MICROAUTORADIOGRAPHIC ANALYSIS OF THE NORMAL ORGAN DISTRIBUTION OF RADIOIODINATED SINGLE-CHAIN FV AND OTHER IMMUNOGLOBULIN FORMS

In Previous studies, we have compared the immunochemical properties, t he in vivo pharmacokinetics, and the tumor penetrance of a radioiodina ted single-chain Fv (sFv) in comparison with other immunoglobulin (Ig) forms (intact IgG, F(ab')2, and Fab') (Cancer Res., 51: 6363-6371, 19 91). Biodistribution studies demonstrated a higher percent injected do se/g in the liver and spleen for the intact IgG and F(ab')2. Renal upt ake was observed with the Fab' and F(ab')2, whereas the sFv demonstrat ed no specific localization in either of these organs. The I-125-label ed sFv also demonstrated a more even distribution throughout the tumor xenografts as compared to the other Ig forms (Cancer Res., 52: 3402-3 408, 1992). Subsequent studies utilizing the sFv conjugated with a rad iometal (Lu-177) demonstrated that the sFv was being metabolized by th e kidney, and a significantly higher percent injected dose/g was obtai ned with a Lu-177 labeled sFv as compared to a I-125-labeled sFv (Canc er Res., 52: 6413-6417, 1992). These previous studies indicated the po tential utility of radioiodinated sFv and other Ig fragments for use i n radioimmunoguided surgery with a hand-held probe, diagnostic imaging , and possibly therapy. The present study compares the distribution in normal tissues of the 4 Ig forms of monoclonal antibody (MAb) CC49, w hich is directed against a pancarcinoma antigen (tumor-associated glyc oprotein-72). I-125-labeled sFv, Fab', F(ab')2, and IgG of MAb CC49 we re administered to athymic mice either bearing or not bearing the tumo r-associated glycoprotein-72 positive human colon carcinoma xenograft (LS-174T). At various intervals following the i.v. injection of the Ig forms, the liver, spleen, kidneys, and lungs were removed for autorad iographic analyses. Dramatic differences were observed in the kidney; the IgG was found only in the renal vasculature, whereas the Fab', F(a b')2, and sFv showed a high density of grains in the cortical tubules. In the liver, the IgG and F(ab')2 were found in association with hepa tocytes, Kupffer cells, and in the sinusoids; the Fab' and sFv were pr imarily associated with the Kupffer cells. In the spleen, the Ig forms localized to the marginal zones surrounding the lymphoid follicles. N o specific accumulation of grains for any of the Ig forms was observed in the lung. In each of the tissues, the clearance rates were related to the size of the Ig form. The localization in the liver and spleen was determined to be antigen-mediated. No specific localization was ob served when I-125-labeled BL-3, an isotype-matched control MAb, was in jected into tumor-bearing mice, or, when the I-125-labeled CC49 IgG wa s administered to non-tumor-bearing mice. By defining the interactions of a MAb and its modified forms not only with tumor lesions but also with normal organs and tissues, more rational protocols for uses of a MAb and its various forms, including sFv's, can be designed for diagno stic and therapeutic applications.