Relationship between the intracellular daunorubicin concentration, expression of major vault protein/lung resistance protein and resistance to anthracyclines in childhood acute lymphoblastic leukemia

In vitro resistance to anthracyclines is related to a poor prognosis in chi ldhood acute lymphoblastic leukemia (ALL), but the underlying mechanisms ar e poorly understood. Using flow cytometry, we studied the contribution of d aunorubicin (DNR) accumulation and retention, cell size, expression of the major vault protein/lung resistance protein (LRP), P-glycoprotein (P-gp) an d multidrug resistance-associated protein (MRP) to the cytotoxicity of DNR (by MTT assay) in childhood ALL. The accumulated and retained DNR content w as not related to the degree of DNR resistance, nor did the content differ between 53 initial and 20 relapse ALL samples (P > 0.05), although the latt er were median two-fold more resistant to DNR (P = 0.004), Leukemic cell vo lume correlated with resistance to the anthracyclines DNR (Rs 0.32, P = 0.0 12) and idarubicin (Rs 0.46, P = 0.011) but not to other classes of drugs s uch as prednisolone, vincristine, L-asparaginase and etoposide. Relapsed pa tients had 1.5-fold larger cells than patients at initial diagnosis of ALL (P = 0.001). After cell volume correction, the intracellular DF IR concentr ation was lower in relapsed compared with initial ALL cells (eg 60 min accu mulation, P = 0.003). Moreover, the intracellular DNR concentration inverse ly correlated with DNR resistance, both in the accumulation (Rs -0.44, P < 0.001) and retention (Rs -0.33, P = 0.016) test condition. The accumulated DNR concentration inversely correlated with expression of LRP (Rs -0.36, P = 0.012) but not with P-gp and MRP. Expression of LRP, but not of P-gp and MRP, significantly correlated with DNR resistance in childhood ALL (Rs 0.33 , P = 0.03). In conclusion, the intracellular DNR concentration and the exp ression level of LRP may contribute to DNR resistance in childhood ALL. The strength of the correlations also indicates that resistance to anthracycli nes can not be explained by one single mechanism.