Therefore, the Food and Drug Administration has recommended that IM administration of testosterone undecanoate be performed slowly by trained personnel in the clinic, and the patient should be observed for at least 30 minutes after injections. IM testosterone therapy was maintained for 3 weeks after enrollment before switching to self-administration of the same dose via the SC route for 8 weeks. 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. B, Schematic illustration of the absorption steps of testosterone esters after intramuscular (left) or subcutaneous (right) injection. Currently, testosterone therapy is indicated for men with unequivocal, organic, or pathologic androgen deficiency to alleviate symptoms and maintain secondary sexual characteristics by raising testosterone into the normal male range (1). Injections with intramuscular (IM) testosterone esters have been available for almost 8 decades and not only result in predictable serum testosterone levels but are also the most inexpensive modality. Agnathans (jawless vertebrates) such as lampreys do not produce testosterone but instead use androstenedione as a male sex hormone.
Selection of the administration route of testosterone is influenced by patient preference, product availability, and the cost of the formulation. More recently, newer formulations of testosterone replacement have become available, which include ultralong-acting testosterone undecanoate for IM injection, transdermal patches and gels, buccal tablets, intranasal sprays, and oral testosterone undecanoate (Table 1), thus providing a range of options to choose from. We have come a long way since the days of Brown-Séquard, who self-administered an extract of animal testes by subcutaneous (SC) injection in 1889 (Fig. 1) (3). Although studies directly comparing the safety of SC vs IM administration of testosterone esters are desirable, clinicians should consider discussing the SC route with their patients because it is easier to self-administer and has the potential to improve patient adherence.
According to this study, the males who were administered the SubQ reported a higher and better quality of life and more satisfactory life and testosterone levels than those administered the IM testosterone injection because the SubQ is painless and easier to self-administer than the IM. When men with low testosterone levels are symptomatic from primary or secondary hypogonadism, they are mostly administered testosterone injections. This blog post will attempt a compThis blog post will attempt a comparative analysis of the Subcutaneous and Intramuscular injections and how they are both administered to improve the testosterone levels of hypogonadism patients. To date, limited data suggest that SC administration of testosterone enanthate and cypionate results in stable and predictable on-treatment concentrations, has good acceptability among patients, and can be self-administered more easily than IM injections. Indeed, long-term compliance among men who are prescribed testosterone therapy with IM injections is low; approximately 69% of men on long-acting esters discontinue treatment within 3 months of therapy, and 95% discontinue it within 12 months (56).
In addition to 6β- and 16β-hydroxytestosterone, 1β-, 2α/β-, 11β-, and 15β-hydroxytestosterone are also formed as minor metabolites. The 6β-hydroxylation of testosterone is catalyzed mainly by CYP3A4 and to a lesser extent CYP3A5 and is responsible for 75 to 80% of cytochrome P450-mediated testosterone metabolism. In addition to conjugation and the 17-ketosteroid pathway, testosterone can also be hydroxylated and oxidized in the liver by cytochrome P450 enzymes, including CYP3A4, CYP3A5, CYP2C9, CYP2C19, and CYP2D6. A small portion of approximately 3% of testosterone is reversibly converted in the liver into androstenedione by 17β-HSD. In the hepatic 17-ketosteroid pathway of testosterone metabolism, testosterone is converted in the liver by 5α-reductase and 5β-reductase into 5α-DHT and the inactive 5β-DHT, respectively. The conjugates of testosterone and its hepatic metabolites are released from the liver into circulation and excreted in the urine and bile.
However, the hydrolysis of testosterone esters by tissue esterases is a slow process because of their high lipophilicity, with negligible spontaneous hydrolysis in water (40). Molecules smaller than 1 kDa, such as testosterone, are preferentially absorbed by the blood capillaries because of the high rate of filtration and reabsorption of fluid across vascular capillaries (39). The physiology of the IM and SC milieu determines the patterns of absorption after administration. Therefore, the formulation and the injection site influence the speed and magnitude of absorption. Testosterone ester is also partly hydrolyzed within the interstitium, with free testosterone entering the circulation directly. With administration using either route, the ester exits the depot via diffusion into the interstitium, from where it enters the lymphatics and subsequently reaches the circulation where it undergoes hydrolysis by intracellular esterases. A, Illustration of the progressive increase in lipophilicity of testosterone esters with increase in number of carbons in the side chain.

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