Evaluation of PET radioligands for in vivo visualization of phosphodiesterase 5 (PDE5)

Advancements in Phosphodiesterase 5 Inhibitors: Unveiling Present and Future Perspectives

Phosphodiesterase 5 (PDE5) inhibitors presented themselves as important players in the nitric oxide/cGMP pathway, thus exerting a profound impact on various physiological and pathological processes. Beyond their well-known efficacy in treating male erectile dysfunction (ED) and pulmonary arterial hypertension (PAH), a plethora of studies have unveiled their significance in the treatment of a myriad of other diseases, including cognitive functions, heart failure, multiple drug resistance in cancer therapy, immune diseases, systemic sclerosis and others. This comprehensive review aims to provide an updated assessment of the crucial role played by PDE5 inhibitors (PDE5-Is) as disease-modifying agents taking their limiting side effects into consideration. From a medicinal chemistry and drug discovery perspective, the published PDE5-Is over the last 10 years and their binding characteristics are systemically discussed, and advancement in properties is exposed. A persistent challenge encountered with these agents lies in their limited isozyme selectivity; considering this obstacle, this review also highlights the breakthrough development of the recently reported PDE5 allosteric inhibitors, which exhibit an unparalleled level of selectivity that was rarely achievable by competitive inhibitors. The implications and potential impact of these novel allosteric inhibitors are meticulously explored. Additionally, the concept of multi-targeted ligands is critically evaluated in relation to PDE5-Is by inspecting the broader spectrum of their molecular interactions and effects. The objective of this review is to provide insight into the design of potent, selective PDE5-Is and an overview of their biological function, limitations, challenges, therapeutic potentials, undergoing clinical trials, future prospects and emerging uses, thus guiding upcoming endeavors in both academia and industry within this domain.

The action of phosphodiesterase-5 inhibitors on β-amyloid pathology and cognition in experimental Alzheimer's disease: A systematic review

Alzheimer's disease (AD) is the most frequent cause of dementia worldwide. The etiology of AD is partially explained by the deposition of β-amyloid in the brain. Despite extensive research on the pathogenesis of AD, the current treatments are ineffective. Here, we systematically reviewed studies that investigated whether phosphodiesterase 5 inhibitors (PDE5i) are efficient in reducing the β-amyloid load in hippocampi and improving cognitive decline in rodent models with β-amyloid accumulation. We identified ten original studies, which used rodent models with β-amyloid accumulation, were treated with PDE5i, and β-amyloid was measured in the hippocampi. PDE5i was efficient in reducing the β-amyloid levels, except for one study that exclusively used female rodents and the treatment did not affect β-amyloid levels. Interestingly, PDE5i prevented cognitive decline in all studies. This study supports the potential therapeutic use of PDE5i for the reduction of the β-amyloid load in hippocampi and cognitive decline. However, we highlight the importance of conducting additional experimental studies to evaluate the PDE5i-related molecular mechanisms involved in β-amyloid removal in male and female animals.

On the Viability of Tadalafil-Based 18F-Radiotracers for In Vivo Phosphodiesterase 5 (PDE5) PET Imaging

Phosphodiesterase 5 (PDE5) is a clinically relevant biomarker and therapeutic target for many human pathologies, yet a noninvasive agent for the assessment of PDE5 expression has yet to be realized. Such agents would improve our understanding of the nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP)/PDE5 pathway in human pathologies and potentially lead to novel uses of PDE5 inhibitors to manage lung conditions like SARS-CoV-2-mediated pulmonary inflammatory responses. In this study, efforts were made to produce an ¹⁸F-labeled analogue of the PDE5 inhibitor tadalafil to visualize PDE5 expression in vivo with positron emission tomography (PET). However, during the late-stage fluorination step, quantitative epimerization of the tadalafil C12a stereocenter occurred, yielding a less active epi-isomer. In vivo dynamic microPET images in mice revealed that the epimerized radiotracer, [¹⁸F]epi-18, rapidly accumulated in the liver with negligible uptake in tissues of known PDE5 expression.

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

Radiosynthesis of a carbon-11 labeled PDE5 inhibitor [11C]TPN171 as a new potential PET heart imaging agent

To develop PET tracers for imaging of heart disease, a new carbon-11 labeled potent and selective PDE5 inhibitor [¹¹C]TPN171 ([¹¹C]9) has been synthesized. The reference standard TPN171 (9) and its corresponding precursor desmethyl-TPN171 (11) were synthesized from methyl 3-oxovalerate and 2-hydroxybenzonitrile in 9 and 10 steps with 31% and 25% overall chemical yield, respectively. The radiotracer [¹¹C]TPN171 was prepared from desmethyl-TPN171 with [¹¹C]CH₃OTf through N-¹¹C-methylation and isolated by HPLC purification followed by SPE formulation in 45-55% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to EOB. The radiochemical purity was >99%, and the molar activity (A_m) at EOB was in a range of 370-740 GBq/μmol.

Radiosynthesis of carbon-11 labeled PDE5 inhibitors as new potential PET radiotracers for imaging of Alzheimer's disease

To develop PET tracers for imaging of Alzheimer's disease, new carbon-11 labeled potent and selective PDE5 inhibitors have been synthesized. The reference standards (5) and (12), and their corresponding desmethylated precursors (6) and (13) were synthesized from methyl 2-amino-5-bromobenzoate and (4-methoxyphenyl)methanamine in multiple steps with 2%, 1%, 1% and 0.2% overall chemical yield, respectively. The radiotracers ([¹¹C]5) and ([¹¹C]12) were prepared from their corresponding precursors 6 and 13 with [¹¹C]CH₃OTf through O-¹¹C-methylation and isolated by HPLC combined with SPE in 40-50% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to EOB. The radiochemical purity was >99%, and the molar activity (A_m) at EOB was in a range of 370-740 GBq/μmol.

Optimization of Extraction Technology of Majun Mupakhi Ela and its Effect on Hydrocortisone-induced Kidney Yang Deficiency in Mice

We used Box-Behnken design-based (BBD) response surface methodology (RSM) in this research to optimize the extraction process of Traditional medicine Majun Mupakhi Ela (MME) and evaluate its effect on hydrocortisone-induced kidney yang deficiency. Three independent parameters were applied to evaluate the maximum phosphodiesterase type 5 (PDE5) inhibition activity of MME extracts in vitro. The optimal processing conditions (extraction time 2 h, solid-liquid ratio 1:16, extraction once) gave a maximum PDE5 inhibition rate of 84.10%, flavonoid content of 0.49 mg/ml, icariin content of 0.028 mg/ml and targeted extraction yield of 26.50%. In animal experiments, MME extracts significantly increased the adrenal mass index, semen weight index, preputial gland weight index, and penis weight index in mice; in the middle and high dose group, the level of serum testosterone increased by 7664.29% and 14207.14% respectively, compared with the model group, and the level of PDE5 decreased by 67.22% and 74.69% respectively compared with the control group. These results indicate that MME has a significant positive effect on the hypothalamus-pituitary-gonadal axis, improve mating ability and not only has inhibits PDE5 activity but also significantly inhibits the expression of PDE5 in penile tissues, potential to become erectile dysfunction (ED) therapies for the clinical management of patients with kidney yang deficiency.

Targeting cyclic nucleotide phosphodiesterase 5 (PDE5) in brain: Toward the development of a PET radioligand labeled with fluorine-18

With the aim to develop a specific radioligand for imaging the cyclic nucleotide phosphodiesterase 5 (PDE5) in brain by positron emission tomography (PET), seven new fluorinated inhibitors (3-9) were synthesized on the basis of a quinoline core. The inhibitory activity for PDE5 together with a panel of other PDEs was determined in vitro and two derivatives were selected for IC₅₀ value determination. The most promising compound 7 (IC₅₀ = 5.92 nM for PDE5A), containing a 3-fluoroazetidine moiety, was further radiolabeled by aliphatic nucleophilic substitution of two different leaving groups (nosylate and tosylate) using [¹⁸F]fluoride. The use of the nosylate precursor and tetra-n-butyl ammonium [¹⁸F]fluoride ([¹⁸F]TBAF) in 3-methyl-3-pentanol combined with the addition of a small amount of water proved to be the best radiolabeling conditions achieving a RCY of 4.9 ± 1.5% in an automated procedure. Preliminary biological investigations in vitro and in vivo were performed to characterize this new PDE5 radioligand. Metabolism studies of [¹⁸F]7 in mice revealed a fast metabolic degradation with the formation of radiometabolites which have been detected in the brain.

Synthesis and in vitro evaluation of new fluorinated quinoline derivatives with high affinity for PDE5: Towards the development of new PET neuroimaging probes

The increasing incidence of Alzheimer's disease (AD) worldwide is a major public health problem. Current treatments provide only palliative solutions with significant side effects. Therefore, new efficient treatment options and novel early diagnosis tools are urgently needed. Recently, strong pre-clinical evidences suggested that phosphodiesterase 5 (PDE5) may be clinically relevant both as biomarker and drug-target in AD. In this study, we intended to develop a new radiofluorinated tracer for the visualisation of PDE5 in brain using PET imaging. Based on currently known PDE5 inhibitors, a series of novel fluorinated compounds bearing a quinoline core have been synthesised via multi-steps reaction pathways. Their affinity for PDE5 and selectivity over other PDE families have been investigated. According to the data collected from this in vitro screening, fluorinated derivatives 24a, b bearing a fluoroethoxy group at the C-3 position of the quinoline core appeared to be the most promising structures and will be further radiolabelled with fluorine-18 for in vitro and in vivo evaluations as PET radiotracer for neuroimaging of PDE5.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Carbon-11 and Fluorine-18 Radiolabeled Pyridopyrazinone Derivatives for Positron Emission Tomography (PET) Imaging of Phosphodiesterase-5 (PDE5)

The cyclic guanosine monophosphate (cGMP) specific phosphodiesterase type 5 (PDE5) plays an important role in various pathologies including pulmonary arterial hypertension and cardiomyopathy. PDE5 represents an important therapeutic and/or prognostic target, but noninvasive assessment of PDE5 expression is lacking. The purpose of this study was to develop and evaluate pyridopyrazinone derivatives labeled with carbon-11 or fluorine-18 as PDE5-specific PET tracers. In biodistribution studies, highest PDE5-specific retention was observed for [¹¹C]-12 and [¹⁸F]-17 in the lungs of wild-type mice and in the myocardium of transgenic mice with cardiomyocyte-specific PDE5 overexpression at 30 min postinjection. In vivo dynamic microPET images in rats revealed that both tracers crossed the blood-brain barrier but brain retention was not PDE5-specific. Both [¹¹C]-12 and [¹⁸F]-17 showed specific binding to PDE5 in myocardium of transgenic mice; however [¹⁸F]-17 showed significantly higher PDE5-specific inhibitable binding than [¹¹C]-12.

Pharmacokinetic investigation of sildenafil using positron emission tomography and determination of its effect on cerebrospinal fluid cGMP levels

Sildenafil (Viagra) is a selective inhibitor of phosphodiesterase type 5 (PDE5), which degrades cyclic guanosine monophosphate to the linear nucleotide. Sildenafil is acutely used in erectile dysfunction and chronically in pulmonary hypertension. Evidence in the last decade shows that sildenafil may have potential as a therapeutic option for Alzheimer's disease or other neurodegenerative disorders. The purpose of this work was to explore whether sildenafil crosses the blood-brain barrier. Pharmacokinetic properties of sildenafil in rodents were investigated using (11) C-radiolabeling followed by in vivo positron emission tomography (PET) and ex vivo tissue dissection and gamma counting. PET results in rats suggest penetration into the central nervous system. Ex vivo data in perfused animals suggest that trapping of [(11) C]sildenafil within the cerebral vascular endothelium limits accumulation in the central nervous system parenchyma. Peroral sildenafil administration to Macaca fascicularis and subsequent chemical analysis of plasma and cerebrospinal fluid (CSF) using liquid chromatography coupled with tandem mass spectrometry showed that drug content in the CSF was high enough to achieve PDE5 inhibition, which was also demonstrated by the significant increases in CSF cyclic guanosine monophosphate levels. Central actions of sildenafil include both relaxation of the cerebral vasculature and inhibition of PDE5 in neurons and glia. This central action of sildenafil may underlie its efficacy in neuroprotection models, and may justify the continued search for a PDE5 ligand suitable for PET imaging. Sildenafil interacts with phosphodiesterase type 5 (PDE5) expressed in the endothelium and/or smooth muscle cells of brain vessels and also crosses the blood-brain barrier to interact with PDE5 expressed in brain cells. At therapeutic doses, the concentration of sildenafil in the cerebrospinal fluid (CSF) is high enough to inhibit PDE5 in the neural cells (neurons and glia). In turn, the concentration of cGMP likely increases in parenchymal cells and, as shown in this report, in the CSF. Read the Editorial Highlight for this article on page 220. Cover Image for this issue: doi: 10.1111/jnc.13302.

Novel Radioligands for Cyclic Nucleotide Phosphodiesterase Imaging with Positron Emission Tomography: An Update on Developments Since 2012

Cyclic nucleotide phosphodiesterases (PDEs) are a class of intracellular enzymes that inactivate the secondary messenger molecules, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Thus, PDEs regulate the signaling cascades mediated by these cyclic nucleotides and affect fundamental intracellular processes. Pharmacological inhibition of PDE activity is a promising strategy for treatment of several diseases. However, the role of the different PDEs in related pathologies is not completely clarified yet. PDE-specific radioligands enable non-invasive visualization and quantification of these enzymes by positron emission tomography (PET) in vivo and provide an important translational tool for elucidation of the relationship between altered expression of PDEs and pathophysiological effects as well as (pre-)clinical evaluation of novel PDE inhibitors developed as therapeutics. Herein we present an overview of novel PDE radioligands for PET published since 2012.

Development of a New Radiofluorinated Quinoline Analog for PET Imaging of Phosphodiesterase 5 (PDE5) in Brain

Phosphodiesterases (PDEs) are enzymes that play a major role in cell signalling by hydrolysing the secondary messengers cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP) throughout the body and brain. Altered cyclic nucleotide-mediated signalling has been associated with a wide array of disorders, including neurodegenerative disorders. Recently, PDE5 has been shown to be involved in neurodegenerative disorders such as Alzheimer's disease, but its precise role has not been elucidated yet. To visualize and quantify the expression of this enzyme in brain, we developed a radiotracer for specific PET imaging of PDE5. A quinoline-based lead compound has been structurally modified resulting in the fluoroethoxymethyl derivative ICF24027 with high inhibitory activity towards PDE5 (IC50 = 1.86 nM). Radiolabelling with fluorine-18 was performed by a one-step nucleophilic substitution reaction using a tosylate precursor (RCY(EOB) = 12.9% ± 1.8%; RCP > 99%; SA(EOS) = 70-126 GBq/μmol). In vitro autoradiographic studies of [(18)F]ICF24027 on different mouse tissue as well as on porcine brain slices demonstrated a moderate specific binding to PDE5. In vivo studies in mice revealed that [(18)F]ICF24027 was metabolized under formation of brain penetrable radiometabolites making the radiotracer unsuitable for PET imaging of PDE5 in brain.