AndroGel Mechanism of Action: Full Molecular Pathway From Skin to Androgen Receptor

What happens in the first minutes after the gel goes on

The 1% product is a hydroalcoholic gel. The testosterone gel 1% prescribing information states that it is applied to clean, dry, intact skin of the shoulders, upper arms and/or abdomen, once daily in the morning, and not to the genitals, chest, armpits, knees or back [3].

In a 227-man study of the 1% gel, the formulation dried within a few minutes when applied to the upper arms, shoulders and abdomen [2].

After application, testosterone moves from the gel through the skin into the bloodstream and reaches target tissues.

Absorption is partial. In the 227-man pharmacokinetic and efficacy study, about 9 to 14 percent of the testosterone applied was bioavailable, a 5 g dose (50 mg testosterone) delivered roughly 5 mg per day, and a 10 g dose (100 mg) delivered roughly 10 mg per day [2]. Serum testosterone rose into the normal adult male range on day 1 with the first application; the authors cite an earlier study in which steady state was reached 48 to 72 hours after the first application [2]. Repeated sampling over 180 days gave very similar total and free testosterone profiles on days 30, 90 and 180 [2].

Testosterone can transfer from treated skin to other people. The testosterone gel 1% label carries a boxed warning: virilization has been reported in children who were secondarily exposed, and children should avoid contact with unwashed or unclothed application sites [3]. The same label notes the gel is flammable until dry [3].

From skin to plasma: binding, distribution and clearance

Circulating testosterone is bound mainly to sex hormone-binding globulin (SHBG) and albumin: about 40% of plasma testosterone is bound to SHBG, 2% is unbound and the rest is bound to albumin and other proteins, according to the testosterone gel 1% label [3]. The Endocrine Society guideline recommends fasting morning total testosterone as the initial diagnostic test, a repeat measurement to confirm, and free or bioavailable testosterone measurement when total testosterone is near the lower limit of normal or when a condition alters sex hormone-binding globulin [7].

The label says inactivation of testosterone occurs primarily in the liver [3]. It also lists liver function tests among the laboratory tests to monitor periodically, along with serum testosterone, PSA, hemoglobin, hematocrit and lipids [3].

Two enzymes split the signal

Aromatase converts androgens to estrogens [1]. The testosterone gel 1% label names estradiol and dihydrotestosterone as the major active metabolites of testosterone and says DHT concentrations increased in parallel with testosterone during treatment, with a mean DHT/T ratio of 0.23 to 0.29 at 50 mg and 0.27 to 0.33 at 100 mg over 180 days [3]. Aromatase is expressed in Leydig cells and other male organs, estradiol is measurable in the blood of men, and estrogen receptor signaling in males acts on brain, adipose tissue, skeletal muscle, bone, and cardiovascular and immune tissue [1].

Estrogens stimulate breast tissue growth while androgens inhibit it, and most gynecomastia reflects deficient androgen action or excess estrogen action in breast tissue [8]. The testosterone gel 1% label lists gynecomastia among adverse reactions, reported in 1 percent of men at 50 mg, none at 75 mg and 3 percent at 100 mg in a 180-day controlled trial [3].

Inside the cell: how the androgen receptor turns hormone into gene activity

Free testosterone or DHT enters the target cell and binds the androgen receptor. In its hormone-free state the receptor sits in a large multiprotein complex with heat shock protein 90; hormone binding promotes dissociation from hsp90 and converts the receptor to its DNA-binding state [5].

Hormone binding prepares the androgen receptor to bind DNA, and the receptor then acts as a transcription factor, binding androgen response elements in the regulatory regions of target genes. Testosterone treatment has been associated with measurable changes in muscle strength, body composition and hematocrit in the men studied [9].

The feedback loop: LH, FSH and sperm production

Exogenous androgens may lead to azoospermia [3]. In a small study of 11 hypogonadal men given 200 mg of intramuscular testosterone cypionate, LH was suppressed after the first injection and stayed suppressed with continued dosing [4].

The Endocrine Society guideline recommends against starting testosterone therapy in men who plan to have children in the near term [7]. Recovery after stopping is possible but variable: in an analysis of 30 studies covering 1,549 healthy men on hormonal male contraception, the median time for sperm concentration to return to 20 million/mL was 3.4 months, with 67 percent recovered by 6 months and 90 percent by 12 months, and recovery was faster with shorter-acting testosterone preparations and shorter treatment [10]. In 66 men treated for infertility after testosterone use with high-dose hCG plus a selective estrogen receptor modulator, 70 percent reached a total motile count above 5 million within 12 months; 64.8 percent of azoospermic men did, compared with 91.7% of cryptozoospermic men, and older age and longer testosterone use predicted slower recovery [11].

What the androgen signal changes, and on what timeline

Different androgen-responsive tissues respond on different clocks. In a 180-day study of 227 hypogonadal men comparing 1% gel at 50 or 100 mg per day with a 5 mg per day testosterone patch, sexual function and mood improved most by day 30 and improved no further through 180 days; leg press strength rose 11 to 13 kg by day 90 in all groups; lean body mass rose 1.28 kg at 50 mg and 2.74 kg at 100 mg by day 90, with fat mass falling 0.90 kg and 1.05 kg respectively; hematocrit and hemoglobin rose as expected [9].

The Testosterone Trials randomized 790 men aged 65 and older with serum testosterone below 275 ng/dL to testosterone gel or placebo gel for one year. Sexual activity increased significantly (P<0.001), as did sexual desire and erectile function. The share of men who improved walking distance by at least 50 m was 20.5 percent on testosterone versus 12.6 percent on placebo (P=0.003) when all three trials were pooled, but the difference was not significant in the physical function trial alone; there was no benefit on the vitality scale, and mood and depressive symptoms improved slightly. Adverse event rates were similar, and the trial was too small to draw conclusions about risks [12].

Bone is the slowest responder. In 211 men from the same program, one year of testosterone gel increased spine trabecular volumetric bone mineral density by 7.5 percent versus 0.8 percent on placebo (treatment effect 6.8 percent, P<0.001), with smaller gains in estimated bone strength at the hip than at the spine [13].

Hematocrit: where the route shows up most clearly

The testosterone gel 1% label states that hematocrit is checked before treatment, re-evaluated 3 to 6 months after starting and then annually, and that therapy is stopped until hematocrit falls if it becomes elevated, because an increase in red cell mass may increase the risk of thromboembolic events [3]. Across 29 placebo-controlled trials in 3,393 men, testosterone gel raised mean hematocrit by 3.0 percent (95 percent CI 1.8 to 4.3) and intramuscular enanthate or cypionate by 4.0 percent (2.9 to 5.1) compared with placebo; the gel and intramuscular figures were not significantly different from each other, while intramuscular enanthate or cypionate rose significantly more than the patch [14]. In a retrospective single-center review of 178 men followed for 3 years, erythrocytosis (hematocrit above 50 percent) occurred in 12.8 percent of men on gels and 66.7 percent on injectables, although the injection group was younger and started with higher hematocrit and the study was not randomized [15]. A review reported that men on testosterone replacement had a 315 percent greater risk of erythrocytosis than controls, with short-acting injectables having the highest incidence and the link to thromboembolic events still inconclusive [16].

Gel versus injection: same receptor, different exposure curve

The receptor pathway is shared across routes, but the route changes the testosterone exposure curve. In 11 hypogonadal men, 200 mg of intramuscular testosterone cypionate produced a threefold rise in serum testosterone peaking on days 2 to 5 and declining back to baseline by days 13 to 14, with wide variation across the dosing interval [4]. The 1% gel instead produced stable average total and free testosterone concentrations through 180 days of daily application [2].

That stability has a practical side. In the same study, raising the dose from 50 to 75 mg per day did not significantly increase average serum testosterone, while lowering 100 mg to 75 mg brought most men into the normal range [2]. The testosterone gel 1% label allows daily doses of 50, 75 or 100 mg, adjusted on the basis of total serum testosterone [3].

In a 4-month ambulatory blood pressure study described in the testosterone gel 1% label, 169 men had usable recordings, and the gel raised mean 24-hour systolic and diastolic pressure by 1.9 and 1.3 mm Hg at week 16, and by 3.0 and 2.2 mm Hg in men already on antihypertensive treatment; the label does not recommend the gel for men with uncontrolled hypertension [3]. The large cardiovascular outcome trial, TRAVERSE, used the 1.62% gel in 5,246 men with cardiovascular disease or high risk: major adverse cardiovascular events occurred in 7.0 percent on testosterone and 7.3 percent on placebo, with a mean systolic rise of 1.0 mm Hg versus a 0.5 mm Hg fall on placebo at 36 months [6][3].