Clinical trials of capromab pendetide assessed the performance of the imaging agent in two populations. The first study population was composed of patients with a new diagnosis of prostate cancer who had tissue confirmation and were considered to be at high risk for metastasis to the lymph nodes. These patients underwent immunoscintigraphy with capromab pendetide before staging via pelvic lymphadenectomy. Sensitivity was 62% and specificity was 72%.5 In addition, positive predictive value (PPV) was 62%, negative predictive value (NPV) was 72%, and overall accuracy relative to histologic results was 68%.5 The second study population included patients with high clinical suspicion of occult recurrent or residual disease after radical prostatectomy. Admission criteria included PSA level higher than 1.0 ng/mL, negative findings on bone scan within 8 weeks before the study, and a prostatic fossa biopsy scheduled for 8 weeks or less after administration of the monoclonal antibody. Patients with a PSA of 1.0 ng/mL or lower were included only if they had a history of increasing PSA level and suspicion of recurrent disease on digital rectal examination.6 Using tissue confirmation from the prostatic fossa as the standard for comparison, yields for capromab pendetide were sensitivity, 49%; specificity, 71%; PPV, 50%; NPV, 70%; and overall accuracy, 63%.5,6
Other clinical studies of posttreatment populations reported various sensitivity, specificity, and accuracy results. A number of factors may be responsible for the different results among the studies, the lack of a perfect standard being foremost. Whereas histologic confirmation via biopsy offers high specificity, sensitivity is limited by sampling error. In addition, scintigrams can be difficult to interpret and readings vary by radiologist.7 Furthermore, PSMA expression has been seen in a variety of tissue samples, including normal tissue from the prostate, bladder, kidney, testis, ovary, fallopian tube, breast, adrenal gland, liver, esophagus, stomach, small intestine, and colon; and cancerous tissue from the bladder, kidney, testis, esophagus, stomach, small intestine, colon, adrenal gland, and lung.8 Expression of PSMA in other tissues can decrease specificity. Conversely, some lines of prostate cancer do not express PSMA, potentially decreasing sensitivity.9
Despite the inconsistent results, immunoscintigraphy with capromab pendetide does offer clinicians an additional tool for planning treatment of prostate cancer. However, as with any medical test, the potential benefit must be weighed against the negative effects and financial cost. Adverse effects were reported in 4% of patients.5 The most common reactions included hypotension, hypertension, elevated bilirubin levels, and elevated liver enzymes. Less common effects included itching, stinging at the infusion site, fever, chills, headache, myalgia, chest pain, and shortness of breath. No deaths have been attributed to capromab pendetide administration. However, the monoclonal antibody is developed from mice; therefore, it is a foreign-body protein and patients may produce human antimouse antibodies. Although uncommon, this response can change the efficacy of future murine-based diagnostic and therapeutic procedures as well as increase the risk of adverse reactions.5,7 Although relatively safe, the cost of the scan is an estimated $2,500, and thus it may not be a cost-effective option for all patients.7
The role of immunoscintigraphy with capromab pendetide in the postprostatectomy population is to aid in finding the cause of an elevated PSA level. Bone scans are more sensitive for evaluating skeletal metastases; therefore, immunoscintigraphy with this agent should not replace bone scans in the evaluation of such lesions. Imaging with capromab pendetide is also not indicated for screening or assessment of response to treatment. Whereas immunoscintigraphy with capromab pendetide seems to be more effective at detecting residual or recurrent disease than CT, MRI, or positron emission tomography, direct comparisons of these more conventional imaging studies to immunoscintigraphy with capromab pendetide, using biopsy confirmation as the standard of comparison, are lacking. Current efficacy data do not provide convincing evidence for clinicians to comfortably trust this modality's findings.
Given the metastatic patterns of prostate cancer, immunoscintigraphy with capromab pendetide should be considered only after bone scan and CT of the pelvis/abdomen are ineffective. No studies indicate that treatment plans should be based solely on capromab pendetide scan findings, therefore, use of conventional imaging studies first is the more costeffective approach. However, as in the case of our patient, immunoscintigraphy with capromab pendetide can be invaluable when no evidence of disease can be found with conventional imaging studies. JAAPA
Brendan Boyer practices in interventional radiology at Brigham and Women's Hospital, Boston, Massachusetts. Martin Boyer works for Radiation Oncology Consultants Ltd, Park Ridge, Illinois. They have indicated no relationships to disclose relating to the content of this article.
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