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Radioimaging of Melanoma Using ^99mTc-labeled Fab Fragment Reactive with a High Molecular Weight Melanoma Antigen¹

Departments of Nuclear Medicine, Clinical Immunology and Biological Therapy and Medical Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, and NeoRx Corporation, Seattle, Washington

Twenty patients with metastatic malignant melanoma were studied with ^99mTc-labeled monoclonal antibody (MoAb) Fab fragment (NR-Ml-05) reactive with a high molecular weight (M_r 240,000) melanoma associated antigen. Patients received 40 mg unlabeled irrelevant MoAb (NR-2AD-IgG) and 7.5 mg unlabeled NR-Ml-05 (whole IgG) prior to infusion of 10 mg ^99mTc-labeled (10–25 mCi) NR-Ml-05 Fab. Unlabeled MoAb were given to block nonspecific and specific binding sites. Gamma camera scans and single photon emission computed tomography were performed at 8 and 24 h postadministration. Of 172 preexisting lesions, 136 were imaged for a sensitivity of detection of 79%. Imaging was site and size dependent with the greatest sensitivity for liver lesions (100%) and the least for bowel (0%). Six sites (2 skin, 1 lung, 3 liver) were detected by single photon emission computed tomography that were missed on routine planar images. Forty-one additional unconfirmed sites were seen. Of these, 7 (17%) have been confirmed as tumor after a median follow-up time of 6 months. False positive scans included scar tissue, areas of chronic inflammation, an infected femoral aneurysm, and septic emboli. Nonspecific uptake of radioactivity occurred in kidney, gallbladder, bowel, thyroid, and myocardium. Human anti-mouse antibodies were detected in up to 69% of patients. In summary, radioimaging with ^99mTc-NR-Ml-05 is a sensitive test, especially for detecting liver lesions. It is safe, simple to administer, and convenient for the patient. Biodistribution and imaging sensitivity differ significantly from studies in which ¹¹¹In-labeled anti-melanoma MoAb have been used.