Detection of Locally Recurrent Colorectal Cancer with Radiolabeled Monoclonal Antibody H‐15

H‐15 (HT‐29‐15) is an IgG1 mouse monoclonal antibody (mAb) to a cell surface antigen (molecular mass, 200,000 daltons) present on virtually all colorectal cancers and also in normal pancreatic ducts and bile ducts, but not in other normal tissues. The biological distribution and imaging characteristics of iodine‐131 ((l31)I)‐labeled mAb H‐15 were studied in 5 primary colorectal cancer patients and 9 patients with local recurrence of colorectal cancer. H‐15 mAb labeled with 0.5‐10 mCi of (131)I was administered 7 to 8 days before surgery at 4 dose levels, ranging from 0.2 to 6 nig. Selective mAb H‐15 localization to tumor tissues was demonstrated in 6 of 12 patients with antigen‐positive tumors: in two patients, recurrent tumors were negative to H‐15 mAb, although the primary tumors were positive. In six patients with positive radioimaging, tumor:normal tissue ratios ranged from 2.05 to 5.35 and tumor:serum ratios from 1,18 to 2.73. The clarity of images seems to correlate well with the latter ratios. Technetium‐99 ((99m)Tc)‐albumin blood pool studies in selected cases showed that local recurrence of colorectal cancers was hypovascular, emphasizing the selective localization of mAb H‐15 despite poor blood flow distribution in the tumors. The results altogether demonstrated that radioimmnnodetection with (131)I mAb H‐15 is valuable for differentiating recurrent colorectal cancer from granuloma formation after surgery.