Quantitative Comparison of Direct Antibody Labeling and Tumor Pretargeting in Uveal Melanoma

SPECT radioimmunoscintigraphy with ^99mTo-labeled anti-melanoma monoclonal antibodies (MAbs) 225.28S is being used to detect uveal melanoma. Recently, pretargeting methods have been described to reduce background activity and perform imaging in a shorter time interval. Methods: We compared the three-step pre-targeting method with conventional radioimmunoscintigraphy in 15 patients with a clinical and laboratory diagnosis of uveal lesion. High-resolution SPECT radioimmunoscintigraphy was performed in all patients with directly labeled MAbs and, 1 wk later, with the three-step pretargeting technique. Eleven patients underwent eye enucleation and specimens of uveal melanoma were available for histology, whereas four patients underwent conservative therapy. The percent injected dose (%ID) delivered to the tumor and the tumor-to-background ratio were calculated. Results: In all three-step radioimmunoscintigraphy studies, there was a reduction of nonspecific nasopharyngeal background. The three-step radioimmunoscintigraphy tumor-to-nontumor ratio was 3.1 ± 1.3 versus 1.5 ± 0.5 of conventional radioimmunoscintigraphy, while the percent injected dose on the tumor was similar for the two methods (4.4 ± 3.0 versus 3.8 ± 2.8) × 10^-3. Conclusion: Improved SPECT imaging with the three-step radioimmunoscintigraphy results from reduced background and from higher counting statistics due to reduction of time interval between radiotracer administration and imaging, whereas the absolute amount of tracer delivered to the tumor by the two methods is comparable.