The potential therapeutic use of phosphodiesterase 10 inhibitors

Pivotal role of phosphodiesterase 10A in the integration of dopamine signals in mice striatal D1 and D2 medium-sized spiny neurones

BACKGROUND AND PURPOSE

Dopamine in the striatum plays a crucial role in reward processes and action selection. Dopamine signals are transduced by D₁ and D₂ dopamine receptors which trigger mirror effects through the cAMP/PKA signalling cascade in D1 and D2 medium-sized spiny neurones (MSNs). Phosphodiesterases (PDEs), which determine the profile of cAMP signals, are highly expressed in MSNs, but their respective roles in dopamine signal integration remain poorly understood.

EXPERIMENTAL APPROACH

We used genetically-encoded FRET biosensors to monitor at the single cell level the functional contribution of PDE2A, PDE4 and PDE10A in the changes of the cAMP/PKA response to transient and continuous dopamine in mouse striatal brain slices.

KEY RESULTS

We found that PDE2A, PDE4 and PDE10A operate on the moderate to high cAMP levels elicited by D₁ or A_2A receptor stimulation. In contrast, only PDE10A is able to reduce cAMP down to baseline in both type of neurones, leading to the dephosphorylation of PKA substrates.

CONCLUSION AND IMPLICATIONS

In both MSN types, PDE10A inhibition blunts the responsiveness to dopamine, whereas PDE2A or PDE4 inhibition reinforces dopamine action.

Synthesis and characterization of novel classes of PDE10A inhibitors - 1H-1,3-benzodiazoles and imidazo[1,2-a]pyrimidines

New compounds containing [1,2,4]triazolo [1,5-a]pyridine (I), pyrazolo [1,5-a]pyridine (II), 1H-1,3-benzodiazole (III) and imidazo [1,2-a]pyrimidine (IV) backbones were designed and synthesized for PDE10A interaction. Among these compounds, 1H-1,3-benzodiazoles and imidazo [1,2-a]pyrimidines showed the highest affinity for PDE10A enzyme as well as good metabolic stability. Both classes of compounds were identified as selective and potent PDE10A enzyme inhibitors.

Inhibitors of phosphodiesterase as cancer therapeutics

Phosphodiesterases (PDEs) are a class of enzymes that hydrolyze cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which is involved in many physiological processes including visual transduction, cell proliferation and differentiation, cell-cycle regulation, gene expression, inflammation, apoptosis, and metabolic function. PDEs are composed of 11 different families and each family contains different subtypes. The distribution, expression, regulation mode and sensitivity to inhibitors of each subtype are different, and they are involved in cancer, inflammation, asthma, depression, erectile dysfunction and other pathological processes of development. A large number of studies have shown that PDEs play an important role in the development of tumors by affecting the intracellular level of cAMP and/or cGMP and PDEs could become diagnostic markers or therapeutic targets. This review will give a brief overview of the expression and regulation of PDE families in the process of tumorigenesis and their anti-tumor inhibitors, which may guide the design of novel therapeutic drugs targeting PDEs for anticancer agent.

[Physiopathology of cAMP/PKA signaling in neurons]

Cyclic adenosine monophosphate (cAMP) and the cyclic-AMP dependent protein kinase (PKA) regulate a plethora of cellular functions in virtually all eukaryotic cells. In neurons, the cAMP/PKA signaling cascade controls a number of biological properties such as axonal growth, synaptic transmission, regulation of excitability or long term changes in the nucleus. Genetically-encoded optical biosensors for cAMP or PKA considerably improved our understanding of these processes by providing a real-time measurement in living neurons. In this review, we describe the recent progresses made in the creation of biosensors for cAMP or PKA activity. These biosensors revealed profound differences in the amplitude of the cAMP signal evoked by neuromodulators between various neuronal preparations. These responses can be resolved at the level of individual neurons, also revealing differences related to the neuronal type. At the subcellular level, biosensors reported different signal dynamics in domains like dendrites, cell body, nucleus and axon. Combining this imaging approach with pharmacology or genetical models points at phosphodiesterases and phosphatases as critical regulatory proteins. Biosensor imaging will certainly help understand the mechanism of action of current drugs as well as help in devising novel therapeutic strategies for neuropsychiatric diseases.

Development of highly potent phosphodiesterase 10A (PDE10A) inhibitors: Synthesis and in vitro evaluation of 1,8-dipyridinyl- and 1-pyridinyl-substituted imidazo[1,5-a]quinoxalines

Herein we report the synthesis of fluorinated inhibitors of phosphodiesterase 10A (PDE10A) which can be used potentially as lead structure for the development of a (18)F-labeled PDE10A imaging agent for positron emission tomography. The use of ortho-fluoropyridines as residues could potentially enable the introduction of (18)F through nucleophilic substitution for radiolabeling purposes. 2-Fluoropyridines are introduced by a Suzuki coupling at different positions of the molecule. The reference compounds, 1,8-dipyridinylimidazo[1,5-a]quinoxalines and 1-pyridinylimidazo[1,5-a]quinoxalines, show inhibitory potencies at best in the subnanomolar range and selectivity factors greater than 38 against other PDE's. 1,8-Dipyridinylimidazo[1,5-a]quinoxalines are more potent inhibitors than 1-pyridinylimidazo[1,5-a]quinoxalines. Using 2-fluoro-3-pyridinyl as residue provided the most potent inhibitors 16 (IC50 = 0.12 nM), 17 (IC50 = 0.048 nM) and 32 (IC50 = 0.037 nM).

A comparative study of binding affinities for 6,7-dimethoxy-4-pyrrolidylquinazolines as phosphodiesterase 10A inhibitors using the linear interaction energy method

The linear interaction energy (LIE) method was used to estimate the free energies of binding for a set of 27 pyrrolidylquinazoline derivatives as phosphodiesterase 10A inhibitors. Twenty-six X-ray crystal structures of phosphodiesterase 10A and two sampling methods, minimization and Hybrid Monte Carlo, were used to assess the affinity models based on the linear interaction energies. The best model was obtained based on the parameters α=0.16 and β=0.04, which represent non-polar and polar interactions, respectively, with a root mean square error (RMSE) of 0.42kcal/mol (R(2)=0.71) and 0.52kcal/mol (R(2)=0.86) for the training and test sets, respectively. In addition, the applicability domain of the model was investigated. After validation of the models, the best model was subsequently used in a virtual screening process, which resulted in a set of optimized compounds. The models developed in this study could be useful as filter for virtual screening and lead optimization processes for phosphodiesterase 10A drug developments.

Discovery of clinical candidate 1-(4-(3-(4-(1H-benzo[d]imidazole-2-carbonyl)phenoxy)pyrazin-2-yl)piperidin-1-yl)ethanone (AMG 579), a potent, selective, and efficacious inhibitor of phosphodiesterase 10A (PDE10A)

We report the identification of a PDE10A clinical candidate by optimizing potency and in vivo efficacy of promising keto-benzimidazole leads 1 and 2. Significant increase in biochemical potency was observed when the saturated rings on morpholine 1 and N-acetyl piperazine 2 were changed by a single atom to tetrahydropyran 3 and N-acetyl piperidine 5. A second single atom modification from pyrazines 3 and 5 to pyridines 4 and 6 improved the inhibitory activity of 4 but not 6. In the in vivo LC-MS/MS target occupancy (TO) study at 10 mg/kg, 3, 5, and 6 achieved 86-91% occupancy of PDE10A in the brain. Furthermore, both CNS TO and efficacy in PCP-LMA behavioral model were observed in a dose dependent manner. With superior in vivo TO, in vivo efficacy and in vivo PK profiles in multiple preclinical species, compound 5 (AMG 579) was advanced as our PDE10A clinical candidate.

PDE10A inhibitors stimulate or suppress motor behavior dependent on the relative activation state of the direct and indirect striatal output pathways

The enzyme phosphodiesterase 10A (PDE10A) regulates the activity of striatal, medium spiny neurons (MSNs), which are divided into a behaviorally stimulating, Gs-coupled D₁ receptor-expressing “direct” pathway and a behaviorally suppressant, Gi-coupled D₂ receptor-expressing “indirect” pathway. Activating both pathways, PDE10A inhibitors (PDE10AIs) combine functional characteristics of D₂ antagonists and D₁ agonists. While the effects of PDE10AIs on spontaneous and stimulated behavior have been extensively reported, the present study investigates their effects on suppressed behavior under various conditions of reduced dopaminergic neurotransmission: blockade of D₁ receptors with SCH-23390, blockade of D₂ receptors with haloperidol, or depletion of dopamine with RO-4-1284 or reserpine. In rats, PDE10AIs displayed relatively low cataleptic activity per se. After blocking D₁ receptors, however, they induced pronounced catalepsy at low doses close to those required for inhibition of apomorphine-induced behavior; slightly higher doses resulted in behavioral stimulant effects, counteracting the catalepsy. PDE10AIs also counteracted catalepsy and related behaviors induced by D₂ receptor blockade or dopamine depletion; catalepsy was replaced by behavioral stimulant effects under the latter but not the former condition. Similar interactions were observed at the level of locomotion in mice. At doses close to those inhibiting d-amphetamine-induced hyperlocomotion, PDE10AIs reversed hypolocomotion induced by D₁ receptor blockade or dopamine depletion but not hypolocomotion induced by D₂ receptor blockade. It is concluded that PDE10AIs stimulate or inhibit motor behavior dependent on the relative activation state of the direct and indirect striatal output pathways.

Pharmacology of JNJ-42314415, a centrally active phosphodiesterase 10A (PDE10A) inhibitor: a comparison of PDE10A inhibitors with D2 receptor blockers as potential antipsychotic drugs

The new phosphodiesterase 10A inhibitor (PDE10AI) JNJ-42314415 [3-[6-(2-methoxyethyl)pyridin-3-yl]-2-methyl-8-morpholin-4-ylimidazo[1,2-a]pyrazine] was compared with three reference PDE10AIs and eight dopamine 2 (D(2)) receptor blockers. Despite displaying relatively low PDE10A activity in vitro, JNJ-42314415 was found to be a relatively potent and specific PDE10AI in vivo. The compound was devoid of effects on prolactin release and of receptor interactions associated with other commonly observed adverse effects of available antipsychotics. Similar to D(2) receptor blockers, the tested PDE10AIs antagonized stimulant-induced behavior and inhibited conditioned avoidance behavior; these effects were observed at doses close to the ED(50) for striatal PDE10A occupancy. Relative to the ED(50) for inhibition of apomorphine-induced stereotypy, PDE10AIs blocked conditioned avoidance behavior and behaviors induced by nondopaminergic stimulants (phencyclidine, scopolamine) more efficiently than did D(2) receptor blockers; however, they blocked behaviors induced by dopaminergic stimulants (apomorphine, d-amphetamine) less efficiently. PDE10AIs also induced less pronounced catalepsy than D(2) receptor blockers. The effects of PDE10A inhibition against dopaminergic stimulants and on catalepsy were potentiated by the D(1) antagonist SCH-23390 (8-chloro-3-methyl-5-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-ol), suggesting that enhancement of D(1) receptor-mediated neurotransmission contributes to the behavioral profile of PDE10AIs. By reducing dopamine D(2) and concomitantly potentiating dopamine D(1) receptor-mediated neurotransmission, PDE10AIs may show antipsychotic activity with an improved side-effect profile relative to D(2) receptor blockers. However, the clinical implications of this dual mechanism must be further explored.

Phosphodiesterase 10A inhibitor MP-10 effects in primates: comparison with risperidone and mechanistic implications

Phosphodiesterase 10A (PDE10A) is highly expressed in striatal medium spiny neurons of both the direct and indirect output pathways. Similar to dopamine D₂ receptor antagonists acting on indirect pathway neurons, PDE10A inhibitors have shown behavioral effects in rodent models that predict antipsychotic efficacy. These findings have supported the clinical investigation of PDE10A inhibitors as a new treatment for schizophrenia. However, PDE10A inhibitors and D₂ antagonists differ in effects on direct pathway and other neurons of the basal ganglia, indicating that these two drug classes may have divergent antipsychotic efficacy and side effect profile. In the present study, we compare the behavioral effects of the selective PDE10A inhibitor MP-10 to those of the clinical standard D₂ antagonist risperidone in rhesus monkeys using a standardized motor disability scale for parkinsonian primates and a newly designed "Drug Effects on Nervous System" scale to assess non-motor effects. Behavioral effects of MP-10 correlated with its plasma levels and its regulation of metabolic activity in striatal and cortical regions as measured by FDG-PET imaging. While MP-10 and risperidone broadly impacted similar behavioral domains in the primate, their effects had a different underlying basis. MP-10-treated animals retained the ability to respond but did not engage tasks, whereas risperidone-treated animals retained the motivation to respond but were unable to perform the intended actions. These findings are discussed in light of what is currently known about the modulation of striatal circuitry by these two classes of compounds, and provide insight into interpreting emerging clinical data with PDE10A inhibitors for the treatment of psychotic symptoms.

Synthesis and in vitro biological evaluation of pyrazole group-containing analogues for PDE10A

Twenty eight new analogues were synthesized by optimizing the structure of MP-10 and their in vitro binding affinities towards PDE10A, PDE3A/B, and PDE4A/B were determined. Among these new analogues, 10a, 10b, 10d, 11a, 11b and 11d are very potent towards PDE10A and have IC₅₀ values of 0.40 ± 0.02, 0.28 ± 0.06, 1.82 ± 0.25, 0.24 ± 0.05, 0.36 ± 0.03 and 1.78 ± 0.03 nM respectively; these six compounds displayed high selectivity for PDE10A versus PDE3A/3B/4A/4B. The promising compounds will be further validated in vivo to identify PDE10A imaging tracers.

Rapid identification of a novel small molecule phosphodiesterase 10A (PDE10A) tracer

A radiolabeled tracer for imaging therapeutic targets in the brain is a valuable tool for lead optimization in CNS drug discovery and for dose selection in clinical development. We report the rapid identification of a novel phosphodiesterase 10A (PDE10A) tracer candidate using a LC-MS/MS technology. This structurally distinct PDE10A tracer, AMG-7980 (5), has been shown to have good uptake in the striatum (1.2% ID/g tissue), high specificity (striatum/thalamus ratio of 10), and saturable binding in vivo. The PDE10A affinity (K(D)) and PDE10A target density (B(max)) were determined to be 0.94 nM and 2.3 pmol/mg protein, respectively, using [(3)H]5 on rat striatum homogenate. Autoradiography on rat brain sections indicated that the tracer signal was consistent with known PDE10A expression pattern. The specific binding of [(3)H]5 to rat brain was blocked by another structurally distinct, published PDE10A inhibitor, MP-10. Lastly, our tracer was used to measure in vivo PDE10A target occupancy of a PDE10A inhibitor in rats using LC-MS/MS technology.

Radiosynthesis and Radiotracer Properties of a 7-(2-[18F]Fluoroethoxy)-6-methoxypyrrolidinylquinazoline for Imaging of Phosphodiesterase 10A with PET

Phosphodiesterase 10A (PDE10A) is a key enzyme of intracellular signal transduction which is involved in the regulation of neurotransmission. The molecular imaging of PDE10A by PET is expected to allow a better understanding of physiological and pathological processes related to PDE10A expression and function in the brain. The aim of this study was to develop a new ¹⁸F-labeled PDE10A ligand based on a 6,7-dimethoxy-4-pyrrolidinylquinazoline and to evaluate its properties in biodistribution studies. Nucleophilic substitution of the 7-tosyloxy-analogue led to the 7-[¹⁸F]fluoroethoxy-derivative [¹⁸F]IV with radiochemical yields of 25% ± 9% (n = 9), high radiochemical purity of ≥99% and specific activities of 110–1,100 GBq/μmol. [¹⁸F]IV showed moderate PDE10A affinity (K_D,PDE10A = 14 nM) and high metabolic stability in the brain of female CD-1 mice, wherein the radioligand entered rapidly with a peak uptake of 2.3% ID/g in striatum at 5 min p.i. However, ex vivo autoradiographic and in vivo blocking studies revealed no target specific accumulation and demonstrated [¹⁸F]IV to be inapplicable for imaging PDE10A with PET.

Preclinical evaluation of 18F-JNJ41510417 as a radioligand for PET imaging of phosphodiesterase-10A in the brain

Phosphodiesterases are enzymes that inactivate the intracellular second messengers 3',5'-cyclic adenosine-monophosphate and/or cyclic guanosine-monophosphate. Of all 11 known phosphodiesterase families, phosphodiesterase-10A (PDE10A) has the most restricted distribution, with high expression in the striatum. PDE10A inhibitors are pursued as drugs for treatment of neuropsychiatric disorders. We have synthesized and evaluated (18)F-JNJ41510417 as a selective and high-affinity radioligand for in vivo brain imaging of PDE10A using PET.

METHODS

The biodistribution of (18)F-JNJ41510417 was evaluated in rats. Rat plasma and perfused brain homogenates were analyzed by high-performance liquid chromatography to quantify radiometabolites. Dynamic small-animal PET was performed in rats and in wild-type and PDE10A knock-out mice and compared with ex vivo autoradiography. Blocking and displacement experiments were performed using the nonradioactive analog and other selective PDE10A inhibitors.

RESULTS

Tissue distribution studies showed predominant hepatobiliary excretion, sufficient brain uptake (0.56 ± 0.00 percentage injected dose at 2 min after tracer injection), and continuous accumulation of the tracer in the striatum over time; rapid washout of nonspecific binding from other brain regions was observed. Polar radiometabolites were detected in plasma and brain tissue. Dynamic small-animal PET showed continuous tracer accumulation in the striatum, with rapid decline in the cortex and cerebellum. Pretreatment and chase experiments with PDE10A inhibitors showed that the tracer binding to PDE10A was specific and reversible. Imaging in PDE10A knock-out and wild-type mice further confirmed that binding in the striatum was specific for PDE10A.

CONCLUSION

Experiments in rats and PDE10A knock-out mice indicate that (18)F-JNJ41510417 binds specifically and reversibly to PDE10A in the striatum, suggesting that this new fluorinated quinoline derivative is a promising candidate for in vivo imaging of PDE10A using PET.

Carbon-11 labeled papaverine as a PET tracer for imaging PDE10A: radiosynthesis, in vitro and in vivo evaluation

Papaverine, 1-(3,4-dimethoxybenzyl)-6,7-dimethoxyisoquinoline, a specific inhibitor of phosphodiesterase (PDE) 10A with IC(50) values of 36 nM for PDE10A, 1,300 nM for PDE3A and 320 nM for PDE4D, has served as a useful pharmaceutical tool to study the physiological role of PDE10A. Here, we report the radiosynthesis of [(11)C]papaverine and the in vitro and in vivo evaluation of [(11)C]papaverine as a potential positron emission tomography (PET) radiotracer for imaging PDE10A in the central nervous system (CNS). The radiosynthesis of papaverine with (11)C was achieved by O-methylation of the corresponding des-methyl precursor with [(11)C]methyl iodide. [(11)C]papaverine was obtained with approximately 70% radiochemical yield and a specific activity >10 Ci/mumol. In vitro autoradiography studies of rat and monkey brain sections revealed selective binding of [(11)C]papaverine to PDE10A enriched regions: the striatum of rat brain and the caudate and putamen of rhesus monkey brain. The biodistribution of [(11)C]papaverine in rats at 5 min demonstrated an initially higher accumulation in striatum than in other brain regions, however the washout was rapid. MicroPET imaging studies in rhesus macaques similarly displayed initial specific uptake in the striatum with very rapid clearance of [(11)C]papaverine from brain. Our initial evaluation suggests that despite papaverine's utility for in vitro studies and as a pharmaceutical tool, [(11)C]papaverine is not an ideal radioligand for clinical imaging of PDE10A in the CNS. Analogs of papaverine having a higher potency for inhibiting PDE10A and improved pharmacokinetic properties will be necessary for imaging this enzyme with PET.

The discovery of potent, selective, and orally bioavailable PDE9 inhibitors as potential hypoglycemic agents

Starting from a non-selective pyrazolo-pyrimidone lead, the sequential use of parallel medicinal chemistry and directed synthesis led to the discovery of potent, highly selective, and orally bioavailable PDE9 inhibitors. The availability of these tools allowed for a thorough evaluation of the therapeutic potential of PDE9 inhibition.