Alprostadil has a wide variety of pharmacological actions; vasodilation and inhibition of platelet aggregation are among the most notable of these effects. In most animal species tested, alprostadil relaxed retractor penis and corpus cavernosum urethrae in vitro. Alprostadil also relaxed isolated preparations of human corpus cavernosum and spongiosum, as well as cavernous arterial segments contracted by either noradrenaline or PGF_2a in vitro. In pigtail monkeys (Macaca nemestrina), alprostadil increased cavernous arterial blood flow in vivo. The degree and duration of cavernous smooth muscle relaxation in this animal model was dose-dependent.

Alprostadil induces erection by relaxation of trabecular smooth muscle and by dilation of cavernosal arteries. This leads to expansion of lacunar spaces and entrapment of blood by compressing the venules against the tunica albuginea, a process referred to as the corporal veno-occlusive mechanism.

Pharmacokinetics

Absorption: For the treatment of erectile dysfunction, alprostadil is administered by injection into the corpora cavernosa. The absolute bioavailability of alprostadil has not been determined.

Distribution: Following intracavernosal injection of 20 mcg alprostadil, mean peripheral plasma concentrations of alprostadil at 30 and 60 minutes after injection (89 and 102 picograms/mL, respectively) were not signifi cantly greater than baseline levels of endogenous alprostadil (96 picograms/mL). Plasma levels of alprostadil were measured using a radioimmunoassay method. Alprostadil is bound in plasma primarily to albumin (81% bound) and to a lesser extent I-globulin IV-4 fraction (55% bound). No signifi cant binding to erythrocytes or white blood cells was observed.

Metabolism: Alprostadil is rapidly converted to compounds, which are further metabolized prior to excretion. Following intravenous administration, approximately 80% of circulating alprostadil is metabolized in one pass through the lungs, primarily by beta- and omega-oxidation. Hence, any alprostadil entering the systemic circulation following intracavernosal injection is very rapidly metabolized. Following intracavernosal injection of 20 mcg alprostadil, peripheral levels of the major circulating metabolite, 13,14-dihydro-15-oxo-PGE₁, increased to reach a peak 30 minutes after injection and returned to pre-dose levels by 60 minutes after injection.

Excretion: The metabolites of alprostadil are excreted primarily by the kidney, with almost 90% of an administered intravenous dose excreted in urine within 24 hours post-dose. The remainder of the dose is excreted in the feces. There is no evidence of tissue retention of alprostadil or its metabolites following intravenous administration.

Pharmacokinetics in Special Populations

Geriatric: The potential effect of age on the pharmacokinetics of alprostadil has not been formally evaluated. In patients with acute respiratory distress syndrome (ARDS), the mean (± SD) pulmonary extraction of alprostadil was 72% ± 15% in 11 elderly patients aged 65 years or older (mean, 71 ± 6 years) and 65% ± 20% in 6 young patients aged 35 years or younger (mean, 28 ± 5 years).

Pediatric: Alprostadil plasma concentrations were measured in 10 neonates (gestational age of 34 weeks in 2 infants and 38 to 40 weeks in 8 infants) receiving steady-state intravenous infusions of alprostadil to treat underlying cardiac malformations. Infusion rates of alprostadil ranged from 5 to 50 (median, 45) nanograms/kilogram/minute, resulting in alprostadil plasma concentrations ranging between 22 and 530 (median, 56) picograms/mL. The wide range of alprostadil plasma concentrations in neonates refl ects high variability in individual clearances of alprostadil in this patient population.

Gender: The potential infl uence of gender on the pharmacokinetics of alprostadil has not been formally studied in healthy subjects. Two studies determined the pulmonary extraction of alprostadil following intravascular administration in 23 patients with ARDS. The mean (± SD) pulmonary extraction was 66% ± 20% in 17 male patients and 69% ± 18% in 6 female patients, suggesting that the pharmacokinetics of alprostadil are not infl uenced by gender.

Race: The potential influence of race in the pharmacokinetics of alprostadil has not been formally evaluated.

Renal and Hepatic Insufficiency: The pharmacokinetics of alprostadil have not been formally studied in patients with renal or hepatic insuffi ciency.

Pulmonary Disease: The pulmonary extraction of alprostadil following intravascular administration was reduced by 15% (66 ± 3.2% vs. 78 ± 2.4%) in patients with ARDS compared with a control group of patients with normal respiratory function who were undergoing cardiopulmonary bypass surgery. Pulmonary clearance was found to vary as a function of cardiac output and pulmonary intrinsic clearance in a group of 14 patients with ARDS or at risk of developing ARDS following trauma or sepsis. In this study, the extraction effi ciency of alprostadil ranged from subnormal (11%) to normal (90%), with an overall mean of 67%.

Drug-Drug Interactions: The potential for pharmacokinetic drug-drug interactions between alprostadil and other agents has not been formally studied.

Clinical Studies

The safety and effi cacy of CAVERJECT Sterile Powder was investigated in men with a diagnosis of erectile dysfunction due to psychogenic, vasculogenic, neurogenic, and/or mixed etiology in two well-controlled studies (Study 1 and Study 2) and in one 6-month open-label study (Study 3).

Study 1: One hundred fi fty-three men with a mean age of 53 years (range 23-69 years) were enrolled. The study had three phases: a 2.5 week double-blind, in-office randomized crossover phase in which each man received placebo or 2.5 mcg, 5 mcg, 7.5 mcg, or 10 mcg of CAVERJECT Sterile Powder; a 2 week open-label, in-offi ce dose-titration phase to identify the optimum home-use dose (the latter dose was defi ned as a dose inducing an erection suffi cient for penetration and lasting < D60 minutes); and a 4-week open-label, self-injection phase. In the double-blind phase, each dose of CAVERJECT was signifi cantly more effective than placebo by clinical evaluation ("full penile rigidity") and by RigiScan criteria (T70% rigidity for at least 10 minutes); there was no response to placebo. The percentage of responders increased with increasing doses of CAVERJECT. The overall response in the dose-ranging phases was 76% (117/153) by clinical evaluation and 51% (78/152) by RigiScan criteria. The optimum dose for self-injection ranged from 1.25 to 65 mcg (median 20 mcg). Seventy-three percent of the injections in 102 men who self-injected CAVERJECT resulted in satisfactory intercourse. Seventy-fi ve percent of the patients remained on the dose identifi ed during the dose-ranging phase; 17% and 8% of the patients slightly decreased or increased the dose, respectively. The mean duration of erection per injection was 70.8 minutes.

Study 2: Two hundred ninety-six men with a mean age of 53.8 years (range 21-74 years) were enrolled in this parallel-design, double-blind study. The men were randomly assigned to one of fi ve groups and received either a single dose of placebo, 2.5 mcg, 5 mcg, 10 mcg, or 20 mcg of CAVERJECT Sterile Powder. No patient responded to placebo. The differences in the response rates in both the clinical and the RigiScan evaluations between each of the doses of CAVERJECT and placebo were statistically signifi cant. There was also a statistically signifi cant dose- response relationship with higher clinical response rates and higher RigiScan response rates with increasing doses of CAVERJECT (with exception of the 10-mcg dose). The mean duration of erection after injection ranged from 12 minutes after the 2.5-mcg dose to 44 minutes after the 20-mcg dose and the relationship was linear (p = .025, linear regression analysis).

Study 3: The safety and effi cacy of CAVERJECT Sterile Powder was evaluated in a 6-month, open-label study in 683 men with a mean age of 58 years (range 20-79 years). The optimum dose of CAVERJECT was established by titration in 89% of men (606/683). Four hundred seventy-one men (69%) completed the 6-month study. At the start of the study, the mean dose was 17.7 mcg of CAVERJECT and at the end of the study it was 20.7 mcg. Eighty-seven percent of the 13,762 injections of CAVERJECT, administered by self-injection by the men in the study, resulted in satisfactory sexual activity. The mean duration of erection was 67.5 minutes.

The formulation of alprostadil contained in CAVERJECT IMPULSE (alprostadil dual chamber system for injection) includes the inactive excipient alpha cyclodextrin. This formulation was compared with CAVERJECT Sterile Powder in 87 men in a single-blind, crossover study designed to evaluate effi cacy and safety. The doses used by the patients in the study ranged from 2.5 mcg to 20 mcg and were the same for both formulations. The effi cacy of the two formulations was shown to be comparable, as assessed by the 30-point erectile function (EF) domain score from the International Index of Erectile Function (IIEF) and by a physician-assessment score for erectile response. The mean EF domain scores for CAVERJECT Sterile Powder and the formulation contained in CAVERJECT IMPULSE (alprostadil dual chamber system for injection) were 26.6 (SD=5.3) and 27.6 (SD=3.8), respectively. The mean physician's assessment scores for CAVERJECT Sterile Powder and the formulation contained in CAVERJECT IMPULSE (alprostadil dual chamber system for injection) were 2.6 (SD=0.6) and 2.7 (SD=0.5), respectively, based on a scale of 0 (no tumescence) to 3 (full rigidity).

Last reviewed on RxList: 3/5/2008
This monograph has been modified to include the generic and brand name in many instances.