Monoclonal antibody (MAb)
B72.3 has been previously shown, by in vitro assays, to have a high degree of specificity for
carcinomas of the colon, ovary and breast versus normal adult tissues.
B72.3 IgG was labelled with 131I and injected i.v. into 20 patients with known or suspected
colorectal cancer. All patients subsequently underwent surgical exploration, with
tumor and selected normal tissues removed for staging purposes. The selective localization of 131I-MAb
B72.3 IgG was demonstrated in biodistribution studies in which the % ID/g of each
tumor was compared with that of the normal tissues, thus providing a relative RI for each lesion. Of the
tumor lesions, 70% (99/142) had an RI of at least 3 (i.e., 3 times greater uptake per gram than normal tissues), and 31% of the
tumor lesions had RIs of over 10. Only 12 of 210 (6%) histologically normal tissues had RIs of greater than 3; either these tissues were adjacent to or draining
tumor masses or, as in the case of 2 patients, the high RI values were apparently due to deposition of
immune complexes in the splenic tissues. Several parameters were studied to determine factors that might influence MAb localization. Whereas
tumors of all histologic types localized the MAb, 31% of the well-differentiated
mucinous carcinomas displayed
tumor-to-normal ratios greater than 10, while less than 5% of the lesions of other
tumor types demonstrated similar localization. The expression of the
antigen (TAG-72) detected by MAb
B72.3 in these
tumors, as studied by immunohistochemical techniques using tissue sections, did not always correlate with the outcome of the MAb distribution. No differences in MAb uptake were observed among the
carcinoma lesions from numerous anatomic locations, demonstrating the ability of i.v. administered
B72.3 to reach all the
tumor sites. Furthermore, autoradiographic studies of
tumors showed good penetration of the MAb into the medial areas of the
tumors, regardless of their size.