The bioavailability of orally administered drugs could be impacted by intestinal and hepatic first-pass metabolism. Testosterone undecanoate (TU), an orally administered ester prodrug of testosterone, is significantly subjected to first-pass metabolism. However, the individual contribution of intestinal and hepatic first-pass metabolism is not well determined. Therefore, the aim of the current study was to predict the metabolic contribution of each site. The hydrolysis–time profiles of TU incubation in human liver microsomes and Caco-2 cell homogenate were used to predict hepatic and intestinal first-pass metabolism, respectively. The in vitro half-life (t1/2 inv) for the hydrolysis of TU in microsomal mixtures was 28.31 ± 3.51 min. By applying the “well-stirred” model, the fraction of TU that could escape hepatic first-pass metabolism (FH) was predicted as 0.915 ± 0.009. The incubation of TU in Caco-2 cell homogenate yielded t1/2 inv of 109.28 ± 21.42 min, which was applied in a “Q gut” model to estimate the fraction of TU that would escape intestinal first-pass metabolism (FG) as 0.114 ± 0.02. Accordingly, only 11% of the absorbed fraction of TU could escape intestinal metabolism, while 91% can pass through hepatic metabolism. Hence, compared to the liver, the intestinal wall is the main site where TU is significantly metabolised during first-pass effect.

Male hypogonadism is often treated by testosterone (T) replacement therapy such as oral administration of the ester prodrug, testosterone undecanoate (TU). However, the systemic exposure to T following oral TU is very low due to esterase-mediated metabolism, particularly in the small intestine. The aim of this work was to examine the esterase-inhibitory effect of natural fruit extract of strawberry (STW) on the intestinal degradation of TU as a potential approach to increasing the oral bioavailability of T. Herein, the hydrolysis of TU was assessed in fasted state simulated intestinal fluid with added esterase activity (FaSSIF/ES) and Caco-2 cell homogenates in the presence of STW extract. It is noteworthy that STW substantially inhibited the degradation of TU in FaSSIF/ES and Caco-2 cell homogenates at concentrations that could be achieved following oral consumption of less than one serving of STW fruit. This can significantly increase the fraction of unhydrolyzed TU in the intestinal lumen as well as in enterocytes. In addition, it was demonstrated that TU has high intestinal lymphatic transport potential as the association of TU with plasma-derived human chylomicrons was in the range of 84%. Therefore, oral co-administration of TU with STW could potentially increase the intestinal stability of TU and consequently the contribution of lymphatically delivered TU to the systemic exposure of T in vivo.