BGC 945 is a cyclopenta [g]quinazoline-based, thymidylate synthase inhibitor specifically transported into A-folate receptor (A-FR)-overexpressing tumors. Affinity of BGC 945 for the A-FR is 70% of the high-affinity ligand folic acid. In contrast to conventional antifolates, BGC 945 has low affinity for the widely expressed reduced-folate carrier (RFC). The K i for isolated thymidylate synthase is 1.2 nmol/L and the IC 50 for inhibition of the growth of A-FR-negative mouse L1210 or human A431 cells is f7 Mmol/L. In contrast, BGC 945 is highly potent in a range of A-FR-overexpressing human tumor cell lines (IC 50 f1-300 nmol/L). Pharmacokinetic variables measured following i.v. injection of 100 mg/kg BGC 945 to KB tumor-bearing mice showed rapid plasma clearance (0.021 L/h) and tissue distribution. The terminal half-lives in plasma, liver, kidney, spleen, and tumor were 2, 0.6, 5, 21, and 28 hours, respectively. Tumor BGC 945 concentration at 24 hours was f1 nmol/g tissue, at least 10-fold higher than that in plasma or normal tissues. Inhibition of thymidylate synthase in tissues leads to increased incorporation of 5-[125 I]-iodo-2V -deoxyuridine ([ 125 I]dUrd) into DNA. Forty-eight hours after injection of 100 mg/kg 6RS-BGC 945 ([ 125 I]dUrd injected at 24 hours), tumor was the only tissue with incorporation above control level (6-fold). The RFC-mediated thymidylate synthase inhibitor plevitrexed also increased uptake of [125 I]dUrd in tumor (10-fold) but, in contrast, also caused increased incorporation in other normal tissues such as spleen and small bowel (4.5-and 4.6-fold, respectively). These data suggest that BGC 945 selectively inhibits thymidylate synthase in A-FR-overexpressing tumors and should cause minimal toxicity to humans at therapeutic doses. (Cancer Res 2005; 65(24): 11721-8)

Thymidylate synthase (EC 2.1.1.45) is a key enzyme for the de novo synthesis of DNA and as such a target for anticancer drug development. There is a need to develop noninvasive methods for assessing thymidylate synthase inhibition in tumors. The aim of this study was to assess the potential of 3 ¶-deoxy-3 ¶-[18 F]fluorothymidine ([ 18 F]FLT) positron emission tomography (PET) for early measurement of thymidylate synthase inhibition and to elucidate the cellular mechanisms involved. Radiation-induced fibrosarcoma-1 tumor-bearing mice were injected with a single i.p. dose of the thymidylate synthase inhibitor 5-fluorouracil (5-FU; 165 mg/kg) and imaged by [18 F]FLT-PET at 1 to 2 hours after treatment. Deoxyuridine, thymidine kinase 1 (cytoplasmic thymidine kinase; EC2.7.1.21), and ATP levels in excised tumors were measured. Cellular assays for membrane transport were also done. There was a 1.8-fold increase in the 60-minute [18 F]FLT tumor/heart radioactivity ratio in drug-treated mice compared with vehicle controls (P = 0.0016). Plasma and tumor deoxyuridine levels increased significantly but thymidine kinase and ATP levels were unchanged. Whole-cell assays implicated a (low level) functional role for the type-1 equilibrative nucleoside transporter (ENT). There was an increase in type-1 ENT-binding sites per cell from 49,110 in untreated cells to 73,142 (P = 0.03) in cells treated with 10 Mg/mL 5-FU for 2 hours, without a change in transporter affinity (P = 0.41). We conclude that [18 F]FLT-PET can be used to measure thymidylate synthase inhibition as early as 1 to 2 hours after treatment with 5-FU by a mechanism involving redistribution of nucleoside transporters to the plasma membrane. (Cancer Res 2006; 66(17): 8558-64)

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The assessment of tissue-specific pharmacodynamics is desirable in the development of tumor-targeted therapies. Plasma deoxyuridine (dUrd) levels, a measure of systemic thymidylate synthase (TS) inhibition, has limited application for studying the pharmacodynamics of novel TS inhibitors targeted to the high affinity alpha-folate receptor (FR). Here, we have evaluated the utility of [(18)F]fluorothymidine positron emission tomography ([(18)F]FLT-PET) for imaging the tissue pharmacodynamics of BGC 945, an FR-targeted antifolate TS inhibitor; the nontargeted antifolate BGC 9331 was used for comparison. TS inhibition by both drugs induced a concentration-dependent increase in [(3)H]thymidine uptake in FR-positive human epidermoid KB cells. Membrane-associated equilibrative nucleoside transporter type 1 levels increased from 55,720 +/- 6,101 to 118,700 +/- 5,193 and 130,800 +/- 10,800 per cell at 100 mug/mL of BGC 9331 and BGC 945, respectively, suggesting this as a potential mechanism of increased nucleoside uptake. In keeping with these in vitro findings, tumor [(18)F]FLT accumulation in KB xenografts increased by >/=2-fold after drug treatment with maximal levels at 1 to 4 hours and 4 to 24 hours after BGC 9331 and BGC 945 treatment, respectively. Of interest to FR targeting, BGC 9331, but not BGC 945, induced accumulation of [(18)F]FLT uptake in intestine, a proliferative and TS-responsive tissue. For both drugs, quantitative changes in tumor [(18)F]FLT uptake were associated with increased tumor dUrd levels. In conclusion, we have validated the utility of [(18)F]FLT-PET to image TS inhibition induced by antifolates and shown the tumor-specific activity of BGC 945. This imaging biomarker readout will be useful in the early clinical development of BGC 945.