A phosphodiesterase inhibitor is a drug that blocks one or more of the five subtypes of the enzyme phosphodiesterase (PDE), thereby preventing the inactivation of the intracellular second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) by the respective PDE subtype(s). The ubiquitous presence of this enzyme means that non-specific inhibitors have a wide range of actions, the actions in the heart, and lungs being some of the first to find a therapeutic use.
A phosphodiesterase (PDE) is an enzyme that breaks a phosphodiester bond. Usually, phosphodiesterase refers to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below. However, there are many other families of phosphodiesterases, including phospholipases C and D, autotaxin, sphingomyelin phosphodiesterase, DNases, RNases, and restriction endonucleases, as well as numerous less-well-characterized small-molecule phosphodiesterases.
A phosphodiesterase type 5 inhibitor is a vasodilating drug which works by blocking the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vessels supplying various tissues. These drugs dilate the corpora cavernosa of the penis, facilitating erection with sexual stimulation, and are used in the treatment of erectile dysfunction (ED). Sildenafil was the first effective oral treatment available for ED. Because PDE5 is also present in the smooth muscle of the walls of the arterioles within the lungs, sildenafil and tadalafil are FDA-approved for the treatment of pulmonary hypertension. As of 2019, the wider cardiovascular benefits of PDE5 inhibitors are being appreciated.
cGMP-specific phosphodiesterase type 5 is an enzyme from the phosphodiesterase class. It is found in various tissues, most prominently the corpus cavernosum and the retina. It has also been recently discovered to play a vital role in the cardiovascular system.
Enoximone is an imidazole phosphodiesterase inhibitor. It is used in the treatment of congestive heart failure and is selective for phosphodiesterase 3.
EHNA is a potent adenosine deaminase inhibitor, which also acts as a phosphodiesterase inhibitor that selectively inhibits phosphodiesterase type 2 (PDE2).
PDE3 is a phosphodiesterase. The PDEs belong to at least eleven related gene families, which are different in their primary structure, substrate affinity, responses to effectors, and regulation mechanism. Most of the PDE families are composed of more than one gene. PDE3 is clinically significant because of its role in regulating heart muscle, vascular smooth muscle and platelet aggregation. PDE3 inhibitors have been developed as pharmaceuticals, but their use is limited by arrhythmic effects and they can increase mortality in some applications.
The PDE2 enzyme is one of 21 different phosphodiesterases (PDE) found in mammals. These different PDEs can be subdivided to 11 families. The different PDEs of the same family are functionally related despite the fact that their amino acid sequences show considerable divergence. The PDEs have different substrate specificities. Some are cAMP selective hydrolases, others are cGMP selective hydrolases and the rest can hydrolyse both cAMP and cGMP.
IBMX (3-isobutyl-1-methylxanthine), like other methylated xanthine derivatives, is both a:
- competitive non-selective phosphodiesterase inhibitor which raises intracellular cAMP, activates PKA, inhibits TNFα and leukotriene synthesis, and reduces inflammation and innate immunity, and
- nonselective adenosine receptor antagonist.
cAMP-specific 3',5'-cyclic phosphodiesterase 4D is an enzyme that in humans is encoded by the PDE4D gene.
cAMP-specific 3',5'-cyclic phosphodiesterase 4A is an enzyme that in humans is encoded by the PDE4A gene.
Cilomilast is a drug which was developed for the treatment of respiratory disorders such as asthma and chronic obstructive pulmonary disease (COPD). It is orally active and acts as a selective phosphodiesterase-4 inhibitor.
Mardepodect is a drug which was developed by Pfizer for the treatment of schizophrenia. It acts as a phosphodiesterase inhibitor selective for the PDE10A subtype. The PDE10A enzyme is expressed primarily in the brain, mostly in the striatum, nucleus accumbens and olfactory tubercle, and is thought to be particularly important in regulating the activity of dopamine-sensitive medium spiny neurons in the striatum which are known to be targets of conventional antipsychotic drugs. Older PDE10A inhibitors such as papaverine have been shown to produce antipsychotic effects in animal models, and more potent and selective PDE10A inhibitors are a current area of research for novel antipsychotic drugs which act through a different pathway to conventional dopamine or 5-HT2A antagonist drugs and may have a more favourable side effects profile. Mardepodect is currently one of the furthest advanced PDE10A inhibitors in development and has progressed through to Phase II clinical trials in humans. In 2017, development of mardepodect for the treatment of schizophrenia and Huntington's disease was discontinued.
BRL-50481 is a drug developed by GlaxoSmithKline which is the first compound that acts as a phosphodiesterase inhibitor selective for the PDE7 family. PDE7 activity is encoded by two genes, PDE7A and PDE7B. BRL-50481 actually shows about an 80-fold preference for the PDE7A subtype, for which it was developed, over PDE7B. BRL-50481 has been shown to increase mineralisation activity in osteoblasts, suggesting a potential role for PDE7 inhibitors in the treatment of osteoporosis.
A phosphodiesterase type 4 inhibitor, commonly referred to as a PDE4 inhibitor, is a drug used to block the degradative action of phosphodiesterase 4 (PDE4) on cyclic adenosine monophosphate (cAMP). It is a member of the larger family of PDE inhibitors. The PDE4 family of enzymes are the most prevalent PDE in immune cells. They are predominantly responsible for hydrolyzing cAMP within both immune cells and cells in the central nervous system.
Piclamilast, is a selective PDE4 inhibitor. It is comparable to other PDE4 inhibitors for its anti-inflammatory effects. It has been investigated for its applications to the treatment of conditions such as chronic obstructive pulmonary disease, bronchopulmonary dysplasia and asthma. It is a second generation compound that exhibits structural functionalities of the PDE4 inhibitors cilomilast and roflumilast. The structure for piclamilast was first elucidated in a 1995 European patent application. The earliest mention of the name "piclamilast" was used in a 1997 publication.
Zaprinast was an unsuccessful clinical drug candidate that was a precursor to the chemically related PDE5 inhibitors, such as sildenafil (Viagra), which successfully reached the market. It is a phosphodiesterase inhibitor, selective for the subtypes PDE5, PDE6, PDE9 and PDE11. IC50 values are 0.76, 0.15, 29.0, and 12.0 μM, respectively.
8-Phenyltheophylline (8-phenyl-1,3-dimethylxanthine, 8-PT) is a drug derived from the xanthine family which acts as a potent and selective antagonist for the adenosine receptors A1 and A2A, but unlike other xanthine derivatives has virtually no activity as a phosphodiesterase inhibitor. It has stimulant effects in animals with similar potency to caffeine. Coincidentally 8-phenyltheophylline has also been found to be a potent and selective inhibitor of the liver enzyme CYP1A2 which makes it likely to cause interactions with other drugs which are normally metabolised by CYP1A2.
Phosphodiesterases (PDEs) are a superfamily of enzymes. This superfamily is further classified into 11 families, PDE1 - PDE11, on the basis of regulatory properties, amino acid sequences, substrate specificities, pharmacological properties and tissue distribution. Their function is to degrade intracellular second messengers such as cyclic adenine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which leads to several biological processes like effect on intracellular calcium level by the Ca2+ pathway.
Atizoram (CP-80633) is a phosphodiesterase 4 inhibitor.
