Colon-specific antigen-p (CSAp). I: Initial clinical evaluation as a marker for colorectal cancer

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Abstract
A radiometric immunoassay for detecting colon-specific antigen-p (CSAp) in the blood of patients suspected of having colorectal cancer has been developed and evaluated in 272 subjects of various disease entities. Using 10 units/ml as the cutoff value for normalcy, the results indicate that the highest number of elevated CSAp titers occurred in patients with advanced colorectal cancer (61%). Only one of 12 colonic adenoma patients had an elevated CSAp titer, and this was slightly above the 10 units/ml cutoff. Other nonneoplastic gastrointestinal disorders showed an 18% abnormal CSAp titer frequency, of which more than half bordered the upper limit of normalcy. CSAp elevations were also found in gastric cancer (20%), pancreatic cancer (20%), breast cancer (5%), and normal individuals (3%). CSAp was compared to carcinoembryonic antigen (CEA) in 44 colorectal cancer patients, in 12 patients with colonic adenomas, and in 62 patients with diverse gastrointestinal disorders. Using a CEA cutoff of 5 ng/ml, CSAp could increase the diagnostic accuracy of the CEA plasma test in colorectal cancer patients by 14%. In patients with colonic adenomas, the CSAp titer was normal when the CEA value was elevated in 25% of the cases. However, both were simultaneously elevated in only 3% of the patients with benign gastrointestinal disorders. Since the CSAp test was less frequently elevated (0–7%) in patients with breast, ovarian, lung, or cervical cancer than was found for CEA (39–75% elevated), it seems that the CSAp blood assay detects colorectal cancer more specifically than the more generally distributed CEA. These results suggest that the combined use of blood CEA and CSAp could enhance the discrimination of colorectal cancers from other nonmalignant gastrointestinal diseases, and could also serve to enhance the colorectal cancer-specificity of the CEA assay. Furthermore, serial monitoring of both markers in four advanced colorectal cancer patients indicated that they reflect disease activity, falling after successful treatment and rising again with recurrence and disease progression.