Background: Prostate cancer (PC) is a radiosensitive disease. In addition to external beam and brachytherapy, systemic bone-seeking radioisotopes have been utilized clinically for many years, with radium-223 leading to overall survival benefits. However, the available beta and alpha emitting particles in the clinic target tumors / stroma in bone only. Biologically targeted radiopharmaceutical treatment has shown promising results in our earlier multiple clinical trials using beta-emitting anti-PSMA therapy with Leutetium-177 radiolabeled anti-PSMA monoclonal antibody (mAb) J591, with accurate targeting and tumor response, but treatment is limited by myelosuppression. Both beta and alpha-small molecule PSMA ligand directed therapy has been utilized anecdotally outside of the US, but has not been systemically studied. Based upon their biodistribution, targeting of salivary glands and potentially proximal renal tubules may pose longer term toxicity issues. mAb-based targeted alpha-particle delivery may be advantageous with less expected myelosuppression than seen with β-emitters.

Experimental procedures: To optimize labeling of humanized DOTA-J591 mAb, actinium-225 (225Ac) nitrate was obtained from Oak Ridge National Laboratories and was dissolved in HCl solution and then mixed with trimethyl ammonium acetate and L-ascorbic acid. This preparation was incubated with DOTA-J591. The labeling efficiency (LE) was determined by ITLC-SG. If the LE is >60%, 225Ac-DOTA-J591 mAb was purified by gel filtration and sterilized by membrane filtration. In a pilot pre-clinical study, we studied the toxicity in non-tumor bearing BALB/c mice.

Results: The radiochemical purity is >95% and the immunoreactivity was >80%. Twenty-five BALB/c mice received from 2.1 to 6.3 KBq of 225Ac-J591. Treatment was well tolerated, with all mice alive and healthy at Day 26.

Conclusions: A stable alpha emitting-antibody complex has been produced with 225Ac-J591. Initial mouse safety experiments have been successful. A RIT dose escalation study of PSMA-targeted alpha emitter in a LNCaP xenograft model is underway.

Citation Format: Jaspreet S. Batra, He Liu, Sae Kim, Vicente N. Navarro, Shankar Vallabhajosula, Scott T. Tagawa, Neil H. Bander. PSMA-targeted alpha radioimmunotherapy for prostate cancer with 225Ac-J591 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5201. doi:10.1158/1538-7445.AM2017-5201