A single dose of 225Ac-RPS-074 induces a complete tumor response in an LNCaP xenograft model

James M. Kelly, Alejandro Amor-Coarasa, Shashikanth Ponnala, Anastasia Nikolopoulou, Clarence Williams, Nikki A. Thiele, David Schlyer, Justin J. Wilson, Stephen G. DiMagno, John W. Babich

Research output: Contribution to journalArticle

11 Scopus citations

Abstract

Promising biochemical responses to 225Ac-prostate-specific membrane antigen (PSMA) 617, even in patients who are refractory to β-particle radiation, illustrate the potential of targeted α-therapy for the treatment of metastatic castration-resistant prostate cancer. However, side effects such as xerostomia are severe and irreversible. To fully harness the potential of targeted α-therapy, it is necessary to increase the therapeutic index of the targeted radioligands. One emerging strategy is to increase clearance half-life through enhanced binding to serum albumin. We have evaluated the albumin-binding PSMA-targeting ligand RPS-074 in a LNCaP xenograft model to determine its potential value to the treatment of prostate cancer. Methods: 225Ac-RPS-074 was evaluated in male BALB/c mice bearing LNCaP xenograft tumors. A biodistribution study was performed over 21 d to determine the dosimetry in tumors and normal tissue. The dose response was measured in groups of 7 mice using 37, 74, and 148 kBq of 225Ac-RPS-074 and compared with positive and negative control groups. Mice were sacrificed when tumor volume exceeded 1,500 mm3. Results: 225Ac-RPS-074 was labeled in greater than 98% radiochemical yield and showed high (.10% injected dose/g) and sustained accumulation in LNCaP tumors from 24 h to beyond 14 d. Signal in blood and highly vascularized tissues was evident over the first 24 h after injection and cleared by 7 d. The tumor-to-kidney ratio was 4.3 ± 0.7 at 24 h and 62.2 ± 9.5 at 14 d. A single injection of 148 kBq induced a complete response in 6 of 7 tumors, with no apparent toxic effects. Treatment with 74 kBq induced a partial response in 7 of 7 tumors, but from 42 d, 6 of 7 experienced significant regrowth. The 37-kBq group experienced a survival benefit relative to the negative control but not compared with the positive control group. Conclusion: A single dose of 148 kBq of 225Ac-RPS-074 induced a complete response in 86% of tumors, with tumor–to–normal-tissue ratios that predict an improved therapeutic index. The use of the macropa chelator enabled quantitative radiolabeling and may facilitate the clinical translation of this promising targeted α-therapeutic.

Original languageEnglish (US)
Pages (from-to)649-655
Number of pages7
JournalJournal of Nuclear Medicine
Volume60
Issue number5
DOIs
StatePublished - May 1 2019

Keywords

  • Ac
  • Complete tumor response
  • PSMA
  • Targeted alpha therapy

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging

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    Kelly, J. M., Amor-Coarasa, A., Ponnala, S., Nikolopoulou, A., Williams, C., Thiele, N. A., Schlyer, D., Wilson, J. J., DiMagno, S. G., & Babich, J. W. (2019). A single dose of 225Ac-RPS-074 induces a complete tumor response in an LNCaP xenograft model. Journal of Nuclear Medicine, 60(5), 649-655. https://doi.org/10.2967/jnumed.118.219592