Safety, tolerability and efficacy of 212Pb-DOTAMTATE as a targeted alpha therapy for subjects with unresectable or metastatic somatostatin receptor-expressing gastroenteropancreatic neuroendocrine tumors (SSTR+ GEP-NETs): A phase 2 study.

4020 Background: 212 Pb-DOTAMTATE is a Targeted Alpha Therapy (TAT) in clinical development for subjects with SSTR+ neuroendocrine tumors. A phase I dose-escalation study has already been completed (Delpassand et al. J Nucl Med 2022). TAT holds the promise to improve outcomes versus Peptide Receptor Radionuclide Therapy (PRRT) with beta-emitters like 177 Lu-DOTATATE, currently considered the standard of care for subjects with GEP-NETs. Methods: ALPHAMEDIX 02 is a Phase II, open-label, multicenter study evaluating the safety, tolerability and efficacy of 212 Pb-DOTAMTATE in PRRT-naïve (Cohort 1, N = 36) and PRRT-refractory (Cohort 2, Target N = 30) subjects with histologically confirmed unresectable or metastatic GEP-NETs, positive somatostatin analogue imaging and at least 1 site of measurable disease per RECIST 1.1. 212 Pb-DOTAMTATE was administered at 67.6 μCi/kg per cycle, with a maximum activity administered per cycle of 5.5 mCi every 8 weeks, for up to 4 cycles. Primary endpoints include overall response rate (ORR) per RECIST1.1, and incidence and severity of adverse events (AEs). Secondary endpoints include progression free survival, overall survival , and health-related quality of life. Initial results of the already completed cohort 1 are presented. Results: In cohort 1, 17 out of 36 subjects with metastatic SSTR+ GEP-NETs achieved a confirmed response (ORR 47.2% (32.0-63.0%)). In the Phase I trial, five out of eight PRRT- naïve subjects with SSTR+ GEP-NETs treated with the same regimen of 212 Pb-DOTAMTATE achieved a response (ORR 62.5% (30.6-86.3%)): the combined ORR from both studies is 50% (22 out of 44, 95%-CI:36% - 64%). Median Duration of Response (DOR) has not been reached in both studies. Four out of five subjects (80%) with confirmed response in Phase I had a DOR of ≥ 12 months. In the ongoing Phase II study the response follow-up for 10 subjects with confirmed response is currently shorter than 6 months: so far 7 out of 17 subjects (41%) with confirmed response had a DOR of ≥ 6 months, and one of these subjects had a DOR of ≥ 12 months. Lymphocytopenia is a lead cause of the 59% grade 3 and 4 AEs reported in subjects in cohort 1. Overall, 4 fatal AEs were reported: death / progressive disease (N = 2), carcinoid syndrome (N = 1) and sepsis (N = 1). Conclusions: In PRRT-naïve subjects with SSTR+ unresectable or metastatic GEP-NETs treatment with up to 4 cycles of ²¹²Pb-DOTAMTATE (67.6 μCi/kg/cycle) was well-tolerated, with a safety profile consistent with the underlying disease and expected toxicities of radioligand therapy, similar to 177 Lu-DOTATATE. The ORR of 47.2% in cohort 1 (50% in the pooled dataset) appears to be substantially higher than the ORR previously reported for 177 Lu-DOTATATE in the pivotal NETTER-1 study (ORR 18% (10–25%)). Clinical trial information: NCT05153772 .