P-glycoprotein mediates resistance to histidine kinase inhibitors.

Histidine kinase inhibitors are being developed as a new class of antimicrobial drugs. We recently demonstrated the activity of a class of histidine kinase inhibitors against a mammalian enzyme, elongation factor-2 kinase (eEF-2K), and the effect of these compounds on cancer cell viability (Arora et al., 2003). To further characterize these compounds, we studied their interaction with ATP-binding cassette transporters, which are known to mediate resistance to a variety of chemotherapeutic agents. The 24 compounds studied belong to three structural series of derivatives of 2-methylimidazolium iodide. We focused this work on a representative compound (NH125) because we found it to be most potent against both histidine kinase and eEF-2K among the series. Cell lines that expressed P-glycoprotein (P-gp) were 2- to 5-fold resistant to NH125. NH125 increased accumulation of P-gp substrates such as paclitaxel and doxorubicin but had no effect on the accumulation of non-P-gp substrates. P-gp modulators verapamil and trans-flupenthixol and MDR1-targeted siRNA increased sensitivity of multidrug-resistant cell lines to NH125. The presence of a benzyl group on the N-3 position of the 2-methylimidazolium iodide was important for the interaction with P-gp. C6-NH, an NH125-resistant cell line, markedly overexpressed P-gp compared with the parental cell line. In animal models, we found that NH125 increased by 129% the survival of sensitive P388 cells bearing mice but had no effect on mice harboring the resistant cell line. These observations indicate that certain histidine kinase inhibitors are substrates for P-gp and hence an important consideration in development of these agents as potential antimicrobial and anticancer agents.