Thymidylate synthase: a target for anticancer drug design.

N10-Propargyl-5,8-dideazafolic acid (CB3717) has proved to be an interesting recent addition to the spectrum of antifolate drugs. Its sole biochemical locus of action appears to be thymidylate synthase, an inhibitory effect which is potentiated by intracellular polyglutamation. The drug has shown a spectrum of clinical activity and toxicity which is unusual for an antimetabolite. It seems likely that the former is attributable to its inhibition of TS, whilst the latter relates to the drug's poor aqueous solubility at physiological pH. Seminal to the discovery of a new generation of more selective thymidylate synthase inhibitors has been the observation that the C2 desamino derivative (CB3804) retains the useful TS-inhibitory and cytotoxic properties of CB3717. It is some two orders of magnitude more water soluble than CB3717 at physiological pH and appears not to produce, in the mouse, the liver and kidney toxicities which have restricted the wider use of CB3717. Thus, in desamino CB3717, it has proved possible to separate the structural features determining antitumor activity from those which are responsible for its systemic toxicities. These encouraging results prompted systematic structure-activity studies of other C2-modified quinazolines, which revealed that the desirable properties of the desamino compound are not unique. Results with two other CB3717 analogues, the C2-methyl (CB3819) and C2-methoxy (CB3828), have been discussed in the present paper. All three CB3717 analogues exhibit TS-inhibitory activities which are broadly comparable to those of the parent drug. In continuous culture CB3828 is as cytotoxic as CB3717, while CB3804 and CB3819 are at least an order of magnitude more potent. As with the desamino derivative (CB3804), so CB3819 is substantially more water soluble than CB3717 and is apparently devoid of its major toxicities. However, the effects of CB3828 on whole cell TS inhibition, both in vitro and in vivo, are rapidly reversible upon removal of exogenous compound, while the inhibition is sustained in similar experiments with the other three compounds. It is likely that these effects relate to the extent to which the various derivatives are converted to polyglutamate species and retained intracellularly. With the exception of CB3828, all are good substrates for FPGS, and the polyglutamate derivatives of CB3717, CB3804 and CB3819 are better TS inhibitors than the corresponding monoglutamates. CB3804 and CB3819 are less toxic and are cleared from the plasma much more rapidly than CB3717, so that the rate and extent of their polyglutamation may be an essential prerequisite of pharmacological activity.(ABSTRACT TRUNCATED AT 400 WORDS)