COMPARISON OF PLASMA AND TISSUE-LEVELS OF ZD1694 (TOMUDEX), A HIGHLY POLYGLUTAMATABLE QUINAZOLINE THYMIDYLATE SYNTHASE INHIBITOR, IN PRECLINICAL MODELS

ZD1694 (Tomudex, raltitrexed) is a specific quinazoline antifolate thy midylate synthase inhibitor that relies on polyglutamation for high po tency. Antibodies to ZD1694 have been used to establish a sensitive ra dioimmunoassay as an alternative to high-performance liquid chromatogr aphy (HPLC). The radioimmunoassay is reproducible, accurate and provid es a means of determining low levels of ZD1694 in plasma (< 1 nM). By virtue of the high cross-reactivity of the antibodies with polyglutama ted forms of ZD1694, it is also possible to measure the total concentr ation of drug in tissues. Results obtained in L1210 mouse leukaemia ce lls and in mouse tissues were similar to those previously determined u sing radiolabelled drug. Pharmacokinetic studies in mice have confirme d that the compound is rapidly eliminated from the plasma and that the re is a prolonged terminal elimination phase. ZD1694 was measured in p lasma (0.56 ng ml(-1), 1.2 pmol ml(-1)) up to 7 days after a single i. p. dose of 100 mg kg(-1) ZD1694. Liver, kidney and gut epithelium had a substantially higher level of ZD1694 immunoreactivity than plasma. F or example, 24 h after a single i.p. dose at 1, 10 and 100 mg kg(-1), total drug levels in the liver were 480, 325 and 152 times higher than plasma levels respectively. In kidney and gut epithelium, total drug levels at these doses were approximately 55 and 34 times those of plas ma. The high tissue to plasma ratios were maintained for at least 7 da ys after administration. Similarly, high tissue to plasma ratios (> 10 0) were found in dogs treated with a clinically relevant dose of ZD169 4. These were maintained for 4 weeks in liver and kidney tissue (> 100 ). Total gastrointestinal concentrations of ZD1694 were approximately 10 times higher than plasma 3 days after administration, but levels we re near to the limit of detection at 4 weeks. These results are consis tent with extensive polyglutamation of ZD1694 within tissues in both m ice and dog and provide further support for the infrequent schedule th at has been used clinically. Although it has not been possible to meas ure individual polyglutamated forms of ZD1694, the radioimmunoassay pr ovides a convenient means of assessing total drug levels in tissues an d is currently the only method suitable for measuring the extent of dr ug retention in normal tissue and tumour biopsies obtained from patien ts treated with ZD1694.