Following deep intramuscular injection, the drug provides a sustained release of testosterone into the bloodstream for up to 2 weeks. Testosterone enanthate and testosterone cypionate are slow-acting injectable forms. A slower release means fewer testosterone injections per year. The solubility in castor oil of hydroxyprogesterone caproate polymorphs was previously reported in the literature as being 278 mg/mL and 301 mg/mL, respectively, at a temperature of 20° , which are comparable to the prodrugs investigated. The solubilities of some short esters have been previously reported, averaging 110 mg/mL for the acetate ester, 175.5 mg/mL for the propionate, 139 mg/mL for the phenylpropionate and 447 mg/mL for the isocaproate . Mean serum testosterone concentrations did not change significantly after switching administration routes (Fig. 4B) (24), confirming similar bioavailability after SC administration. B, Total testosterone concentrations after intramuscular (IM) and SC administration of testosterone enanthate in 14 transgender men (24). A, Mean trough concentrations of testosterone in hypogonadal men on weekly 75 mg subcutaneous (SC) testosterone enanthate (29). Mean total testosterone concentrations gradually increased from predose values of 224 ng/dL to 374 ng/dL, 479 ng/dL, and 541 ng/dL at weeks 1, 6, and 12, respectively (29) (Fig. 4A). During week 7 of the study, the dose of testosterone was either reduced to 50 mg/week or increased to 100 mg/week with the aim of maintaining on-treatment serum testosterone levels within the normal range (27). In another study, 150 hypogonadal men were started on SC testosterone enanthate 75 mg/week for 52 weeks, which was administered via a novel SC autoinjector (27). As different muscle groups have variable blood flow (eg, the blood flow to the deltoids is higher than the glutei) (44), which further varies with physical activity (45), serum on-treatment testosterone concentrations after IM injections are dependent on these characteristics. This type of injection is painful and doesn’t have a timed release, making it ineffective at balancing levels over time. It’s typically injected as an intramuscular injection, and it only remains in the body for a few hours. An aqueous Testosterone suspension doesn’t include an ester. Asymmetric unit of Tena presenting non-hydrogen atoms at 50% probability level (a); overall packing viewed along b-axis (b). Overall packing shows that steroid molecules were assembled roughly diagonal with respect to the aoc plane (Figure 2b). They were bridged via C-H⋯O bonding between the O3B carbonyl acceptor of enanthate tail and CH2 donor (C21A-H21B⋯O3B interaction with an interaction magnitude of 45.1 kJ/mol). This feature can be attributed to a certain degree to the preferred orientation of crystallites in some samples (TDec and TUnd are pronounced in this regard). Once injected, testosterone stays in its esterified form in the muscle tissue. This level of solubility is named "the partition coefficient," which means the higher the solubility in oil, the higher the partition coefficient. The higher the level of carbon groups in the ester, the more soluble in oil it is, and less soluble in water.
