Targeting Tumour Proliferation with a Small-Molecule Inhibitor of AICAR Transformylase Homodimerization

Aminoimidazole carboxamide ribonucleotide transformylase/ inosine monophosphate cyclohydrolase (ATIC) is a bifunctional homodimeric enzyme that catalyzes the last two steps of de novo purine biosynthesis. Homodimerization of ATIC, a protein-protein interaction with an interface of over 5000 A(2) , is required for its aminoimidazole carboxamide ribonucleotide (AICAR) transformylase activity, with the active sites forming at the interface of the interacting proteins. Here, we report the development of a small-molecule inhibitor of AICAR transformylase that functions by preventing the homodimerization of ATIC. The compound is derived from a previously reported cyclic hexapeptide inhibitor of AICAR transformylase (with a K(i) of 17 ?M), identified by high-throughput screening. The active motif of the cyclic peptide is identified as an arginine-tyrosine dipeptide, a capped analogue of which inhibits AICAR transformylase with a K(i) value of 84 ?M. A library of nonnatural analogues of this dipeptide was designed, synthesized, and assayed. The most potent compound inhibits AICAR transformylase with a K(i) value of 685 nM, a 25-fold improvement in activity from the parent cyclic peptide. The potential for this AICAR transformylase inhibitor in cancer therapy was assessed by studying its effect on the proliferation of a model breast cancer cell line. Using a nonradioactive proliferation assay and live cell imaging, a dose-dependent reduction in cell numbers and cell division rates was observed in cells treated with our ATIC dimerization inhibitor.