Alprostadil has a wide variety of pharmacological actions; vasodila- tion and inhibition of platelet aggregat ion 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 PGF2α 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.
Distribution: Following intracavernosal injection of 20 micrograms alprostadil, mean peripheral plasma concentrations of alprostadil at 30 and 60 minutes after injection (89 and 102 picograms/ millili ter, respect ively) were not significantly greater than baseline levels of endogenous alprostadil (96 picograms/milliliter). Alprostadil is bound in plasma primarily to albumin (81% bound) and to a lesser extent α-globulin IV-4 fraction (55% bound). No significant 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 circulat ion following intracavernosal injection is very rapidly metabolized. Following intracavernosal injection of 20 micrograms 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 predose 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 malformat ions. 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/milliliter. The wide range of alprostadil plasma concentrations in neonates reflects high variability in individual clearances of alprostadil in this patient population.
Gender: The potential influence 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 influenced by gender.
Race: The potential influence of race on the pharmacokinetics of alprostadil has not been formally evaluated.
Renal and Hepatic Insufficiency: The pharmacokinetics of alprostadil have not been formally examined in patients with renal or hepat ic insufficiency.
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 efficiency of alprostadil ranged from subnormal (11%) to normal (90%), with an overall mean of 67%.
The potential for pharmacokinetic drug-drug interactions between alprostadil and other agents has not been formally studied.
Last reviewed on RxList: 3/3/2008
This monograph has been modified to include the generic and brand name in many instances.
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