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Edex

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Edex

CLINICAL PHARMACOLOGY

Alprostadil (PGE1 ) is one of the prostaglandins, a family of naturally occurring acidic lipids with various pharmacological effects. Endogenous PGE1 is derived from dihomo-gamma-linolenic acid, a fatty acid found within the phospholipids of cellular membranes. As an endogenous substance, PGE1 exerts its biological effects either directly or indirectly by regulating and modifying the synthesis and effects of other hormones and mediators.

Mode of Action

Alprostadil is a smooth muscle relaxant. Precontracted isolated preparations of the human corpus cavernosum, corpus spongiosum and cavernous artery are relaxed by alprostadil. Alprostadil has been shown to bind to specific receptors in human penile tissue. Two types of receptors that differ in their PGE1 binding affinity have been identified. The binding of alprostadil to its receptors is accompanied by an increase in intracellular cAMP levels. Human cavernous smooth muscle cells respond to alprostadil by releasing intracellular calcium into the surrounding medium. Smooth muscle relaxation is associated with a reduction of the cytoplasmic free calcium concentration. Alprostadil also attenuates presynaptic noradrenaline release in the corpus cavernosum which is essential for the maintenance of a flaccid and non-erect penis.

Alprostadil induces erection by relaxation of trabecular smooth muscle and by dilation of cavernous 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

Alpha-Cyclodextrin

After reconstitution, PGE1 immediately dissociates from the α-cyclodextrin inclusion complex; the in vivodisposition of both components occurs independently after administration. After intravenous infusion of radiolabeled α-cyclodextrin to healthy volunteers, the radiolabeled components were rapidly eliminated within 24-hours, urine accounting for 81-83% of radioactivity and feces for 0.1%. There was no evidence of significant accumulation of radiolabeled α-cyclodextrin in the body even after 7 days of repeated intravenous injection. After intracavernous administration in monkeys, radiolabeled α-cyclodextrin was rapidly distributed from the injection site with less than 0.1% of the dose remaining in the penis 1 hour after administration. There was no evidence of tissue retention of radiolabeled α-cyclodextrin in monkeys.

Alprostadil Absorption: After intracavernous injection of 20 mcg of edex® (alprostadil for injection) in 24 patients with erectile dysfunction, mean systemic plasma concentrations of PGE1 increased from baseline of 0.8 ± 0.6 pg/mL to a peak (Cmax) of 16.8 ± 18.9 pg/mL (corrected for baseline) within 2 to 5 minutes and dropped to endogenous plasma levels within 2 hours (Table 1). The absolute bioavailability of alprostadil estimated from systemic exposure was about 98% as compared to the same dose given by a short-term intravenous infusion.

Distribution: The volume of distribution for PGE1 was not estimated. Approximately 93% of PGE1 found in plasma is protein-bound.

Metabolism: PGE1 is metabolized in the corpus cavernosum after intracavernous administration. PGE1 entering the systemic circulation is rapidly and extensively metabolized in the lungs with a first-pass pulmonary elimination of 60 to 90% of PGE1 . Enzymatic oxidation of the C15-hydroxy group followed by reduction of the C13, 14-double bond produces the primary metabolites, 15-keto-PGE1 , 15-keto-PGE0 , and PGE0 . 15-keto-PGE1 has only been detected in vitroin homogenized lung preparations, whereas 15-keto-PGE0 and PGE0 have been measured in plasma. Unlike the 15-keto metabolites which are less pharmacologically active than the parent compound, PGE0 is similar in potency to PGE1 in vitrousing isolated animal organs.

After intracavernous injection of 20 mcg of edex® (alprostadil for injection) to 24 patients with erectile dysfunction, mean systemic plasma 15-keto-PGE0 levels increased within 7 minutes from endogenous levels of 12.9 ± 11.8 pg/mL to a Cmax of 421 ± 337 pg/mL (corrected for baseline) followed by a decrease to baseline levels in several hours. Mean systemic plasma PGE0 levels increased within 20 minutes from endogenous levels of 0.6 ± 0.5 pg/mL to a Cmax of 3.9 ± 2.3 pg/mL (corrected for baseline) followed by a decrease to baseline levels in several hours.

Excretion: After further degradation of PGE1 by beta and omega oxidation, the main metabolites are excreted primarily in urine (88%) and feces (12%) over 72 hours, and total excretion is essentially complete (92%) within 24 hours after administration. No unchanged PGE1 has been found in the urine and there is no evidence of tissue retention of PGE1 and its metabolites. After intracavernous injection of 20 mcg of edex® (alprostadil for injection) in patients with erectile dysfunction, the terminal half-lives (t½) of 15-keto-PGE0 and PGE0 were calculated to be 40.9 ± 16.5 minutes and 63.2 ± 31.1 minutes, respectively. The terminal half-life of PGE1 in healthy volunteers was calculated to be around 9-11 minutes which is consistent with that reported in the literature (8 minutes).

Mean total body clearance of PGE1 in patients with erectile dysfunction was calculated to be around 115 L/min after an intravenous infusion of 20 mcg alprostadil. The above value exceeded cardiac output indicating extensive and rapid elimination of PGE1 in the lungs and/or blood.

Special Populations

Geriatric: The potential effect of age on the pharmacokinetics of alprostadil has not been formally evaluated.

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

Hepatic Insufficiency: In a study in symptomatic subjects with impaired hepatic function and age/weight/sex-matched healthy volunteers, 120 mcg of alprostadil was administered by intravenous infusion over 2 hours. The mean Cmax value of PGE1 in hepatically impaired patients was 96% higher than in healthy volunteers. Mean Cmax values of both 15-keto-PGE0 and PGE0 increased 65% as compared to those in healthy volunteers. The terminal half-lives of PGE1 , PGE0 , and 15-keto-PGE0 and plasma albumin levels were similar in patients compared to healthy volunteers. Due to the fact that PGE1 is primarily metabolized in the lung, the observed differences between hepatically impaired subjects and healthy volunteers were not anticipated; the mechanism responsible for the observed discrepancies is not known.

Renal Impairment: In a study in symptomatic subjects with end-stage renal disease undergoing hemodialysis and age/weight/sex-matched healthy volunteers, 120 mcg of alprostadil was administered by intravenous infusion over 2 hours. The mean Cmax value of PGE1 in renally impaired patients was 37% lower as compared to that in healthy volunteers whereas mean Cmax values of 15-keto-PGE0 and PGE0 in these patients increased 104% and 145%, respectively as compared to those in healthy volunteers. The terminal half-lives of PGE1 , PGE0 and 15-keto-PGE0 and plasma albumin levels were similar in these patients vs healthy volunteers. The mechanism responsible for the observed discrepancies between renally impaired subjects and healthy volunteers is not known.

Pulmonary Disease: The pulmonary extraction of alprostadil following intravascular administration was reduced by 15% (66 ± 3.6% vs 78 ± 2.3%) in patients with acute respiratory distress syndrome (ARDS) compared with a 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 pulmonary extraction efficiency of alprostadil ranged from subnormal (11%) to normal (90%), with an overall mean of 67%.

Drug-Drug Interactions: In clinical trials, concomitant use of agents such as antihypertensive drugs, diuretics, antidiabetic agents (including insulin), or nonsteroidal anti-inflammatory drugs had no apparent effect on the efficacy or safety of edex® (alprostadil for injection) .

Aspirin, Warfarin, Digoxin, Glyburide: Several drug-drug interaction studies have been conducted with alprostadil alone or in combination with aspirin, digoxin or warfarin in healthy volunteers and with glyburide in subjects with stable, non-insulin dependent diabetes mellitus. The pharmacokinetic profiles of aspirin, warfarin, digoxin, and glyburide were not affected by concomitant administration of alprostadil. There were no clinically important changes or trends in pharmacodynamic parameters for these drugs.

Heparin: The pharmacokinetic and pharmacodynamic interaction between alprostadil intravenous infusion, 90 mcg over 3 hours, and heparin (5,000 IU) was evaluated in 12 healthy volunteers. Alprostadil had a significant effect on the pharmacodynamics of heparin resulting in a 140% increase in partial thromboplastin time and a 120% increase in thrombin time. Therefore, caution should be exercised with concomitant administration of heparin and edex® (alprostadil for injection) .

TABLE 1

Study No. Participants Route and
Dose
Administration
Drug/Metabolites Cmax1[pg/mL] Tmax[min] AUC2
[pg•min/mL]
Total
Clearance3
[L/min]
4
[min]
PHAKI 848 Erectile Dysfunction Patients 20 mcg/0.5 hr IV PGE1 7.09 ± 3.12 25.5 ± 4.8 174 ± 101 115 -
15-keto-PGE0 471 ± 88 30.0 ± 1.2 13705 ± 2559 - 15.6 ± 5.6
PGE0 7.10 ± 2.19 32.2 ± 2.4 380 ± 115 - 39.8 ± 26.3
20 mcg/IC PGE1 16.8 ± 18.9 4.8 ± 3.3 173 ± 115 - -
15-keto-PGE0 421 ± 337 9.7 ± 7.7 10500 ± 4101 - 40.9 ± 16.5
PGE0 3.9 ± 2.3 20.3 ± 12.6 252 ± 134 - 63.2 ± 31.1
1 Baseline-corrected data.
2 AUC0-150 for IV infusion and AUC0-120 for IC injection.
3 Calculated as IV dose/AUC0-150 (IV).
4 Apparent terminal half-life.

Clinical Studies

In two studies [Protocol numbers KU-620-001 (Study 1) and KU-620-002 (Study 2)], the safety and efficacy of edex® (alprostadil for injection) were evaluated in 347 men with a diagnosis of erectile dysfunction due to vasculogenic, neurogenic and/or mixed etiology. Each study consisted of three phases: an in-office dose-titration phase, a two-week double-blind cross-over phase at home, and an open-label at home treatment phase that lasted for 12 months (Study 1) or six months (Study 2).

During the dose-titration phase, individualized optimum doses of edex® (alprostadil for injection) were established. Erectile response was measured by the Buckling Test to assess axial penile rigidity. A positive Buckling Test was achieved if the erect penis was able to support an axial load of 1.0 kg without buckling of the penile shaft. During the subsequent two-week double-blind, cross-over phase, patients self-injected edex® (alprostadil for injection) or placebo at home. Thereafter, patients continued to perform self-injections of open-label edex® (alprostadil for injection) for six or 12 months, and the occurrence of an erection sufficient for sexual intercourse was documented following each injection.

Results

Study 1: One hundred fourteen men with a mean age of 53 years (range 22 to 65 years) were enrolled in the first phase. The mean optimum dose was 13.8 mcg (range 1 to 20 mcg). Seventy-six percent (87/114) of patients had an erection with a positive penile Buckling Test. Among the 71% (81/114) of patients who entered the placebo-controlled phase, an erection sufficient for sexual intercourse was achieved in 74% (60/81) of patients following edex® (alprostadil for injection) injection compared to 7% (6/81) of patients following placebo injection. The mean duration of erection following edex® (alprostadil for injection) was 56.9 minutes compared to 4.0 minutes following placebo. Among the 65% (74/114) of patients who entered the open-label treatment phase, the mean rate of response with an erection sufficient for sexual intercourse was 88.9% through 12 months. The average dose of edex® (alprostadil for injection) remained essentially unchanged throughout the study duration.

Study 2: Two hundred thirty-three men with a mean age of 59.8 years (range 23 to 74 years) were enrolled in the first phase. The mean optimum dose was 25.9 mcg (range 1 to 40 mcg). Seventy-three percent (17½33) of patients had an erection with a positive penile Buckling Test. Among the 60% (14½33) of patients who entered the placebo-controlled phase, an erection sufficient for sexual intercourse was achieved in 73% (103/141) of patients following edex® (alprostadil for injection) injection compared to 13% (18/141) of patients following placebo injection. The mean duration of erection following edex® (alprostadil for injection) was 59.0 minutes compared to 7.6 minutes following placebo. Among the 60% (139/233) of patients who entered the open-label treatment phase, the mean rate of response with an erection sufficient for intercourse was 85.3% through six months. The average dose of edex® (alprostadil for injection) remained essentially unchanged throughout the study duration.

Last reviewed on RxList: 12/9/2008
This monograph has been modified to include the generic and brand name in many instances.

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