pH-dependent regulation of camptothecin-induced cytotoxicity and cleavable complex formation by the antimalarial...

Two classes of drugs interact with DNA topoisomerase (topo) I, namely topoI poisons such as the camptothecins, which create DNA single-strand breaks and the catalytic inhibitors, which do not. Here, we demonstrate that the antimalarial agent chloroquine is a catalytic inhibitor of eukaryote topoI, as the drug inhibited topoI-mediated DNA relaxation. Chloroquine is known to be a topoII catalytic inhibitor and as such is able to inhibit the activity of a topoII poison, i.e. etoposide. We now show that chloroquine also inhibits the topol poison camptothecin as camptothecin-stimulated nicking of plasmid DNA was inhibited by chloroquine. These observations also apply to endogenous topoI in whole cells. Accordingly, camptothecin-induced single-strand breaks as well as cytotoxicity were antagonised by chloroquine. Further, in a band depletion assay in whole cells, chloroquine prevented camptothecin-mediated topoI trapping, indicating that chloroquine inhibits topoI by interfering with the DNA binding step of the enzyme. In contrast to camptothecin, chloroquine is a weak base and therefore does not enter the cell if the extracellular fluid is acidic, as is the case in most solid tumors. This leads to the possibility of directing cytotoxicity to solid tumors with low extracellular pH by combining a neutral anticancer agent, i.e. camptothecin with a weak base antagonist, i.e. chloroquine. To test the feasibility of this principle, we investigated the drug combination at varying extracellular pH. We found that the antagonising effect of chloroquine on camptothecin-mediated trapping of topoI and DNA single-strand break formation was abolished at acidic extracellular pH. In a clonogenic assay, camptothecin in combination with chloroquine selectively killed cells at low pH (6.2), while camptothecin cytotoxicity was antagonised by chloroquine at normal pH (7.2). In conclusion, we show that the topoI catalytic inhibitor chloroquine inhibits camptothecin and that chloroquine can target the cytotoxic effect of camptothecin to tumor cells in acidic environments.