Gabr A, Kuin A, Aalders M, El-Gawly H, Smets L A
Division of Experimental Therapy, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Amsterdam.
Cancer Res. 1997 Nov 1;57(21):4811-6.
pH-mediated conversions in the structure of the topoisomerase (topo) I inhibitors camptothecin (CPT) and its analogues have strong implications for the pharmacokinetics and pharmacodynamics of these novel anticancer agents. Because the cell-penetrating and biologically active lactone isomers predominate at acidic conditions, we have tested if low pH potentiates the cytotoxic and antitumor effects of CPT and its water-soluble derivative topotecan (TPT). In L1210 leukemia cells, rapid initial uptake of radiolabeled CPT and TPT was followed by a gradual release 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 increased proportionally, up to 5-fold, with decreasing pH of the incubating medium (from 7.4 to 6.0). With TPT, a maximum 3-fold increase was observed at pH 6.8 to 6.4. By contrast, the cellular pharmacokinetics of the topoisomerase II inhibitor etoposide (ETP) were independent of the ambient pH. The large increases in intracellular CPT and TPT levels caused only moderate potentiation of cytotoxicity in short-term incubations. Conditions of very low pH < or =6.2 even antagonized the cytotoxicity of the topo I and topo II inhibitors, due to inhibition of DNA synthesis by intracellular acidification. However, in clinically relevant schedules of prolonged exposures at low drug concentration, low pH potentiated the cytotoxicity of CPT and TPT by 2-3-fold. To investigate the effect of local pH in vivo, the basal interstitial pH of 6.8 of RIF-1 tumors was selectively lowered by i.p. injection of the host animals with the mitochondrial inhibitor meta-iodobenzylguanidine (32 mg/kg) and glucose (1.5 g/kg). In accordance with the pH 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 intervention significantly potentiated the response of tumors to CPT. The results indicate that local pH is an important determinant of the cellular pharmacokinetics and the antitumor activity of CPT and analogues.
拓扑异构酶（topo）I抑制剂喜树碱（CPT）及其类似物结构中由pH介导的转变对这些新型抗癌药物的药代动力学和药效动力学有重要影响。由于细胞穿透性且具有生物活性的内酯异构体在酸性条件下占主导，我们测试了低pH是否会增强CPT及其水溶性衍生物拓扑替康（TPT）的细胞毒性和抗肿瘤作用。在L1210白血病细胞中，放射性标记的CPT和TPT最初迅速摄取，随后在生理pH 7.4时从细胞中逐渐释放，而在pH 6.2时细胞内维持高药物水平。随着孵育培养基pH值降低（从7.4降至6.0），CPT的稳态摄取水平成比例增加，最高可达5倍。对于TPT，在pH 6.8至6.4时观察到最大增加3倍。相比之下，拓扑异构酶II抑制剂依托泊苷（ETP）的细胞药代动力学与周围pH无关。细胞内CPT和TPT水平的大幅增加在短期孵育中仅引起细胞毒性的适度增强。极低pH≤6.2的条件甚至会拮抗topo I和topo II抑制剂的细胞毒性，这是由于细胞内酸化抑制了DNA合成。然而，在低药物浓度下延长暴露的临床相关方案中，低pH使CPT和TPT的细胞毒性增强了2至3倍。为了研究体内局部pH的影响，通过腹腔注射线粒体抑制剂间碘苄胍（32 mg/kg）和葡萄糖（1.5 g/kg）选择性降低了RIF-1肿瘤6.8的基础间质pH。与TPT在pH 6.8至6.4摄取的最佳pH一致，肿瘤酸化对该类似物的抗肿瘤作用没有影响。相比之下，该干预显著增强了肿瘤对CPT的反应。结果表明，局部pH是CPT及其类似物细胞药代动力学和抗肿瘤活性的重要决定因素。
