Nonsteroidal ¹⁸F-Labeled PET Tracer for Imaging Androgen Receptors

Androgen receptor (AR) signaling is the key driver of prostate cancer. Thus, standard-of-care treatments focus on androgen-deprivation therapy followed by AR signaling inhibitors. Even when initially responsive, patients eventually develop resistance to therapies targeting AR signaling, leading to disease progression. To study lesion-to-lesion AR occupancy, the radiofluorinated analog of the endogenous androgen [¹⁸F]16β-fluoro-5α-dihydrotestosterone ([¹⁸F]FDHT) has been investigated as a potential AR imaging agent. In the literature, [¹⁸F]FDHT demonstrates slow clearance from healthy tissue, poor plasma stability in vivo, and nonapplicability in mouse studies. To overcome these limitations, we explored nonsteroidal selective androgen receptor modulator (SARM)-based pharmacophores as potential PET tracers. Here, we describe the performance of a ¹⁸F-radiolabeled AR tracer, [¹⁸F]F-SARM3. Methods: [¹⁸F]F-SARM3 was synthesized via 1-step copper-catalyzed radiofluorination, and its AR affinity (half-maximal inhibitory concentration [IC₅₀]) and biological activity (half-maximal effective concentration [EC₅₀]) were studied in various prostate cancer cell lines. The tracer's stability in vitro was also evaluated. [¹⁸F]F-SARM3 was evaluated in LNCaP and 22Rv1 (AR-positive) tumor-bearing male mice to assess its AR specificity and clearance profile, and its performance was compared with that of [¹⁸F]FDHT. Results: [¹⁸F]F-SARM3 was synthesized with a radiochemical yield and purity of 2.7 ± 1.4% and 97.9 ± 2.3%, respectively. In in vitro cell-binding assays, [¹⁸F]F-SARM3 demonstrated high affinity for ARs, similar to that of dihydrotestosterone (IC₅₀, 20.2 ± 14.6 nM vs. 9.6 ± 4.0 nM, respectively, in 22Rv1 cells). [¹⁸F]F-SARM3 functions as a partial AR agonist, with an EC₅₀ of 15.0 ± 12.0 nM. Furthermore, [¹⁸F]F-SARM3 is highly stable in phosphate-buffered saline and mouse blood (98.5% ± 1.2% and 98.6% ± 1.5% intact, respectively). The tracer's in vivo specificity was confirmed with the observed uptake in tumors and prostate glands of castrated mice, where uptake was blockable. In vivo specificity was not observed with [¹⁸F]FDHT. Conclusion: We developed a first-in-class SARM-based AR tracer that displays high affinity and selectivity for AR in vitro and in vivo. This represents a suitable PET tracer to image AR status in rodent models and provides a strong rationale for clinical translation of [¹⁸F]F-SARM3 as a high-affinity AR agonist PET imaging agent.