analogue at co ncentrations equal to or higher than 3 mu M yielded 70% inhibition of P-gp functionality. Our data provide the first evidence that the bindi ng of a verapamil analogue to P-gp is not sufficient to completely inh ibit the efflux of this anthracycline. The cells were, subsequently, c ultured for several days. Resistance was progressively recovered with time, with the treated cells being just as resistant as before photola beling after 6 days.