Previous studies have shown that (a) single-chain antibody binding proteins, or sFvs, localize experimental tumor xenografts (D. E. Milenic et al., Cancer Res., 51: 6363–6371, 1991) and (b) the administration of γ-interferon (IFN-γ) increases the expression of a high molecular weight glycoprotein, tumor-associated glycoprotein 72 (TAG-72), which improves mAb-based tumor targeting as well as radioimmunotherapy (J. W. Greiner et al., Cancer Res., 53: 600–608, 1993). The present experimental study was designed to determine whether exploiting those two observations in combination could augment tumor detection. Initial results revealed significant localization of a single-chain antibody binding protein of CC49 (i.e., CC49 sFv), a second generation anti-TAG-72 mAb, to human colon tumor xenografts (HT-29), which express low constitutive TAG-72 levels. IFN-γ treatment of mice bearing HT-29 tumors significantly increased TAG-72 levels in the tumor xenografts. Increased TAG-72 expression was accompanied by a 2–4-fold augmentation of CC49 sFv localized to the HT-29 tumors, measured by direct quantitation of 125I-labeled CC49 sFv tumor deposition as well as tumor:normal tissue ratios. Enhanced CC49 sFv tumor localization improved HT-29 tumor visualization by external scintigraphy as well as when using a hand-held gamma-detecting probe to discriminate between normal (i.e., heart, hind leg) and tumor tissue. The gamma-detecting probe was the same as that used intraoperatively with 125I-labeled CC49 IgG to identify occult tumors in patients. The present experimental findings indicate that the efficiency by which 125I-labeled CC49 sFv localizes tumor in vivo can be enhanced with IFN-γ. Results of the present study suggest that (a) the incorporation of an IFN-γ treatment schema prior to radioimmunoscintigraphy may increase the signal from the tumor site(s), thus providing a better discrimination between tumor and background, and (b) combining 125I-labeled CC49 sFv with IFN-γ will not only reduce the time interval between antibody injection and surgery, but will also increase the efficiency of tumor localization using the intraoperative gamma-detecting probe.

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