An alpha-folate receptor (aFR)-mediated inhibitor of thymidylate synthase (TS), with potential antineoplastic activity. Upon intravenous infusion, idetrexed selectively targets and binds to aFR-expressing tumor cells. Upon uptake by aFR, this agent binds to and inhibits TS. This reduces thymine nucleotide synthesis, inhibits both DNA synthesis and cell division, and leads to tumor cell apoptosis. TS catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), an essential precursor for DNA synthesis, and plays a key role in cell growth and division. aFR, a cell-surface receptor glycoprotein, is overexpressed on various tumor cell types, but is minimally expressed by normal, healthy tissue.