Quantitation of thymidylate synthase in human tumors using an ultrasensitive enzyme-linked immunoassay

A thymidylate synthase ternary complex-specific antibody, FTS, permits functional monitoring of fluoropyrimidines dosing

5-Fluorouracil (5FU) and similar fluoropyrimidines induce covalent modification of thymidylate synthase (TS) and inhibit its activity. They are often used to treat solid cancers, but drug resistance and toxicity are drawbacks. Therefore, there is an unmet need for a functional assay to quantify fluorouracil activity in tissues, so as to individually tailor dosing. It is cumbersome to separately quantify unmodified and 5FU-modified TS using currently available commercial anti-TS antibodies because they recognize both forms. We report here the first monoclonal antibody (FTS) specific to 5FU-modified TS. By immunoblot assay, the FTS antibody specifically recognizes modified TS in a dose-dependent manner in 5FU-treated cells, in cancer xenograft tissues of 5FU-treated mice, and in the murine tissues. In the same assay, the antibody is nonreactive with unmodified TS in untreated or treated cells and tissues. Speculatively, a high-throughput assay could be enabled by pairing anti-TS antibodies of two specificities, one recognizing only modified TS and another recognizing both forms, to structurally quantify the TS-inhibiting effect of fluorouracil at a cellular or tissue level without requiring prior protein separation. Such a development might aid preclinical analytic studies or make practical the individual tailoring of dosing.

Transgenic expression of human thymidylate synthase accelerates the development of hyperplasia and tumors in the endocrine pancreas

Thymidylate synthase (TS) is an essential enzyme for DNA synthesis and repair and elevated levels of TS have been identified as an important prognostic biomarker for colorectal cancer and several other common human malignancies. In addition, TS gene expression has been linked with cell-cycle regulation and cell proliferation through the ability of retinoblastoma protein to repress the transcriptional activation of E2F target genes such as TS. Therefore, overproduction of TS could participate in the progression to a neoplastic phenotype. Consistent with this model, a recent study has suggested that ectopic TS expression can induce a transformed phenotype in mammalian cells. To investigate the role of deregulated TS activity in tumor development, we generated transgenic mice that express high levels of catalytically active human TS (hTS) exclusively in the pancreas and low levels of hTS in multiple other tissues. Analyses of pancreatic tissue in TS transgenic mice revealed abnormalities within the endocrine pancreas, ranging from pancreatic islet hyperplasia to the detection of islet cell tumors. Overexpression of hTS in murine islets provides a new model to study genetic alterations associated with the progression from normal cells to hyperplasia to islet cell tumors, and suggests that this mouse model may be useful for regulating TS activity in vivo for development of cancer prevention and new therapies.

In vivo antitumor efficacy of CW252053, a folate-based thymidylate synthase inhibitor

Previous studies have demonstrated that CW252053, a quinazoline antifolate, exhibits potent inhibitory activity against thymidylate synthase (TS) as well as cytotoxic activity against tumor cell lines in vitro. In this study, we evaluated the in vivo antitumor efficacy of CW252053 in the mouse tumor model. Female B6D2F1 mice were injected with LY3.7.2C TK-/- (thymidine kinase deficient mouse lymphoma) cells into the gastrocnemius muscle. Then, CW252053 was administered twice daily by intraperitoneal injection for 10 days, and tumor growth was monitored daily by leg diameter measurement. All animals in the vehicle, 5-FU, and low dose (30 mg/kg) CW252053 treated groups died between days 12 and 23 because of the tumor burden. In contrast, dosing with 60 mg/kg of CW252053 produced a cure rate against tumor growth of 37.5% and a survival rate of 50%. Even more significantly, a higher dose of CW252053 (120 mg/kg) elicited both a 100% cure rate and a 100% survival rate at the termination of the study, confirming that this compound has very potent in vivo antitumor activity against tumor growth. During the experimental period of this study no signs of toxicity were observed even at the high CW252053 dosage rate of 120 mg/kg.

Diagnostic and prognostic tumor markers in the gastrointestinal tract

The gastrointestinal tract is the most common site of malignancies of any anatomic system in the body. An early detection of primary tumors of the bowel, pancreas, liver, stomach, and esophagus is often difficult in asymptomatic patients and for this reason these tumors are often detected at a relatively advanced stage, when symptoms lead to a diagnostic evaluation. Furthermore, gastrointestinal tract tumors have an extremely variable prognosis; thus, the identification of new prognostic parameters may be useful for selecting patients to more tailored therapies. In this work, the main molecular, genetic, tissular, and circulating tumor markers proposed for diagnosis and prognosis of gastrointestinal malignancies are reviewed and discussed.

Lack of correlation between thymidylate synthase levels in primary colorectal tumours and subsequent response to chemotherapy

The increasing interest in 5-fluorouracil (5-FU) modulation and the development of new antifolates has focused attention in recent studies on the expression of the target enzyme thymidylate synthase (TS) as a determinant of drug sensitivity and resistance. Resistance to TS-directed drugs has been shown to occur in vitro and in vivo with increased expression of the enzyme (determined by enzymatic assays as well as protein and gene expression assays). Several studies have evaluated the role of TS as a prognostic indicator of clinical response to chemotherapy containing TS-directed drugs. We have used a polyclonal antibody to recombinant human TS to establish a silver-enhanced immunogold staining method to localize TS in human tumours. Human tumour cell lines with acquired resistance to TS inhibitors owing to increased levels of TS were used to confirm the specificity of immunostaining. Stained sections were evaluated by image analysis. Immunostaining in tumour sections was greatly reduced (>80%) by preabsorption of the antiserum with recombinant TS. The method was used to determine the extent of TS immunostaining in 134 primary human colorectal tumours. The results were then compared with the clinical outcome and response to chemotherapy for the treatment of subsequent metastatic disease. A wide range (approximately 100-fold) of TS immunostaining was observed in these primary tumour sections. Normal mucosal tissue levels were 5-10 times lower than those observed in the adjacent tumour tissue. The values for TS immunostaining did not correlate with clinical endpoints, such as time from diagnosis to relapse, response to chemotherapy for disseminated disease, nor with Dukes' staging. This lack of correlation may be because this group of patients was selected on the basis of their need for palliative chemotherapy and did not include patients who were cured of their disease. Also, primary tumour TS expression may not give a good indication of the TS expression in metastatic lesions. The prognostic significance of TS protein expression in primary and metastatic lesions requires further evaluation.

The relationship between intrinsic thymidylate synthase expression and sensitivity to THYMITAQ in human leukaemia and colorectal carcinoma cell lines

Thymidylate synthase (TS) expression has been characterized for a panel of eight human colorectal carcinoma and five human leukaemia cell lines, to relate differences in intrinsic TS activity, protein and mRNA levels to growth inhibition caused by continuous exposure to THYMITAQ, a specific non-classical antifolate TS inhibitor. Although a 20-fold variation in sensitivity to THYMITAQ was found within the colorectal cell line panel (IC50 0.12-2.7 microM), sensitivity was not related to TS activity, TS protein or TS mRNA levels. For the leukaemic cell lines, only a twofold range in sensitivity to THYMITAQ was observed (IC50 0.87-2.3 microM), and this did not correlate with TS activity, TS protein or TS mRNA levels. Across all of the cell lines, TS activity was linearly related to TS protein levels (r2 = 0.87, P < 0.0001). However, for both the colorectal and leukaemia cell line panels, no relationship was found between TS mRNA/18S rRNA ratios and either TS activity or TS protein, consistent with the importance of post-transcriptional mechanisms in regulating TS activity. Two of the colorectal cell lines (BE and HCT116) and one of the human leukaemic cell lines (HL60), were intrinsically resistant to THYMITAQ (IC50 > 2 microM) in the absence of TS overexpression, suggesting that, subsequent to TS inhibition, events such as DNA repair and tolerance to apoptotic stimuli are also important determinants of sensitivity to THYMITAQ.

Thymidylate synthase and drug resistance

Thymidylate synthase is an important target for both fluorinated pyrimidines and for new folate analogues. Resistance to 5-fluorouracil (5FU) can be related to insufficient inhibition of thymidylate synthase. The 5FU-nucleotide FdUMP induces inhibition of thymidylate synthase which is enhanced and retained for longer in the presence of increased folate pools, for which leucovorin is a precursor. In a murine model system, 5FU treatment caused a 4-fold induction of thymidylate synthase levels which may have contributed to resistance. Addition of leucovorin to this treatment prevented this induction and increased the antitumour effect 2-3-fold. In the clinical setting, 5FU administration to patients resulted in approximately 50% inhibition of TS after 48 h. The combination with leucovorin resulted in a more pronounced inhibition after 48 h (approximately 70%). A significant relationship was observed with outcome of treatment; when thymidylate synthase levels were high and inhibition was low, no response was observed. A separate study showed that low thymidylate synthase levels appeared to be an independent prognostic factor for adjuvant therapy.

The effect of different routes of administration of 5-fluorouracil on thymidylate synthase inhibition in the rat

A rat colon tumour model of liver metastases was used to administer 5-fluorouracil (5FU) by intraperitoneal (i.p.) bolus injection (50 mg/kg), isolated liver perfusion (ILP, 150 mg/kg) and hepatic artery infusion (HAI, 50 mg/kg). The biochemical effect of 5FU, delivered by different routes, on its target enzyme thymidylate synthase (TS) was studied in both tumour and normal tissues of the rat. In tumour tissue, only small differences were observed in the extent of TS inhibition. A pronounced inhibition of TS was observed 3 h after 5FU administration by all routes, but was followed by a recovery of TS activity within 24 and 48 h. Effects of 5FU on normal tissues were diverse. In liver, TS activity increased 6-fold after ILP and HAI administration of 5FU, and a 2-fold increase of FdUMP binding to TS was seen for all routes of administration. In intestinal mucosa, both induction of TS activity (by ILP) and inhibition of TS activity (by HAI) were observed, while i.p. injection did not cause major changes. TS activity and FdUMP binding to TS in bone marrow was strongly inhibited after administration of 5FU by all routes, but administration by ILP seemed slightly advantageous, since a smaller extent of TS inhibition was observed compared to the other routes of administration. 5FU given by ILP had a small antitumour effect in this colon tumour model, while HAI administration had no antitumour activity. Since this difference in antitumour activity could not be related to differences in TS inhibition in the tumour, the RNA-directed mechanism of action of 5FU could be involved. Focusing on the effects of TS, we may conclude that the ILP administration of 5FU offered the important advantage of a lack of severe TS inhibition in normal tissues, which corresponds with the low systemic toxicity observed.