Folate based inhibitors of thymidylate synthetase in experimental and clinical cancer

CB3717 (N-(4-(N-((2-amino-4-hydroxy-6-quInazolinyl) methyl) prop-2-ynylamino) benzoyl)-L-glutamic acid) is a novel quinazoline folate analogue developed at the Institute of Cancer Research and selected on the basis of its potent inhibition of thymidylate synthetase. The introduction of a compound capable of producing a pure "thymineless" death was sought in the belief that this might confer it with advantageous properties and hence the departure from the traditional enzymic locus of classical antifolates, dihydrofolate reductase. Fluorinated pyrimidine compounds, though capable of metabolism to the thymidylate synthetase inhibitor, 5-fluorodeoxyuridine monophosphate, probably owe their cytotoxicity to a combination of factors including incorporation into RNA and this cannot be used to test the effects of an uncomplicated thymineless state. Pre-clinical studies confirmed that the cytotoxic effect of CB3717 in tissue culture was associated with the inhibition of thymidylate synthetase and significant antitumour activity was demonstrated in mice. At lethal doses in mice, renal and hepatic toxicity occurred but no evidence of gastrointestinal or bone marrow toxicity was found. An extensive phase I clinical trial incorporating 99 patients is described, with a dose escalation from 100 to 600mg/m2 and treatments being administered by a one hour or 12 hour infusion at 3 weekly intervals. The dose limiting toxicity was found to be renal at doses in the region of 500-600mg/m2 when substantial reductions in glomerular filtration rates were noted. A dose of 400mg/m2 was recommended for phase II evaluation using this schedule. Transient abnormalities in serum alanine transaminase levels typified the hepatic toxicity which occurred in -80% of patients at all dose levels and the association of this with variable malaise led to attempts at amelioration by prolongation of the infusion to 12 hours or the administration of prednisolone for one week after treatment. Neither manoeuvre was successful in preventing the enzyme elevation, but steroid therapy improved malaise in some patients. Several patients who received multiple courses of CB3717 (>4) appeared to develop tolerance with a return of transaminase levels towards normal values. Other toxicities observed were rashes (-10%) and sporadic myelotoxicity affecting leucocytes and platelets with a 10-12 day nadir. Neither of these effects were dose related. Gastrointestinal toxicity was not observed. Responses were seen in the following tumours: ovary (1 CR, 1 PR), breast (3 PR), large cell bronchus (1 PR) and mesothelioma (1 PR)