Thymidylate Synthase Expression in Patients with Colorectal Carcinoma Using a Polyclonal Thymidylate Synthase Antibody in Comparison to the TS 106 Monoclonal Antibody

Colorectal cancer is one of the most common human cancers, for which 5-fluorouracil (5FU) is usually part of the treatment. Thymidylate synthase (TS), the target enzyme for 5FU, can be predictive for the outcome of 5FU-based therapy. TS levels in tumor samples can be determined with radiochemical enzyme assays, RT-PCR, and immunohistochemical staining. We validated TS immunohistochemistry with a polyclonal rabbit anti-human TS antibody using the avidin-biotin method. This antibody can be used on paraffin-embedded, formalin-fixed material using an antigen retrieval method with citrate buffer and microwave treatment. The antibody shows a granular cytosolic staining pattern. The reproducibility in cross-sections from colorectal tumors from 50 patients was 90% and the interobserver variability was acceptable with a kappa of 0.45. On Western blotting it detects purified TS at 36 kD, while in 5FU-treated cells the ternary complex between FdUMP, TS, and 5,10-methylene-tetrahydrofolate is clearly visible at 38 kD, with no other interfering bands. In a separate set of tumors, immunostaining was compared with enzyme levels; Western blots correlated with enzyme levels. Because both this polyclonal antibody and the monoclonal antibody TS-106 are being used for large-scale studies, we also determined whether they could be used interchangeably. No differences were observed. This polyclonal antibody is specific and gives reproducible results. A study on a larger scale is ongoing to determine the role of TS as a predictive parameter in patients with colorectal cancer treated either with postoperative adjuvant 5FU/levamisole or with surgery only.