Injectable testosterone undecanoate has more favourable pharmacokinetics and pharmacodynamics than testosterone enanthate
Authors: Partsch Carl-Joachim , Weinbauer Gerhard F , Fang Ruiying , Nieschlag Eberhard 
Published: December 2008

Partsch C-J, Weinbauer GF, Fang R, Nieschlag E. Injectable testosterone undecanoate has more favourable pharmacokinetics and pharmacodynamics than testosterone enanthate. Eur J Endocrinol 1995;132:514–19. ISSN 0804–4643
Testosterone preparations producing constant physiological testosterone serum levels are desirable for long-term treatment of androgen deficiency. However, all injectable testosterone esters used clinically for substitution of male hypogonadism are characterized by unfavourable pharmacokinetics. We therefore tested two groups of five long-term orchidectomized cynomolgus monkeys (Macaca fascicularis), which received a single intramuscular injection of 10 mg/kg body weight of an injectable testosterone undecanoate (TU) preparation or testosterone enanthate (TE) in a preclinical study to assess the pharmacokinetic and pharmacodynamic characteristics of TU in comparison to TE. The dose was equivalent to 6.3 and 7.2 mg of pure testosterone per kilogram body weight in the TU and TE group, respectively. Following injection of TU, mean serum testosterone rose to 58 ± 18 nmol/l on day 1 and remained at moderately supraphysiological levels of 40–68 nmol/l for 45 days. Thereafter, testosterone levels were maintained in the normal range of intact monkeys for another 56 days. The TE injection resulted in highly supraphysiological levels of 100–177 nmol/l from immediately after the injection to day 5. A rapid decline followed and testosterone levels reached the lower limit of normal after 31 days. Serum testosterone levels were significantly higher in the TEthan in the TU-treated animals on days 0.5–7 (p < 0.05). Significantly lower testosterone levels were seen in the TE than in the TU group on days 16, 22, 25 and 31 (p < 0.05). Pharmacokinetic analysis of serum testosterone levels showed a significantly higher area under the curve for TU (4051 ± 939 vs 1771 ±208 nmol·h/l; p < 0.045), a longer residence time (40.7 ±4.1 vs 11.6 ±1.1 days; p <0.00012), a longer terminal half-life (25.7 ± 4.0 vs 10.3 ± 1.1 days; p < 0.0069), and a lower maximal testosterone concentration (73 ± 12 vs 177 ± 21 nmol/l; p < 0.0027). Following TU injection, oestradiol levels increased from 48 ± 8 pmol.l to a plateau of 80–118 pmol/l from day 1 to day 59. In contrast, TE injection resulted in a rapid increase of oestradiol levels to a maximum of 166 ± 29 pmol/l after 4 days (p < 0.05 vs TU- treated group). In the TU and TE groups levels below 80 pmol/l were reached after 66 and 16 days, respectively. Ejaculatory response was induced for 14 weeks in the TU animals in contrast to 7 weeks in the TE animals. Ejaculate weight reached a maximum of 533 ± 163 mg at day 52 in the TU group (p < 0.05 vs TE group). In the TE animals, the maximal ejaculate weight of 41 ± 17 mg was seen at day 16. Thus, with respect to androgen substitution therapy, TU showed pharmacokinetic and pharmacodynamic properties clearly superior to those of TE and may provide an important improvement in the substitution of male androgen deficiency and also for male contraception.
E Nieschlag, Institute of Reproductive Medicine of the University, Steinfurter Str. 107, D-48149 Münster, Germany