PSA-Targeted Alpha-, Beta-, and Positron-Emitting Immunotheranostics in Murine Prostate Cancer Models and Nonhuman Primates.

Purpose: Most patients with prostate cancer treated with androgen receptor (AR) signaling inhibitors develop therapeutic resistance due to restoration of AR functionality. Thus, there is a critical need for novel treatment approaches. Here we investigate the theranostic potential of hu5A10, a humanized mAb specifically targeting free PSA ().

Experimental Design: LNCaP-AR (LNCaP with overexpression of wildtype AR) xenografts (NSG mice) and _Hi- transgenic mice were imaged with Zr- or treated with Y- or Ac-labeled hu5A10; biodistribution and subcellular localization were analyzed by gamma counting, PET, autoradiography, and microscopy. Therapeutic efficacy of [Ac]hu5A10 and [Y]hu5A10 in LNCaP-AR tumors was assessed by tumor volume measurements, time to nadir (TTN), time to progression (TTP), and survival. Pharmacokinetics of [Zr]hu5A10 in nonhuman primates (NHP) were determined using PET.

Results: Biodistribution of radiolabeled hu5A10 constructs was comparable in different mouse models. Specific tumor uptake increased over time and correlated with PSA expression. Treatment with [Y]/[Ac]hu5A10 effectively reduced tumor burden and prolonged survival ( ≤ 0.0054). Effects of [Y]hu5A10 were more immediate than [Ac]hu5A10 (TTN, < 0.0001) but less sustained (TTP, < 0.0001). Complete responses were observed in 7 of 18 [Ac]hu5A10 and 1 of 9 mice [Y]hu5A10. Pharmacokinetics of [Zr]hu5A10 were consistent between NHPs and comparable with those in mice. [Zr]hu5A10-PET visualized the NHP-prostate over the 2-week observation period.

Conclusions: We present a complete preclinical evaluation of radiolabeled hu5A10 in mouse prostate cancer models and NHPs, and establish hu5A10 as a new theranostic agent that allows highly specific and effective downstream targeting of AR in PSA-expressing tissue. Our data support the clinical translation of radiolabeled hu5A10 for treating prostate cancer.