Obstructive respiratory diseases (OPD), such as asthma or the chronic obstructive pulmonary disease (COPD), have been one of the earliest marketed indications, associated with the PDE inhibitors. Asthma and COPD are common heterogeneous, typically inflammation-mediated diseases of the airways, characterized by the airflow limitation (Rabe and Watz, 2017; Papi et al., 2018). Non-selective theophylline and its derivatives aminophylline, dyphylline and oxtriphylline had been widely used in managing bronchospasm in asthma and COPD under multiple trade names (Table 2); xanthines are characterized by multiple mechanisms of action, including adenosine receptor antagonism, phosphoinositide-3-kinase inhibition and histone deacetylase activation (Spina and Page, 2017). Other marketed PDE inhibitors in respiratory disorders include PDE 4-selective roflumilast, approved by the FDA (Daliresp^®, 2011) and EMA (Daxas^®, 2010) in COPD; PDE 4-selective ibudilast, marketed in Japan and several other Asian markets for the use in asthma as Ketas^® and Pinatos^®, and non-selective enprofylline, marketed in asthma as Nilyph^®.

The rationale behind targeting PDEs to manage the airway diseases is based on the involvement of several families in the inflammatory and structural cells within the respiratory system. Among the individual PDE families, PDE 3, PDE 4, PDE 5, PDE 7, PDE 8, and PDE 9 have been described to hold the highest degree of relevance (Ntontsi et al., 2019). Inhibiting these enzymes result in a variety of cellular responses, including the airway smooth muscle relaxation, bronchodilation and inhibition of inflammatory pathways. Particularly, PDEs 4 and 5 inhibition has also been described to affect the process of airway wall remodeling (Fan Chung, 2006).

In the clinical setting, the evidence is scarce and limited to PDE 4 inhibitors in COPD, showing a benefit in improving lung function and reducing the likelihood of exacerbations, but without a significant impact on the quality-of-life, based on the results of a systematic review of clinical trials involving cilomilast, roflumilast, and tetomilast (Janjua et al., 2020). A 1-year study of roflumilast in severe COPD has specifically shown a post-bronchodilator forced expiratory flow increasing by 39 ml versus placebo without significant changes in the COPD exacerbation rate (mean values 0.86 vs. 0.92 exacerbations per patient with roflumilast vs. placebo) (Calverley et al., 2007). However, a subset of patients in stage IV disease has shown a greater decrease in the rate (1.01 vs. 1.59 exacerbations per patient). The common adverse drug reactions (ADRs) in PDE 4 inhibitors include diarrhea, nausea, vomiting, dyspepsia, and headache (Janjua et al., 2020). Roflumilast, in particular, is described to be additionally associated with insomnia, weight loss and depressed mood. In asthma, the sole published evidence on ibudilast has shown an improvement in airway hypersensitivity and reduction in asthma attack severity (Kawasaki et al., 1992).

To summarize, PDE inhibitors are currently recognized as an add-on therapy in patients with COPD showing persistent symptoms or exacerbations (Janjua et al., 2020). The major obstacles in their wider clinical use and development are safety profile negatively affecting compliance in patients with COPD and lack of clinical data in patients with severe asthma (Rabe and Watz, 2017). A currently Recruiting Phase 1 trial of roflumilast in patients with severe asthma (NCT04108377) could provide further insights on the clinical applicability of PDE 4 in asthma. Additionally, two novel PDE inhibitors, ensifentrine, and tanimilast, are currently in clinical trials for COPD and asthma; ClinicalTrials.gov records mention two Recruiting Phase 3 studies of tanimilast in COPD (NCT04636814, NCT04636801), while ensifentrine is in two Ongoing Phase 3 trials in COPD and a Phase 2 study in asthma that has shown a dose-dependent bronchodilation similar to salbutamol, but without impacting potassium levels and having a less significant impact on heart rate and pulse rate (Bjermer et al., 2019).

Some of the currently most widely marketed PDE inhibitors fall under the diverse category of cardiovascular drugs, primarily including cardiotonics, vasodilatory agents, as well as antiaggregants. Cardiovascular diseases (CVD) are a heterogeneous group of disorders affecting heart and blood vessels; the (World Health Organization, 2022) includes coronary heart disease, cerebrovascular disease, peripheral arterial disease, rheumatic heart disease, congenital heart diseases and venous thromboembolism into the term. The approved indications in this category include erectile dysfunction (ED), congestive heart failure (CHF), intermittent claudication, hypertension, and thrombosis-related complications (Table 3).

The earliest FDA-approved ⁷ cardiovascular PDE inhibitors are PDE 3-selective dipyridamole (Persantine^®, 1961), amrinone (Inocor^®, 1984) and milrinone (Primacor^®, 1987), and non-selective pentoxifylline (Trental^®, 1984). Dipyridamole is approved as an adjunct to coumarin anticoagulants in the prevention of postoperative thromboembolic complications of cardiac valve replacement; in combination with aspirin (Aggrenox^®, 1999), it is indicated for the stroke risk reduction. Amrinone and milrinone are both marketed in CHF, while pentoxifylline is approved in intermittent claudication. Other identified FDA approvals within this category include PDE 3-selective anagrelide (Agrylin^®, 1997; also authorized as Xagrid^® by the EMA in 2004), PDE 5-selective sildenafil (Viagra^®, 1998) and its derivatives avanafil (Stendra^®, 2012; also authorized as Spedra^® by the EMA in 2013), tadalafil (Cialis^®, 2003) and vardenafil (Levitra^®, 2003 and Staxyn^®, 2010; also authorized as Vivanza^® by the EMA in 2003), and PDE 3-selective cilostazol (Pletal^®, 1999). Anagrelide is approved for treating thrombocythemia, secondary to myeloproliferative malignancies, as well as to reduce risks of thrombosis-related events. Sildenafil, along with avanafil, tadalafil, and vardenafil, are all marketed in ED; sildenafil and tadalafil are also marketed in pulmonary arterial hypertension as Revatio^® (2005) and Adcirca^® (2009), respectively. Cilostazol is approved in intermittent claudication. Additionally, a number of PDE inhibitors, such as cGMP-PDE-specific cicletanine, PDE 3-selective enoximone and olprinone, non-selective papaverine, ibudilast, and PDE 5-specific mirodenafil, udenafil, and lodenafil, have received marketing authorization in other regions. Cicletanine is authorized in France for the treatment of hypertension (Tenstaten^®, 1986). Enoximone is marketed across Europe for the treatment of CHF, initially authorized in France (Perfan^®, 1987); olprinone is authorized for the same indication in Japan (Coretec^®, 1986). Papaverine is another PDE inhibitor, used in ED, as well as vascular spasm-associated cerebral and peripheral ischemia; it is marketed worldwide under various trade names (Table 2). Ibudilast is marketed in post-stroke dizziness (Ketas^® and Pinatos^®). Mirodenafil (Mvix^®, 2007) and udenafil (Zydena^®, 2005) are both approved in the Asian markets for the treatment of ED. Lodenafil (Helleva^®) is marketed for ED in Brazil. PDE 1-specific vinpocetine (Cavinton^®) is marketed as a dietary supplement in managing cerebral vascular disorders and cognitive impairment. At least one cardiovascular PDE inhibitor is also used in veterinary practice; PDE 3-specific pimobendan is marketed under various trade names, including Acardi^® (1997) for treatment of CHF in dogs.

The cardiac PDE families include PDEs 1, 2, 3, 4, 5, 8, 9, and 10; dysregulation in their expression patterns, activation and subcellular localization is often associated with CVDs (Chen and Yan, 2021). The bulk of currently marketed agents target PDEs 3 and 5, with PDEs 1, 4, 9, and 10 being additional targets. The PDE 3 inhibition was initially found to regulate the contractile function of the heart; global PDE 3A knockout increases cardiac contractility and relaxation through cAMP-dependent elevations of Ca²⁺ transient amplitudes and Ca²⁺ contents in the sarcoplasmic reticulum (Sun et al., 2007), which subsequently became the theoretical basis for its usage in congestive heart failure. Another important effect of PDE 3 is on the cardiomyocyte survival; chronic inhibition of its activity or reduction of PDE 3A expression induces apoptosis associated with a persistent induction of inducible cAMP early repressor (ICER), while preventing PDE 3A reduction is able to disrupt the PDE 3A-ICER feedback loop and protect cardiomyocytes from apoptosis (Ding et al., 2005). Furthermore, PDE 3 inhibition stimulates vascular relaxation, reducing peripheral and pulmonary vascular resistance and enhancing coronary blood flow (Chen and Yan, 2021). The PDE 5 inhibition has been shown to exert cardioprotective effects; a study involving sildenafil and vardenafil on a rabbit model of ischemia has shown a mitochondrial ATP-sensitive K channel opening-dependent protective effect against reperfusion injury (Ockaili et al., 2002). Additionally, studies involving sildenafil have showed a RhoA/Rho-kinase pathway inhibition-mediated attenuation of the left ventricular dysfunction (Chau et al., 2011) and the inhibition of the hypertrophy progression, fibrosis, and chamber remodeling, also improving basal and β-stimulated contractility and relaxation via protein kinase G activity, as well as Ca²⁺ handling (Nagyama et al., 2009).

Clinically, the bulk of published evidence is on the PDE 5 inhibitors. In ED, these agents are the first-line treatment in patients with primary disease with efficacy relating to placebo ranging from 0.47 in sildenafil to 0.26 in mirodenafil, based on the results of a trade-off network meta-analysis of 82 trials (n = 47,626 patients) (Chen et al., 2015). Vardenafil and lodenafil both have the relative efficacy of 0.35, udenafil and tadalafil—0.33 and avanafil—0.29. The most common ADRs, according to the analysis of 72 trials (n = 20,325) (Chen et al., 2015), are headache, flushing, dyspepsia, and nasal congestion; the frequency at therapeutic doses is highest in sildenafil (18.42%) and lowest in mirodenafil (10.23%). A recent meta-analysis of 44 trials (n = 3,853 patients) (Mykoniatis et al., 2021) has shown that combining PDE 5 inhibitors with other interventions, such as antioxidants, daily tadalafil, shockwaves or a vacuum device, improves the outcome, especially in the refractory and/or hard-to-treat disease. In pulmonary hypertension, a systematic review of 36 trials (n = 2,999 patients) (Barnes et al., 2019) has shown that treatment with PDE 5 inhibitors is associated with an improvement in WHO functional class and 6-min walking distance (up to 48 m), as well as reduced mortality, in patients with Group 1 pulmonary arterial hypertension.

PDE 3 inhibitors are another class of CVD-active inhibitors that has been explored clinically. In CHF, milrinone is described as an important option to treat patients with refractory disease (Chen and Yan, 2021); it functions by improving cardiac contractility and relaxation, induces vasodilation and has the overall effect of increased cardiac output, improvement of left ventricle-arterial coupling and enhanced cardiac mechanical efficiency (Ayres and Maani, 2022). ADRs associated with milrinone, such as an increased risk of arrhythmias and hypotension, are often dose-limiting, however (Chong et al., 2018). In intermittent claudication, according to the results of a systematic review of 16 trials (n = 3,972 patients) (Brown et al., 2021), the use of cilostazol is associated with improvements in functional status (mean initial claudication distance (ICD) of 26.49 m higher in cilostazol group vs. placebo group) and quality-of-life; comparison with pentoxifylline did not show any differences in functional status (mean ICD of 20 m higher in cilostazol group vs. pentoxifylline group). In cilostazol, the common ADRs are headache, diarrhea, dizziness, pain, and palpitations, headache being the most common (Brown et al., 2021). In thrombocytopenia, anagrelide is used in essential thrombocytopenia typically as a second-line therapy, preferentially in younger patients (Birgegård, 2016). A recently published longitudinal study (n = 150 patients) (Mazzucconi et al., 2020) have showed a response rate of 85.4%; ADRs included palpitations, peripheral vasodilation, anemia, diarrhea, gastric distress and thrombotic events, suggesting the importance of assessing thrombotic risk and monitoring cardiac function. The drug was also found to be characterized by nephrotoxicity, increasing the risk of renal function impairment, according to a recent retrospective study (Kwiatkowski et al., 2021).

Here, PDE inhibitors have been shown to be valuable tools to improve the functional status in patients with vascular and CVDs. It is suggested that developing isoform-specific inhibitors and/or activators could improve the clinical value of PDE modulators in CVDs, while modulating isoform-specific protein-protein interactions within specific signalosome could provide a strategy to improve the molecules’ specificity (Chen and Yan, 2021). In HF, an interesting novel approach is PDE 1 inhibition, investigated with ITI-214 (lenrispodun); it was shown to induce inodilator effects after a single oral dose, increase mean left ventricular power index and cardiac output, and reduce systemic vascular resistance (Gilotra et al., 2021).

Several PDE inhibitors have been marketed in inflammatory disorders affecting the skin and joints (Table 4). The FDA approvals in this category include PDE 4-specific amlexanox (Aphthasol^®, 1996), apremilast (Otezla^®, 2014), and crisaborole (Eucrisa^®, 2016). Amlexanox was originally approved for the treatment of recurrent aphthous ulcerations in the oral mucosa; however, current FDA records list the drug as discontinued, presumably due to termination of licensing agreement between the drug’s developer, ULURU Inc., and Discus Dental Inc., the company that held exclusive rights for sales and marketing in the United States. Apremilast is approved for the treatment of active psoriatic arthritis and moderate-to-severe plague psoriasis, and crisaborole is approved in mild-to-moderate atopic dermatitis. Ibudilast, in addition to its previously mentioned uses in asthma and post-stroke dizziness, is marketed for allergic conjunctivitis as Ketas^® and Eyevinal^®. Recently, difumilast was marketed in Japan as Moizerto^® (2021).

Recurrent aphthous ulcerations is a common chronic disease in the oral mucosa, characterized by solitary or multiple, recurrent, small ulcers with erythematous halos and yellow or gray pseudomembranes (Lau and Smith, 2022). Psoriasis is an immune-mediated genetic skin disease that can also manifest in joints; chronic plague psoriasis represents the majority of cases (Boehncke and Schön, 2015). Atopic dermatitis is a common inflammatory skin disorder of multifactorial etiology characterized by recurring and intensely pruritic lesions (Tollefson and Bruckner, 2014). In the skin, the PDE 4 enzymes have been shown to be primarily expressed in keratinocytes, neutrophils, Langerhans cells, and T-cells (Li et al., 2018). Additionally, peripheral blood mononuclear cells demonstrate higher PDE 4B and PDE 4D mRNA levels in psoriatic patients (Schafer et al., 2016). Earlier studies have also noted potential disturbances in adenylyl cyclase signalling, associated with psoriasis (Wright et al., 1973). In atopic dermatitis, overexpression and overactivity of PDE 4 enzymes leads to the production of inflammatory cytokines and imbalance of T-cell activity and polarization, causing skin inflammation and disease exacerbation (Li et al., 2018).

Clinically, amlexanox, apremilast, crisaborole, and difumilast have all showed positive safety and efficacy. Amlexanox, originally formulated as a 5% topical oral paste, was shown to accelerate complete ulcer healing and the time to resolution of pain, and reduce the ulcer size (Bell, 2005). Treatment with apremilast, according to the most recent Phase 3 trial in patients with recurrent psoriatic arthritis (n = 505 patients) (Edwards et al., 2016), resulted in 40% achieving 20% functional improvement (as compared to 18% taking placebo) and 41% achieving 50% reduction in skin involvement (vs. 24% taking placebo). As an orally active drug, its use can be associated with systemic ADRs, such as diarrhea, nausea, headache, upper respiratory tract infection, vomiting, nasopharyngitis, and abdominal pain (Cada et al., 2014). Additionally, its use is associated with a risk of depression and weight loss. Crisaborole, based on the results of two Phase 3 trials in atopic dermatitis (n₁ = 759 patients, n₂ = 763 patients) (Paller et al., 2016), has showed an improvement in skin involvement (3 2-grade improvement; 32.8% and 31.4% vs. 25.4% and 18.0% taking the vehicle-control), as well as a pruritus relieve. The only ADR associated with the treatment was application site pain (Paller et al., 2016). Difamilast, based on the results of a Phase 3 long-term study in the Japanese adult and pediatric patients (n _adult = 166 patients, n _pediatric = 200 patients) (Saeki et al., 2022), has shown a cumulative success EASI-75 (Eczema Area and Severity index) rates of 55.4% and 73.5%. The most common ADRs included dermatitis, acne and pigmentation disorder.

Here, PDE inhibitors are considered interesting additions to the established treatment strategies. Amlexanox is a first-line treatment option in apthous stomatitis (Lau and Smith, 2022). Apremilast is active in both psoriatic arthritis and cutaneous manifestations of the disease; however, its overall efficacy is low compared to other available options (Boehncke and Schön, 2015). Crisaborole is an option to treat mild-to-moderate disease, characterized by low systemic toxicity; it is reported, nevertheless, that it is not yet clear if it is characterized by lower toxicity than the other available topical therapeutic options (Li et al., 2018). Difamilast has showed attractive results in the Japanese population, more evidence is needed on patients of other ethnicities to discuss its wider use.

Three PDE inhibitors have been marketed in central and peripheral nervous system disorders (Table 4): drotaverine (No-Spa^®), tofisopam (Grandaxin^®) and caffeine (Cafcit^®). Drotaverine is marketed in functional bowel disorders due to the smooth muscle spasm, while tofisopam is marketed in anxiety. Additionally, caffeine is approved by the FDA in 1999 for use in the apnea of prematurity.

Drotaverine, a structural analogue of papaverine, acts as a direct smooth muscle relaxant through PDE inhibition and Ca²⁺ channel blocking (Patai et al., 2016). Caffeine is a centrally acting methylxanthine with a complex mechanism of action; however, it is believed that the principal mechanism involves adenosine receptors antagonism, due to PDE inhibition requiring very high concentrations (Atik et al., 2017). Tofisopam is described as an atypical benzodiazepine that doesn’t interact with γ-aminobutyric acid receptors, but instead acts as a selective PDE inhibitor with the highest affinities to PDE 4A1 (0.42 μM) and PDE 10A1 (0.92 μM) (Rundfeldt et al., 2010).

The published clinical evidence on these drugs is generally scarce. Drotaverine had been studied as a spasmolytic in irritable bowel syndrome (IBS) and labor augmentation. In IBS, drotaverine was shown to improve abdominal symptoms, including pain frequency and severity, and stool frequency, based on the results of a double-blind placebo-controlled study (Rai et al., 2014). In labor augmentation, it reduced the duration of first and second stages of labor, but with a limited impact on pain (Singh et al., 2004). In apnea of prematurity, caffeine reduces the incidence of bronchopulmonary dysplasia and improves the rate of survival without neurodevelopmental disability in long-term use, in addition to reducing the frequency of apnea of prematurity and the need for mechanical ventilation during the first 7 days of therapy (Schmidt et al., 2006; Schmidt et al., 2007). On tofisopam, literature sources state that it is potent in alleviating vegetative symptoms accompanying anxiety disorders as an anxiolytic without sedative-hypnotic, muscle relaxant and anticonvulsive properties (Szegó et al., 1993).

To summarize, the nervous system disorders currently represent a niche use for PDE inhibitors. Drotaverine and tofisopam lack clinical evidence to discuss its wider applicability. Caffeine is being considered as the mainstay of apnea of prematurity pharmacotherapy; however, it is not considered to act primarily as a PDE inhibitor.

Among the investigational CNS disorders, neurodegenerative and psychiatric conditions, such as Alzheimer’s disease (AD) and schizophrenia, are particularly prevalent. In schizophrenia, PDE 10A inhibition is reported as a plausible approach due to the expression pattern of PDE 10A in striatal medium spiny neurons and functional similarity of its inhibition to the dopamine D₂ receptor antagonism (Snyder and Vanover, 2017). These agents include MK-8189, TAK-063 (also known as balipodect), FRM 6308 (EVP-6308), MP-10 (mardepodect) and OMS 824. Out of all these agents, published evidence on clinical studies exists for MK-8189 and TAK-063; both agents have showed acceptable tolerability, but the efficacy was reported to be low (Krogmann et al., 2019). As of 2022, only MK-8189 appears to be in the ongoing development, with a Phase 1 multiple ascending dose study of safety, tolerability, pharmacokinetics and the effect on QTc currently in the Recruiting stage (NCT05406440). In addition to PDE 10A, at least one PDE 9A inhibitor is undergoing development in psychiatric and neurodegenerative disorders, BI-409306 (osoresnontrine). The drug is currently in an Ongoing Phase 1 trial of the effect on ketamine-induced cognitive deficit in healthy male subjects (NCT04602221). It was previously investigated in cognitive impairment due to schizophrenia (Brown et al., 2019) and prodromal-to-mild AD (Frölich et al., 2019), but failed to demonstrate efficacy.

In AD, PDE4 inhibitors represent the bulk of investigational approaches (Klyucherev et al., 2022). The rationale here stems from the effect of cAMP on memory formation and cognition; an intracellular increase in cerebral cAMP activates protein kinase A associated with cAMP response element binding protein which is vital for synaptic plasticity (Tibbo et al., 2019). The drugs include BPN14770 (putative generic name zatolmilast), HT-0712, EHT 202 (etazolate), MEM1414, and DG071. BPN14770 has so far been investigated in three Phase 1 trials (NCT02648672, NCT02840279, and NCT03030105) in AD and was found to be associated with an improvement in working memory in healthy elderly subjects at low-to-mid doses, as reported in the developer’s press release (Tetra Discovery, 2016) ⁸ . Its ongoing development, however, goes predominantly in the direction of fragile X syndrome, a genetic neurodevelopmental disorder, characterized by intellectual disability, hyperactivity, sensory hypersensitivity, autistic-like behavior and susceptibility to seizures (Gurney et al., 2019). According to the results of a placebo-controlled Phase 2 study, BPN14770 resulted in a significant cognitive improvement related to language and improvements in caregiver scales rating language and daily functioning (Berry-Kravis et al., 2021). Additionally, there is one Recruiting Phase 2/3 study in male adolescents (NCT05163808), a Phase 3 Not yet recruiting study in male adults (NCT05358886) and an open-label extension Phase 3 Not yet recruiting study to assess the long-term safety and tolerability in subjects of the former studies. HT-0712 was reported to improve long-term memory in mice (Bourtchouladze et al., 2003; Peters et al., 2014) and facilitate behavioral recovery and cortical reorganization after an ischemic insult (MacDonald et al., 2007); however, no results from a clinical study could be identified. EHT 202 was investigated in a Phase 2 study as an adjunct to an acetylcholinesterase inhibitor in patients with mild-to-moderate AD (NCT00880412); no results had been published. Another CNS indication under ongoing investigation is Parkinson’s disease (PD); a previously mentioned PDE 1 inhibitor ITI-214 has shown reduction in neuroinflammation and enhancement of memory performance and cognition in preclinical models (Snyder et al., 2016; Pekcec et al., 2018; O’Brien et al., 2020). Clinically, it was reported to improve the motor functions, according to a Phase 1/2 trial in patients with mild to moderate PD (Davis et al., 2019).

Another notable investigational indication are neoplasms which is developing rapidly across numerous targets (reviewed in Attwood et al., 2021; Sokolov et al., 2021). In cancer, the PDE overexpression is observed in numerous types of cancer and its inhibition has shown positive effects on preclinical models (Peng et al., 2018). Antineoplastic PDE inhibitors in clinical development include PBF-999, exisulind, CEL-031 and CC-1088. PBF-999, a dual adenosine A2a receptor antagonist/PDE 10 inhibitor, is investigated in a Phase 1 Recruiting trial (NCT03786484) in patients with immunotherapy-naïve and pretreated advanced solid tumors. According to the developer’s website (PaloBioPharma Pipeline, 2022) ⁹ , colon cancer appears to be the primary indication. Exisulind is the sulfone derivative of sulindac, a selective apoptotic antineoplastic drug (Exisulind: Aptosyn, 2004). It had been evaluated in a wide range of tumors, the most recent being prostate (Weight et al., 2012), lung (Govindan et al., 2009), and melanoma (Curiel-Lewandrowski et al., 2012); however, no further development could be identified, possibly due to insufficient efficacy. CEL-031 is another apoptotic antineoplastic drug, developed for bladder cancer ¹⁰ ; lack in its recent development could be due to poor oral bioavailability. CC-1088, a thalidomide analogue, was originally developed for myelodysplastic syndromes (Dredge, 2005). According to an in vitro study on myeloma cells (Molostvov et al., 2004), CC- 1088 acts by inhibiting production of vascular endothelial growth factor and interleukin (IL)- 6, and by inducing apoptosis ¹¹ .

Among the other investigational indications of note are blood disorders, retinitis pigmentosa, necrobiosis lipoidica, autoimmune diseases, mycobacterial diseases, the novel coronavirus disease (COVID-19) and metabolic diseases. In blood disorders, tovinontrine is evaluated for sickle cell anemia in a Phase 2a Ongoing study to evaluate the long-term safety, tolerability and pharmacodynamics (NCT04053803). A gene therapy using an adeno-associated virus vector AAV2/5-hPDE6 is in a Phase 1/2 Ongoing trial (NCT03328130) in patients with retinitis pigmentosa. CC-11050 is currently evaluated in a Phase 2 Recruiting study in patients with leprosy and erythema nodosum leprosum (NCT03807362). The drug has also been studied in cutaneous lupus erythematosus, tuberculosis (TB) and HIV infection; published evidence in TB suggests that PDE inhibition can enhance the lung function recovery (Subbian et al., 2016; Wallis et al., 2021) and improve responsiveness to the antibacterial therapy (Subbian et al., 2016). In HIV, CC-11050 was shown to have no impact on CD4 counts or plasma viremia, but led to a decrease in natural killer cells and plasma IL-8 level, as well as to an increase in IL-6 (Boulougura et al., 2019). In COVID-19, ensifentrine had been investigated in a Phase 2 study to evaluate the efficacy and safety in the recovery of patients hospitalized with the disease (NCT04527471); the study is in an Unknown status since 2021, however. Nevertheless, tanimilast, another PDE 4 inhibitor, was found to exert a modulatory effect on the SARS-CoV-2 genomic ssRNA-induced pro-inflammatory and Th1- polarising potential of dendritic cells (Nguyen et al., 2022). MN-001 (tipelukast) was previously investigated in idiopathic pulmonary fibrosis (Phase 2, no results published) and currently is in a Not yet recruiting Phase 2 study in patients with non-alcoholic fatty liver disease, type 2 diabetes mellitus (T2DM), and hypertriglyceridemia (NCT05464784).

In addition to the PDE inhibitors in clinical development, mentioned above, we have identified several experimental drugs. Among the novel indications here is Duchenne muscular dystrophy, potentially manageable using a PDE 4 and PDE 5 inhibitor combination therapy. According to an in vivo study of a novel PDE 4 RP 73401 (piclamilast) in the mdx mice, it reduces the mRNA level of profibrotic genes in the gastrocnemius and diaphragm of the animal, and significantly reduced the percentage affected by fibrosis (Nio et al., 2017).

Many of experimental agents are PDE 10 inhibitors. JNJ-42314415, MR 1916, SEP-39, and ASP9436 are novel PDE 10 inhibitors with an antipsychotic activity; characterization of JNJ- 42314415, in particular, have showed that the effects of PDE 10A inhibition against dopaminergic stimulants and on catalepsy were potentiated by a D₁ antagonist, providing a potential strategy to improve safety profile of PDE 10A inhibitors through reducing dopamine D₂ and concomitantly potentiating dopamine D₁ receptor-mediated neurotransmission (Megens et al., 2014). In addition, MR-1916, when combined with risperidone, resulted in a significant enhancement of the conditioned avoidance response in rats without affecting extrapyramidal side effects (Arakawa and Maehara, 2020).

Several agents target novel PDE targets. BRL-50481, a PDE 7 inhibitor, have showed neuroprotector (Chen et al., 2020) and anti-inflammatory properties (Kim et al., 2022) on animal models. Lu AF33241, a dual PDE 2A/PDE 10A inhibitor, was found to attenuate sub-chronic phencyclidine-induced deficits in novel object recognition and displayed antipsychotic-like activity (Redrobe et al., 2015). EHNA, a PDE 2 inhibitor, was recently shown to inhibit proliferation and stimulate migration of human osteosarcoma cells (Murata et al., 2019) and to inhibit growth and invasion of human malignant melanoma cells (Hiramoto et al., 2014). Interestingly, a PDE 3/4 inhibitor zardaverine was recently shown to act as a potent cytotoxic agent in embryonal rhabdomyosarcoma and cervical carcinoma cell lines (Cartledge et al., 2017), as well as in hepatocellular carcinoma (Sun et al., 2014); the mechanism, however, is independent of PDE inhibition.

One more agent to discuss is resveratrol, a naturally occurring polyphenol marketed as a dietary supplement due to its antioxidant properties (Salehi et al., 2018). Several preclinical studies have showed that some of its biological properties are mediated via PDE inhibition. Resveratrol was found to impart neuroprotection in brain ischemia by inhibiting PDEs and regulating the cAMP/AMPK/SIRT1 pathway, which reduces ATP energy consumption (Wan et al., 2016). Additionally, it was found to enhance glucose-stimulated insulin secretion by pancreatic β-cells (Rouse et al., 2014) and ameliorate aging-associated metabolic disorders by multiple mechanisms, including an increase in mitochondrial biogenesis and protection against diet-induced obesity and glucose intolerance (Park et al., 2012).

Several studies involving resveratrol were done on human subjects as well, such as AD, Gulf War syndrome, T2DM, postmenopausal osteoporosis and COVID-19. In AD, it was found to alter the Aβ40 levels and the brain volume loss increase (Turner et al., 2015), decrease MMP9 levels, modulate neuro-inflammation and induce adaptive immunity (Moussa et al., 2017); the mechanism involved may not be related to the PDE inhibition, however. In Gulf War syndrome, a chronic multi-symptom disorder of an unknown etiology that affects veterans of the Gulf War, it reduced the symptom severity (Hodgin et al., 2021). In T2DM, a systematic review of three trials has reported a neutral impact on glycosylated hemoglobin A1c (mean difference 0.1% vs. placebo), fasting blood glucose levels (2 mg/dl difference vs. placebo), and insulin resistance (−0.35 difference vs. placebo) (Jeyaraman et al., 2020). In postmenopausal osteoporosis, resveratrol has showed the potential to slow bone loss in the lumbar spine and femoral neck (Wong et al., 2020). In COVID-19, resveratrol was shown to reduce angiotensin-converting enzyme 2 expression in the adipose tissue, a potential SARS-CoV-2 reservoir contributing to massive viral spread in COVID-19 patients with obesity (de Ligt et al., 2021). The direction and status of overall clinical development for this compound is uncertain, however; among the major barriers reported for its development is poor bioavailability and solubility (Sun et al., 2014).