Phosphodiesterase 5 inhibitor

Question 1

What are common indications?

Answer 1

Erectile dysfunction
Primary pulmonary hypertension

Question 2

What is its mechanism of action to achieve an erection?

Answer 2

Nitric oxide is the key local mediator in initiation and maintenance of an erection.

NO activates soluble guanylyl cyclase which produces cGMP, which leads vasodilation.

PDE-inhibitors works by preventing the degrading cGMP to GMP, increasing the likelihood of forming an erection and prolonging its duration.

Question 3

What are important side effects?

Answer 3

Headache is most common
flushing
dizziness
nasal congestion
hypotension-uncommon
arrhythmias-uncommon
priapism

Question 4

What group of people should be prescribed with caution?

Answer 4

Recent stroke
Recent ACS
Recent significant cardiovascular
Patients taking other vasodilators; alpha blockers and CCB

Avoided or used at lower dose in:
hepatic or renal impairment due to reduced metabolism and excretion

Question 5

What are important interactions?

Answer 5

-Any drug that increases NO concentration ;nitrates Nicorandil (their combined use can cause marked arterial vasodilation and cardiovascular collapse)

• Plasma concentrations and side effects are enhanced by CYP450 enzymes

Question 6

What are dosages for both indications?

Answer 6

ED-50mg slidenafil (1 hour before administration)
Pulmonary hypertension-
20mg TDS

Question 7

Why may tadalafil be preferred over sildenafil for ED?

Answer 7

Tadalafil has a longer duration of action (36 hours) sildenafil had 6 hours so timing of administration is less important.

The absorption of oral sildenafil and onset effect is delayed if taken with food.

Question 8

What recreational drug should be avoided along side PDEi adminstration?

Answer 8

Amyl nitrite: poppers
combination can cause sudden and severe drop in BP