Targeting the α-folate receptor with cyclopenta[g]quinazoline-based inhibitors of thymidylate synthase

The α-FR has been reported to be overexpressed in many carcinomas, in particular those of the ovary and uterus. The high expression of α-FR in some tumours compared with normal tissues has been exploited over the last decade for folate-mediated targeting of macromolecules, anticancer drugs, imaging agents and nucleic acids to cancer cells. CB300638, a cyclopenta[g]quinazoline-based inhibitor of thymidylate synthase (TS), has been reported to have high affinity for the receptor and selectivity for α-FR overexpressing tumour cell lines. In this study, the structural features of the molecule, in particular modifications at the 2-position, have been investigated with respect to TS inhibition, affinity for the α-FR and reduced folate carrier (RFC) and activity in A431-FBP cells (transfected with human α-FR) compared with neo-transfected A431 cells. Compounds 1a,b, 2a,b and 3a,b were synthesised utilising multistep sequences. It was found that the 2-substituent does not affect the affinity for the α-FR; however, it greatly affects selectivity for A431-FBP cells, and suggests that there are factors other than TS inhibition and α-FR affinity that are important for the activity of these compounds. Compound 2b (2-CH2OH derivative) displayed the highest selectivity for the A431-FBP cells compared with A431 cells.

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