Absorption — Testosterone topical solution delivers physiologic circulating testosterone that approximate normal concentration range (i.e., 300 to 1050 ng/dL) seen in healthy men following application to the axilla.

On the skin, the ethanol and isopropyl alcohol evaporate leaving testosterone and octisalate. The skin acts as a reservoir from which testosterone is released into the systemic circulation over time (see Figure 1). In general, steady-state serum concentrations are achieved by approximately 14 days of daily dosing.

Figure 1: Mean (±SD) Serum Testosterone Concentrations on Day 7 in Patients Following Testosterone Topical Solution Once-Daily Application of 30 mg, 60 mg, or 90 mg of Testosterone

When testosterone topical solution treatment is discontinued after achieving steady-state, serum testosterone concentrations returned to their pretreatment concentrations by 7 to 10 days after the last application.

Distribution — Circulating testosterone is primarily bound in the serum to sex hormone-binding globulin (SHBG) and albumin. Approximately 40% of testosterone in plasma is bound to SHBG, 2% remains unbound (free) and the rest is bound to albumin and other proteins.

Metabolism —Testosterone is metabolized to various 17-keto steroids through two different pathways. The major active metabolites of testosterone are estradiol and dihydrotestosterone (DHT).

DHT concentration increased in parallel with testosterone concentration during testosterone topical solution treatment. The mean steady-state DHT/T ratio remained within normal limits and ranged from 0.17 to 0.26 across all doses on Days 15, 60, and 120.

Excretion — There is considerable variation in the half -life of testosterone as reported in the literature, ranging from 10 to 100 minutes. About 90% of a dose of testosterone given intramuscularly is excreted in the urine as glucuronic and sulfuric acid conjugates of testosterone and its metabolites; about 6% of a dose is excreted in the feces, mostly in the unconjugated form. Inactivation of testosterone occurs primarily in the liver.

Potential for testosterone transfer: The potential for testosterone transfer from males dosed with testosterone topical solution to healthy females was evaluated in a clinical study conducted with a 2% testosterone formulation. 10 males were treated with 60 mg of testosterone in each axilla (the maximum testosterone dose of 120 mg). At 2 hours after the application of testosterone topical solution to the males, the females rubbed their outer forearms for 15 minutes on the axilla of the males. The males had covered the application area with a T-shirt. Serum concentrations of testosterone were monitored in the female subjects for 72 hours after the transfer procedure. Study results show a 13% and 17% increase in testosterone exposure (AUC[0 24]) and maximum testosterone concentration (Cmax), respectively, compared to baseline in these females. In a prior clinical study conducted with a 1% testosterone formulation under similar study conditions, direct skin-to-skin transfer showed a 131% and 297% increase in testosterone exposure (AUC[0-72]) and maximum testosterone concentration (Cmax), respectively, compared to when men had covered the application area with a T-shirt.

In a clinical study conducted with a 2% testosterone formulation to evaluate the effect of washing on the residual amount of testosterone at the axilla, 10 healthy male subjects received 60 mg of testosterone to each axilla (the maximum testosterone dose of 120 mg). Following 5 minutes of drying time, the left axilla was wiped with alcohol towelettes which were assayed for testosterone content. Subjects were required to shower with soap and water 30 minutes after application. The right axilla was then wiped with alcohol towelettes which were assayed for testosterone content. A mean (SD) of 3.1 (2.8) mg of residual testosterone (i.e., 92.6% reduction compared to when axilla was not washed) was recovered after washing this area with soap and water. [see Dosage and Administration (2.2) and Warnings and Precautions (5.2)].

Use of deodorants and anti-perspirants: In a parallel designed clinical study evaluating the effect of deodorants and antiperspirants in healthy premenopausal females dosed with testosterone topical solution, each subject applied either a combined deodorant/antiperspirant spray (6 subjects) or stick (6 subjects) or a deodorant spray (6 subjects) to a single axilla 2 minutes before the application of 30 mg of testosterone to the same axilla. A control group of 6 subjects only applied 30 mg of testosterone to a single axilla. Blood samples were collected for 72 hours from all subjects following testosterone topical solution administration. Although a decrease of up to 33% of testosterone exposure (AUC[0-72]) was observed when antiperspirants or deodorants are used 2 minutes prior to testosterone topical solution application, underarm deodorant or antiperspirant spray or stick products may be used 2 minutes prior to testosterone topical solution application as part of normal, consistent, and daily routine. [see Dosage and Administration (2.2), and Patient Counseling Information (17.4)].

Effect of showering/washing: In a parallel designed clinical study to evaluate the effect of washing on the testosterone systemic exposure, two groups of 6 healthy premenopausal female subjects were each dosed with 30 mg of testosterone to a single axilla. The application sites of each group were washed with soap and water 2 hours or 6 hours after the application of testosterone topical solution. A control group of 6 female subjects applied 30 mg of testosterone to a single axilla and did not wash the application site. Blood samples were collected for 72 hours from all subjects following dosing with testosterone topical solution. A decrease of up to 35% of testosterone exposure (AUC[0-72]) was observed when applications sites were washed 2 hours and 6 hours after testosterone topical solution application. Patients should be advised to avoid swimming or washing the application site until 2 hours following application of testosterone topical solution. [see Dosage and Administration (2.2) and Patient Counseling Information (17.4)].