Quinone reductase 2: A novel target of the quinolines

For years, the quinoline drugs have been used for the treatment of malaria, lupus and arthritis, however the exact mechanism of their action is still unknown. We utilized a functional proteomics approach that took advantage of the similarities between the quinolines and the ATP purine ring to identify targets of the quinolines. The purine-binding proteomes of human red blood cells (RBC) and Plasmodium falciparum were captured on gamma-phosphate linked ATP and screened with quinoline drugs by displacement affinity chromatography. Two proteins were identified through screening of the RBC purinome---quinone reductase 2 (QR2) and aldehyde dehydrogenase 1 (ALDH1). Surprisingly, no proteins were recovered from the P. falciparum purinome. To test this result, we applied RBC and P. falciparum lysates to primaquine- and hydroxychloroquine-Sepharose. Again, the only two proteins recouped were QR2 and ALDH1 from the RBC proteome while no proteins were recovered from P. falciparum. Additionally, we have shown that QR2 is inhibited by the quinolines in vitro. Quinone reductase 2 is involved in maintaining redox status of the cell. We therefore propose that inhibition of QR2 by the quinolines leads to an inhospitable environment for the parasite causing parasite death. Computational molecular modeling identified the binding site of the quinolines to QR2 and was used to synthesize novel QR2 inhibitors based on the structure of the quinolines. These novel inhibitors potently inhibited QR2 activity in vitro and showed antimalarial activity in a cell based and mouse model of malaria. However, these compounds were not effective against drug resistant malaria strains. To further test the role of QR2 in malaria, a knockout mouse of QR2 was developed. Infection of these mice with a murine malaria parasite revealed that there was no difference in rates of infection nor was there any difference in response to quinoline treatment, indicating that QR2 is not involved in malaria infection. However, an immune phenotype was identified and further analysis revealed that QR2 is involved in the regulation of autoimmunity. The results presented here establish QR2 as a target of the quinolines and shed light on the mechanism of action of the quinoline drugs.