Camptothecin analogs with enhanced activity against human breast cancer cells. II. Impact of the tumor pH gradient

Abstract

Human breast tumors often exist in an acidic and hypoxic microenvironment, which can promote resistance to radiation and chemotherapies. A tumor-selective pH gradient arises in these tumors which favors uptake and retention of drugs like camptothecin that are weak acids. We evaluated the effect of alkyl substitutions at the 7 position in seven CPTs with varying groups at the 10 position on modulation by acidic extracellular pH in three human breast cancer cell lines. Growth inhibition was assessed by propidium iodide staining of nucleic acids in human breast cancer cells cultured at either extracellular pH 6.8 or 7.4 that were (1) hormone-sensitive (MCF-7/wt), (2) hormone insensitive (MDA-MB-231), or (3) alkylator-resistant (MCF-7/4-hc). Over 10-fold pH modulation was observed in 7-halomethyl analogs of methylenedioxy-CPT and in 7-alkyl analogs of 10-amino-CPT. Of 39 analogs tested, the overall pattern of activity across breast tumor cell lines was similar with some notable exceptions. For example, 7-propyl-10-amino-CPT was modulated 16- to 20-fold by acidic extracellular pH in the MCF-7 cell lines, but only 6-fold in MDA-MB-231 cells. One mechanism that can contribute to pH modulation is enhanced cellular drug uptake and retention. In MCF-7/wt cells, uptake of 10-amino-CPT increased 4-fold, while retention increased over 10-fold at acidic extracellular pH. In addition, gene expression analysis of MCF-7/wt cells indicated that expression of a number of genes changed under acidic culture conditions, including down-regulation of the CPT efflux protein pump breast cancer resistance protein (BCRP). Interestingly, expression of topoisomerase I, the molecular target of CPT, was not affected by acidic growth conditions. These results highlight the importance of maintaining key features of tumor physiology in cell culture models used to study cancer biology and to discover and develop new anticancer drugs. While several substitutions at the 7 and 10 positions enhance potency, 7-halomethyl and 10-amino CPT analogs show selective activity at the acidic pH common to the microenvironment of most solid tumors.

Effect of a Novel Vacuolar-H+-ATPase Inhibitor on Cell and Tumor Response to Camptothecins

“…It is unlikely that the sensitization of colon carcinoma cells was mediated by interaction of NiK-12192 with the transporters (BCRP or P-glycoprotein), because it was detected in cells expressing either low (parental) or high levels (resistant sublines) of these proteins. However, because changes of acidification could influence the expression of these transport systems (Adams et al, 2006), an indirect effect mediated by protein down-regulation could not be ruled out.…”

Overcoming Physiologic Barriers to Cancer Treatment by Molecularly Targeting the Tumor Microenvironment

“…Pretreating animals with bicarbonate before administering mitoxantrone or doxorubicin has been shown to increase the response of model CH3 or MCF7 breast cancers, respectively, without significantly increasing the pHe of normal muscle (90,94). The chemical structure of camptothecan has even been modified to take advantage of this pH-dependent partitioning across the plasma membrane to increase its antiproliferative effect (95,96). Although these approaches were successful in treating animal tumors, it is not clear if these acute approaches will yield long-term effectiveness without systemic acid base side effects during multicycle chemotherapy in humans.…”

“…Gene expression changes have been identified that are a response to the acidotic environment of the tumor that may contribute to tumor progression. These induced genes include the acid-responsive angiogenic stimulator interleukin-8 (103), the bone remodeling parathyroid hormone/parathyroid hormone receptor pathway (104), the breast cancer resistance protein (96), and tumor necrosis factor-a (105). Additional mechanisms of gene regulation in response to acidity include induction of the Sp1 transcription factor (106) and specific increases in mRNA stability (107).…”

Acidic pH of Tumor Microenvironment Enhances Cytotoxicity of the Disulfiram/Cu²⁺ Complex to Breast and Colon Cancer Cells

“…The reduction potential of a ROS species formed in these reactions is generally lower in alkaline than in acidic conditions [36]. Previous studies have shown that lowering the pHe from 7.4 to 6.7 resulted in lowering of intracellular pH (pHi) from 7.4 to 6.9/7.0 [37,38]. The question whether the pHi difference in the range of 0.5-0.4 might result in the changes of intracellular ROS stability/reactivity deserves further investigation.…”