PDE5 selective inhibitors
Mechanism of action:
In response to sexual stimulation,
Nitric oxide released from nonadrenergic-noncholinergic neurotransmission and the
endothelium of the cavernous smooth muscle is probably the principal
neurotransmitter for penile erection.
Within the muscle, nitric oxide activates a guanylyl cyclase that raises intracellular
concentrations of cyclic guanosine monophosphate (GMP).
Cyclic GMP in turn activates a specific protein kinase which results in the opening of
the potassium channels and hyperpolarization and causes sequestration of intracellular
calcium and blocks calcium influx. As a result of this drop in cytosolic calcium,
smooth muscle relaxation occurs leading to erection.
 Activation of the NO-cGMP pathway results in trabecular smooth muscle
relaxation, leading to increased blood flow into the penis with pooling of
blood in sinuses; and increase in corpus cavernosum pressure, resulting in
penile erection.
 On return to the flaccid state, cyclic GMP is hydrolyzed to guanosine
monophosphate by phosphodiesterase type 5.
PDE-5 inhibitors do not increase the nitric oxide level, but they potentiate
the nitric oxide effect to stimulate erection. Without sexual arousal,
this effect activates the nerve-nitric oxide pathway, these inhibitors
are ineffective
Adverse reactions
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Indications
Oral Phosphodiesterase-5 Inhibitor therapy as medically necessary in
males for the treatment of erectile dysfunction when ANY of the
following criteria are met:
1- Age 60 or older
2- Hormonally-induced erectile dysfunction with EITHER of the
following:

Erectile dysfunction persists despite correction of an abnormal
testosterone, prolactin, or thyroid level

correction of the hormonal deficiency is contraindicated due to
comorbidity (e.g., a low testosterone in a man with prostate cancer)
3- Neurogenic erectile dysfunction such as resultant from spinal cord
injury, multiple sclerosis, pituitary microadenoma with
hyperprolactinemia, cerebral vascular accident (CVA), diabetes,
radical prostatectomy or surgically induced impotence
4- Vasculargenic erectile dysfunction such as resultant from aortic
aneurysm, atherosclerosis, hypertension, hyperlipidemia, or
peripheral vascular disease (PVD)
5- Pelvic trauma-induced erectile dysfunction such as resultant from
compression injuries or radiation.
6- Pharmacologic-induced erectile dysfunction where the patient has
tried ONE alternate, non-erectile dysfunction-causing medication and
erectile dysfunction persists, OR there is a contraindication to making
medication changes.
safety concerns
 This has been the area of primary concern due to reports of
myocardial infarction and death.
These concerns have however been ameliorated as studies have not
shown an increased rate of myocardial infarction, ischemic heart
disease or mortality.
BUT..Treatment of ED in a patient with cardiovascular disease is
complicated by a small increase in risk of myocardial infarction
related to sexual activity.
 In some cases, the sudden hearing loss was accompanied by
tinnitus and/or dizziness.
 Non-arteritic anterior ischemic optic neuropathy (NAION) has
been reported rarely in temporal relationship with the use of
PDE5I.
 rare reports of prolonged erections more than four hours and
priapism (painful erections more than six hours in duration) for
this class of compounds
PDE5 Inhibitors & Erection dysfunction(ED)
According to the 2006 American Urological Association (AUA) Guidelines:

PDE5 inhibitors are considered to be the first line of therapy in the
management of ED, unless contra-indicated.

the management of ED begins with the identification of organic
comorbidities and psychosexual dysfunctions; both should be
appropriately treated.

For patients with a definite endocrinopathy, endocrine therapy for
hypogonadism, hyperprolactinemia, and thyroid disorders is an
appropriate intervention.
Contraindications& drug interaction
 The PDE5I can potentiate the hypotensive effects of nitrates.
Therefore, PDE5I are contraindicated in patients who are using
any form of organic nitrate, either regularly and/or
intermittently because of the risk of severe hypotension. PDE5
inhibitors can potentiate the vasodilatation of nitrates and result
in potentially dangerous hypotension.
 Caution is advised when PDE5I are co-administered with alpha
blocking agents and/or anti-hypertensives because both have
vasodilatory effects and an additive effect on blood pressure
lowering may be anticipated.
 The longer half-life of tadalafil imposes increased risk for drug
interactions in patients taking potent cytochrome P450 (CYP450)
inhibitors e.g: erythromycin, ketoconazole, cimetidine or the
protease inhibitors.
Patients Failing to Respond to their Initial PDE5 Inhibitor

Patients who have failed to respond to a PDE5 inhibitor will not
necessarily fail to respond to other drugs in the class.

Patients should not be deemed true treatment failures until they
have failed to respond to maximum dose medication on at least
eight occasions.

Non-responding patients may derive benefit from a regular dosing
schedule, suggesting that in some patients reversal of ED may
result from more continuous exposure to PDE5 inhibitors.

Some patients also fail to respond to ED treatment for other
reasons, such as associated androgen deficiency. This may be
overcome by instituting treatment with androgen replacement
therapy
Recommendations for treatment optimisation with PDE5- inhibitors

Control medical and lifestyle risk factors.

Emphasize the need for sexual stimulation.

Titrate dose if lower doses are not effective.

Counsel patients on how to take the drug properly.

Explain that excessive use of alcohol should be avoided, as
this can have a negative impact on erectile function.

Manage expectations and inform patient that most patients
respond to treatment after one or two doses; some patients
may need seven or eight attempts before they are successful.
The first few attempts at intercourse may be emotionally
charged for both the patient and partner, especially if patients
have not been sexually active for a long time.

Discuss side effects.
Uses of PDE-5 inhibitors beyond ED
Premature ejaculation (PE):
The concomitant use of SSRIs and sildenafil may represent an
appropriate approach for PE.
The possible mechanism may be that an improved erection (firmness,
duration or both) resulting from the PDE-5 inhibitor provides
inhibition of ejaculation via down-regulation of receptors involved in
somatosensory latency times.
A reduction in performance anxiety may exist on a subconscious level.
Anorgasmia in women:
PDE5 is expressed in clitoral corpus cavernosum and in vaginal smooth
muscle and epithelium. Therefore it is possible that PDE5 inhibitors
could affect female sexual arousal disorder.
Increased levels of cGMP have been shown to occur in human-cultured
vaginal smooth muscle cells treated with a PDE5 inhibitor suggesting
involvement of the NO/cGMP axis in the female sexual response.
SSRI antidepressants commonly produce iatrogenic sexual dysfunction.
Sildenafil (100 mg) and vardenafil (10 mg) have been used to reverse
SSRI-induced anorgasmia in women.
Pulmonary hypertension
Pulmonary hypertension is the result of upregulation of PDE5 gene
expression, causing vasoconstriction in the lung.
PDE5 inhibitors are used as potent pulmonary vasodilators reducing
Pulmonary hypertension and inhibiting vascular remodelling.
Raynaud's phenomenon
Sildenafil has been shown to be effective in treating severe Raynaud's
phenomenon associated with systemic sclerosis and digital ulceration.
Heart failure
Sildenafil has shown promise in the treatment of congestive cardiac
failure.
It has also been shown to reduce aortic pressure through vasodilation,
reduced arterial stiffness and wave reflection and could be used in the
management of systemic hypertension.
Vascular disease
Sildenafil has been shown to significantly improve neurovascular
coupling without affecting overall cerebral blood flow by increasing
brain levels of cGMP. This data suggest that PDE5 inhibitors may
have a role in promoting recovery from stroke.
Irritable bowel syndrome (IBS)
NO is a major inhibitory neurotransmitter in gastrointestinal tract.
Sildenafil causes relaxation of smooth muscles in various organs by
increasing levels of NO.