Current Pharmaceutical Design - Volume 21, Issue 3, 2015

Phosphodiesterase 2 (PDE2) is a ubiquitous enzyme whose major role is to hydrolyze the important second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). In the central nervous system, pharmacological inhibition of PDE2 results in boosted cAMP and/or cGMP signaling, which is responsible for series of changes in protein expression relevant to psychiatric and learning and memory disorders, such as depression, anxiety, and cognition deficits in Alzheimer’s disease. In the periphery, inhibition of PDE2 exhibits beneficial effects in the diseased cardiovascular system, the respiratory system, skeletal muscles and Candida albicans-caused systemic infections. Even though blood-brain barrier penetration properties and selectivity of currently available PDE2 inhibitors have hindered them from entering clinical trials, PDE2 is still of great potential therapeutic values in different categories of diseases, and there is demand for development of new generation drugs targeting PDE2 for treatment of diseases in central nervous and peripheral systems.

Phosphodiesterases (PDEs) are a super family of 11 enzyme families responsible for the hydrolysis of the intracellular secondary messengers cyclic AMP (cAMP) and cyclic GMP (cGMP). PDE4, in particular, is highly expressed in brain regions involved with regulation of memory, anxiety, and depression, including the hippocampus, amygdala, and nucleus accumbens. Senescence has been shown to result in extreme dysregulation of the cAMP pathway in various brain regions. Thus, as a critical controller of intracellular cAMP levels, PDE4 may be a potential target for the treatment of senescence-related cognitive disorders, which could be pathological and/or non-pathological in origin. While there is great potential in the development of novel PDE4 inhibitors for treatment of senescentcognition impairment, there are also currently many pitfalls that need to be overcome. PDE4 has four subfamilies (PDE4A, B, C, and D) that are differentially expressed throughout the brain and body, as well as at least 25 splice variants derived from alternative splicing and multiple promoter sites. PDE4 subtypes have been shown to have differential effects on behavior, and cAMP itself has also been shown to play a contrasting role in behavior in different brain regions. This review will focus on what is currently understood about PDE4 in aging, the potential for PDE4 modulation as a cognitive therapy, and current pitfalls and limitations that need to be overcome in the PDE4 field. Overall, furthering our understanding of this incredibly complex pathway may one day assist with the development of novel therapeutics for both pathological and non-pathological cognitive disorders associated with senescence.

Phosphodiesterases (PDEs) are the only known enzymes to degrade intracellular cyclic AMP and/or cyclic GMP. The PDE superfamily consists of 11 families (PDE1– PDE11), each of which has 1 to 4 subtypes. Some of the subtypes may have multiple splice variants (e.g. PDE4D1–PDE4D11), leading to a total of more than 100 known proteins to date. Growing attention has been paid to the potential of PDEs as therapeutic targets for mood disorders and/or diseases affecting cognitive activity by controlling the rate of hydrolysis of the two aforementioned second messengers in recent years. The loss of cognitive functions is one of the major complaints most patients with CNS diseases face; it has an even more prominent negative impact on the quality of daily life. Cognitive dysfunction is usually a prognosis in patients suffering from neuropsychiatric and neurodegenerative diseases, including depression, schizophrenia, and Alzheimer’s disease. This review will focus on the contributions of PDEs to the interface between cognitive deficits and neuropsychiatric and neurodegenerative disorders. It is expected to make for the understanding and discovery that selective PDE inhibitors have the therapeutic potential for cognitive dysfunctions associated with neuropsychiatric and neurodegenerative disorders.

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Since both cAMP and cGMP signaling are essential in a variety of cellular functions, including neuroplasticity and neuroprotection, PDE-Is are receiving increased attention as possible targets for treatment of age-related cognitive decline as well as Alzheimer’s disease (AD). In this review we will give a translational overview of the preclinical and clinical data on PDE-Is and cognition enhancement focusing on aging and AD. PDE2, 4 and 5 inhibitors improved memory performance in both aged animals and models of AD. Treatment with a PDE3-I or PDE7-I has not been tested in aged animals yet, but in mouse models of AD both PDE-Is improved memory performance. Unfortunately, there are no peer-reviewed studies on the effects of PDE-I treatment in aged human subjects except the possible positive effect on memory impairment of the PDE1-I vinpocetine. Three other types of PDE-Is have been tested on cognition in mild to moderate AD patients: the PDE3-I cilostazol is being tested as a co-treatment to the acetylcholinesterase inhibitor donepezil, but with inconsistent results; the PDE4-I MK-0952 has been tested, although the outcome has not been disclosed yet; and the PDE9-I PF- 04447943 was reported to have no effects on cognition. Obviously, the demonstration of clinical proof of concept for cognition enhancing effects of PDE-Is and the generation of isoform selective PDE-Is are the final hurdles to overcome in developing safe and efficacious novel PDE-Is for the treatment of age-associated cognitive decline or AD.

Developing therapeutics for traumatic brain injury remains a challenge for all stages of recovery. The pathological features of traumatic brain injury are diverse, and it remains an obstacle to be able to target the wide range of pathologies that vary between traumatic brain injured patients and that evolve during recovery. One promising therapeutic avenue is to target the second messengers cAMP and cGMP with phosphodiesterase inhibitors due to their broad effects within the nervous system. Phosphodiesterase inhibitors have the capability to target different injury mechanisms throughout the time course of recovery after brain injury. Inflammation and neuronal death are early targets of phosphodiesterase inhibitors, and synaptic dysfunction and circuitry remodeling are late potential targets of phosphodiesterase inhibitors. This review will discuss how signaling through cyclic nucleotides contributes to the pathology of traumatic brain injury in the acute and chronic stages of recovery. We will review our current knowledge of the successes and challenges of using phosphodiesterase inhibitors for the treatment of traumatic brain injury and conclude with important considerations in developing phosphodiesterase inhibitors as therapeutics for brain trauma.

Stroke is the fourth leading cause of mortality and neurological disability. It is caused by an intricate interplay of environmental and genetic factors. Genes not only influence susceptibility to stroke but have also been found to alter the response to pharmacological agents and may also influence the clinical outcome of the disease. Current treatment strategies for stroke include tissue plasminogen activator, antiplatelet agents and lipid lowering drugs. These act via diverse mechanisms of actions and are centered around the management of modifiable risk factors to prevent the recurrent stroke events. However, a significant number of patients experience poor clinical outcome due to recurrent stroke events and drug induced adverse reactions. Therefore, accurate risk management and targeted prevention strategies remain yet to be explored at the level of individual patients with stroke. Pharmacogenetic based research studies have identified the relation between genetic factors and inter-individual variability towards drug treatment. Several single nucleotide polymorphisms in genes encoding for metabolizers, transporters and target receptors have been reported to influence the pharmacokinetics and pharmacodynamics of drugs used in the treatment of stroke. Many candidate gene studies have investigated the role of genetic variants in association with altered drug response in stroke treatment. However, these results are limited to clinical trials and should be replicated in Genome Wide Association (GWAS) Studies. In addition to this long term follow up prospective studies would be helpful in predicting drug induced risk/benefit ratio. Pharmacogenetic studies will reveal the correlation between variation in drug responses on the basis of the individual’s genomic profile better known as Personalized or Individualized Medicines. This will also optimize risk assessment and will stratify the population requiring careful attention before prescribing a particular medicine to achieve maximum therapeutic benefit. Moreover, this will help in designing the novel therapeutic agents with a targeted approach. In this concern, the Genomics and Randomized Trials Network (GARNET) has been created, which is a Pharmacogenomics Consortium aimed to identify genetic variants affecting an individual's response to treatment with the help of advanced technology. This review will address the major issues of therapeutic failures concerned with existing drugs used in the treatment of stroke and the need for exploring new and targeted therapeutic strategies based on pharmacogenetics.

The harmful effects caused by misuse of psychoactive substances have raised both medical and social problems. Substance dependence is a chronic relapsing disorder, which appears to involve neuroadaptive changes in cellular signaling and downstream gene expression. The unchanged consumption of present substances and increasing demand for new psychostimulants make the development of novel management/treatment strategies challenging. Emerging evidence has shown that the cyclic AMP (cAMP) signaling cascade plays a critical role in the initiation and development of dependence. Thus, phosphodiesterase 4 (PDE4), the primary hydrolytic enzyme for intracellular cAMP, is considered a potential target for future therapeutics dealing with prevention and intervention of substance dependence. This implication is supported by recent data from preclinical studies, and the rapid development of PDE4 inhibitors. Taken together, specific inhibitors of PDE4 and its subtypes possibly represent a novel class of pharmacotherapies for the prevention and abstinence of substance dependence. Here we discuss the modulatory role of cAMP signal transduction in the process of substance dependence and highlight recent evidence that PDE4 inhibitors might be a promising approach to substance dependence therapy.

Huntington's disease (HD) is an autosomal-dominant inherited neurodegenerative disorder characterized by motor dysfunction, cognitive decline, and emotional and psychiatric disturbances. The genetic mutation is characterized by a CAG expansion, resulting in the formation of a mutant huntingtin protein with an expanded polyglutamine repeat region. Mutated huntingtin has been shown to impair a number of physiological activities by interacting with several factors. In particular, cAMP response element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF) are severely affected by mutant huntingtin. In this view, drugs targeted at counteracting CREB loss of function and BDNF decrease have been considered as powerful tools to treat HD. Recently, cyclic nucleotide phosphodiesterase (PDE) inhibitors have been used successfully to increase levels of CREB and BDNF in HD models. Indeed, PDE4, 5 or 10 inhibitors have been shown to afford neuroprotection and modulation of CREB and BDNF. In this review, we will summarize the data supporting the use of PDE inhibitors as the therapeutical approach to fight HD and we will discuss the possible mechanisms of action underlying these effects.

Cyclic AMP and cyclic GMP are essential second messengers that regulate multiple signaling pathways in virtually all cell types. Their accumulation in cells is finely regulated by cyclic nucleotide phosphodiesterases (PDEs), the only enzymes that can degrade these signaling molecules and thus provide exquisite control over intracellular signaling processes. One PDE family, PDE10A, is highly enriched in the brain and its unique expression profile in specific brain regions of interest, in particular to antipsychotic treatment, has made it an attractive therapeutic target for the treatment of schizophrenia. However, after a Phase II trial failure of a selective PDE10A inhibitor for the treatment of schizophrenia, it has encouraged the field to reexamine the role of this enzyme in the brain, and the possible CNS disorders in which PDE10A inhibition could be therapeutic. We will review the localization of PDE10A, both within the brain and the neuron and discuss how its role in regulating cAMP and cGMP accumulation modulates intracellular signaling pathways. Since this cellular signaling has best been documented in the striatum, we will focus our discussion of PDE10A in the context of disorders that affect the basal ganglia, including psychiatric disorders such as bipolar disorder and autism spectrum disorders and the movement disorders, including Parkinson’s disease and Huntington’s disease.

Phosphodiesterase 11A (PDE11A) is the most recently discovered 3’, 5’-cyclic nucleotide phosphodiesterase. By breaking down both cAMP and cGMP, PDE11A is a critical regulator of intracellular signaling. To date, PDE11A has been implicated to play a role in tumorigenesis, brain function, and inflammation. Here, we consolidate and, where necessary, reconcile the PDE11A literature to evaluate this enzyme as a potential therapeutic target. We compare the results and methodologies of numerous studies that report conflicting tissue expression profiles for PDE11A. We conclude that PDE11A expression is relatively restricted in the body, with reliable expression reported in tissues such as the brain (particularly the hippocampus), the prostate, and the adrenal gland. Each of the four PDE11A splice variants (PDE11A1-4) appears to exhibit a distinct tissue expression profile and has a unique N-terminal regulatory region, suggesting that each isoform could be individually targeted with a small molecule or biologic. Progress has been made in identifying a tool PDE11A inhibitor as well as an activator; however, the functional effects of these pharmacological tools remain to be determined. Importantly, PDE11A knockout mice do exist and appear healthy into late age, suggesting a potential safety window for targeting this enzyme. Considering the implication of PDE11A in disease-relevant biology, the potential to selectively target specific PDE11A variants, and the possibility of either activating or inhibiting the enzyme, we believe PDE11A holds promise as a potential future therapeutic target.