ABSTRACT

A phosphodiesterase inhibitor is a drug that blocks one or more of the five subtypes of the enzyme phosphodiesterase (PDE) selectively in the brain. PDE3 inhibitors can be thought of as a backdoor approach to cardiac stimulation, whereas β-agonists go through the front door to produce the same cardiac effects. PDE3 was identified as a potential therapeutic target in cardiovascular disease and asthma, and indeed, PDE3 inhibitors have subsequently been shown to relax vascular and airway smooth muscle, inhibit platelet aggregation and induce lipolysis. It was recognised that papaverine and pentoxifylline mediated vasorelaxation by a number of mechanisms including non-selective PDE inhibition and these drugs can be considered as forerunners to the clinically successful PDE5 inhibitors used today for the treatment of erectile dysfunction. Phosphodiesterase inhibitors (PDIs) have important vascular and myocardial protective effects and thus have shown therapeutic usefulness in the clinical settings for treatment of patients with heart failure, pulmonary hypertension, and coronary artery disease. By increasing our understanding of the physiological roles of the individual PDE isoforms, in parallel with the development of even more selective inhibitors of these enzymes, it is highly likely that better therapeutically active drugs will emerge.

PHOSPHODIESTERASES

The homeostatic role of phosphodiesterases (PDEs) as related to the intracellular levels of cAMP and cGMP was first described by Sutherland (Sutherland, Rall, 1958) who, due to this, was awarded the Nobel Prize for Physiology and Medicine in 1971. Phosphodiesterases (PDEs) hydrolyze the phosphodiester bond of cAMP and cGMP to form the inactive 5′-AMP and 5′-GMP. In optimization of the intracellular levels of cAMP and cGMP, breakdown is predominant over synthesis. PDEs consists of a superfamily with 11 subfamilies, which have been characterized on the basis of amino acid sequence, substrate specificity, pharmacological properties and allosteric regulation. Within these families, more than 40 isoforms are expressed either by different genes or as expression of the same gene through alternative splicing (Francis et al. 2001). The substrate specificities include the enzymes which are specific for cAMP hydrolysis, those for cyclic GMP hydrolysis (Mehats et al., 2002) and those that hydrolyze both. The significance of PDEs as regulators of signaling is evident from their development as drug targets in diseases such as asthma and obstructive pulmonary disease, cardiovascular diseases such as heart failure and atherosclerotic peripheral disease, neurological disorders, erectile dysfunction. Table 1 summarizes the functions of each PDE and the cardiovascular effects of specific inhibitors.

PHOSPHODIESTERASE INHIBITORS
Phosphodiesterase inhibitors are the drugs which blocks one or more of the five subtypes of the enzyme phosphodiesterase (PDE) selectively in the brain (Weiss 1975; Fertel and Weiss, 1976) therefore preventing the inactivation of the intracellular second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) by the respective PDE subtype(s). The potential activity for selective phosphodiesterase inhibitors as therapeutic agents was predicted as early as 1977 by Weiss and Hait (Weiss and Hait, 1977).

CLASSIFICATION

Non-selective phosphodiesterase inhibitors
Methylated xanthines and derivatives: (Essayan, 2001)
• Caffeine, a minor stimulant.
• Aminophylline.
• IBMX (3-isobutyl-1-methylxanthine), used as investigative tool in pharmacological research.
• Paraxanthine.
• Pentoxifylline, a drug which has the potential to enhance circulation and may have applicability in treatment of diabetes, fibrotic disorders, peripheral nerve damage, and microvascular injuries (Deree et al., 2008).
• Theobromine.
• Theophylline, a bronchodilator.

Table 1.Functions of PDEs and cardiovascular effects of specific inhibitors. Click here to view full image

Methylated xanthines act as both:
1. Competitive nonselective phosphodiesterase inhibitors (Essayan, 2001) which raise intracellular cAMP, activate PKA, inhibit TNF-alpha (Deree et al., 2008, Marques et al., 1999) and leukotriene (Peters-Golden M et al., 2005) synthesis, and thereby reduce inflammation and innate immunity (Peters-Golden M et al., 2005) and
2. Nonselective adenosine receptor antagonists (Daly et al., 1987). But different analogues which show varying potency at the numerous subtypes, and a wide range of synthetic xanthine derivatives (some nonmethylated) have been developed in the search for compounds with greater selectivity for phosphodiesterase enzyme or adenosine receptor subtypes (MacCorquodale, 1929; Daly et al., 1986; Daly et al., 1987; Shamim et al., 1989; Daly et al., 1991; Ukena et al., 1993; Daly, 2000; Daly 2007; González et al., 2007; Baraldi et al., 2008).
PDE1 selective inhibitors
• Vinpocetine
PDE2 selective inhibitors
• EHNA (erythro-9-(2-hydroxy-3-nonyl)adenine).
• Anagrelide.
PDE3 selective inhibitors
• Enoximone and milrinone, used clinically for short-term treatment of cardiac failure. These drugs mimic sympathetic stimulation and thereby increase cardiac output.
PDE3 is sometimes referred to as cGMP-inhibited phosphodiesterase.
PDE4 selective inhibitors
• Mesembrine, an alkaloid from the herb Sceletium tortuosum.
• Rolipram, used as investigative tool in pharmacological research.
• Ibudilast, a neuroprotective and bronchodilator drug used mainly in the treatment of asthma and stroke. It inhibits PDE4 to a greatest extent, but also shows significant inhibition of other PDE subtypes, and so acts as a selective PDE4 inhibitor or a non-selective phosphodiesterase inhibitor, depending on the dose.
• Piclamilast, a more potent inhibitor than rolipram (de Visser et al., 2007).
• Luteolin, supplement extracted from peanuts which also possesses IGF-1 properties (Yu et al., 2010).
PDE5 selective inhibitors
Sildenafil, tadalafil, vardenafil, and the newer udenafil and avanafil selectively inhibit PDE5, which is a cGMP-specific and responsible for the degradation of cGMP in the corpus cavernosum. These phosphodiesterase inhibitors are used primarily as remedies for erectile dysfunction but also having some other medical applications such as treatment of pulmonary hypertension (Huang and Lie, 2013).

Figure 1.Diagram: Endothelial molecular pathways controlling NO activity and smooth muscle relaxation and site of action of PDE5 inhibitors. Inside the smooth muscle cell, NO binds to guanylyl cyclase which converts GTP into cGMP. cGMP then initiates a cascade of reactions in part mediated by protein kinase G (PKG) and subsequent K+ channel opening leading to reductions in intracellular Ca2+ concentration. Click here to view full image

Therapeutic efficacy of PDE inhibitors in myocardial protection
The therapeutic efficacy of PDE inhibitors in myocardial protection after ischemic preconditioning (IPC) is well documented (Murry CE, 1986 Wang Y et al., 2007). The fundamental concept of IPC is that a short period of ischemia and reperfusion delimits myocardial cellular damage during a subsequent event of prolonged lethal ischemia. The effects of IPC have been reproduced by using preconditioning mimetics like mitochondrial ATP-sensitive K+ (mitoKATP) channel openers (Wang Y et al., 2007). Unfortunately, the protective effects of IPC are thus short-lived. The search for pharmacological agents which can keep the heart in a sustainable state of protection will be a right step in the right direction. BMS-191095, a specific KATP channel opener, given at an interval of 20 h,thus kept the heart in protective state (Wang et al., 2007). The emerging concept of chronic intermittent preconditioning in which the heart can be constantly protected through chronic exposure to morphine (Peart, 2004) could be extended to the clinic by using promising pharmacological agents that can provide long-lasting protective effects to the heart before surgical procedures.
Phosphodiesterase inhibitors (PDIs) have important vascular and myocardial protective effects and thus have shown therapeutic usefulness in the clinical settings for treatment of patients with heart failure, pulmonary hypertension, and coronary artery disease (Boswell-Smith, 2006 Guazzi et al., 2007). PDIs selectively antagonize phosphodiesterase 5 (PDE5), which is present in high abundance in a variety of cells in humans (Kass et al., 2007) including canine and mouse cardiomyocytes (Das et al., 2006, Senzaki et al., 2001). The pharmacodynamics of PDIs in general and sildenafil in particular were consistent due to the tissue distribution profile of PDE-5 and its isoform PDE-3. PDIs prevent the breakdown of nitric oxide (NO)-driven cGMP, primarily in vascular smooth muscle cells, and thus act as potent vasodilators.
In vitro, PDIs have been shown to protect adult cardiomyocytes during ischemia-reperfusion (I/R) injury with a possible role for cGMP-dependent protein kinase-1 (Das et al., 2006). Sildenafil has also shown production of a marked preconditioning-like effect in the intact rabbit hearts (Ockaili et al., 2002). It appears that the protection incurred by sildenafil lasts only for a limited period. Kukreja and colleagues (Das et al., 2004) have also shown that the potent effect of sildenafil-induced cardioprotection was due to the early translocation of protein kinase C (PKC) from cytosol to the membrane (Das et al., 2004). These data provided direct evidence of an essential role of PKC in sildenafil-induced cardioprotection. Since one of the PDE-5 inhibitors, tadalafil, has long-lasting effects, hence tadalafil can be used for longer-term protective effects.

Systemic Circulation:
Effectiveness of PDE3 inhibitors for the treatment of congestive heart failure
PDE3 has a high affinity for cAMP but it can also hydrolyse cGMP. However, it hydrolyses cAMP ten times more efficiently than cGMP. Therefore, cGMP effectively acts as a competitive inhibitor for cAMP and consequently for PDE3 (Lugnier et al., 1983) As a result of its high expression in both the vasculature and the airways, PDE3 was identified as a potential therapeutic agent in cardiovascular disease and asthma, and subsequently vascular and airway smooth muscle relaxation, inhibit platelet aggregation (Barnes PJ, 1988) and induce lipolysis (Manganiello, 1995). However, the unequivocal effect of PDE3 inhibitors as positive inotropic agents provided a strong rationale for developing such drugs in the treatment of chronic heart disease (Nicholson, 1991). A number of PDE3 selective inhibitors, were also developed to treat patients with heart failure. However, chronic treatment with PDE3 selective inhibitors like milrinone was associated with increased risk of mortality and has consequently somewhat jaundiced the view of PDE3 as a drug target (Packer, 1991). Nonetheless, milrinone is still used in the acute treatment of heart failure, and cilostazol, another PDE3 inhibitor like Enoximone is used in the treatment of intermittent claudication.

Effectiveness of PDE4 inhibitors for the treatment of inflammatory airways disease
PDE4, formerly known as cAMP-PDE, is a cAMP-specific PDE and hence it is the predominant isoenzyme in a number of inflammatory cells except platelets, implicated in inflammatory airways disease. It is expressed in the airways smooth muscle, brain and cardiovascular tissues (Muller, 1996) and hence it is the largest PDE subfamily having over 35 different isoforms identified so far. The molecular structure, compartmentalisation and function of PDE4 have been extensively investigated (Houslay, 2001) and therefore PDE4 is the most widely characterised PDE isoenzyme. In the early 1970s, rolipram, a cAMP-PDE inhibitor, was earlier developed as a potential drug to treat depression because it was demonstrated that elevation of cAMP could enhance noradrenergic neurotransmission in the central nervous system. In addition, due to the worldwide succession of serotonin selective reuptake inhibitors in the treatment of depression, PDE4 inhibitors as a potential therapy got terminated. Nonetheless, there is a current resurgence underway in PDE4 inhibitors (Renau, 2004).
Effectiveness of PDE5 inhibitors for the treatment of erectile dysfunction
A variety of treatments have been historically used to treat erectile dysfunction which influence the cyclic nucleotide signalling pathway in vascular smooth muscle including PGE1, papaverine and pentoxifylline. It was recognised that papaverine and pentoxifylline mediated vasorelaxation including non-selective PDE inhibition, hence these drugs can be considered as forerunners to the clinically successful PDE5 inhibitors used today in the treatment of erectile dysfunction, although at the time, PDE5 inhibition as a mechanism to account for the actions of these particular drugs was not established (Allenby, 1991, Huang and Lie, 2013).

Table 2.Disease targets for isoenzyme selective PDE inhibitors. Click here to view full image

CONCLUSION
Non-selective PDE inhibitors including theophylline and papaverine have been used as therapeutic agents for over 70 years for a range of diseases. However, it is only in the last 10 years, that potent PDE selective drugs have been proved as effective therapeutic agents in the treatment of disease, and the worldwide success of sildenafil in the treatment of erectile dysfunction is evidence of the effect of such drugs. Selective PDE inhibitors investigated in a wide range of diseases (Table 2) including the use of PDE2 inhibitors in sepsis; PDE5 inhibitors in the treatment of sexual dysfunction in females, cardiovascular disease and pulmonary hypertension; and PDE4 inhibitors to treat asthma, COPD, allergic rhinitis, psoriasis, multiple sclerosis, depression, Alzheimer’s disease and schizophrenia. By investigating more on the physiological roles of the individual PDE isoforms, in parallel with the development of even more selective inhibitors of these enzymes, it is more likely that better therapeutically active drugs will emerge.

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