CELLULAR PHARMACOKINETICS AND CYTOTOXICITY OF CAMPTOTHECIN AND TOPOTECAN AT NORMAL AND ACIDIC PH

pH-mediated conversions in the structure of the topoisomerase (topo) I inhibitors camptothecin (CPT) and its analogues have strong implicati ons for the pharmacokinetics and pharmacodynamics of these novel antic ancer agents, Because the cell-penetrating and biologically active lac tone isomers predominate at acidic conditions, we have tested if low p H potentiates the cytotoxic and antitumor effects of CPT and its water -soluble derivative topotecan (TPT). In L1210 leukemia cells, rapid in itial uptake of radiolabeled CPT and TPT was followed by a gradual rel ease from cells at physiological pH 7.4, whereas high drug levels were maintained in cells at pH 6.2. Steady-state uptake levels of CPT incr eased proportionally, up to 5-fold, with decreasing pH of the incubati ng medium (from 7.4 to 6.0). With TPT, a maximum 3-fold increase was o bserved at pH 6.8 to 6.4. By contrast, the cellular pharmacokinetics o f the topoisomerase II inhibitor etoposide (ETP) were independent of t he ambient pH, The large increases in intracellular CPT and TPT levels caused only moderate potentiation of cytotoxicity in short-term incub ations, Conditions of very low pH less than or equal to 6.2 even antag onized the cytotoxicity of the topo I and topo II inhibitors, due to i nhibition of DNA synthesis by Intracellular acidification, However, in clinically relevant schedules of prolonged exposures at low drug conc entration, low pH potentiated the cytotoxicity of CPT and TPT by 2-3-f old. To investigate the effect of local pH in vivo, the basal intersti tial pH of 6.8 of RIF-1 tumors was selectively lowered by i.p. injecti on of the host animals with the mitochondrial inhibitor meta-iodobenzy lguanidine (32 mg/kg) and glucose (1.5 g/kg). In accordance with the p H optimum for TPT uptake at pH 6.8 to 6.4, tumor acidification had no effect on the antitumor effect of this analogue, By contrast, the inte rvention significantly potentiated the response of tumors to CPT, The results indicate that local pH is an important determinant of the cell ular pharmacokinetics and the antitumor activity of CPT and analogues.