Discussion

Banusha Rajeenthira, Peter Coombs, Keith Ansell, David Whalley, Hayley Jones, Chido Mpamhanga, Simon Osborne, Jonathan Large, Ela Smiljanic-Hurley, Denise Harding, Nils Visser (NKI), Daniel Peeper (NKI) and Debra Taylor

MRC Technology, Centre for Therapeutics Discovery, 1-3 Burtonhole Lane, Mill Hill, London, NW7 1AD

Netherlands Cancer Institute (NKI), Department of Molecular Oncology, Plesmanlaan 121, 1066 CX, Amsterdam, The Netherlands

Cancer cells are known to rely heavily on de novo purine biosynthesis and targeting this pathway is a recognised strategy for the development of anti-cancer therapeutics. PAICS, (phosphoribosylaminoimidazole carboxylase / phosphoribosylaminoimidazole succinocarboxamide synthetase), catalyses two consecutive steps of the de novo purine biosynthetic pathway via two separate domains. The carboxylase domain is responsible for the conversion of AIR to CAIR, followed by aspartylation of CAIR to SAICAR in an ATP-dependent reaction mediated by the synthetase domain. PAICS has been shown to be over-expressed in a number of cancer cell lines and higher levels of PAICS expression in breast cancer patients is associated with poor prognosis; suggesting the enzyme as an attractive target for cancer therapy.

Hits identified from a fragment screen targeting the synthetase domain of PAICS have been progressed in a medicinal chemistry program to generate lead compounds with low nanomolar potencies. In the absence of structural information, a homology model was created and used to predict the binding modes of compounds. A panel of mutants were designed and created by site-directed mutagenesis to explore the contributions of specific residues and explore the binding mode in detail. Selected inhibitors were screened in biochemical and biophysical assays against the mutants to confirm the binding mode and inform on the SAR, providing important information for their chemical optimization.