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.