Differences in biodistribution of indium-111-and iodine-131-labeled B72.3 monoclonal antibodies in patients with colorectal cancer.

We have compared the biodistributions of [131I]B72.3 and 111In-SCN-Bz-DTPA B72.3 monoclonal antibody (MoAb) in patients with metastatic colon cancers. B72.3 is an IgG1 that recognizes a mucin-like colon cancer associated antigen. Eight patients were infused with 3-5 mCi and 0.36-20 mg of 111In-labeled B72.3 prepared with a bifunctional chelate, isothiocyanatobenzyl-DTPA (SCN-Bz-DTPA). The biodistribution was compared with that of 13 patients previously studied as part of a separate trial, with 1-10 mCi and 0.16-1.35 mg of [131I]B72.3. The Beta T1/2 in serum was 63 +/- 5 hr for 111In-SCN-Bz-DTPA B72.3 and 52 +/- 10 hr for [131I]B72.3. Whole-body retention of the 111In (T1/2 = 11.8 days) was significantly longer than for [131I]B72.3 (T1/2 = 3.3 days), p less than 0.000001. The 131I was excreted primarily through the urine. Urinary excretion of 111In was low and gamma camera images confirmed that some 111In was excreted in the bowel. Tumor localization was seen in one of seven evaluable patients receiving 111In-SCN-Bz-DTPA B72.3. Gamma camera images showed that the liver concentrates 111In but not 131I. We conclude that 111In-SCN-Bz-DTPA B72.3 is metabolized in a different manner from the iodinated B72.3. The high concentration and prolonged retention of 111In by the liver interferes with tumor imaging of metastases.