Folate Receptor Structures Provide Molecular Determinants for Folate and Antifolate Binding and Facilitate Rational Drug Design Strategies for Cancer Therapeutics

Human folate receptors (hFR) are cell surface glycosylphosphatidylinositol (GPI-) anchored glycoproteins that characteristically bind the vitamin folic acid as well as the physiological folate, 5-methyltetrahydrofolate (5-MTHF), with dissociation constant in the subnanomolar range and subsequently transport metabolites by endocytosis. The physiological aspects of FR has made the receptor a significant drug vector due to its ability to bind and transport a chemically diverse range of antifolate drugs, folate-conjugates, and immunological agents. Immense interest has resulted in the development of therapeutic and imaging agents that selectively target FR-positive cells for the treatment of cancer and inflammatory diseases in humans. However, design of drugs for FR-mediated therapy has mainly been empirical and efficacy is still in question due to transport of the drugs via the ubiquitously expressed reduced folate carrier (RFC), which causes dose limiting toxic effects to normal tissue. Here we show the three dimensional structure models of hFR pertinent during stages of endocytosis and particularly hFR in complex to the natural folate metabolite and antifolate drugs at atomic resolution. These structures elucidate the molecular details of key determinants in hFR ligand binding. Ultimately, these findings may aid in rational drug design strategies to develop novel antifolates based on structural rather than strictly empirical principles. Optimized design strategies will aim to find novel drugs with specificity toward hFRs relative to the RFC and more importantly toward a single type of hFR protein either overexpressed in cancerous or inflammatory disease states. Utilizing a structure based strategy, optimized drug efficacy can be achieved wherein drug uptake can occur at lower doses in targeted pathological cells and collateral damage to normal cells is substantially diminished.