THE MODULATING EFFECT OF PSC833, CYCLOSPORINE-A, VERAPAMIL AND GENISTEIN ON IN-VITRO CYTOTOXICITY AND INTRACELLULAR CONTENT OF DAUNORUBICININ CHILDHOOD ACUTE LYMPHOBLASTIC-LEUKEMIA

Resistance to anthracyclines is related to a poor prognosis in childho od acute lymphoblastic leukemia (ALL). Resistance to this class of dru gs may (partly) be reversed by modulating agents, as has been demonstr ated in a variety of cell lines. However, it is unknown which modulato rs may be of clinical benefit in childhood ALL. Therefore, we studied the modulating effect of PSC 833, cyclosporin A (CsA), verapamil (Vp) and genistein on daunorubicin (DNR) cytotoxicity, accumulation and ret ention in childhood ALL cells, DNR cytotoxicity was determined using t he MTT assay; DNR accumulation, DNR retention and the expression of P- glycoprotein (P-gp), multidrug resistance-associated protein (MRP) and major vault protein/lung resistance protein (LRP) were determined by flow cytometry. In the majority of samples PSC 833 (19/26), CsA (22/26 ) and Vp (15/18) sensitized the cells to DNR whereas genistein made 25 out of 26 samples more resistant to DNR. The sensitizing effect on th e cytotoxicity of DNR was median 1.2-fold using 2 mu M PSC 833 (P=0.02 5), 1.5-fold using 4 mu M CsA (P=0.003) and 1.6-fold using 6 mu M Vp ( P=0.012) whereas the adverse effect of 25 mu M genistein was median 1. 8-fold (P<0.0001). No relationship was found between the sensitizing e ffect of PSC 833, CsA or Vp and the degree of DNR resistance. In contr ast, the adverse effect of genistein was largest in DNR sensitive samp les (P=0.003). The effect of each modulator on the cytotoxicity of DNR did not differ between initial and relapse ALL samples although the l atter were median 1.4-fold more resistant to DNR (P=0.005). Modulation of DNR cytotoxicity was not correlated with changes in the accumulate d and retained intracellular DNR content or with the expression of P-g p, MRP and LRP. Besides genistein, PSC 833, CsA and Vp incidentally ma de ALL cells more resistant to DNR. CsA stimulated the leukemic cell s urvival in seven out of 26 samples, a phenomenon that was not related to the degree of DNR resistance. In conclusion, PSC 833, CsA and Vp bu t not genistein may be used to sensitize cells to DNR in childhood ALL . The data also indicate that not all patients may have a therapeutic benefit from these modulators. Therefore, an in vitro culture assay ma y be necessary to screen for patients who may benefit by a modulator i n their therapy.