The Antitumour Activity of ZD9331, a Non-Polyglutamatable Quinazoline Thymidylate Synthase Inhibitor

Abstract

Thymidylate synthase (TS) is regarded as a good target for the development of quinazoline (folate-based) anticancer agents. The quinazoline-based TS inhibitor, D1694 or Tomudex (trade mark of Zeneca PLC) demonstrated exciting activity against colorectal tumours in the worldwide Phase II¹ clinical studies and is currently in Phase III study for this tumour type. Tomudex is an excellent substrate for folylpolyglutamate synthetase (FPGS) and is almost completely metabolised to polyglutamate forms (mainly tetra and pentaglutamates) that are not readily effluxed from the cell². Prolonged intracellular drug retention is thus a feature of this drug. This has the advantage of showing antitumour activity by bolus administration and the current clinical protocol is a 15min infusion once every 3 weeks. The polyglutamates of Tomudex are ~ 60-fold more active than the parent drug as TS inhibitors². Polyglutamation is thus a requirement for antitumour activity and may offer some tumour selectivity. However a mechanism of acquired and possibly intrinsic resistance to Tomudex is the failure of cells to polyglutamate the drug³. In order to broaden the spectrum of tumours responsive to TS inhibition by antifolates we synthesised and evaluated compounds unable to undergo such metabolism. As drug retention is not a general feature of this class of compound, infusion protocols were employed to evaluate their activity in mice⁴. Since TS inhibition is only achieved during drug administration this offers a high degree of control over the length of time that DNA synthesis is inhibited. Prevention of polyglutamation is possible through modification of the quinazoline ring (7-methylation)⁵ or the glutamate residue. Compound design also focused on developing TS inhibitors with potency at least equal to the polyglutamates of Tomudex (Ki tetraglu = 1nM). Acid-containing (water-soluble) and highly lipophilic analogues were synthesised but we concentrated on those that had high water-solubility and used the reduced-folate/MTX carrier (RFC) for cell entry. One such compound, ZD9331, was chosen as the most promising compound with activity against a number of experimental tumours (murine and human xenografts) in mice^{6, 7, 8}. The in vitro activity of ZD9331 is summarised below and, where appropriate, is compared with other TS inhibitors of known biochemical profile. Thus N¹⁰-propargyl-5,8-dideazafolic acid (CB3717), the prototype quinazoline TS inhibitor⁹, uses the RFC poorly and is slowly polyglutamated when compared with ICI 198583¹⁰ or Tomudex². AG337, represents a newer class of lipophilic quinazolines discovered by Agouron Pharmaceuticals that neither uses the RFC nor is a substrate for FPGS¹¹.