Lysosomotropic properties of weakly basic anticancer agents promote cancer cell selectivity in vitro.

Drug distribution in cells is a fundamentally important, yet often overlooked, variable in drug efficacy. Many weakly basic anticancer agents accumulate extensively in the acidic lysosomes of normal cells through ion trapping. Lysosomal trapping reduces the activity of anticancer drugs, since anticancer drug targets are often localized in the cell cytosol or nucleus. Some cancer cells have defective acidification of lysosomes, which causes a redistribution of trapped drugs from the lysosomes to the cytosol. We have previously established that such differences in drug localization between normal and cancer cells can contribute to the apparent selectivity of weakly basic drugs to cancer cells in vitro. In this work, we tested whether this intracellular distribution-based drug selectivity could be optimized based on the acid dissociation constant (pKa) of the drug, which is one of the determinants of lysosomal sequestration capacity. We synthesized seven weakly basic structural analogs of the Hsp90 inhibitor geldanamycin (GDA) with pKa values ranging from 5 to 12. The selectivity of each analog was expressed by taking ratios of anti-proliferative IC50 values of the inhibitors in normal fibroblasts to the IC50 values in human leukemic HL-60 cells. Similar selectivity assessments were performed in a pair of cancer cell lines that differed in lysosomal pH as a result of siRNA-mediated alteration of vacuolar proton ATPase subunit expression. Optimal selectivity was observed for analogs with pKa values near 8. Similar trends were observed with commercial anticancer agents with varying weakly basic pKa values. These evaluations advance our understanding of how weakly basic properties can be optimized to achieve maximum anticancer drug selectivity towards cancer cells with defective lysosomal acidification in vitro. Additional in vivo studies are needed to examine the utility of this approach for enhancing selectivity.

药物在细胞内的分布是药物疗效中一个极其重要但却常常被忽视的变量。许多弱碱性抗癌药物通过离子捕获大量积聚在正常细胞的酸性溶酶体中。溶酶体捕获降低了抗癌药物的活性，因为抗癌药物的靶点通常位于细胞质或细胞核中。一些癌细胞的溶酶体酸化存在缺陷，这导致捕获的药物从溶酶体重新分布到细胞质中。我们先前已经确定，正常细胞和癌细胞之间这种药物定位的差异可能有助于弱碱性药物在体外对癌细胞表现出选择性。在这项工作中，我们测试了这种基于细胞内分布的药物选择性是否可以根据药物的酸解离常数（pKa）进行优化，pKa是溶酶体隔离能力的决定因素之一。我们合成了7种热休克蛋白90（Hsp90）抑制剂格尔德霉素（GDA）的弱碱性结构类似物，其pKa值在5到12之间。每种类似物的选择性通过计算抑制剂在正常成纤维细胞中的抗增殖半数抑制浓度（IC50）值与在人白血病HL - 60细胞中的IC50值之比来表示。在一对由于小干扰RNA（siRNA）介导的液泡质子ATP酶亚基表达改变而溶酶体pH不同的癌细胞系中进行了类似的选择性评估。对于pKa值接近8的类似物观察到了最佳选择性。对于具有不同弱碱性pKa值的商业抗癌药物也观察到了类似的趋势。这些评估增进了我们对如何优化弱碱性特性以在体外对溶酶体酸化缺陷的癌细胞实现最大抗癌药物选择性的理解。还需要进行更多的体内研究来检验这种方法在提高选择性方面的实用性。