In this study, we have shown that elevated levels of s-TATI independently predict an adverse outcome in CRC patients and this was also the case in separate analysis of patients with Stage II and III disease.
We have previously shown that high expression of TATI in tumour tissue correlates with an adverse prognosis in two independent cohorts of CRC patients, including the cohort studied here, in which t-TATI was also significantly associated with a reduced time to metachronous liver metastasis . The lack of any association between s-TATI levels and t-TATI is therefore somewhat surprising, given the significant relationship between s-TATI and an adverse prognosis demonstrated in this study. Indeed, s-TATI and t-TATI remained independent prognostic factors when both were included in multivariate analysis. In addition, while s-TATI levels were significantly higher in patients presenting with distant metastasis at diagnosis, we could not confirm any relationship between s-TATI and time to liver or lung metastases.
To our knowledge, the relationship between s-TATI and t-TATI has only been investigated in one previous study on renal cell carcinoma, which also failed to show a significant association between these variables . However, in that study, it was also shown that an impaired renal function could contribute to elevated s-TATI levels in patients with renal cancer. A limitation to this study is the lack of information on renal function for the patients included. The relationship between s-TATI and renal function in CRC patients is an important issue that should be addressed in future studies. In this study, age at diagnosis was significantly associated with high s-TATI levels, which is in line with the study by Solakidi et al. . As high age is associated with a poor DFS and OS in CRC, and also with an impaired renal function, this relationship could to some extent contribute to the adverse prognostic impact of high s-TATI levels. Notably, the independent prognostic value of s-TATI remained significant for both DFS and OS when adjusted for age in the multivariate Cox regression analysis.
Elevation of s-TATI may also be caused by several non-malignant conditions such as severe inflammatory disease , pancreatitis and hepatobiliary disease , and tissue destruction [24, 25]. TATI is produced at high concentration in the pancreas, from which it leaks into serum in pancreatic disease . However, normal s-TATI levels have been found after total pancreatectomy showing that TATI is produced in several tissues [26, 27]. Solakidi et al suggested that leakage from the tumour cells into the circulation might be the mechanism behind elevated s-TATI levels , but the lack of a correlation between s-TATI and t-TATI in this study implies that leakage from the tumour into the circulation is not a major cause of elevated s-TATI levels. Apart from its trypsin inhibiting function, TATI has also been ascribed properties promoting a more malignant phenotype in CRC  and in prostate cancer cells . Interestingly, TATI has been shown to stimulate the EGF receptor and thus increase tumour aggressiveness . Therefore, it could well be that t-TATI contributes to the biological aggressiveness of the tumour per se, whereas s-TATI levels reflect a local and/or systemic response to the disease.
Notably, while there was no association between t-TATI and tumour location, s-TATI levels were significantly lower in left-sided tumours compared to right-sided tumours and in rectal cancers compared to colonic cancers, the majority of which had received preoperative RT. None of these associations were seen for s-CEA. The significant association between lower s-TATI levels and RT indicates that treatment might directly affect s-TATI levels. This would be of interest to investigate in future studies, comparing levels of s-TATI before, during and after RT. In this study, s-TATI was an independent prognostic factor in patients not receiving RT, whereas it was only prognostic in univariate analysis in patients treated with RT. As the number of patients that had received RT was relatively small (n = 85) and the number of patients that had not received RT was even smaller (n = 23), no conclusions can be drawn regarding the value of s-TATI as a predictor of response to preoperative RT in rectal cancer patients. This should be investigated in a prospective setting, preferably a randomised trial.
Notably, the significantly lower s-TATI levels in left-sided compared to right-sided colonic tumours indicate that differences in s-TATI levels according to tumour location might also be due to differences in biological characteristics of the tumours, e.g., microsatellite instability (MSI), which is more frequently present in tumours arising in the right colon [30, 31]. Information on MSI status was not available for the tumours in this study, but should be considered in future studies.
Earlier studies on s-TATI in colorectal cancer have only concerned its diagnostic value, and for this purpose s-TATI has not been found to provide information additional to that obtained with s-CEA [13, 14]. The prognostic value of s-TATI in CRC has not been studied before. In the present study s-TATI was found to be a prognostic biomarker that was independent of conventional clinicopathological factors, and also independent of s-CEA. Furthermore, in most settings, the value of s-TATI was stronger than that of s-CEA.
Individualised treatment is still not available for patients with CRC in the adjuvant setting; currently the TNM stage is used for selection of patients for adjuvant treatment. In stage II patients about 20% will develop cancer recurrence, and there is modest evidence that these patients benefit from adjuvant chemotherapy . Several high risk factors have been suggested for selecting stage II patients for adjuvant chemotherapy, i.e few lymph nodes examined, T4 disease, obstruction and tumour perforation . Stage III CRC patients, on the other hand, have a well documented benefit from adjuvant chemotherapy  although it is estimated that up to 80% will relapse despite adjuvant treatment. It is therefore obvious that more accurate prognostic factors are needed when selecting patients for adjuvant treatment in CRC. The data presented here show that s-TATI is an independent prognostic biomarker in the whole cohort and in separate analysis of patients with Stage II and III disease.