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Shalini Prabhakar
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PHOSPHODIESTERASE INHIBITORS - A SHORT NOTE

24 September 2021

Phosphodiesterase (PDE) is an enzyme which regulate the degradation of secondary messengers include c-AMP and c-GMP in the cell. The production of secondary messenger is regulate by Nitric Oxide (NO). There are 11 families of PDE wherein PDE-5, PDE-4, and PDE-3 are the targets of most drugs.

Phosphodiesterase inhibitors exert their action in Heart ,Lungs and Genitals.

Example of drugs inhibiting the PDE .

PDE-5 inhibitor - Sildenafil, tadalafil and vardenafil.

PDE-4 inhibitor - Apremilast, roflumilast

PDE-3 inhibitor - cilostazol, milrinone

Non selective PDE inhibitor – theophyilline, methyxanthanine

 

HOW IT ACTS?

The drug which inhibit the enzyme thereby inhibiting the degradation. Increased available of c-AMP & c-GMP inside the cell cause vasodilation activity of smooth vessels.

PDE-5 inhibitor class of drugs are used in the treatment erectile dysfunction and pulmonary hypertension by targeting the penis and lungs respectively.

In erectile dysfunction, it mainly inhibit the degradation of c-GMP, decrease the level of ca2+ ion inside the cell, cause vasodilation of smooth muscles and increase the blood flow to the penis leading to erection.

In pulmonary hypertension where it trigger vasodilation of blood vessels and regulate the pressure.

The action to treat pulmonary hypertension is seen when used as lower dose compared to treat erectile dysfunction.

 

PDE-4 inhibitor targets the lungs, brain , skin and immune. It increases the c-AMP levels causing smooth muscle relaxation the airway therefore use in treatment of COPD and asthma. In addition it is used in treatment of inflammatory condition that affect the skin and other tissues. Researches regarding their use in conditions such as depression and anxiety are being conducted.

PDE-3 mainly target the heart, it works by increasing the c-AMP leads to the cardiac contractility, relax smooth muscles and makes them to be useful in heart failure condition in addition they can also help to inhibit the platelet aggregation hence they can also be used in myocardial infarction.

In case of non specific phosphodiesterase inhibitors like theophylline there will be dilation of bronchioles of lungs,with makes them an ideal candidate for COPD and Asthma.

 

Is Sildenafil a competitive inhibitor?

Yes sildenafil is a competitive inhibitor of the enzyme phosphodiesterase which prevents the breakdown of cGMP , They are considered reversible because they bind with PDE for a limited period of time , after which we can see a reverse of action.

 

Is Coffee a PDE inhibitor ?

In fact caffeine in Coffee is a PDE inhibitor it has shown effect by increasing the level of cAMP which results in smooth muscle relaxation .

 

Then what about Theophylline ?

Theophylline is a methylxanthine coming under the class non phosphodiesterase inhibitor which is an ideal candidate for pulmonary diseases.

 

What is Priapism ?

Erection lasting for more than 4 hours is termed as priapism.Priapism requires immediate medical intervention,Terbutaline and pseudoephedrine are the preferred first line agent . If erection persist even after that intra cavernous phenylephrine or needle aspiration of blood from the penis may be needed.

 

What are the ADR associated with Phosphodiesterase inhibitors?

  • Head ache

  • Nasal congestion

  • Dyspepsia

  • Flushing

  • Nausea

  • Diarrhea

  • Ventricular arrhythmias

  • Hypotension

  • Upset stomach

  • Restlessness

 

 

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