Androgen pharmacology encompasses the physiology and pathophysiology of testosterone and its active metabolites, the clinical pharmacology of testosterone replacement therapy (TRT) across multiple delivery systems, the mechanism and organ-specific indications of the two isoforms of the 5 alpha-reductase (5AR) enzyme and their inhibitors, the receptor pharmacology and clinical profiles of competitive androgen receptor (AR) antagonists used in prostatic disease and androgen-dependent skin conditions, and the pharmacotoxicology of anabolic-androgenic steroids (AAS) in non-medical use. Each of these areas carries high examination relevance because the drug interactions, adverse effect profiles, and monitoring requirements for androgen-active compounds are distinctive, clinically actionable, and form the basis for multiple T3 and T4 case-based questions in endocrinology, urology, and sports medicine contexts.
Testosterone is the principal circulating androgen in men, produced predominantly (approximately 95%) by the Leydig cells of the testes under the stimulation of luteinizing hormone (LH). The remaining 5% is derived from peripheral conversion of adrenal androgen precursors, principally dehydroepiandrosterone (DHEA) and androstenedione. Testosterone biosynthesis proceeds through the steroidogenic pathway from cholesterol via pregnenolone to progesterone to 17-hydroxyprogesterone to androstenedione, with the final step being the 17-ketosteroid reductase (17-beta-hydroxysteroid dehydrogenase type 3, encoded by the HSD17B3 gene) reduction of androstenedione to testosterone in Leydig cells. In women, testosterone is produced in smaller quantities by the ovarian theca cells (under LH stimulation), the adrenal cortex (from DHEA and androstenedione), and peripheral tissues, with circulating testosterone levels approximately 15 to 20 times lower than in men.1
Testosterone circulates predominantly bound to plasma proteins: approximately 60% to 70% is tightly bound to sex hormone-binding globulin (SHBG), approximately 25% to 35% is loosely bound to albumin, and only 1% to 3% is free (unbound). The free and albumin-bound fractions constitute the biologically active ("bioavailable") testosterone, because SHBG-bound testosterone is not available for tissue uptake. Clinical interpretation of testosterone levels requires accounting for SHBG concentration: conditions that increase SHBG (aging, estrogen administration, hyperthyroidism, hepatic disease) produce falsely reassuring total testosterone values despite reduced bioavailable testosterone, while conditions that decrease SHBG (obesity, insulin resistance, hypothyroidism, exogenous androgens) may lower total testosterone values while bioavailable testosterone remains adequate. Calculated free testosterone or a direct analog assay for free testosterone is therefore essential in patients where SHBG disturbance is suspected.1,2
Intracellularly, testosterone exerts its effects through two principal mechanisms: direct binding to the androgen receptor (AR) and conversion to more potent active metabolites. The AR is a member of the nuclear receptor superfamily and resides in the cytoplasm in an inactive complex with heat shock proteins. Testosterone binding to the AR causes dissociation of heat shock proteins, AR dimerization, translocation to the nucleus, and binding to androgen response elements (AREs) in the promoter regions of androgen-regulated genes, driving transcription of genes controlling skeletal muscle protein synthesis, erythropoiesis, bone mineralization, sexual differentiation, and secondary sex characteristic development. In target tissues expressing high levels of the enzyme 5 alpha-reductase (5AR), testosterone is converted intracellularly to dihydrotestosterone (DHT), which binds the AR with approximately 3 to 5 times greater affinity and produces a more stable androgen receptor-DHT (AR-DHT) complex than testosterone alone. DHT is the primary mediator of androgenic effects in the prostate, skin, and scalp, explaining why finasteride and dutasteride (5AR inhibitors) selectively reduce prostatic and dermal androgenic stimulation without abolishing muscle and erythropoietic androgenic effects that are mediated predominantly by testosterone itself.1,3
The second major intracellular metabolic pathway for testosterone is aromatization to estradiol by aromatase (CYP19A1). Approximately 80% of circulating estradiol in men is derived from peripheral aromatization of testosterone and androstenedione in adipose tissue, muscle, brain, and other sites; only 20% is secreted directly by the testes. This peripheral estradiol production serves essential physiological roles in men: estradiol is required for normal bone mineral density maintenance (AR-independent effect via ER alpha in osteoblasts), for closure of the epiphyseal growth plates, for regulation of gonadotropin secretion via negative feedback at the hypothalamus and pituitary, and for several aspects of cardiovascular function and sexual interest. The clinical importance of aromatization in testosterone replacement therapy (TRT) is that supraphysiological testosterone doses increase aromatization proportionally, raising circulating estradiol and potentially causing gynecomastia, fluid retention, and mood instability.1
Testosterone: primary mediator of muscle protein synthesis, bone density, erythropoiesis, libido, and mood. DHT: primary mediator of prostate growth, male-pattern hair loss (androgenetic alopecia), beard and body hair growth, external genital virilization during fetal development, and sebaceous gland activity. 5AR inhibitors reduce DHT without substantially reducing testosterone, producing a tissue-selective reduction in androgenic stimulation that targets DHT-mediated effects while preserving testosterone-mediated effects on muscle, bone, and erythropoiesis.
Testosterone replacement therapy (TRT) is indicated in men with confirmed hypogonadism defined by consistently low morning serum total testosterone below 300 ng/dL (10.4 nmol/L) combined with symptoms attributable to testosterone deficiency, including reduced libido, erectile dysfunction, diminished energy, loss of morning erections, depressive mood, reduced muscle mass, and increased adiposity. The diagnosis requires at least two fasting morning testosterone measurements below the established threshold, as testosterone levels exhibit circadian variation with peak concentrations in the early morning. The clinical pharmacology of TRT is substantially determined by the formulation, which governs route of administration, pharmacokinetic profile, serum testosterone fluctuation, conversion to dihydrotestosterone (DHT) and estradiol, ease of dose adjustment, and risk of accidental transfer to others.2
Intramuscular testosterone esters are the oldest and most widely used TRT formulations outside the United States. Testosterone enanthate (200 mg per mL) and testosterone cypionate (200 mg per mL), both oily depot formulations, are administered by intramuscular (IM) injection at doses of 100 to 200 mg every 1 to 2 weeks or 75 to 100 mg weekly. Both are long-chain ester prodrugs: esterification of the 17-beta hydroxyl group of testosterone with the enanthate or cypionate fatty acid delays absorption from the injection site depot, producing a prolonged release with peak serum testosterone typically at 24 to 72 hours post-injection and a half-life of approximately 7 to 8 days (enanthate) or 8 to 10 days (cypionate). The principal pharmacokinetic drawback of biweekly IM injection is the pronounced supraphysiological peak in the first 24 to 72 hours followed by a trough that may fall below the normal range before the next injection, producing cyclical symptom variation. Weekly injection reduces this peak-to-trough fluctuation substantially.
Testosterone undecanoate in castor oil (Nebido, 1,000 mg per 4 mL) has a markedly longer half-life of approximately 21 days and is administered every 10 to 14 weeks after an initial loading injection 6 weeks later, producing more stable serum levels but requiring 4 mL deep IM injections with a mandatory 30-minute post-injection observation period for pulmonary oil microembolism risk.2,3
Transdermal testosterone formulations deliver testosterone through the skin directly into the systemic circulation, bypassing hepatic first-pass metabolism and producing more stable serum testosterone concentrations than biweekly IM injections. Testosterone gels (AndroGel, Testim, Fortesta) are applied daily to the upper arms, shoulders, or abdomen, with doses of 25 to 100 mg per day producing dose-proportional increases in serum testosterone. Gel formulations carry a significant risk of accidental transfer to female partners or children through skin contact, requiring the patient to wash hands after application, cover the application site with clothing, and avoid skin-to-skin contact for several hours. Testosterone patches (Androderm) are applied to the non-scrotal skin daily at doses of 2 to 4 mg per day and produce steady-state serum testosterone within the normal range, but local skin reactions occur at the application site in 35% to 50% of users. Nasal testosterone gel (Natesto) is applied intranasally three times daily and produces rapid absorption with a shorter duration of activity that preserves pulsatile gonadotropin secretion and may be preferred in men concerned about fertility, though the three-times-daily dosing schedule requires high adherence.2
Subcutaneous testosterone pellets (Testopel) are implanted subcutaneously in the hip or buttock region through a minor office procedure every 3 to 6 months, releasing testosterone at a controlled rate from compressed pellet matrices. The principal clinical advantage is compliance independence for the treatment duration; disadvantages include the need for a minor surgical procedure, the inability to adjust dose after implantation, and occasional pellet extrusion. Oral testosterone undecanoate (Jatenzo, Tlando) is absorbed via the intestinal lymphatic system (chylomicron pathway), bypassing hepatic first-pass because its long ester chain directs absorption into the lymphatics rather than the portal circulation. Earlier oral testosterone formulations (methyltestosterone, now largely discontinued) were absorbed through the portal system and produced severe hepatotoxicity because of alkylation at the 17-alpha position required to resist first-pass metabolism. Contemporary oral testosterone undecanoate must be taken with a meal containing at least 19 grams of fat to ensure adequate lymphatic absorption, and blood pressure elevation is the most clinically relevant adverse effect of the oral formulation.2,3
Pulmonary oil microembolism (POME) is a rare but potentially life-threatening complication of testosterone undecanoate 1,000 mg IM (Nebido), arising from inadvertent intravascular injection or rapid vascular absorption of the castor oil vehicle. Symptoms include cough, dyspnea, chest pain, dizziness, and syncope occurring within minutes to 30 minutes of injection. All Nebido injections must be administered by a healthcare professional with a 30-minute post-injection monitoring period. This restriction does not apply to testosterone enanthate or cypionate, which use smaller injection volumes and different vehicles.
Erythrocytosis (polycythemia) is the most common dose-dependent adverse effect of testosterone replacement therapy (TRT), occurring in approximately 20% to 40% of men on injectable formulations. Testosterone stimulates erythropoiesis through multiple mechanisms: direct stimulation of erythroid progenitor cells in bone marrow via androgen receptor (AR) activation, suppression of hepcidin (a hepatic peptide that limits iron availability for erythropoiesis) through testosterone-driven erythropoietin upregulation, and increased serum erythropoietin production. The clinical threshold for action is a hematocrit above 54% (or hemoglobin above 18.5 g/dL), at which point TRT dose reduction, formulation change to a transdermal preparation (which produces lower and more stable testosterone levels and less erythrocytosis than injectable formulations), or therapeutic phlebotomy is indicated. Untreated erythrocytosis from TRT increases whole-blood viscosity and produces a prothrombotic state, elevating the risk of venous thromboembolism and arterial events. Hematocrit and hemoglobin should be measured at baseline, at 3 months after initiation of TRT, and annually thereafter.2,4
The cardiovascular risk profile of TRT has been a subject of substantial clinical debate since the premature termination of the Testosterone in Older Men with Mobility Limitations (TOM) trial in 2010, which was stopped after observing an excess of cardiovascular events in testosterone-treated men. Subsequent large observational studies and meta-analyses have produced conflicting results, with some showing increased cardiovascular risk and others showing neutral or even favorable effects. The most comprehensive evidence to date comes from the Testosterone Replacement Therapy for Assessment of Long-term Vascular Events and Efficacy Response in Hypogonadal Men (TRAVERSE) trial, a randomized controlled trial of 5,246 men with hypogonadism and elevated cardiovascular risk followed for a mean of 33 months, which found no significant difference in the rate of major adverse cardiovascular events (MACE) between testosterone and placebo, establishing non-inferiority with respect to cardiovascular safety in this population. Testosterone did increase the risk of atrial fibrillation (3.5% vs 2.4%), pulmonary embolism (0.9% vs 0.5%), and acute kidney injury in the TRAVERSE trial, findings that inform current monitoring recommendations. Testosterone is contraindicated in men with recent (within 6 months) myocardial infarction or stroke, and in patients with New York Heart Association (NYHA) class III or IV heart failure.4
TRT suppresses spermatogenesis by inhibiting the hypothalamic-pituitary-gonadal (HPG) axis through negative feedback: exogenous testosterone suppresses hypothalamic GnRH pulsatility and pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, reducing intratesticular testosterone (which is required at 50 to 100 times the concentration of circulating testosterone to maintain spermatogenesis) and directly suppressing the FSH-dependent Sertoli cell support of germ cell development. This pharmacological effect is dose-dependent and formulation-independent; virtually all TRT regimens produce profound oligospermia or azoospermia in most men within 3 to 6 months of treatment initiation. Spermatogenesis typically recovers over 6 to 24 months after TRT cessation, but recovery may be incomplete in some men, particularly those with pre-existing borderline spermatogenic function or those who have been on TRT for many years. Men desiring fertility who also have hypogonadal symptoms should be offered alternatives to TRT, specifically human chorionic gonadotropin (hCG) or selective estrogen receptor modulator (SERM) therapy (clomiphene citrate), which stimulate endogenous testosterone production through different mechanisms while preserving spermatogenesis.52,5
Monitoring of TRT requires a structured follow-up schedule. Serum total testosterone should be measured at 3 to 6 months after initiation and periodically thereafter, with the target range typically 400 to 700 ng/dL (13.9 to 24.3 nmol/L) for most formulations; the timing of the sample relative to the last injection dose must be specified (mid-cycle for biweekly injection, 2 hours post-application for gels). Hematocrit, hemoglobin, and complete blood count (CBC) at baseline and 3 months, then annually. Prostate-specific antigen (PSA) at baseline and 3 to 12 months for men over 40 or at elevated prostate cancer risk; TRT is contraindicated in men with active prostate cancer or untreated high-grade prostatic intraepithelial neoplasia. Bone mineral density (BMD) by dual-energy X-ray absorptiometry (DEXA) at baseline in men with osteoporosis risk. Fasting lipid panel, glucose, and blood pressure monitoring are also part of standard TRT follow-up given testosterone's modest effects on high-density lipoprotein (HDL) cholesterol (slight reduction with injectable formulations) and insulin sensitivity.2
TRT suppresses spermatogenesis by driving HPG axis suppression and reducing intratesticular testosterone far below spermatogenic threshold. For hypogonadal men who want to preserve fertility, hCG (typically 500 to 2,000 IU subcutaneously 2 to 3 times weekly) maintains intratesticular testosterone by providing LH receptor stimulation without suppressing the HPG axis, and can be combined with recombinant FSH if spermatogenesis is inadequate on hCG alone. Clomiphene citrate stimulates endogenous LH and FSH by blocking hypothalamic estrogen receptor feedback, raising both testosterone and gonadotropin levels while preserving the fertility axis. Neither of these approaches is as effective as TRT for all hypogonadal symptoms, but both maintain sperm production.
5 alpha-reductase (5AR) exists as two principal isoforms with distinct tissue distributions and pharmacological relevance. Type 1 5AR (encoded by SRD5A1) is expressed predominantly in the skin, sebaceous glands, liver, and other peripheral tissues. Type 2 5AR (encoded by SRD5A2) is expressed predominantly in the prostate, seminal vesicles, epididymis, and hair follicles, and is the isoform responsible for the majority of intraprostatic dihydrotestosterone (DHT) production. The central role of type 2 5AR in prostate development is illustrated by the phenotype of men with congenital SRD5A2 deficiency: they are born with ambiguous or female-appearing external genitalia (because DHT is required for male external genital virilization in utero) but have normal internal male structures (because testosterone, not DHT, drives wolffian duct development), and undergo dramatic phallic virilization at puberty when rising testosterone levels partially compensate for deficient DHT signaling in peripheral tissues.3
Finasteride is a competitive inhibitor of type 2 5AR with approximately 100-fold selectivity for the type 2 over the type 1 isoform. At the therapeutic dose of 5 mg per day for benign prostatic hyperplasia (BPH), finasteride reduces serum DHT by approximately 70% and intraprostatic DHT by over 90%, producing a sustained reduction in prostate volume of approximately 20% to 30% over 6 to 12 months, which reduces lower urinary tract symptoms (LUTS) attributable to prostatic obstruction and reduces the risk of acute urinary retention and the need for surgical intervention over the long term.2 At the lower dose of 1 mg per day for male-pattern hair loss (androgenetic alopecia), finasteride reduces serum DHT by approximately 60% and produces scalp DHT reduction of approximately 65%, stabilizing hair loss and producing modest regrowth in the vertex scalp in most treated men. Because finasteride reduces DHT substantially, it reduces serum prostate-specific antigen (PSA) by approximately 50% after 6 months of treatment; this predictable PSA reduction must be accounted for when using PSA as a prostate cancer screening tool in men on finasteride: the measured PSA should be doubled to estimate the equivalent true PSA, and failure to do so risks missing significant PSA elevations.63,6
Dutasteride is a non-selective dual inhibitor of both type 1 and type 2 5AR isoforms. Because dutasteride inhibits both the peripheral (type 1) and the prostatic (type 2) enzyme, it achieves greater DHT suppression: serum DHT is reduced by approximately 90% to 95% (compared to 70% with finasteride). Intraprostatic DHT suppression is comparable to finasteride. The greater serum DHT reduction of dutasteride does not translate into meaningfully superior BPH outcomes compared to finasteride in head-to-head trials, and the additional type 1 inhibition means a greater degree of peripheral DHT reduction that may increase the risk of systemic anti-androgenic effects. Both finasteride and dutasteride are absolutely contraindicated in women who are pregnant or may become pregnant because DHT is required for normal male fetal external genital development; women of childbearing potential must not handle crushed finasteride or dutasteride tablets and should not be exposed to the semen of men taking dutasteride, which carries measurable dutasteride concentrations that persist for up to 6 months after cessation given dutasteride's very long half-life of approximately 3 to 5 weeks.3
The sexual adverse effects of 5AR inhibitors are the most clinically relevant concern for individual patients. Both finasteride and dutasteride are associated with decreased libido (approximately 5% to 7% of patients), erectile dysfunction (approximately 5% to 9%), ejaculatory dysfunction including reduced ejaculate volume and orgasmic dysfunction (approximately 3% to 8%), and gynecomastia (approximately 1% to 2%), all attributable to the reduction in DHT-mediated peripheral androgenic signaling and the shift in androgen-to-estrogen ratio. The Post-Finasteride Syndrome (PFS) is a controversial clinical entity in which some men report persistent sexual, neurological, and psychological symptoms (persistent erectile dysfunction, reduced libido, depression, cognitive impairment) after cessation of finasteride therapy. The mechanistic basis of PFS is incompletely understood; proposed mechanisms include persistent alterations in neurosteroid levels (DHT and its metabolites serve as neuroactive steroids modulating gamma-aminobutyric acid type A (GABA-A) receptor function in the central nervous system), epigenetic changes in AR signaling, and psychological factors. While PFS is not universally accepted as a defined pharmacological entity, its documentation in clinical series warrants discussion with patients before initiating 5AR inhibitor therapy.9
Finasteride (5 mg/day) and dutasteride both reduce PSA by approximately 50% within 3 to 6 months through reduction of PSA gene transcription in prostatic epithelium. A man on finasteride with a measured PSA of 1.5 ng/mL has an estimated true PSA of approximately 3.0 ng/mL. Failing to double the measured PSA in this setting would cause a clinically significant PSA elevation (indicative of prostate cancer) to be missed. Any failure of PSA to fall by at least 50% after 6 months on 5AR inhibitor therapy, or any upward trend in PSA despite therapy, should prompt investigation regardless of the absolute value.
Anti-androgens are agents that block androgenic signaling at its various pharmacological targets: at the androgen receptor (AR), through competitive blockade; at the level of steroid biosynthesis (the steroidal anti-androgens and abiraterone acetate, covered separately in oncology modules); and by reducing androgen precursor availability through mineralocorticoid receptor (MR)-mediated mechanisms (spironolactone). The clinical applications of AR antagonists span a range from polycystic ovary syndrome (PCOS) and hirsutism (spironolactone) to castration-resistant prostate cancer (bicalutamide, enzalutamide) and gender-affirming feminizing hormone therapy (spironolactone, cyproterone acetate).7
Spironolactone is a synthetic steroidal compound that is primarily a mineralocorticoid receptor (MR) antagonist used as a potassium-sparing diuretic and antihypertensive, but that also blocks the AR with moderate affinity at higher doses and reduces adrenal androgen synthesis. At the doses used clinically for anti-androgenic indications (100 to 200 mg per day for hirsutism, acne, or PCOS), spironolactone produces its anti-androgenic effect through competitive AR blockade, reduction in testosterone biosynthesis by inhibiting the 17-hydroxylase/17,20-lyase enzyme (CYP17A1) at high doses, and possibly through increased metabolic clearance of testosterone. Its active metabolites, particularly canrenone, contribute to both the MR antagonism and some AR antagonism. Because spironolactone reduces androgen activity without fully suppressing hypothalamic-pituitary-gonadal (HPG) axis estrogen production, it is used in women with androgen-excess conditions such as PCOS and hirsutism, and as a component of feminizing hormone therapy in transgender women. The major adverse effects in this context are menstrual irregularity (due to the anti-androgenic and anti-mineralocorticoid effects on the hypothalamic-pituitary-ovarian axis), hyperkalemia (due to MR antagonism reducing renal potassium excretion), and hypotension. Spironolactone is teratogenic in males: animal studies demonstrate feminization of male fetuses, and contraception is required for women of childbearing potential.9
Bicalutamide is a non-steroidal AR antagonist with high binding affinity for the AR and no intrinsic agonist activity at physiologically relevant concentrations. Unlike the steroidal anti-androgens (cyproterone acetate, spironolactone), bicalutamide does not suppress gonadotropin secretion, meaning that its AR blockade in the hypothalamus and pituitary allows luteinizing hormone (LH) secretion to rise (because testosterone can no longer feed back negatively), and circulating testosterone levels increase approximately 1.5-fold above baseline during bicalutamide monotherapy. This testosterone rise partially offsets the AR blockade in peripheral tissues and is the pharmacokinetic rationale for combining bicalutamide with GnRH agonist or antagonist therapy in prostate cancer treatment to achieve combined androgen blockade (CAB). Bicalutamide at 150 mg per day has been studied as monotherapy for prostate cancer and for hirsutism. At 50 mg per day it is used as flare protection during GnRH agonist initiation in prostate cancer. Hepatotoxicity is the most serious adverse effect of bicalutamide, occurring in approximately 1% to 3% of patients; liver function tests should be monitored at baseline and periodically during treatment. Gynecomastia and breast tenderness occur in most men on bicalutamide monotherapy due to the combination of elevated testosterone (aromatized to estradiol) and AR blockade in breast tissue.8
Cyproterone acetate (CPA) is a steroidal AR antagonist and progestin that combines AR blockade with potent progestogenic and anti-gonadotropic activity, reducing both endogenous testosterone production (via HPG suppression) and AR signaling. CPA is used in Europe and other regions for prostate cancer, severe hirsutism, and as a feminizing hormone component in transgender women, but is not approved in the United States. Its progestogenic activity causes feedback suppression of LH and follicle-stimulating hormone (FSH), reducing testicular testosterone production in addition to blocking the AR, making it a more complete androgen suppressant than bicalutamide alone. Hepatotoxicity is more serious with CPA than with bicalutamide: meningioma risk is an additional significant concern with long-term CPA use at doses of 25 mg per day or higher, with French pharmacovigilance data demonstrating a dose-dependent and duration-dependent elevation in meningioma incidence that has led to regulatory restrictions in several countries. Venous thromboembolism is another CPA-specific risk, attributed to its progestogenic activity.9
Enzalutamide is a second-generation, high-affinity non-steroidal AR antagonist developed specifically to overcome resistance to earlier anti-androgens including bicalutamide. It binds the AR with approximately 5 to 8 times greater affinity than bicalutamide, inhibits nuclear translocation of the AR-ligand complex, and inhibits deoxyribonucleic acid (DNA) binding and coactivator recruitment by the AR even when nuclear translocation occurs. Enzalutamide is approved for castration-resistant prostate cancer (CRPC) and for metastatic castration-sensitive prostate cancer (mCSPC).8 The principal resistance mechanism to enzalutamide in CRPC is the emergence of constitutively active AR splice variants, particularly AR-V7 (androgen receptor splice variant 7), which lacks the ligand-binding domain and therefore cannot be blocked by any ligand-competitive AR antagonist. AR-V7 positivity in circulating tumor cells predicts resistance to both enzalutamide and abiraterone and signals the need for alternative treatment such as taxane chemotherapy. The central nervous system (CNS) adverse effects of enzalutamide, including fatigue, cognitive impairment, and a seizure risk (approximately 0.5% per year) attributable to enzalutamide's activity as a negative allosteric modulator of gamma-aminobutyric acid type A (GABA-A) receptors, are distinct from bicalutamide and require caution in patients with seizure history.7,8
AR-V7 is a truncated, constitutively active splice variant of the AR that is transcribed from the AR gene but lacks the C-terminal ligand-binding domain (LBD). Because all competitive AR antagonists (bicalutamide, enzalutamide, apalutamide, darolutamide) bind to the LBD, AR-V7 is intrinsically resistant to this entire class. In CRPC tumors that express AR-V7, enzalutamide treatment provides minimal benefit. Circulating tumor cell AR-V7 testing is clinically available and, when positive, guides the transition from AR-pathway-directed therapy to taxane chemotherapy (docetaxel or cabazitaxel), which retains activity regardless of AR-V7 status.
Anabolic-androgenic steroids (AAS) are synthetic derivatives of testosterone engineered to dissociate anabolic effects (muscle protein synthesis, bone density, erythropoiesis) from androgenic effects (prostate growth, sebaceous gland stimulation, virilization) or to modify pharmacokinetics for specific routes of administration. In practice, no currently available AAS achieves complete dissociation of anabolic from androgenic properties, because both effects are mediated through the same androgen receptor (AR); structural modifications that enhance anabolic activity tend to reduce hepatic metabolism and increase activity at peripheral AR-expressing tissues, but complete separation remains a theoretical rather than achieved pharmacological goal. AAS misuse for performance enhancement and body composition involves doses 10 to 100 times higher than therapeutic testosterone replacement therapy (TRT) doses, typically administered in stacking (multiple AAS simultaneously) and cycling (on-off protocols) patterns to maximize anabolic effects while attempting to evade testing or limit adverse effects.9
The primary pharmacokinetic classification of AAS distinguishes C17 (carbon-17) alpha-alkylated oral steroids from injectable ester formulations. C17-alpha-alkylated AAS (methyltestosterone, stanozolol, oxandrolone, oxymetholone) contain a methyl or ethyl group at the 17-alpha carbon that prevents first-pass hepatic oxidation at the 17-beta hydroxyl, conferring oral bioavailability. This modification also dramatically impairs the hepatocyte's normal mechanisms for conjugating and excreting the steroid, producing dose-dependent intrahepatic cholestasis, peliosis hepatis (blood-filled cysts in the hepatic parenchyma), and with prolonged high-dose use, hepatocellular adenoma and hepatocellular carcinoma. The C17-alpha-alkylation hepatotoxicity is the most serious distinctive organ toxicity of oral AAS and is not seen with injectable testosterone esters because the ester-modified steroids undergo normal hepatic metabolism once the ester is cleaved. Injectable AAS (nandrolone decanoate, boldenone undecylenate, trenbolone acetate) avoid hepatotoxicity but retain full systemic androgenic and anabolic receptor activity.9,10
Cardiovascular toxicity represents the most life-threatening consequence of non-medical AAS use and is the principal cause of AAS-related premature mortality. The principal cardiovascular mechanisms include: left ventricular hypertrophy (LVH) from the anabolic/hypertrophic effect on cardiomyocytes, which, unlike physiological athletic hypertrophy, produces pathological concentric hypertrophy with reduced diastolic compliance and predisposition to arrhythmia; adverse lipid profile changes, specifically marked reduction in high-density lipoprotein cholesterol (HDL-C) and variable increase in low-density lipoprotein cholesterol (LDL-C), producing an atherogenic lipid environment; accelerated coronary artery disease demonstrated by premature calcification in young AAS users on coronary computed tomography (CT) imaging; and direct cardiomyopathic effects with myocardial fibrosis and dilatation seen at autopsy in AAS-using athletes dying of sudden cardiac death. Thrombotic risk is elevated through hematocrit elevation (erythrocytosis from supraphysiological testosterone) and possible prothrombotic effects on platelet aggregation and coagulation factor synthesis.109,10
Endocrine consequences of AAS misuse are predictable from the pharmacology of hypothalamic-pituitary-gonadal (HPG) axis suppression. Exogenous AAS suppress gonadotropin secretion through negative feedback, producing testicular atrophy, azoospermia, and, in female users, menstrual disruption and virilization (clitoromegaly, voice deepening, androgenic alopecia). In men, recovery of the HPG axis and spermatogenesis after cessation of high-dose AAS may take 6 to 24 months and may be incomplete, particularly with prolonged or heavy use. Human chorionic gonadotropin and SERMs (clomiphene, tamoxifen) are used post-cycle as stimulants of HPG axis recovery in the AAS-using community. Gynecomastia in male AAS users occurs because supraphysiological androgens drive aromatization to estradiol, and the shift in androgen-to-estrogen ratio, combined with desensitized AR in breast tissue, produces glandular breast tissue proliferation. Additional adverse effects include acne vulgaris (from sebaceous gland AR stimulation), tendon rupture (from AAS-augmented muscle mass exceeding tendon tensile strength), psychiatric effects including mood elevation, aggression, impulsivity, and hypomania during use, and withdrawal-related dysphoria and hypogonadal symptoms during the off-cycle period.9
Normal testosterone is metabolized at the C17-beta hydroxyl by hepatic 17-hydroxysteroid dehydrogenases, converting it to androstenedione for conjugation and excretion. The C17-alpha methyl group in oral AAS sterically blocks this oxidation, prolonging intrahepatic residence and impairing bile acid transport, producing cholestasis. Over time, accumulated hepatocyte toxicity from repeated cholestatic episodes produces peliosis hepatis, and decades of use is associated with hepatocellular adenoma and carcinoma. Injectable AAS esters (e.g., nandrolone decanoate) undergo normal ester cleavage and hepatic metabolism without this problem. This is a structural chemistry-based toxicity distinction that explains why route of administration dramatically alters the hepatotoxicity profile of androgenic steroids.
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