Pharmacokinetics and Acceptability of Subcutaneous Injection of Testosterone Undecanoate

Can injectable testosterone undecanoate (TU) be administered effectively and acceptably by the subcutaneous (SC) route?

To investigate the acceptability and pharmacokinetics (PK) of SC injection of TU.

Randomized sequence, crossover clinical study of SC vs IM TU injections.

Ambulatory clinic of an academic andrology center.

Twenty men (11 hypogonadal, 9 transgender men) who were long-term users of TU. injections. Intervention: Injection of 1000 mg TU (in 4 mL castor oil vehicle) by SC or IM route. Main Outcome Measures: Patient-reported pain, acceptability, and preference scales. PK by measurement of serum testosterone, dihydrotestosterone (DHT), and estradiol (E₂) concentrations with application of population PK methods and dried blood spot (DBS) sampling.

Pain was greater after SC compared with IM injection 24 hours (but not immediately) after injection but both routes were equally acceptable. Ultimately 11 preferred IM, 6 preferred SC, and 3 had no preference. The DBS-based PK analysis of serum testosterone revealed a later time of peak testosterone concentration after SC vs IM injection (8.0 vs 3.3 days) but no significant route differences in model-predicted peak testosterone concentration (8.4 vs 9.6 ng/mL) or mean resident time (183 vs 110 days). The PK of venous serum testosterone, DHT, and E₂ did not differ according to route of injection.

We conclude that SC TU injection is acceptable but produces greater pain 24 hours after injection that may contribute to the overall majority preference for the IM injection. The PK of testosterone, DHT, or E₂ did not differ substantially between SC and IM routes. Hence whereas further studies are required, the SC route represents an alternative to IM injections without a need to change dose for men for whom IM injection is not desired or recommended.