4128 Background: Targeted Alpha-Emitter Therapy (TAT) with 212Pb-DOTAMTATE has been shown to be safe and effective in subjects with neuroendocrine tumors (NET) who have not received previous PRRT, however, data is lacking for the use of TAT once progression occurs. Herein, we present the safety and efficacy of 212Pb-DOTAMTATE in subjects with recurrent NETs following prior 177Lu/90Y-PRRT. Methods: Subjects with biopsy-proven unresectable or metastatic SSTR expressing NETs from different primary sites with at least one measurable lesion who had received progressed after receiving prior PRRT were enrolled and received up to four 8-week cycles of 212Pb-DOTAMTATE at 67.6 μCi/kg/cycle. Response to treatment was measured per RECIST 1.1 criteria and by 68Ga/64Cu-DOTATATE PET/CT. Safety parameters were also obtained. Results: A total of 11 PRRT subjects were enrolled regardless of primary tumor location (pancreas (4), small bowel (3), midgut (1), ileum (1), thymus (1), and lung (1)). 8/11 subjects (73%) completed all four cycles. The mean cumulative dose was 20.9 mCi. As of January 2022, an objective radiological response (ORR) was demonstrated in 30% of evaluable subjects (1CR, 2PR, and 7 SD). In addition, 70% (7/10) evaluable subjects demonstrated a response per 68Ga/64Cu-DOTATATE PET/CT SSTR imaging. One hundred forty-five AEs were reported, with Grade 1 (79%). There were 23 (16%) Grade 2 AEs and 5 (3%) Grade 3/ 4 AEs. Of the AEs reported, 84 (58%) were considered possibly related, and 61 (42%) were considered not related or unlikely related. The most frequent AEs (reported in ≥ 4 subjects) include: alopecia (100%), fatigue (100%), nausea (91%), anemia (36%), alanine aminotransferase increased (36%), aspartate aminotransferase increased (36%) and lymphopenia (46%). Three SAEs were reported (achalasia, asthma exacerbation, and septic shock) one of which resulted in the death of the subject (septic shock). No SAEs were considered related to study drug. Conclusions: This is the first clinical trial of 212Pb-targeted alpha-emitter therapy in subjects with NETs who progressed following prior PRRT. The use of 212Pb-DOTAMTATE in the recurrent setting is highly effective with manageable toxicity and warrants further investigation. Clinical trial information: NCT03466216.