## 1 Introduction  
Testosterone undecanoate (TU, Figure 1) is an ester prodrug of the anabolic steroid, testosterone. TU was introduced in the late 1970s as an effective oral testosterone replacement therapy [1, 2]. In fact, testosterone is well absorbed from the intestinal lumen. However, therapeutic plasma levels following oral administration of testosterone cannot be achieved due to extensive first-pass metabolism [3]. Therefore, TU and other testosterone prodrugs have been proposed to reach effective levels of testosterone by avoiding pre-systemic metabolism and/or to provide more flexible therapeutic options in terms of the route of administration, and stability of plasma levels attained [4]. Once absorbed to the systemic circulation, the undecanoate side chain of TU is cleaved by the action of non-specific plasma esterase to release testosterone [5]. Currently, TU is commercially available as oral capsules, Andriol Testocaps(r) and Jatenzo(r), and intramuscular injections, Aveed(r) and Nebido(r). These formulationsare commonly used in the treatment of male hypogonadal disorders characterized by low serum levels of testosterone [6].  
Several animal studies have explored the mechanisms involved in the oral bioavailability of TU. [PERSON] and colleagues have demonstrated that the metabolism of TU starts in the intestinal wall of rats, and part of the fraction that escapes intestinal metabolism is absorbed via the intestinal lymphatics [7]. [PERSON] et al. have also shown that TU is metabolised in the intestinal wall, as metabolites of TU could be detected in the portal vein of rats [8]. In lymph duct-cannulated dogs, it was shown that the bioavailability of orally administered TU was as low as 3%, almost all of which was absorbed through intestinal lymphatics [9]. In humans, [PERSON] et al. have placed thoracic duct catheters in four participants following neck dissection for the resection of left mouth tumours [10]. In that study, following the oral administration of radiolabelled TU, the thoracic lymph was rich with TU and its main metabolite 5\(\alpha\)-dihydrotestosterone undecanoate (5\(\alpha\)-DHTU). However, both were undetectable in the systemic circulation. It should be mentioned that 5\(\alpha\)-DHTU is a product of the metabolic activity of 5\(\alpha\)-reductase on TU. Several other human studies have also suggested that lymphatically transported TU is the main source of testosterone in the systemic circulation [11; 12; 13], and the bioavailability of testosterone increases as the fat-content of the co-administered food increases [14]. To note, fatty-food increases intestinal lymphatic delivery of lipophilic drugs that have high lymphatic transport potential [15]. However, the systemic bioavailability of testosterone following the oral administration of TU in conditions that facilitate intestinal lymphatic transport was still as low as 6.8% in humans [1].  
Taking together the above-mentioned observations, it is now well established that the intestinal wall contributes to the metabolism of TU. The fraction that survives intestinal first-pass metabolism is partly absorbed through the lymphatic system while the remaining part is delivered via the portal vein to the liver where it is substantially metabolised [16]. Both intestinal and hepatic first-pass metabolism contribute to the low bioavailability of TU. However, the relative contribution of each metabolic site (intestinal wall versus liver) is still unclear. Once the major site of TU metabolism is revealed, a clear insight for an approach to improve the bioavailability of TU could be achieved. Therefore, the aim of the current study was to predict the individual impact of intestinal and hepatic first-pass metabolism on the bioavailability of orally administered TU.