Radiolabeled Monoclonal Antibody Indium 111-Labeled CYT-356 Localizes Extraprostatic Recurrent Carcinoma After Prostatectomy

Objectives. The sites of recurrent carcinoma of the prostate were localized with radiolabeled monoclonal antibody, and these sites were correlated with the response of patients treated with pelvic radiation after prostatectomy. Methods. Radionuclide scans were performed with indium 111-labeled CYT-356, a monoclonal antibody that binds to prostate epithelial cells, in 48 men diagnosed with recurrent carcinoma detected by prostate-specific antigen (PSA) screening after radical retropubic prostatectomy. Results. In 48 patients with recurrent carcinoma detected by PSA screening following radical retropubic prostatectomy, 73% had monoclonal antibody activity beyond the prostatic fossa, and only 3 patients (6%) had activity in the prostatic fossa alone; 65% had monoclonal antibody activity in pelvic lymph nodes despite the fact that lymph node dissections were pathologically negative at the time of prostatectomy in 90% of the patients; and 23% of patients had monoclonal antibody activity in abdominal and extrapelvic retroperitoneal nodes. Of 48 patients, 13 underwent external beam radiation therapy after monoclonal antibody scans. Six patients had scans showing activity beyond the field of radiation, and radiation therapy failed in 4 of these patients. Seven patients had scans with no activity beyond the field of radiation therapy, and radiation therapy failed in only 2 of these patients. Conclusions. The scans frequently show monoclonal antibody uptake in pelvic, abdominal, and extrapelvic retroperitoneal sites beyond the region of limited obturator node dissections and may account for the understaging and subsequent failure of radical prostatectomy in some patients. The monoclonal antibody scan seems to be a good predictor of which patients will respond to radiation therapy after radical prostatectomy, but because these patients often have nodal activity beyond the radiated field, this initial response may not be curative.