Alprostadil: Mechanism of Action and Side Effects
Mechanism of Action
Alprostadil is a synthetic prostaglandin E1 that produces erections by relaxing smooth muscle in the corpus cavernosum and increasing cavernous artery diameter, while also causing systemic vasodilation through decreased peripheral resistance and inhibition of platelet aggregation. 1, 2
Local Penile Effects
- Smooth muscle relaxation in the corpus cavernosum is the primary mechanism for erectile function 1, 3
- Increased diameter of cavernous arteries enhances blood flow to erectile tissue 3
- Long-term use may increase baseline penile blood flow, though clinical significance remains unclear 3
Systemic Vascular Effects
- Vasodilation occurs through decreased peripheral resistance at doses of 1-10 mcg/kg IV 2
- Inhibition of platelet aggregation contributes to its anticoagulant properties 2
- Reflex increases in cardiac output and heart rate accompany blood pressure reduction 2
Pharmacokinetics
- Rapid metabolism: approximately 80% is metabolized in one pass through the lungs via β- and ω-oxidation 2
- Local metabolism occurs at the injection site before systemic circulation 3
- Metabolites are excreted primarily by the kidney within 24 hours 2
- No unchanged alprostadil appears in urine and no tissue retention occurs 2
Side Effects
Common Adverse Effects
Penile pain is the most frequent side effect, occurring in approximately one-third of patients overall but in only 11% of individual injections, causing 3-5% of patients to discontinue therapy. 4
Penile/Local Effects
- Penile pain: transient and usually mild, reported in 50% of men at some point during treatment 4
- Hematoma or ecchymosis: occurs in 8% of patients 4
- No fibrosis reported with intraurethral administration, distinguishing it from intracavernosal injection 5, 6
Systemic Effects
- Hypotension: occurs in approximately 3% of patients after the first dose 7
- Syncope: affects roughly 3% of patients, necessitating supervised first administration 7, 1
- No typical systemic effects observed with oral ED treatments 6
Serious Adverse Effects
Priapism (erection >4 hours) occurs in 1-4% of patients and requires immediate intervention to prevent irreversible corporal fibrosis and permanent erectile dysfunction. 7, 4
Priapism Risk Stratification
- Alprostadil monotherapy carries lower risk of ischemic priapism compared to papaverine/phentolamine combinations 5
- Prolonged erections occurred in 5% of men in clinical trials 4
- Delayed treatment beyond 36 hours causes corporal fibrosis and penile shortening 5
- Early intracavernosal phenylephrine within 4 hours typically reverses the condition 7
Penile Fibrosis
- Intracavernosal injection: reported incidence ranges from <1% to >20%, occurring in 2-8% in major trials 5, 4
- Intraurethral administration: fibrosis not listed among adverse effects 5, 6
- Fibrosis results from untreated ischemic priapism, not from alprostadil itself 5
Route-Specific Considerations
Intracavernosal Injection
- Penile pain in 50% of patients 4
- Priapism in 1% 4
- Fibrotic complications in 2% 4
- Hematoma/ecchymosis in 8% 4
Intraurethral Administration
- No priapism reported in clinical studies 6
- No fibrosis documented 6
- Discomfort possible in patients with pre-existing lower-limb varicosities 7
- Fast onset with good safety profile 6
Critical Safety Requirements
Mandatory Supervision
- First dose must be administered under direct healthcare supervision to monitor for syncope and prolonged erections 7, 1
- Dose titration required to establish minimal effective dose before home use 7, 1
- Vital signs monitoring essential, particularly in cardiovascular disease patients 7
Patient Education Mandates
- Emergency plan for prolonged erections must be established before prescribing 7
- Patients must report any erection lasting ≥4 hours immediately 7
- Proper injection technique training is mandatory to reduce complications 7
- Treatment must not exceed once per 24-hour period 7