(UroToday.com) Antibodies and small molecule ligands target PSMA with different kinetics and biodistribution, with certain sites of PSMA expression such as salivary/lacrimal glands, kidneys, and small bowel less accessible to large antibodies. Alpha emitters such as 225Ac have high potency, but short range; 225Ac-J591 has previously completed radiochemistry and xenograft studies. At the 2021 ASCO virtual annual meeting, Dr. Scott Tagawa and colleagues presented results of a dose-escalation plus expansion cohort of a first in human study of 225Ac-J591. The hypothesis is that mAb J591 will be able to deliver a potent alpha emitter to tumors without dose-limiting toxicity to other organs.