Cell cycle effects of CB30865, a lipophilic quinazoline-based analogue of the antifolate thymidylate synthase inhibitor ICI 198583 with an undefined mechanism of action

CB30865 (p-[N-(7-bromo-3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl+ ++)-N-(prop-2-ynyl)amino]-N-(3-pyridylmethyl)benzamide) is a quinazoline-based pyridine-containing compound that emerged from a programme aimed at the development of Thymidylate Synthase (TS) inhibitors as Anticancer agents. Its structure is based on the Antifolate ICI 198583, but with a pyridine ring replacing the glutamate. Despite its structure, CB30865 does not act in vitro via inhibition of TS or, apparently, other known folate-dependent pathways, and extensive mechanistic studies suggest that it acts via a novel locus with respect to conventional antitumour agents. However, CB30865 is highly potent against a variety of human tumour cell lines (e.g., 50%-inhibitory concentration [IC50] values in the 1-10 nM range). Thus, the cell cycle effects of CB30865 were investigated. DNA histogram analysis of W1L2 human lymphoblastoid, L1210 murine leukaemia, and CH1 human ovarian cells (propidium iodide staining) has demonstrated that CB30865 does not cause a phase-specific arrest at concentrations that have been shown to inhibit colony formation. This is unexpected for an Anticancer agent. In W1L2 cells, using bromodeoxyuridine (BrdUrd) labelling and bivariate Hoechst/ propidium iodide staining, it was revealed that 0.003-0.15 microM CB30865 (1-50 x 72 h IC50) caused cells to arrest in all phases of the cell cycle simultaneously after 20-24 h exposure. This effect was also observed in CH1 and L1210 cells, though the arrest was at slightly different times. Thus, using this technique, it has been demonstrated that CB30865 induces an unusual and delayed cell cycle arrest, which provides further evidence for a novel locus of action for this compound.