Design,synthesis and pharmacological evaluation of novel polycyclic heteroarene ethers as PDE10A inhibitors: Part I

We report analogue-based rational design and synthesis of two novel series of polycyclic heteroarenes, pyrrolo[3,2-b]quinolines and pyrido[2,3-b]indoles, tethered to a biaryl system by a methyl-, ethyl- or propyl ether as PDE10A inhibitors. A number of analogues were prepared with variable chain length and evaluated for their ability to block PDE10A enzyme using a radiometric assay. Detailed SAR analyses revealed that compounds with an ethyl ether linker are superior in potency compared to compounds with methyl or propyl ether linkers. These compounds, in general, showed poor metabolic stability in rat and human liver microsomes. The metabolic profile of one of the potent compounds was studied in detail to identify metabolic liabilities of these compounds. Structural modifications were carried out that resulted in improved metabolic stability without significant loss of potency.     

N-Acylhydrazones as inhibitors of PDE10A

Cyclic nucleotide phosphodiesterases (PDEs) are represented by a large superfamily of enzymes. A series of hydrazone-based inhibitors was synthesized and shown to be novel, potent, and selective against PDE10A. Optimized compounds of this class were efficacious in animal models of schizophrenia and may be useful for the treatment of this disease.     

Synthesis and preliminary biological evaluation of potent and selective 2-(3-alkoxy-1-azetidinyl) quinolines as novel PDE10A inhibitors with improved solubility

We report the discovery of a novel series of 2-(3-alkoxy-1-azetidinyl) quinolines as potent and selective PDE10A inhibitors. Structure–activity studies improved the solubility (pH 7.4) and maintained high PDE10A activity compared to initial lead compound 3, with select compounds demonstrating good oral bioavailability. X-ray crystallographic studies revealed two distinct binding modes to the catalytic site of the PDE10A enzyme. An ex vivo receptor occupancy assay in rats demonstrated that this series of compounds covered the target within the striatum.     

Design and synthesis of potent and selective pyridazin-4(1H)-one-based PDE10A inhibitors interacting with Tyr683 in the PDE10A selectivity pocket

Utilizing structure-based drug design techniques, we designed and synthesized phosphodiesterase 10A (PDE10A) inhibitors based on pyridazin-4(1H)-one. These compounds can interact with Tyr683 in the PDE10A selectivity pocket. Pyridazin-4(1H)-one derivative 1 was linked with a benzimidazole group through an alkyl spacer to interact with the OH of Tyr683 and fill the PDE10A selectivity pocket. After optimizing the linker length, we identified 1-(cyclopropylmethyl)-5-[3-(1-methyl-1H-benzimidazol-2-yl)propoxy]-3-(1-phenyl-1H-pyrazol-5-yl)pyridazin-4(1H)-one (16f) as having highly potent PDE10A inhibitory activity (IC₅₀ = 0.76 nM) and perfect selectivity against other PDEs (>13,000-fold, IC₅₀ = >10,000 nM). The crystal structure of 16f bound to PDE10A revealed that the benzimidazole moiety was located deep within the PDE10A selectivity pocket and interacted with Tyr683. Additionally, a bidentate interaction existed between the 5-alkoxypyridazin-4(1H)-one moiety and the conserved Gln716 present in all PDEs.     

Discovery of triazines as selective PDE4B versus PDE4D inhibitors

In this study we report a series of triazine derivatives that are potent inhibitors of PDE4B. We also provide a series of structure activity relationships that demonstrate the triazine core can be used to generate subtype selective inhibitors of PDE4B versus PDE4D. A high resolution co-crystal structure shows that the inhibitors interact with a C-terminal regulatory helix (CR3) locking the enzyme in an inactive ‘closed’ conformation. The results show that the compounds interact with both catalytic domain and CR3 residues. This provides the first structure-based approach to engineer PDE4B-selective inhibitors.     

Novel,potent, selective,and brain penetrant phosphodiesterase 10A inhibitors

Herein we report the discovery of a novel series of phosphodiesterase 10A inhibitors. Optimization of a HTS hit (17) resulted in potent, selective, and brain penetrant 23 and 26; both exhibited much lower clearance in vivo and decreased volume of distribution (rat PK) and have thus the potential to inhibit the PDE10A target in vivo at a lower efficacious dose than the reference compound WEB-3.     

Synthesis and biological evaluation of Vinpocetine derivatives

A new series of Vinpocetine derivatives were synthesized and evaluated for their inhibitory activity on PDE1A in vitro. Seven compounds with higher inhibitory activity were selected for surface plasmon resonance (SPR) binding experiments. Compared with Vinpocetine, these high potency compounds presented a higher binding affinity with PDE1A, which was consistent with inhibitory activity. After further screening, compounds 5, 7, 21, 34 and Vinpocetine were selected to examine the vasorelaxant effects on endothelium-intact rat thoracic aortic rings. The study suggested that the effects of compounds 7 and 21 were the most significant with the maximum value of 93.46 ± 0.77% and 92.90 ± 0.78% (n = 5) at a concentration of 100 μM respectively. Based on these studies, compounds 7 and 21 were considered for further development as hit compounds.     

Novel 1-alkynyl substituted 1,2-dihydroquinoline derivatives from nimesulide (and their 2-oxo analogues): a new strategy to identify inhibitors of PDE4B

A number of novel 1-(3-arylprop-2-ynyl) substituted 1,2-dihydroquinoline derivatives related to nimesulide and their 2-oxo analogues have been designed as potential inhibitors of PDE4. All these compounds were synthesized by using Sonogashira coupling as a key step. In vitro PDE4B inhibitory properties and molecular modeling studies of some of the compounds synthesized are presented.     

Radiosyntheses and in vivo evaluation of carbon-11 PET tracers for PDE10A in the brain of rodent and nonhuman primate

The radiosyntheses and in vivo evaluation of four carbon-11 labeled quinoline group-containing radioligands are reported here. Radiolabeling of [¹¹C]1–4 was achieved by alkylation of their corresponding desmethyl precursors with [¹¹C]CH₃I. Preliminary biodistribution evaluation in Sprague-Dawley rats demonstrated that [¹¹C]1 and [¹¹C]2 had high striatal accumulation (at peak time) for [¹¹C]1 and [¹¹C]2 were 6.0-fold and 4.5-fold at 60 min, respectively. Following MP-10 pretreatment, striatal uptake in rats of [¹¹C]1 and [¹¹C]2 was reduced, suggesting that the tracers bind specifically to PDE10A. MicroPET studies of [¹¹C]1 and [¹¹C]2 in nonhuman primates (NHP) also showed good tracer retention in the striatum with rapid clearance from non-target brain regions. Striatal uptake (SUV) of [¹¹C]1 reached 1.8 at 30 min with a 3.5-fold striatum:cerebellum ratio. In addition, HPLC analysis of solvent extracts from NHP plasma samples suggested that [¹¹C]1 had a very favorable metabolic stability. Our preclinical investigations suggest that [¹¹C]1 is a promising candidate for quantification of PDE10A in vivo using PET.     

Design and synthesis of novel benzimidazole derivatives as phosphodiesterase 10A inhibitors with reduced CYP1A2 inhibition

A novel class of phosphodiesterase 10A (PDE10A) inhibitors with reduced CYP1A2 inhibition were designed and synthesized starting from 2-{[(1-phenyl-1H-benzimidazol-6-yl)oxy]methyl}quinoline (1). Introduction of an isopropyl group at the 2-position and a methoxy group at the 5-position of the benzimidazole ring of lead compound 1 resulted in the identification of 2-{[(2-isopropyl-5-methoxy-1-phenyl-1H-benzimidazol-6-yl)oxy]methyl}quinoline (25b), which exhibited potent PDE10A inhibitory activity with reduced CYP1A2 inhibitory activity compared to compound 1.     

Triazoloquinazolines as a novel class of phosphodiesterase 10A (PDE10A) inhibitors

Novel triazoloquinazolines have been found as phosphodiesterase 10A (PDE10A) inhibitors. Structure-activity studies improved the initial micromolar potency which was found in the lead compound by a 100-fold identifying 5-(1H-benzoimidazol-2-ylmethylsulfanyl)-2-methyl-[1,2,4]triazolo[1,5-c]quinazoline, 42 (PDE10A IC₅₀ = 12 nM) as the most potent compound from the series. Two X-ray structures revealed novel binding modes to the catalytic site of the PDE10A enzyme.     

Novel benzimidazole derivatives as phosphodiesterase 10A (PDE10A) inhibitors with improved metabolic stability

In this study, we report the identification of potent benzimidazoles as PDE10A inhibitors. We first identified imidazopyridine 1 as a high-throughput screening hit compound from an in-house library. Next, optimization of the imidazopyridine moiety to improve inhibitory activity gave imidazopyridinone 10b. Following further structure–activity relationship development by reducing lipophilicity and introducing substituents, we acquired 35, which exhibited both improved metabolic stability and reduced CYP3A4 time-dependent inhibition.     

Docking and 3-D QSAR studies of dual PDE4-PDE7 inhibitors†

Small dual-specificity molecules inhibiting PDE4 and PDE7 can be used to treat inflammatory diseases. To design and synthesize dual PDE4 and PDE7 inhibitors, we carried out the target-based docking and the 3D QSAR study using CoMFA. Three compounds were synthesized. We predicted their inhibitory activities using our 3D QSAR model and tested their activities against PDE4 and PDE7 in vitro.     

Synthesis of 2H-1,3-benzoxazin-4(3H)-one derivatives containing indole moiety: Their in vitro evaluation against PDE4B

A number of 2H-1,3-benzoxazin-4(3H)-one derivatives containing indole or benzofuran moieties were synthesized by using Pd/C–Cu mediated coupling-cyclization strategy as a key step. The o-iodoanilides or o-iodophenol were coupled with 3-{2-(prop-2-ynyloxy)ethyl}-2H-benzo[e][1,3]oxazin-4(3H)-one using 10%Pd/C–CuI–PPh₃ as a catalyst system and Et₃N as a base to give the target compounds. All the synthesized compounds were tested for their PDE4B inhibitory potential in vitro using a cell based cAMP reporter assay. Some of them showed fold increase of the cAMP level when tested at 30 μM. A representative compound showed encouraging PDE4B inhibitory properties that were supported by its docking results.     

Synthesis of 2,2,4-trimethyl-1,2-dihydroquinolinyl substituted 1,2,3-triazole derivatives: Their evaluation as potential PDE 4B inhibitors possessing cytotoxic properties against cancer cells

The 2,2,4-trimethyl-1,2-dihydroquinolinyl substituted 1,2,3-triazole derivatives were designed as potential inhibitors of PDE4B. These compounds were synthesized via a multi-step sequence consisting of copper-catalyzed azide-alkyne cycloaddition (CuAAC) as a key step in aqueous media. The required alkynes were prepared from nimesulide via N-propargylation and then nitro group reduction followed by a CAN mediated modified Skraup reaction of the resulting amine. All the synthesized compounds showed PDE4B inhibitory properties in vitro at 30 μM with two compounds showing >50% inhibition that were supported by the in silico docking results of these compounds at the active site of PDE4B. Three of these PDE4 inhibitors showed promising cytotoxic properties against A549 human lung cancer cells in vitro with IC₅₀ ∼8–9 μM.     

Local anti-inflammatory effect and behavioral studies on new PDE4 inhibitors

Phosphodiesterase 4 (PDE4) inhibitors are effective anti-inflammatory drugs, although some adverse effects are observed in animals and humans. These effects have forced researchers to find new PDE4 inhibitors with less adverse effects. We recently reported the synthesis of novel heterocyclic-fused pyridazinones that inhibit PDE4. As a first step in the study of the anti-inflammatory properties of these compounds, we studied the effects of local administration of these pyridazinone derivatives in a mouse model of acute inflammation. We found that 6-Benzyl-3-methyl-4-phenylpyrazolo[3,4-d]pyridazin-7(6H)-one (CC4), ethyl 6,7-dihydro-6-ethyl-3-methyl-7-oxo-4-phenyl-thieno[2,3-d]pyridazine-2-carboxylate (CC6) and ethyl 6,7-dihydro-6-ethyl-3-methyl-4-phenyl-1H-pyrrolo[2,3-d]pyridazine-2-carboxylate (CC12) reduced the paw edema induced by zymosan in mice as rolipram (the PDE4 inhibitor prototype with anti-inflammatory activity) and indomethacin did. It is well known that rolipram locally administered induces some adverse effects such as hyperalgesia. Thus, we studied this effect after local administration of CC4, CC6 and CC12 in the formalin test. We found that CC6 induced hyperalgesic effects, whereas CC4 and CC12 did not change the nociceptive threshold. Furthermore, we found that rolipram and CC6 reduced locomotor activity, whereas CC4 and CC12 did not change locomotor performance of the mice. Since CC4 and CC12 neither affected the nociceptive threshold nor changed the locomotor performance of mice, they appear more suitable than CC6 for future studies on animals and could be developed as an anti-inflammatory drug for humans.     

Synthesis and biological evaluation of nimesulide based new class of triazole derivatives as potential PDE4B inhibitors against cancer cells

A new class of 1,2,3-triazol derivatives derived from nimesulide was designed as potential inhibitors of PDE4B. Synthesis of these compounds was carried out via a multi-step sequence consisting of copper-catalyzed azide–alkyne cycloaddition (CuAAC) as a key step in aqueous media. The required azide was prepared via the reaction of aryl amine (obtained from nimesulide) with α-chloroacetyl chloride followed by displacing the α-chloro group by an azide. Some of the synthesized compounds showed encouraging PDE4B inhibitory properties in vitro that is >50% inhibition at 30 μM that were supported by the docking studies of these compounds at the active site of PDE4B enzyme (dock scores ～ ?28.6 for a representative compound). Two of these PDE4 inhibitors showed promising cytotoxic properties against HCT-15 human colon cancer cells in vitro with IC₅₀ ～ 21–22 μg/mL.     

Synthesis,SAR study,and biological evaluation of novel 2,3-dihydro-1H-imidazo[1,2-a]benzimidazole derivatives as phosphodiesterase 10A inhibitors

Phosphodiesterase 10A (PDE10A) inhibitors were designed and synthesized based on the dihydro-imidazobenzimidazole scaffold. Compound 5a showed moderate inhibitory activity and good permeability, but unfavorable high P-glycoprotein (P-gp) liability for brain penetration. We performed an optimization study to improve both the P-gp efflux ratio and PDE10A inhibitory activity. As a result, 6d was identified with improved P-gp liability and high PDE10A inhibitory activity. Compound 6d also showed satisfactory brain penetration, suppressed phencyclidine-induced hyperlocomotion and improved MK-801-induced working memory deficit.     

Increased social interaction in mice deficient of the striatal medium spiny neuron-specific phosphodiesterase 10A2

Cyclic nucleotide phosphodiesterase 10A (PDE10A) is a member of phosphodiesterase families that degrade cAMP and/or cGMP in distinct intracellular sites. PDE10A has a dual activity on hydrolysis of both cAMP and cGMP, and is prominently expressed in the striatum and the testis. Previous studies suggested that PDE10A is involved in regulation of locomotor activity and potentially related to psychosis, but concrete physiological roles of PDE10A remains elusive yet. In this study, we genetically inactivated PDE10A2, a prominent isoform of PDE10A in the brain, in mice, and demonstrate that PDE10A2 deficiency results in increased social interaction without any major influence on different other behaviors, along with increased levels of striatal cAMP. We also demonstrate that PDE10A2 is selectively distributed in medium spiny neurons, but not interneurons, of the striatal complex. Thus, our results establish a physiological role for PDE10A2 in regulating cAMP pathway and social interaction, and suggest that cAMP signaling cascade in striatal medium spiny neurons might be involved in regulating social interaction behavior in mice.     

Synthesis and bioactivity of 3,5-dimethylpyrazole derivatives as potential PDE4 inhibitors

A series of 3,5-dimethylpyrazole derivatives containing 5-phenyl-2-furan moiety were designed and synthesized as phosphodiesterase type 4 (PDE4) inhibitors. Bioassay results showed that the title compounds exhibited considerable inhibitory activity against PDE4B and blockade of LPS-induced TNFα release. Among the designed compounds, compound If showed the best inhibitory activity against PDE4B with the IC₅₀ value of 1.7?μM, which also showed good in vivo activity in animal models of asthma/COPD and sepsis induced by LPS. The primary structure–activity relationship (SAR) study and docking results suggested that introduction of the substituent groups to the phenyl ring at the para-position, especially methoxy group, was helpful to enhance inhibitory activity against PDE4B.