Sustanon 250 Metabolites: How the Body Breaks Down the Drug

Sustanon 250 enters the body as a mixture of four esters and is excreted as dozens of different metabolites. Between these points lies a multi-step path through the blood, target tissues, and the liver. Understanding this path explains the drug's effects, its side effects, and the principles of laboratory diagnostics. The editorial team traced how the body breaks down Sustanon 250.

Step One: Release and Hydrolysis of the Esters

One milliliter of Sustanon 250 contains 30 mg of testosterone propionate, 60 mg of phenylpropionate, 60 mg of isocaproate, and 100 mg of decanoate, dissolved in peanut oil. In total this corresponds to approximately 176 mg of pure testosterone - the rest of the mass is the ester 'tails.'

After an intramuscular injection, the solution forms an oil depot. The esters gradually diffuse from it into the interstitial fluid and blood. The longer and more lipophilic the ester chain, the slower this process. Propionate is released fastest, decanoate slowest.

In the blood and tissues, esterase enzymes cleave the ester bond. Free testosterone, identical to the endogenous one, is released along with the corresponding carboxylic acid: propionic, phenylpropionic, isocaproic, or decanoic. These acids are then metabolized like ordinary fatty acids and have no hormonal activity.

It is the rate of release from the depot, not hydrolysis, that determines the duration of action of each component. Hydrolysis in the blood occurs quickly, so esters are present in circulation in small amounts, and the main part of the hormone exists as free or protein-bound testosterone.

Active Metabolites: DHT and Estradiol

A small but biologically very important portion of testosterone is converted into two active hormones. The first is dihydrotestosterone (DHT), formed by the action of 5-alpha-reductase. DHT binds to the androgen receptor more tightly than testosterone and is not aromatized.

The enzyme 5-alpha-reductase exists in several isoforms. Type 2 predominates in the prostate, hair follicles, and genitals; type 1 in the skin and liver. It is the local formation of DHT in these tissues that explains why androgens affect the prostate, hair, and sebaceous glands more strongly than could be expected from the blood testosterone level alone.

The second active metabolite is estradiol. It is formed by the enzyme aromatase (CYP19A1), which is present in adipose tissue, the brain, bones, and testes. In men, estradiol is necessary for bone health, libido, and regulation of the hormonal axis, but its excess at supraphysiological testosterone doses causes gynecomastia and fluid retention.

Importantly, both of these pathways do not saturate immediately: the more testosterone that arrives, the more DHT and estradiol are formed. Therefore, with non-medical use of high doses, both androgenic and estrogenic side effects increase.

Hepatic Metabolism and Inactivation

The main portion of testosterone is inactivated in the liver. The first step is usually oxidation of the 17-hydroxyl group by the enzyme 17-beta-hydroxysteroid dehydrogenase, forming androstenedione - a much weaker androgen.

Next comes reduction of the A ring by the enzymes 5-alpha- and 5-beta-reductase and 3-alpha-hydroxysteroid dehydrogenase. The result is two main end metabolites: androsterone (the 5-alpha isomer) and etiocholanolone (the 5-beta isomer). Both have practically no androgenic activity.

Injectable testosterone, unlike oral 17-alpha-alkylated steroids, is metabolized by ordinary pathways and does not create 'chemical resistance' to liver enzymes. That is why it is considered much less hepatotoxic, although supraphysiological hormone levels affect the liver's synthesis of lipoproteins and transport proteins.

Part of the testosterone in the blood is bound to sex hormone-binding globulin (SHBG) and albumin. Androgens reduce the synthesis of SHBG in the liver, so with the use of Sustanon 250 the proportion of free hormone may change - this should be taken into account when interpreting tests.

Conjugation and Excretion

For a substance to be able to leave in the urine, it must become water-soluble. For steroids this is achieved by conjugation - the addition of glucuronic acid or a sulfate group. The predominant part of testosterone metabolites is excreted precisely as glucuronides.

The key enzyme of testosterone glucuronidation is UDP-glucuronosyltransferase 2B17 (UGT2B17). Androsterone and etiocholanolone are also conjugated with the participation of other isoforms of this family. Sulfated metabolites make up a smaller share.

The main route of excretion is the kidneys: most of the administered dose leaves in the urine as conjugates. A small part is excreted in the bile. The carboxylic acids released from the esters are oxidized to carbon dioxide and water or incorporated into general fatty acid metabolism.

Because of the depot effect of decanoate, complete excretion of exogenous testosterone after the last injection of Sustanon 250 takes weeks. During this time the hormone level gradually decreases, while the body's own hormonal axis remains suppressed.

Individual Differences and Laboratory Diagnostics

Genetics significantly affect testosterone metabolism. The most striking example is the common deletion of the UGT2B17 gene. People who have it in both copies produce almost no testosterone glucuronide. A study by Schulze and colleagues (2008) showed that this radically changes the parameters of the urinary steroid profile.

The frequency of this deletion differs between populations. That is why the modern anti-doping system uses not only fixed thresholds but also an individual approach - the steroidal module of the athlete biological passport, which compares a person's parameters with their own history.

The main parameters that laboratories assess when analyzing the steroid profile:

• the testosterone/epitestosterone (T/E) ratio;
• the concentrations of androsterone and etiocholanolone and their ratio;
• metabolites of 5-alpha- and 5-beta-androstanediol;
• the carbon isotope ratio by IRMS, which makes it possible to distinguish synthetic testosterone from endogenous.

For clinical medicine, knowledge of metabolism is important for another reason: it explains why, against the background of Sustanon 250, the levels of estradiol, DHT, SHBG, LH, and FSH change, and helps the doctor correctly interpret test results.

Editorial Conclusions

The metabolism of Sustanon 250 begins with the gradual release of four esters from the oil depot and their hydrolysis to ordinary testosterone. From then on, the hormone acts the same as endogenous testosterone.

Part of the testosterone is converted into active DHT and estradiol, which explains the androgenic and estrogenic effects. The main mass is inactivated in the liver to androsterone and etiocholanolone and excreted in the urine as glucuronides.

Genetic variants of the enzymes, in particular UGT2B17, significantly change the metabolic profile, which is taken into account by modern methods of laboratory analysis.

We also recommend reading our articles on the effect of Sustanon 250 on the prostate and hair, where DHT plays a key role, as well as on the drug's interaction with alcohol and medications.

References

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6. Sustanon 250 mg/ml solution for injection. Summary of Product Characteristics (SmPC).