Emerging role of thymidylate synthase for the pharmacogenomic selection of patients with thoracic cancer.

The use of antifolate drugs, and particularly of pemetrexed, is emerging as a relevant therapeutic strategy for the treatment of some specific thoracic cancers. Pemetrexed is a novel multitargeted antifolate which has been shown in vitro to inhibit at least three different enzymes in the folate pathway: thymidylate synthase (TS), dihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyl transferase (GARFT) of which TS is its main target [1]. These enzymes are involved in the synthesis of nucleotides and interference with their function will ultimately hinder the synthesis of RNA and DNA. Pre-clinical studies have demonstrated the cytotoxic activity of this agent in a broad range of tumor types including non-small cell lung cancer (NSCLC). In vitro studies indicated that tumor cell lines expressing high levels of TS or DHFR have reduced sensitivity to pemetrexed, suggesting that increased expression levels might correlate with reduced clinical efficacy [2]. In NSCLC, which accounts for more than 80% of all lung cancer, two recent independent Phase III trials showed that pemetrexed alone or in combination with cisplatin demonstrated a statistically superior activity in patients with non-squamous histotypes [3, 4]. Although histology has not consistently been associated with clinical outcomes in almost every trial testing the role of platinum-based chemotherapy in advanced NSCLC, it has now emerged as a potential predictive factor due to the evidence of the differential expression of TS between the different tumor histotypes, which seems to play an important role [3]. Preliminary molecular evidence, in fact, has previously shown that TS expression is significantly higher in squamous cell carcinoma compared with adenocarcinoma by means of both Real-Time PCR gene quantification and of immunohistochemistry for the detection of the protein levels [5]. In undifferentiated large cell carcinoma, a subsequent analysis consistently reported significantly higher TS levels in desmocollin 3 (DSC-3)-positive as compared to DSC-3negative tumors (indicating a squamous and adenocarcinoma origin of the tumors, respectively), reporting that lack of DSC-3 immunoreactivity in this histologic subtype is associated with lower TS expression [8]. In the same experimental setting, TS expression levels were tested in 22 cases having a cytological diagnosis of NSCLC with no clear histological definition, and in the corresponding histological samples obtained at the time of subsequent surgical resection. As a result, a significantly high degree of concordance in terms of TS protein expression in matched specimens was detected. This result raises the possibility of using the protein levels, evaluated by immunohistostaining on cytological specimens, to select the patients with unresectable advanced disease to be treated with TS-inhibiting agents. On the other hand, the small cell carcinoma (SCLC) group of patients, comprising approximately 13% of all lung cancers, has shown minimal response to pemetrexed in several studies including a large Phase III study [6]. Again, the explanation for these results may be found at the molecular level, in that SCLC has been shown to have very high TS expression when compared with either well-differentiated pulmonary neuroendocrine tumors or NSCLC [7]. These data once again support the hypothesis that elevated TS expression decreases the response to pemetrexed, and that the determination of TS levels can be used in to exclude those patients who are most likely to be unaffected from the treatment. A clear example of this is found in a very recent retrospective study from our group carried out on malignant pleural mesothelioma (MPM), a disease in which pemetrexed has been previously shown to be clinically active. We showed that TS protein and gene expression levels are significantly correlated with the clinical outcome in a consecutive series of 60 patients with advanced MPM treated with pemetrexed, with or without plat