Abstract
Abstract Background: J591 is a monoclonal antibody that selectively binds to prostate-specific membrane antigen (PSMA). Prospective phase I and II trials with therapeutic radiolabeled 177Lu-J591 have shown favorable PSA decline rates. Additionally, our group has previously reported that non-invasive evaluation of PSMA expression by imaging with planar gamma camera imaging is associated with response and survival. Initial anecdotal experience showed declines in CTC counts which led to prospective study. Methods: Three clinical trials of 177Lu-J591 in men with metastatic CRPC have included prospective evaluation of CTC counts (CellSearch methodology) prior to and following therapy (single-dose 177Lu-J591, fractionated-dose 177Lu-J591, and fractionated-dose 177Lu-J591 + docetaxel). Based upon prior experience of J591-based therapy without an effector molecule as immunotherapy, and an anecdotal report of CTC count decline following a low (imaging) dose of J591 without an effector molecule, a cohort of patients who received J591 without 177Lu labeling was also retrospectively analyzed. Results: 44 pts unselected for PSMA expression received 177Lu-J591 therapy had prospectively measured CTC counts. 39 of 44 pts (88.6%) had imaging evidence of PSMA expression. 57% of pts had baseline unfavorable (≥ 5 cells/7.5 mL) CTC counts. Of overall evaluable pts, 72.4% had decline in CTC counts (7.5 to 100% decline), with 50% of those with unfavorable baseline counts converting to favorable counts at 4-6 weeks. 17.1% with undetectable CTC counts remained undetectable at follow up. 7 pts with mCRPC and elevated CTC counts who received low-dose (naked J591) prior to imaging were also identified, and 4 (57%) of these pts demonstrated CTC count decline. Conclusion: In addition to previously reported PSA declines, treatment with 177Lu-J591 is associated with favorable CTC count decline in men with metastatic CRPC. Infusion with unlabeled J591 may also lead to CTC count clearance. Based on this finding, we are planning a prospective clinical trial with unlabeled (naked) J591 in men with metastatic CRPC and unfavorable CTC counts. Citation Format: Pravin Date, Beerinder S. Karir, Jaspreet S. Batra, Yuliya Jhanwar, Himisha Beltran, David M. Nanus, Neil H. Bander, Scott T. Tagawa. Radiolabeled anti-PSMA antibody J591 immunotherapy is associated with favorable circulating tumor cell (CTC) count control in men with castration-resistant prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr CT306. doi:10.1158/1538-7445.AM2015-CT306
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