Mechanism of action¹

About alprostadil (PGE₁)

Anatomy of a penis; showing the corpus spongiosum, corpus cavernosa and urethra

The active ingredient in MUSE is alprostadil, which is chemically identical to the naturally occurring eicosanoid, prostaglandin E₁ (PGE₁).

Prostaglandin E₁ is a naturally occurring acidic lipid that is synthesized from fatty acid precursors by most mammalian tissues and has a variety of pharmacologic effects. Human seminal fluid is a rich source of prostaglandins, including PGE₁ and PGE₂, and the total concentration of prostaglandins in ejaculate has been estimated to be approximately 100-200 mcg/mL.

In vitro, alprostadil (PGE₁) has been shown to cause dose-dependent smooth muscle relaxation in isolated corpus cavernosum and corpus spongiosum preparations. Additionally, vasodilation has been demonstrated in isolated cavernosal artery segments that were pre-contracted with either norepinephrine or prostaglandin F_2∝. When alprostadil was injected into the corpus cavernosum of pigtail monkeys in vivo, dose-dependent increases in cavernosal artery blood flow were observed.

Human study results

In human studies using Doppler duplex ultrasonography, intraurethral administration of 500 mcg of MUSE resulted in an increase in cavernosal artery diameter and a 5- to 10-fold increase in peak systolic flow velocities. These results suggest that intraurethral alprostadil is absorbed from the urethra, transported throughout the erectile bodies by communicating vessels between the corpus spongiosum and corpora cavernosa, and able to induce vasodilation of the targeted vascular beds.

Vasodilatory effects

The vasodilatory effects of alprostadil on the cavernosal arteries and the trabecular smooth muscle of the corpora cavernosa result in rapid arterial inflow and expansion of the lacunar spaces within the corpora. As the expanded corporal sinusoids are compressed against the tunica albuginea, venous outflow through subtunical vessels is impeded and penile rigidity develops. This process is referred to as the corporal veno-occlusive mechanism.

Notable systemic effects

The most notable systemic effects of alprostadil are vasodilation, inhibition of platelet aggregation, and stimulation of intestinal and uterine smooth muscle. Intravenous doses of 1 to 10 micrograms per kilogram of body weight lower blood pressure in mammals by decreasing peripheral resistance. Reflex increases in cardiac output and heart rate may accompany these effects. About 80% of alprostadil administered by MUSE is absorbed within 10 minutes and is rapidly cleared from the systemic circulation by the lungs, leaving barely detectable systemic levels.

Important Risk Information

CONTRAINDICATIONS

MUSE is contraindicated in men with any of the following:

Known hypersensitivity to alprostadil.

Abnormal penile anatomy: MUSE is contraindicated in patients with urethral stricture, balanitis (inflammation/infection of the glans of the penis), severe hypospadias and curvature, and in patients with acute or chronic urethritis.

Sickle cell anemia or trait, thrombocythemia, polycythemia, multiple myeloma: MUSE is contraindicated in patients who are prone to venous thrombosis or who have a hyperviscosity syndrome and are therefore at increased risk of priapism (rigid erection lasting 6 or more hours).

MUSE should not be used in men for whom sexual activity is inadvisable (see Precautions below).

MUSE should not be used for sexual intercourse with a pregnant woman unless the couple uses a condom barrier.

PRECAUTIONS

A complete medical history and physical examination should be undertaken to exclude reversible causes of erectile dysfunction prior to the initiation of MUSE therapy. In addition, underlying disorders that might preclude the use of MUSE (see Contraindications above) should be sought.

Cardiovascular effects: During in-clinic dosing, patients should be monitored for symptoms of hypotension, and the lowest effective dose of MUSE should be prescribed.

Hematologic effects: Patients administering MUSE improperly may be at risk of urethral abrasion resulting in minor bleeding or spotting. Patients on anticoagulant therapy or with bleeding disorders may be at higher risk of bleeding. Patients on anticoagulant therapy have been safely treated with MUSE; however, the risk/benefit ratio in these patients should be considered prior to prescribing MUSE.

Resumption of sexual activity: Sexual intercourse is considered a vigorous physical activity, and it increases heart rate as well as cardiac work. Physicians may want to examine the cardiac fitness of patients prior to treating erectile dysfunction.

Priapism and prolonged erection: In clinical trials of MUSE, priapism (rigid erection lasting ≥ 6 hours) and prolonged erection (rigid erection lasting 4 hours and < 6 hours) were reported infrequently (< 0.1% and 0.3% of patients, respectively). Nevertheless, these events are a potential risk of pharmacologic therapy and can cause penile injury. Physicians should lower the dose or consider discontinuing MUSE treatment in any patient who develops priapism or prolonged erection.

Drug-drug interactions: Because there are low or undetectable (< 2 picograms/mL) amounts of alprostadil found in the peripheral venous circulation following MUSE administration, systemic drug-drug interactions with MUSE are unlikely. Although formal studies have not been conducted, the concomitant use of MUSE and antihypertensive medications may increase the risk of hypotension. It is therefore advised that caution be used in the administration of MUSE to individuals on antihypertensive medications. In addition, the presence of medications in the circulation that attenuate erectile function may influence the response to MUSE.

Drug-device interactions: Use of MUSE in patients with penile implants has not been studied.

Sexual preference: There is no experience in homosexual men and no experience with other than vaginal intercourse.

WARNINGS: Because of the potential for symptomatic hypotension and syncope, which occurred in 3% and 0.4%, respectively, of patients during in-clinic dosing, MUSE titration should be carried out under medical supervision. During post-marketing surveillance, syncope occurring within 1 hour of administration has been reported. Patients should be cautioned to avoid activities, such as driving or hazardous tasks, where injury could result if hypotension or syncope were to occur after MUSE administration.

INFORMATION FOR PATIENTS: Patients should be informed that MUSE offers no protection from the transmission of sexually transmitted diseases. Patients and partners who use MUSE need to be counseled about the protective measures that are necessary to guard against the spread of sexually transmitted agents, including the human immunodeficiency virus (HIV).

Although unreported in clinical trials, there is the possibility that an overdosage of MUSE can cause priapism, a painful erection of the penis sustained for hours and unrelieved by sexual intercourse or masturbation. This condition is serious and, if untreated, it can lead to permanent inability to have an erection. Patients who experience a prolonged erection should seek prompt medical attention. Patients should be instructed how to administer MUSE. A patient package insert must be given to each patient at the initiation of MUSE therapy.

ADVERSE REACTIONS

Most frequently reported drug-related side effects include penile pain, urethral burning, urethral bleeding/spotting, testicular pain, hypotension and dizziness.

Mechanism of action1

About alprostadil (PGE1)