Metabolites of Testosterone Propionate: How the Body Breaks Down the Drug

After injection, testosterone propionate goes through a long path of transformations: from cleavage of the ester to dozens of metabolites excreted in the urine. Both the desired effects and the side effects depend on these transformations — from acne to gynecomastia — as does what the laboratory sees. The editorial team analyzed the drug's metabolism step by step.

Step one: cleavage of the ester

Testosterone propionate is testosterone to whose hydroxyl group at the 17β position a propionic acid residue is attached. The ester itself has no significant hormonal activity: it serves as a “transport form” that slows the release from the oil depot in the muscle.

After the molecule enters the blood and tissues from the depot, it is cleaved by nonspecific esterases. As a result, free testosterone and propionic acid are formed. Since the propionate chain is short, the drug leaves the depot comparatively quickly, so the testosterone level after injection rises within the first day and declines relatively quickly.

Propionic acid is an ordinary metabolite for the body. It is formed, for example, during the breakdown of some amino acids and fatty acids with an odd number of carbon atoms, as well as by intestinal bacteria. It is then converted into propionyl-CoA and, through succinyl-CoA, enters the Krebs cycle. In the amounts supplied by the drug, it has no independent clinical significance.

Thus, from the moment of hydrolysis, the drug's further fate is the fate of ordinary testosterone. The differences from other esters lie only in the speed of hormone delivery, that is, in the shape of the concentration curve, not in the set of metabolites.

Active metabolites: DHT and estradiol

A small but biologically very important fraction of testosterone in the target tissues is converted into dihydrotestosterone (DHT) under the action of 5α-reductase. There are several isoforms of this enzyme: type 2 predominates in the prostate and hair follicles, type 1 in the skin and liver. DHT binds to the androgen receptor more strongly than testosterone and is not aromatized.

It is with DHT that the androgenic effects in the skin (enhanced secretion of the sebaceous glands), in the hair follicles (androgenetic baldness in predisposed people), and in the prostate are associated. Therefore drugs that block 5α-reductase are used in the treatment of benign prostatic hyperplasia and baldness.

Another part of testosterone, under the action of aromatase (CYP19A1), is converted into estradiol. Aromatase is active in adipose tissue, the brain, bones, and liver. Estradiol in men is necessary for the bones, lipid metabolism, and libido, but in excess it causes gynecomastia. The more testosterone comes from outside, the more estradiol is formed.

These two pathways explain why the side effects of testosterone are heterogeneous: some people complain mainly of acne and hair loss, others of fluid retention and breast tenderness. The activity of the enzymes depends on genetics, age, and the amount of adipose tissue.

Inactivation in the liver and excretion

The bulk of testosterone is inactivated in the liver. The enzyme 17β-hydroxysteroid dehydrogenase oxidizes testosterone to androstenedione, and then 5α- and 5β-reductases and 3α-hydroxysteroid dehydrogenases convert the steroids into androsterone and etiocholanolone. These are the main quantitative metabolites found in the urine.

For the substances to be able to leave the body with the urine, they need to be made water-soluble. For this, glucuronic acid (glucuronidation) or a sulfate group is attached to them. The key enzyme for the glucuronidation of testosterone itself is the UDP-glucuronosyltransferase UGT2B17.

Alongside the liver, part of the metabolism is carried out by other tissues — the skin, prostate, and adipose tissue. A separate pathway involving cytochrome CYP3A4 leads to the formation of hydroxylated derivatives, in particular 6β-hydroxytestosterone. It is precisely through CYP3A4 that interactions are possible with drugs that induce or inhibit this enzyme.

Metabolism genetics and laboratory indicators

The metabolism of testosterone differs significantly between people. A vivid example is the UGT2B17 gene. In some people both copies of this gene are absent (deletion polymorphism), and they excrete significantly less testosterone glucuronide in the urine. The frequency of the deletion differs between populations: in residents of East Asia it is significantly higher than in Europeans.

Schulze and colleagues (2008) showed that this genotype affects the ratio of testosterone to epitestosterone (T/E) in the urine after testosterone administration. For anti-doping laboratories this became one of the arguments in favor of the transition to an individual “steroid profile” within the athlete biological passport and confirmation by the isotope mass spectrometry method.

In clinical practice, metabolism is important for interpreting tests. The level of total testosterone in the blood depends on the time since injection: with a short ester like propionate, a test taken on different days can give very different results. Therefore the doctor always takes into account when the drug was administered.

Similarly, the level of estradiol and DHT reflects not only the dose but also the enzyme activity in a specific person. Two patients with the same therapy regimen can have different concentrations of metabolites — and different side effects.

• total and free testosterone — taking into account the time since injection;
• estradiol — with symptoms from the breast glands or fluid retention;
• SHBG (sex hormone-binding globulin) — for calculating the free fraction;
• liver enzymes — as a general marker of the load on the liver.

What metabolism means for health

Understanding metabolism helps explain why supraphysiological doses of testosterone have such a wide palette of side effects. Increasing the substrate proportionally increases the formation of both DHT and estradiol, so androgenic and estrogenic effects are enhanced simultaneously.

Attempts to intervene in metabolism on one's own — for example, to block aromatase or 5α-reductase without medical indications — create new imbalances. Too low estradiol harms the bones, lipid profile, and libido, and pharmacological suppression of DHT has its own side-effect profile.

The liver, although it is the main organ of testosterone inactivation, usually does not experience such a pronounced load for injectable esters as with oral 17α-alkylated steroids. However, liver diseases change hormone metabolism, in particular they enhance aromatization, and this should be taken into account.

Finally, metabolism also explains the suppression of one's own hormonal axis: the hypothalamus and pituitary respond both to testosterone and to estradiol, and both signals reduce the secretion of LH and FSH. Because of this, endogenous testosterone production and spermatogenesis are suppressed.

Editorial conclusions

Testosterone propionate, after injection, is broken down by esterases into testosterone and propionic acid; thereafter the drug behaves like ordinary testosterone.

The key active metabolites are DHT and estradiol, which determine a significant part of both the desired and the undesirable effects. The bulk of the hormone is inactivated in the liver and excreted in the urine in the form of glucuronides of androsterone, etiocholanolone, and testosterone.

Genetic features, in particular the UGT2B17 polymorphism, noticeably affect the profile of metabolites, so the interpretation of tests is always individual.

The editorial team also recommends familiarizing yourself with our materials on the interaction of testosterone propionate with alcohol and drugs, on its legal status, and on its effect on the bone and joint apparatus.

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