Objective

Targeting de novo nucleotide biosynthesis is an established approach for cancer therapy, with the reliance of cancer cells upon de novo versus salvage pathways conferring susceptibility to anti-metabolite strategies. The enzyme PAICS catalyses two sequential reactions in this pathway, and is up-regulated in many breast cancer cells lines; elevated expression also correlates to a poor prognosis in breast cancer patients. PAICS inhibition may therefore provide a novel approach in breast cancer therapy.

A fragment screen and subsequent hit to lead optimisation programme led to Compound 3, an ATP competitive inhibitor which shows biochemical IC₅₀ = 3nM and a clean profile against a panel of kinases. Cellular target engagement studies reported here using a HaloTag pull-down approach demonstrate selectivity for PAICS. Further mechanistic studies also revealed that compound exposure causes cells to arrest in S phase, providing additional evidence that metabolic inhibition lies behind the mechanism of action. A chorioallantoic membrane cell culture model was used for target validation: LM-2 breast cancer cells with PAICS knock-out exhibited reduced cell proliferation. Finally, reduced growth of LM-2 cells was also observed in an in vivo murine xenograft model.

This work demonstrates that Compound 3 is a highly specific and potent inhibitor of PAICS, and exhibits anti-tumour activity.