Comparison of plasma and tissue levels of ZD1694 (Tomudex), a highly polyglutamatable quinazoline thymidylate synthase inhibitor, in preclinical models

Abstract

Summary ZD1 694 (Tomudex, raltitrexed) is a specific quinazoline antifolate thymidylate synthase inhibitor that relies on polyglutamation for high potency. Antibodies to ZD1694 have been used to establish a sensitive radioimmunoassay as an alternative to high-performance liquid chromatography (HPLC). The radioimmunoassay is reproducible, accurate and provides a means of determining low levels of ZD1694 in plasma (< 1 nM). By virtue of the high cross-reactivity of the antibodies with polyglutamated forms of ZD1 694, it is also possible to measure the total concentration of drug in tissues. Results obtained in L1210 mouse leukaemia cells and in mouse tissues were similar to those previously determined using radiolabelled drug. Pharmacokinetic studies in mice have confirmed that the compound is rapidly eliminated from the plasma and that there is a prolonged terminal elimination phase. ZD1694 was measured in plasma (0.56 ng ml-1; 1.2 pmol ml-') up to 7 days after a single i.p. dose of 100 mg kg-' ZD1 694. Liver, kidney and gut epithelium had a substantially higher level of ZD1 694 immunoreactivity than plasma. For example, 24 h after a single i.p. dose at 1, 10 and 100 mg kg-', 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 plasma. The high tissue to plasma ratios were maintained for at least 7 days after administration. Similarly, high tissue to plasma ratios (> 100) were found in dogs treated with a clinically relevant dose of ZD1 694. 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 were near to the limit of detection at 4 weeks. These results are consistent with extensive polyglutamation of ZD1 694 within tissues in both mice and dog and provide further support for the infrequent schedule that has been used clinically. Although it has not been possible to measure individual polyglutamated forms of ZD1 694, the radioimmunoassay provides a convenient means of assessing total drug levels in tissues and is currently the only method suitable for measuring the extent of drug retention in normal tissue and tumour biopsies obtained from patients treated with ZD1 694.

The plasma pharmacokinetics and cerebrospinal fluid penetration of the thymidylate synthase inhibitor raltitrexed (Tomudex TM ) in a nonhuman primate model

“…Triexponential elimination of raltitrexed and a prolonged terminal elimination phase has also been found in mice and dogs [3], but no terminal half-life was calculated. The pharmacokinetic parameters obtained in nonhuman primates in this study are comparable to those reported in adult patients with colorectal cancer who received raltitrexed as a 15-min bolus infusion.…”

Population pharmacokinetics of raltitrexed in patients with advanced solid tumours

“…Blood samples were collected over 24–72 h in the Phase I studies and up to 29 days in the studies where the radiolabelled drug was administered and where the effect of renal function was investigated. Raltitrexed plasma concentrations were determined using a sensitive radioimmunoassay which uses sheep antiserum raised against raltitrexed [6, 22]. Assay performance was monitored using control samples.…”

“…Assay performance was monitored using control samples. The intra‐ and interassay coefficient of variation for plasma measurements was 10.7% and 12.9%, respectively, and the reported lower limit of detection of the assay was 0.2 ng ml −1 [22].…”

Novel chemotherapy agents for colorectal cancer: Oral fluoropyrimidines, oxaliplatin, and raltitrexed

“…Raltitrexed is transported into cells by the reduced folate carrier, metabolized to polyglutamated species, and retained intracellularly, resulting in prolonged TS inhibition [40].…”