Targeting the BB2 receptor in prostate cancer using a Pb-203 labeled peptide.

Objectives: The objective of this study is to prepare and assess the efficacy of in vivo tumor targeting of ²⁰³Pb-RM2, a BB2r antagonist, for targeting BB2r expressing prostate cancer in PC-3 xenografted animal models.

Methods: The BB2r antagonist, RM2 (RM2= DOTA- 4-amino-1-carboxymethyl-piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂), was radiolabeled with commercially available ²⁰³Pb by the addition of 80μg of RM2 dissolved in 0.5M NaOAc buffer at pH=6.5. The reaction mixture was maintained at 85°C for 1 hour. Prior to radiolabeling, free Fe present in the ²⁰³PbCl₂ samples was removed via ion exchange chromatography. Quality control of the final product was performed using radio-HPLC. Pharmacokinetic analysis of ²⁰³Pb-RM2 was conducted at 15 min, 1, 4, 24, and 48 hrs post administration in PC-3 xenografted ICR-SCID male mice. Pharmacokinetic analysis of free ²⁰³PbCl₂ was performed in male CF1 mice for in vivo evaluation of radio-complex stability. Micro-SPECT imaging of ²⁰³Pb-RM2 tumor targeting potential was performed at 4 and 24 hrs post administration in PC-3 xenografted mice.

Results: ²⁰³Pb-RM2 was prepared in 90.4 + 3.8% yield and identified as a single species using radio-HPLC. UV-HPLC detection demonstrated the presence of non-radioactive Pb-RM2. PC-3 tumor uptake values obtained at 15 min, 1, 4, 24, and 48 hrs post administration were 6.4 + 0.7, 9.8 + 1.7, 7.7 + 2.3, 4.4 + 1.0, and 3.1 + 0.7 %ID/g, respectively. BB2r blocking studies performed at 4 hours post administration were successful at blocking 86% of tumor uptake when compared to the non-blocked uptake at the same time point. ²⁰³Pb-RM2 uptake in the liver, kidney, and bone at 24 hrs was 0.2 + 0.1, 2.3 + 1.1, and 0.1 + 0.1 %ID/g, respectively. Free ²⁰³Pb-Cl₂ demonstrated values in the same organs of 7.5 + 2.2, 34.6 + 4.3, and 17.2 + 2.6 %ID/g, respectively. Micro-SPECT imaging revealed tumor uptake and clearance patterns consistent with pharmacokinetic data.

Conclusion: ²⁰³Pb-RM2 can be prepared as a single species in high yield. ²⁰³Pb-RM2 targets the BB2r expressed on the surface of PC-3 prostate xenografts and demonstrates long term tumor retention with no evidence of radio-complex instability assessed over 24 hours post administration. In vitro and in vivo validation of ²⁰³Pb-RM2 to serve as the imaging companion for ²¹²Pb-RM2 therapy agents are in progress. Research Support: VA Merit Review Award # 1I01BX001699 and VA Research Career Scientist Award to TJH, NIH Initiative for Maximizing Student Diversity (IMSD EXPRESS) 5-R25-GM05601-16 supporting RMW