Preliminary results of a phase I/II dose-escalation study of fractionated dose 177Lu-PSMA-617 for progressive metastatic castration resistant prostate cancer (mCRPC)

Abstract

Background: PSMA is overexpressed in PC with limited expression in other organs. PC is radiosensitive with dose-response. Dose-fractionation allows delivery of higher total dose per cycle, may result in less radioresistance due to repopulation compared with doses 6-12 wks apart. Initial Ph I safety results of the dose-escalation portion of this study [ESMO 2018] were without DLT at all dose levels. Methods: Entry criteria: progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and taxane (or unfit/refuse chemo) without limit of # prior therapies, adequate organ function, ECOG performance status 0-2. No preselection for PSMA expression. Treatment was a single cycle of fractionated dose 177Lu-PSMA-617 on D1 and D15. In the Ph I dose-escalation cohort, men received 7.4 to 22 GBq. In Ph II, a Simon 2 stage design enrolled pts at the 22.2 GBq dose level. Pre- and post-treatment 68Ga-PSMA11 PET/CT and post-treatment 177Lu-PSMA-617 imaging was performed in addition to standard serial CT and bone scans. Cellsearch CTC count at baseline and 12 wks. Results: 44 men (29 in Ph I, 15 in Ph II; total 21 at 22.2 GBq) with median age 69 (range 55-91), median PSA 182.97 (range 0.89-5541) were treated. 93% with bone, 45% nodal, 18% lung, 9% liver, 9% other visceral metastases. 55% with at least 1 prior chemo regimen, 52% >1 prior potent AR therapy, 27% with Ra223, 30% sip-T, 5% 177Lu-J591; 66% Halabi poor risk, 30% intermed risk. With follow up ongoing, 61% with >50% PSA decline (71.4% at 22.2 GBq), median overall survival 16 months (95% CI 11-NR). Of 26 with paired CTC counts, 57.7% decreased, 7.7% stable, 34.6% increased; 34.6% converted from detectable to undetectable at 12 weeks). 61.4% with all grade xerostomia, 29.5% fatigue, 25% thrombocytopenia, 25% anemia, 25% pain, 15.5% nausea. While not required for eligibility, all pts had some PSMA uptake in at least 1 site on PSMA PET, with 2.2% highest lesion SUV < liver SUV, 4.5% 1-2.5x, 13.6% 2.5-5x, and 79.5% highest lesion SUV > 5x liver. Conclusions: A single fractionated cycle of up to 22.2 GBq of 177Lu-PSMA-617 is safe, with encouraging early efficacy signals, even without selection for PSMA expression by imaging. A trend for dose-response was observed. Clinical trial identification: NCT03042468. Legal entity responsible for the study: Well Cornell Medicine. Funding: Weill Cornell Medicine, Prostate Cancer Foundation, US Department of Defense, US National Institutes of Health, Endocyte. Disclosure: S.T. Tagawa: Advisory / Consultancy, Research grant / Funding (institution): Endocyte. H. Beltran: Advisory / Consultancy: Endocyte. S. Vallabhajosula: Research grant / Funding (institution): Endocyte; Full / Part-time employment: NCM USA. O. Sartor: Research grant / Funding (institution): Endocyte. All other authors have declared no conflicts of interest.