Abstract 3220: Preliminary evaluation of 212Pb BB2r targeted radiotherapy in prostate cancer

Introduction: Treating castration resistant prostate cancer (CRPC) continues to be a challenge for clinicians. We have recently shown that gamma emitting 203 Pb-RM2 (DOTA-4-amino-1-carboxymethyl-piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH 2 ) efficiently targets the BB2r expressed on CRPC cells in vivo. We are now exploring the potential of using the alpha-emitting radioisotope 212 Pb to generate the radiotherapeutic 212 Pb-RM2. Using 212 Pb-RM2, we have performed in vitro cell studies to examine treatment effects in terms of DNA damage, clonogenic potential, and cell viability and preliminary in vivo xenograft targeting proof of principle studies. Methods: 212 Pb-RM2 was prepared using an automated radiosynthesizer using 212 Pb eluted from a 212 Ra/ 212 Pb generator. DNA damage, colony formation, and cell viability were evaluated in PC-3 cells following treatment with 212 Pb-RM2 at 0.51, 1, 2, and 4uCi/160uL. DNA damage was assessed using 53BP1 foci following a 4hr incubation with 212 Pb-RM2 (T0) and at 24hrs and 48hrs. Colony formation assays were performed on cells treated with TRT for 4hrs, and analyzed at 12-14 days. The MTT assay was conducted up to 144hrs. All cell experiments were performed in triplicate on different occurrences. PC-3 xenografted SCID mice were administered 212 Pb-RM2 and organs/tissues were harvested and counted at 15 min, 1hr, 4hr, and 24hrs post injection. Results: DNA damage was found to be sustained over time with 53BP1 foci/cell at 48hrs of 14.8+1.6, 33.8+2. 6, 29.6+1.1, and 40.6+1.6 (avg+SEM) at 0.51, 1, 2, and 4uCi of 212 Pb-RM2/160uL, respectively. Colony formation followed a dose dependent response with surviving fraction decreasing as TRT increased with 17.7+3.2, 4.4+2.9, 0.5+0.5, and 0.0+0.0 (avg+SEM) at 0.51, 1, 2, and 4uCi of 212 Pb-RM2/160uL, respectively. 212 Pb-RM2 tumor uptake in PC-3 xenografts was found to be 5.2+0.7, 5.0+0.9, 3.9+0.7, and 2.6+0.3 %ID/g at 15min, 1hr, 4hr, and 24hrs respectively. In vivo BB2r blocking studies confirmed 212 Pb-RM2 targeting of the BB2r with > 72% blocking of normal pancreas uptake measured at 4hr post injection. Conclusions: Treatment with alpha emitting 212 Pb-RM2 TRT showed sustained levels of DNA damage which was confirmed by impaired colony formation at all dose levels of 212 Pb-RM2 TRT studied. Preliminary in vivo tumor targeting and retention of 212 Pb-RM2 was observed out to 24hrs post injection warranting further evaluation of 212 Pb-RM2 in multiple CRPC cell line models assessing potential therapeutic efficacy. Acknowledgements: US Veterans Administration (VA) BX001699 and USVA Research Career Scientist (TJH) Citation Format: Tammy L. Rold, Nkemakonam C. Okoye, Hoffman Timothy. Preliminary evaluation of 212 Pb BB2r targeted radiotherapy in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3220.