MON-261 Characterization of d3-Testosterone, A Novel, Non-Aromatizing Androgen

Abstract Disclosure: P.M. Tarantino: None. J.A. Boice: None. R.E. Dudley: None. B.C. Sippy: None. Background: Substitution of hydrogen with deuterium, a stable non-toxic isotope of hydrogen, can alter the pharmacokinetic profile of a molecule while maintaining its pharmacodynamic properties. d3-Testosterone (d3-T) is a structurally identical, deuterium-substituted isotopologue of testosterone (T) designed to resist aromatization to estradiol (E2). Conversion of T into E2 is linked with potential safety (breast cancer) and tolerability (gynecomastia) concerns associated with T therapy. Therefore, prior to human clinical testing of d3-T, an assessment was conducted to determine the metabolic and pharmacodynamic similarities and differences between d3-T and T.Materials and Methods: The metabolic stability of d3-T relative to T in the presence of recombinant human aromatase (CYP19) was evaluated in vitro using two different testing conditions, 4.0 pmol/ml of enzyme and 0.5 µM substrate or 10.0 pmol/ml and 2.0 µM, for CYP19 and d3-T/T concentrations, respectively, with each condition run in triplicate. Additionally, the metabolic stability and downstream metabolites of d3-T and T were evaluated in vitro using both human and rat hepatocytes, each run in duplicate. The pharmacodynamic effects of T and d3-T were compared in an in vitro androgen receptor translocation assay, in triplicate, and using 5α-dihydrotestosterone (5α-DHT) as a positive control.Results: d3-T was substantially more resistant to aromatization than T in the presence of recombinant human CYP19; d3-T had a half-life between 4 and 7 times longer than that of T (55.9 or 79.9 minutes versus 7.7 or 18.5 minutes for d3-T and T, respectively). In rat and human hepatocytes, d3-T and T were metabolized at nearly identical rates and produced similar metabolite profiles. The potency of d3-T and T in inducing androgen receptor translocation in vitro was similar, with EC50 values of 0.53 nM (95% CI = 0.38 to 0.69 nM) and 0.74 nM (95% CI = 0.58 to 0.95 nM), respectively; both were approximately 10-fold less potent than the positive control 5α-DHT.Conclusions: d3-T and T have similar metabolic profiles in hepatocytes and d3-T retains T’s potency in inducing androgen receptor translocation. However, unlike T, d3-T is resistant to aromatization to E2. d3-T may be a useful alternative to T in clinical situations where the aromatization of T limits its therapeutic potential, such as hormone replacement therapy in postmenopausal women, the treatment of ER+ breast cancer, or in men on T therapy who develop gynecomastia. Presentation: Monday, July 14, 2025