Choline kinase inhibition as a treatment strategy of cancers with deregulated lipid metabolism

Abstract

Aberrant choline metabolism is a characteristic shared by many human cancers. It is predominantly caused by elevated expression of choline kinase alpha, which catalyses the phosphorylation of choline to phosphocholine, an essential precursor of membrane lipids. In this thesis, a novel choline kinase inhibitor has been developed and its therapeutic potential evaluated. Furthermore the probe was used to elaborate choline kinase biology. A lead compound, ICL-CCIC-0019 (IC50 of 0.27 ± 0.06 μM), was identified through a focused library screen. ICL-CCIC-0019 was competitive with choline and non-competitive with ATP. In a selectivity screen of 131 human kinases, ICL-CCIC-0019 inhibited only 5 kinases more than 20% at a concentration of 10 μM (< 35% in all 131 kinases). ICL- CCIC-0019 potently inhibited cell growth in a panel of 60 cancer cell lines (NCI-60 screen) with a median GI50 of 1.12 μM (range: 0.00389–16.2 μM). Importantly, proliferation of normal cells was only minimally affected (MCF-10A, ST-T1b and CCD-18Co; GI50 30–120 μM). In HCT116 cells, ICL-CCIC-0019 potently inhibited the formation of phosphocholine (EC50 0.67 ± 0.28 μM), which consequently decreased the formation of phosphatidylcholine. The compound arrested cells in the G1 phase of the cell cycle, and induced endoplasmic reticulum stress and apoptosis. A single injection of ICL-CCIC-0019 at 10 mg/kg decreased tumour uptake of the choline kinase specific PET tracer [18F]fluoromethyl-[1,2-2H4]-choline at 24 hours (AUC0–60 –23%). Treatment of HCT116 colon cancer cell xenograft bearing mice with 5 mg/kg ICL-CCIC-0019 i.p. resulted in strong tumour growth inhibition. Human breast cancer cell lines oncogenically transformed by HER2 exhibit increased levels of phosphocholine and are therefore more likely respond to CHKA inhibition. To identify such patients more readily, a novel, non-invasive, PET-imaging-based HER2- targeting diagnostic tool, [18F]GE-226, was developed. [18F]GE-226 (KD = 76 pM) uptake was 11 to 67-fold higher in 10 HER2 positive versus negative cell lines in vitro. Tumour uptake correlated with HER2 expression in 5 different tumour models (r2 = 0.78), and a fluorophore-labelled tracer analogue co-localised with HER2 expression. Tracer uptake was not influenced by short-term or continuous treatment with trastuzumab, but reflected HER2 degradation by short-term HSP90 inhibition. Taken together, these data further validate CHKA as a drug target and warrant the further development of ICL-CCIC-0019, potentially in the setting of HER2 positive cancers.