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

Opportunities and perspectives of small molecular phosphodiesterase inhibitors in neurodegenerative diseases

Phosphodiesterase (PDE) is a superfamily of enzymes that are responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). PDE inhibition promotes the gene transcription by activating cAMP-response element binding protein (CREB), initiating gene transcription of brain-derived neurotrophic factor (BDNF). The procedure exerts neuroprotective profile, and motor and cognitive improving efficacy. From this point of view, PDE inhibition will provide a promising therapeutic strategy for treating neurodegenerative disorders. Herein, we summarized the PDE inhibitors that have entered the clinical trials or been discovered in recent five years. Well-designed clinical or preclinical investigations have confirmed the effectiveness of PDE inhibitors, such as decreasing Aβ oligomerization and tau phosphorylation, alleviating neuro-inflammation and oxidative stress, modulating neuronal plasticity and improving long-term cognitive impairment.

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.

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.

Challenges on Cyclic Nucleotide Phosphodiesterases Imaging with Positron Emission Tomography: Novel Radioligands and (Pre-)Clinical Insights since 2016

Cyclic nucleotide phosphodiesterases (PDEs) represent one of the key targets in the research field of intracellular signaling related to the second messenger molecules cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP). Hence, non-invasive imaging of this enzyme class by positron emission tomography (PET) using appropriate isoform-selective PDE radioligands is gaining importance. This methodology enables the in vivo diagnosis and staging of numerous diseases associated with altered PDE density or activity in the periphery and the central nervous system as well as the translational evaluation of novel PDE inhibitors as therapeutics. In this follow-up review, we summarize the efforts in the development of novel PDE radioligands and highlight (pre-)clinical insights from PET studies using already known PDE radioligands since 2016.

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.

PET Imaging of the Adenosine A2A Receptor in the Rotenone-Based Mouse Model of Parkinson's Disease with [18F]FESCH Synthesized by a Simplified Two-Step One-Pot Radiolabeling Strategy

The adenosine A_2A receptor (A_2AR) is regarded as a particularly appropriate target for non-dopaminergic treatment of Parkinson’s disease (PD). An increased A_2AR availability has been found in the human striatum at early stages of PD and in patients with PD and dyskinesias. The aim of this small animal positron emission tomography/magnetic resonance (PET/MR) imaging study was to investigate whether rotenone-treated mice reflect the aspect of striatal A_2AR upregulation in PD. For that purpose, we selected the known A_2AR-specific radiotracer [¹⁸F]FESCH and developed a simplified two-step one-pot radiosynthesis. PET images showed a high uptake of [¹⁸F]FESCH in the mouse striatum. Concomitantly, metabolism studies with [¹⁸F]FESCH revealed the presence of a brain-penetrant radiometabolite. In rotenone-treated mice, a slightly higher striatal A_2AR binding of [¹⁸F]FESCH was found. Nonetheless, the correlation between the increased A_2AR levels within the proposed PD animal model remains to be further investigated.

One-step radiosynthesis of the MCTs imaging agent [18F]FACH by aliphatic 18F-labelling of a methylsulfonate precursor containing an unprotected carboxylic acid group

Monocarboxylate transporters 1 and 4 (MCT1 and MCT4) are involved in tumour development and progression. Their level of expression is particularly upregulated in glycolytic cancer cells and accordingly MCTs are considered as promising drug targets for treatment of a variety of human cancers. The non-invasive imaging of these transporters in cancer patients via positron emission tomography (PET) is regarded to be valuable for the monitoring of therapeutic effects of MCT inhibitors. Recently, we developed the first ¹⁸F-radiolabelled MCT1/MCT4 inhibitor [¹⁸F]FACH and reported on a two-step one-pot radiosynthesis procedure. We herein describe now a unique one-step radiosynthesis of this radiotracer which is based on the approach of using a methylsulfonate (mesylate) precursor bearing an unprotected carboxylic acid function. With the new procedure unexpected high radiochemical yields of 43 ± 8% at the end of the radiosynthesis could be obtained in a strongly reduced total synthesis time. Moreover, the radiosynthesis was successfully transferred to a TRACERlab FX2 N synthesis module ready for future preclinical applications of [¹⁸F]FACH.

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.

Synthesis and In Vitro Evaluation of 8-Pyridinyl-Substituted Benzo[e]imidazo[2,1-c][1,2,4]triazines as Phosphodiesterase 2A Inhibitors

Phosphodiesterase 2A (PDE2A) is highly expressed in distinct areas of the brain, which are known to be related to neuropsychiatric diseases. The development of suitable PDE2A tracers for Positron Emission Tomography (PET) would permit the in vivo imaging of the PDE2A and evaluation of disease-mediated alterations of its expression. A series of novel fluorinated PDE2A inhibitors on the basis of a Benzoimidazotriazine (BIT) scaffold was prepared leading to a prospective inhibitor for further development of a PDE2A PET imaging agent. BIT derivatives (BIT1–9) were obtained by a seven-step synthesis route, and their inhibitory potency towards PDE2A and selectivity over other PDEs were evaluated. BIT1 demonstrated much higher inhibition than other BIT derivatives (82.9% inhibition of PDE2A at 10 nM). BIT1 displayed an IC₅₀ for PDE2A of 3.33 nM with 16-fold selectivity over PDE10A. This finding revealed that a derivative bearing both a 2-fluoro-pyridin-4-yl and 2-chloro-5-methoxy-phenyl unit at the 8- and 1-position, respectively, appeared to be the most potent inhibitor. In vitro studies of BIT1 using mouse liver microsomes (MLM) disclosed BIT1 as a suitable ligand for ¹⁸F-labeling. Nevertheless, future in vivo metabolism studies are required.

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.

Investigation of an 18F-labelled Imidazopyridotriazine for Molecular Imaging of Cyclic Nucleotide Phosphodiesterase 2A

Specific radioligands for in vivo visualization and quantification of cyclic nucleotide phosphodiesterase 2A (PDE2A) by positron emission tomography (PET) are increasingly gaining interest in brain research. Herein we describe the synthesis, the ¹⁸F-labelling as well as the biological evaluation of our latest PDE2A (radio-)ligand 9-(5-Butoxy-2-fluorophenyl)-2-(2-([¹⁸F])fluoroethoxy)-7-methylimidazo[5,1-c]pyrido[2,3-e][1,2,4]triazine (([¹⁸F])TA5). It is the most potent PDE2A ligand out of our series of imidazopyridotriazine-based derivatives so far (IC₅₀ hPDE2A = 3.0 nM; IC₅₀ hPDE10A > 1000 nM). Radiolabelling was performed in a one-step procedure starting from the corresponding tosylate precursor. In vitro autoradiography on rat and pig brain slices displayed a homogenous and non-specific binding of the radioligand. Investigation of stability in vivo by reversed-phase HPLC (RP-HPLC) and micellar liquid chromatography (MLC) analyses of plasma and brain samples obtained from mice revealed a high fraction of one main radiometabolite. Hence, we concluded that [¹⁸F]TA5 is not appropriate for molecular imaging of PDE2A neither in vitro nor in vivo. Our ongoing work is focusing on further structurally modified compounds with enhanced metabolic stability.

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.

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.