Abstract

Hu11B6 is a monoclonal antibody that internalizes in cells expressing androgen receptor (AR)-regulated prostate specific enzyme human kallikrein 2 (hK2; KLK2). In multiple rodent models, Actinium-225 labeled hu11B6-IgG1 ([225Ac]hu11B6-IgG1) has shown promising treatment efficacy. In the current study we investigated options to enhance and optimize [225Ac]hu11B6 treatment. Firstly, we evaluated the possibility of exploiting IgG3, the immunoglobulin G (IgG) subclass with superior activation of complement and ability to mediate FC-gamma-receptor binding, for immunotherapeutically enhanced hK2 targeted alpha-radioimmunotherapy. Secondly, we compared the therapeutic efficacy of a single high activity vs. fractionated activity. Finally, we used RNA sequencing to analyze the genomic signatures of prostate cancer that progressed after targeted alpha therapy. [225Ac]hu11B6-IgG3 was a functionally enhanced alternative to [225Ac]hu11B6-IgG1 but offered no improvement of therapeutic efficacy. Progression free survival was slightly increased with a single high activity compared to fractionated activity. Tumor free animals succumbing after treatment revealed no evidence of treatment associated toxicity. In addition to upregulation of canonical aggressive prostate cancer genes, such as MMP7, ETV1, NTS and SCHLAP1, we also noted a significant decrease in both KLK3 (PSA) and FOLH1 (PSMA) but not in AR and KLK2, demonstrating efficacy of sequential [225Ac]hu11B6 in a mouse model.