Discovery of 2-(4-Pyridin-2-ylpiperazin-1-ylmethyl)-1H-benzimidazole (ABT-724), a Dopaminergic Agent with a Novel Mode of Action for the Potential Treatment of Erectile Dysfunction

Uncovering the mechanisms of leech and centipede granules in the treatment of diabetes mellitus-induced erectile dysfunction utilising network pharmacology

ETHNOPHARMACOLOGICAL RELEVANCE

Diabetes mellitus-induced erectile dysfunction (DMED) is one of the most common complications of diabetes mellitus. Leech and centipede granules (LCG) have traditionally been used as blood-activating agents in various ethnomedicinal systems of East Asia, especially in China. It is often used to regulate bodily functions and considered as adjuvant therapy for promoting blood circulation, alleviating blood coagulation, activating meridians, and relieving stasis.

AIM OF THE STUDY

This study aimed to identify potential genes and mechanisms of LCG on DMED from the network pharmacological perspective.

MATERIALS AND METHODS

The active components of LCG were identified by UHPLC-Q-TOF-MS, TCMID, and the BATMAN-TCM databases, and the disease targets of DMED were obtained from the DisGeNET, CooLGeN, GeneCards databases. After identifying DMED targets of LCG, a protein-protein interaction (PPI) network was constructed. Hub genes and significant modules were identified via the MCODE plug-in of Cytoscape software. Then, significant signaling pathways of the modules were identified using the Metascape database. The probable interaction mode of compounds-hub genes is examined using Molecular Operating Environment (MOE) docking software. Besides, we investigated the effects and mechanisms of LCG on improving erectile function in the streptozotocin (STZ)-induced diabetic rats model.

RESULTS

Combined UHPLC-Q-TOF-MS analysis with network pharmacology study, 18 active compounds were selected for target prediction. There are 97 common target genes between LCG and DMED. Enrichment of the KEGG pathway mainly involves in the calcium signaling pathway, NF-kappa B signaling pathway, cGMP-PKG signaling pathway, HIF-1 signaling pathway, PI3K-Akt signaling pathway, and mTOR signaling pathway. Nine hub genes were regulated by LCG in DMED, including CXCL8, NOS3, CRH, TH, BDNF, DRD4, ACE, CNR1, and HTR1A. The results of molecular docking analysis showed that the tyrosin, ursolic acid, and L-Histidine has a relatively stable interaction with corresponding hub genes via generating hydrogen bonds, H-π, and π-π interactions. Significantly, the results in docking predicted a higher affinity of vardenafil to the hub genes compared to the tyrosin, ursolic acid, and L-Histidine. Furthermore, LCG increased the testosterone, erection frequency, the ratio of ICP and MAP, SOD, cGMP, cAMP as well as decreased the MDA, and AGEs expression levels. And, LCG ameliorated the histological change of penile tissues in DMED rats. Hence, LCG attenuates oxidative stress, increases NO production; For the mechanism exploration, LCG could significantly upregulate the mRNA and protein expression of CNR1, NOS3, CRH, TH, BDNF, and DRD4, whereas CXCL8, ACE, and HTR1A levels were significantly higher than those in the DMED group. Moreover, LCG activates the NO/cGMP/PKG pathway, PI3K/Akt/nNOS pathway, cAMP/PKA pathway, and inhibits the HIF-1α/mTOR pathway to improve erectile function.

CONCLUSIONS

Our results suggest that LCG maybe offer a new therapeutic basis for the treatment of DMED via altering the gene expression of involved metabolic pathways.

Radiosynthesis and evaluation of 18F-labeled dopamine D4-receptor ligands

INTRODUCTION

The dopamine D4 receptor (D4R) has attracted considerable attention as potential target for the treatment of a broad range of central nervous system disorders. Although many efforts have been made to improve the performance of putative radioligand candidates, there is still a lack of D4R selective tracers suitable for in vivo PET imaging. Thus, the objective of this work was to develop a D4-selective PET ligand for clinical applications.

METHODS

Four compounds based on previous and new lead structures were prepared and characterized with regard to their D4R subtype selectivity and predicted lipophilicity. From these, 3-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-1H-pyrrolo[2,3-b]pyridine I and (S)-4-(3-fluoro-4-methoxybenzyl)-2-(phenoxymethyl)morpholine II were selected for labeling with fluorine-18 and subsequent evaluation by in vitro autoradiography to assess their suitability as D4 radioligand candidates for in vivo imaging.

RESULTS

The radiosynthesis of [18F]I and [18F]II was successfully achieved by copper-mediated radiofluorination with radiochemical yields of 7% and 66%, respectively. The radioligand [18F]II showed specific binding in areas where D4 expression is expected, whereas [18F]I did not show any uptake in distinct brain regions and exhibited an unacceptable degree of non-specific binding.

CONCLUSIONS

The compounds studied exhibited high D4R subtype selectivity and logP values compatible with high brain uptake, but only ligand [18F]II showed low non-specific binding and is therefore a good candidate for further evaluation.

ADVANCES IN KNOWLEDGE

The discovery of new lead structures for high-affinity D4 ligands opens up new possibilities for the development of suitable PET-radioligands.

IMPLICATIONS FOR PATIENT

PET-imaging of dopamine D4-receptors could facilitate understanding, diagnosis and treatment of neuropsychiatric and neurodegenerative diseases.

Recent findings leading to the discovery of selective dopamine D4 receptor ligands for the treatment of widespread diseases

Since its discovery, the dopamine D₄ receptor (D₄R) has been suggested to be an attractive target for the treatment of neuropsychiatric diseases. Novel findings have renewed the interest in such a receptor as an emerging target for the management of different diseases, including cancer, Parkinson's disease, alcohol or substance use disorders, eating disorders, erectile dysfunction and cognitive deficits. The recently resolved crystal structures of D₄R in complexes with the potent ligands nemonapride and L-745870 strongly improved the knowledge on the molecular mechanisms involving the D₄R functions and may help medicinal chemists in drug design. This review is focused on the recent development of the subtype selective D₄R ligands belonging to classical or new chemotypes. Moreover, ligands showing functional selectivity toward G protein activation or β-arrestin recruitment and the effects of selective D₄R ligands on the above-mentioned diseases are discussed.

Central Mechanisms of Apomorphine and m-Chlorophenylpiperazine on Synergistic Action for Ejaculation in Rats

BACKGROUND

We previously reported that the combination of the dopamine (DA) receptor agonist apomorphine and the 5-hydroxytryptamine (5-HT₂) receptor agonist m-chlorophenylpiperazine (m-CPP) in rats potently and selectively facilitates the ejaculatory response through activation of D₂-like and 5-HT_2C receptors, respectively.

AIM

The aim of this study was to clarify the target level of the proejaculatory effects induced by combination of these agonists.

METHODS

For in vivo behavioral studies, apomorphine and m-CPP were given intracerebroventricularly and intrathecally alone or in combination with either drug administered systemically. Male rats were acclimated to observational cages bedded in paper towels, and the occurrence of ex copula ejaculation was assessed by evaluating the presence and weight of ejaculatory plugs dropped from the tip of the penis to the paper towels or adhered to the tip of the penis at 30 min after drug administration. For in vitro contraction studies, seminal vesicles isolated from rats were suspended in an organ bath to test contractile responses to drug combinations, and the effects of the combined drugs on the contractile response of noradrenaline were also tested.

MAIN OUTCOME MEASURES

The presence and weight of ejaculatory plugs produced by drug-induced ejaculation and the contractile responses of the seminal vesicle were evaluated.

RESULTS

Intrathecal m-CPP (10 μg), but not intracerebroventricular m-CPP, evoked the synergistic effects on ejaculation when used in combination with systemically administered apomorphine (0.1 mg/kg, subcutaneous). Moreover, the synergy between m-CPP and apomorphine was completely abolished by the intrathecal 5-HT_2C receptor antagonist SB242084 (10 μg). Intrathecal or intracerebroventricular apomorphine (1-10 μg) evoked proejaculatory effects in combination with systemically administered m-CPP (0.3 mg/kg, intraperitoneal). The selective peripherally acting D₂-like receptor agonist carmoxirole did not evoke ejaculation when used in combination with m-CPP. Furthermore, isolated rat seminal vesicles were completely insensitive to the combination of apomorphine and m-CPP.

CONCLUSION

These results indicated that the synergistic effects of the drugs on ejaculation were induced at the central level but not at peripheral sites. Our findings also suggested that the 5-HT_2C receptor mediated the stimulation of the spinal ejaculatory pattern generator and was synergistically potentiated by the spinal DA receptor and that activation of the supraspinal DA receptor was also involved in mediating these synergistic effects. Yoshizumi M, Yonezawa A, Kimura Y, et al. Central Mechanisms of Apomorphine and m-Chlorophenylpiperazine on Synergistic Action for Ejaculation in Rats. J Sex Med 2021;18:231-239.

Therapeutic Strategies for Erectile Dysfunction With Emphasis on Recent Approaches in Nanomedicine

This review addressed erectile dysfunction, regarding pathophysiology and therapeutic strategies. The line of treatment includes phosphodiesterase type-5 inhibitors and other types of therapy like topical and stem-cell transplant. Scientific literature was assessed to investigate the impact of nanotechnology on erectile dysfunction therapy. Various nanotechnology approaches were applied, like vesicular systems, lipid-based carriers, nanocrystals, dendrimers, liquid crystalline systems and nanoemulsions. Smart nano-systems can alter the landscape of the modern pharmaceutical industry by re- investigation of pharmaceutically suboptimal but biologically active entities for treatment of erectile dysfunction which were previously considered undeveloped.

Emerging Treatments for Erectile Dysfunction: a Review of Novel, Non-surgical Options

PURPOSE OF REVIEW

To review novel, non-surgical therapies for erectile dysfunction (ED).

RECENT FINDINGS

Recently, a landmark study identified the SIM1 locus, involved in the leptin-melanocortin pathway, as an independent risk factor for ED and a potential target for novel therapies. The recent literature otherwise has focused on low-intensity shock wave therapy (LiSWT), with several randomized trials and meta-analyses suggesting therapeutic efficacy. There are few novel oral agents for ED. There is growing evidence suggesting efficacy of intracavernosal stem cells therapy and low-intensity shock wave therapy (LiSWT), although these therapies are still investigational. A better understanding of the pathophysiologic spectrum of ED will offer new opportunities for novel, non-surgical therapies for ED.

Research in pharmacotherapy for erectile dysfunction

Although oral phosphodiesterase-5 (PDE5) inhibitors are generally accepted as an effective therapy for erectile dysfunction (ED), men with ED from diabetes or radical prostatectomy respond poorly to these drugs. Many researchers have tried to develop novel therapeutics that target alternative molecular pathways. A group of therapeutics belongs to centrally acting agents that target dopamine and melanocortin receptors. The other one is the peripherally acting agents that target soluble guanylate cyclase, Rho-kinase pathway, and Maxi-K channel, etc. Also, a variety of preclinical studies by the application of biotherapies in the concept of therapeutic angiogenesis or neural regeneration as well as anti-fibrosis to regenerate damaged erectile tissue have been reported. This article will address the current therapeutic targets for ED under clinical or preclinical development, including pharmacotherapy and biotherapy which comprises protein therapy and gene therapy. In spite of numerous clinical trials that target alternative pathways, these agents have yet to reach the market. The results from preclinical studies targeting therapeutic angiogenesis, neural regeneration, and anti-fibrosis are promising.

Advances in the treatment of erectile dysfunction: what's new and upcoming?

Erectile dysfunction adversely affects up to 20% of all men and is the most commonly treated sexual disorder. The public health implications of this condition are significant and represent a challenge for our healthcare system. The physiological pathways responsible for erections have been extensively studied, and much advancement has been made since the introduction of phosphodiesterase 5 inhibitors. Newer agents, such as dopaminergic and melanocortin receptor agonists, which target central erectogenic pathways, are under investigation. Newer formulations and delivery methods of existing medications such as alprostadil will also be introduced in the near future. Furthermore, low-intensity shockwave lithotripsy and stem cell regenerative techniques are innovative approaches to the treatment of erectile dysfunction.

Emerging drugs for the treatment of erectile dysfunction

INTRODUCTION

Erectile dysfunction adversely affects the lives of millions of men, and is the most commonly treated sexual disorder today. The erectile process has been extensively investigated, with major advances made in elucidating many of the complex molecular pathways involved. These advances have allowed researchers to design and study drug formulations that target various aspects of this complex process. The initial culmination of this research was the introduction of phosphodiesterase 5-inhibitors. While effective in many patients, they are not satisfactory for all afflicted men. As a result, researchers are developing novel drugs that target different molecular pathways.

AREAS COVERED

The paper will review these pathways, and the potential agents that target them. More specifically, first dopaminergic and melanocortin receptor agonists that act centrally will be covered. Then, the paper will examine the "second-generation" phosphodiesterase 5-inhibitors, soluble guanylate cyclases, rho-kinase inhibitors, and maxi-k channel activators that act peripherally.

EXPERT OPINION

Most of these novel drugs have yet to reach Phase III studies. However, it is likely that in years to come, patients will be selectively treated with these novel agents as a monotherapy or in combination with others acting in a synergistic manner.

Design and synthesis of benzimidazole analogs endowed with oxadiazole as selective COX-2 inhibitor

New molecules of benzimidazole endowed with oxadiazole were designed and synthesized from 2-(2-((pyrimidin-2-ylthio)methyl)-1H-benzo[d]imidazol-1-yl)acetohydrazide as 1-((5-substituted alkyl/aryl-1,3,4-oxadiazol-2-yl)methyl)-2-((pyrimidin-2-ylthio)methyl)-1H-benzimidazoles (5a-r) with the aim to acquire selective cyclooxygenase (COX-2) inhibitor activity. The synthesized compounds were screened by in vitro cyclooxygenase assays to determine COX-1 and COX-2 inhibitory potency and the results showed that they had good-to-remarkable activity with an IC50 range of 11.6-56.1 µM. The most active compounds were further screened for their in vivo anti-inflammatory activity by using the carrageenan-induced rat paw edema model. In vitro anticancer activities of the hybrid compounds were assessed by the National Cancer Institute (NCI), USA, against 60 human cell lines, and the results showed a good spectrum. Compound 5l exhibited significant COX-2 inhibition with an IC50 value of 8.2 µM and a percent protection of 68.4%. Compound 5b evinced moderate cytotoxicity toward the UO-31 cell line of renal cancer. A docking study was performed using Maestro 9.0, to provide the binding mode into the binding sites of the cyclooxygenase enzyme. Hopefully, in the future, compound 5l could serve as a lead compound for developing new COX-2 inhibitors.

Drug induced increases in CNS dopamine alter monocyte, macrophage and T cell functions: implications for HAND

Central nervous system (CNS) complications resulting from HIV infection remain a major public health problem as individuals live longer due to the success of combined antiretroviral therapy (cART). As many as 70 % of HIV infected people have HIV associated neurocognitive disorders (HAND). Many HIV infected individuals abuse drugs, such as cocaine, heroin or methamphetamine, that may be important cofactors in the development of HIV CNS disease. Despite different mechanisms of action, all drugs of abuse increase extracellular dopamine in the CNS. The effects of dopamine on HIV neuropathogenesis are not well understood, and drug induced increases in CNS dopamine may be a common mechanism by which different types of drugs of abuse impact the development of HAND. Monocytes and macrophages are central to HIV infection of the CNS and to HAND. While T cells have not been shown to be a major factor in HIV-associated neuropathogenesis, studies indicate that T cells may play a larger role in the development of HAND in HIV infected drug abusers. Drug induced increases in CNS dopamine may dysregulate functions of, or increase HIV infection in, monocytes, macrophages and T cells in the brain. Thus, characterizing the effects of dopamine on these cells is important for understanding the mechanisms that mediate the development of HAND in drug abusers.

Selective syntheses of no-carrier-added 2- and 3-[18F]fluorohalopyridines through the radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates

No-carrier-added 2- and 3-[(18)F]fluorohalopyridines were readily synthesized as potentially useful labeling synthons for prospective PET radiotracers through the selective radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates, themselves conveniently prepared in a single pot from iodohalopyridines.

Recent advances in the search for D3- and D4-selective drugs: probes, models and candidates

Dopamine D(2)-like receptors (including D(2), D(3) and D(4)) belong to the 'rhodopsin-like' family of G protein-coupled receptors (GPCRs), which represent the largest group of targets for bioactive molecules. Due to their high sequence similarity, the design of subtype-selective ligands requires rational and effective strategies. The general formula of 1,4-disubstituted aromatic piperidines and piperazines (1,4-DAPs) was extracted from classical dopaminergic drugs. The biological properties of this compound family are encoded by an aromatic head group that controls intrinsic activity, an amine moiety and a lipophilic appendage. D(3)- and D(4)-selective molecular probes and drug candidates were generated from the general formula of 1,4-DAP. Formal structural rearrangement led to investigational drugs beyond the 1,4-DAP structure. The very recent publication of the X-ray crystal structure of D(3) should facilitate efficient discovery of unprecedented chemotypes. However, the development of D(3)-selective agonists, functionally selective ligands and the exploitation of homo- and heteromers remain challenging.

The dopamine D4 receptor: biochemical and signalling properties

Dopamine is an important neurotransmitter that regulates several key functions in the brain, such as motor output, motivation and reward, learning and memory, and endocrine regulation. Dopamine does not mediate fast synaptic transmission, but rather modulates it by triggering slow-acting effects through the activation of dopamine receptors, which belong to the G-protein-coupled receptor superfamily. Besides activating different effectors through G-protein coupling, dopamine receptors also signal through interaction with a variety of proteins, collectively termed dopamine receptor-interacting proteins. We focus on the dopamine D4 receptor, which contains an important polymorphism in its third intracellular loop. This polymorphism has been the subject of numerous studies investigating links with several brain disorders, such as attention-deficit hyperactivity disorder and schizophrenia. We provide an overview of the structure, signalling properties and regulation of dopamine D4 receptors, and briefly discuss their physiological and pathophysiological role in the brain.

Transmembrane segment five serines of the D4 dopamine receptor uniquely influence the interactions of dopamine, norepinephrine, and Ro10-4548

Conserved serines of transmembrane segment (TM) five (TM5) are critical for the interactions of endogenous catecholamines with alpha(1)- and alpha(2)-adrenergic, beta(2)-adrenergic, and D1, D2, and D3 dopamine receptors. The unique high-affinity interaction of the D4 dopamine receptor subtype with both norepinephrine and dopamine, and the fact that TM5 serine interactions have never been studied for this receptor subtype, led us to investigate the interactions of ligands with D4 receptor TM5 serines. Serine-to-alanine mutations at positions 5.42 and 5.46 drastically decreased affinities of dopamine and norepinephrine for the D4 receptor. The D4-S5.43A receptor mutant had substantially reduced affinity for norepinephrine, but a modest loss of affinity for dopamine. In functional assays of cAMP accumulation, norephinephrine was unable to activate any of the mutant receptors, even though the agonist quinpirole displayed wild-type functional properties for all of them. Dopamine was unable to activate the S5.46A mutant and had reduced potency for the S5.43A mutant and reduced potency and efficacy for the S5.42A mutant. In contrast, Ro10-4548 [RAC-2'-2-hydroxy-3-4-(4-hydroxy-2-methoxyphenyl)-1-piperazinyl-propoxy-acetanilide], a catechol-like antagonist of the wild-type receptor unexpectedly functions as an agonist of the S5.43A mutant. Other noncatechol ligands had similar properties for mutant and wild-type receptors. This is the first example of a dopamine receptor point mutation selectively changing the receptor's interaction with a specific antagonist to that of an agonist, and together with other data, provides evidence, supported by molecular modeling, that catecholamine-type agonism is induced by different ligand-specific configurations of intermolecular H-bonds with the TM5 conserved serines.

The identification of structurally novel, selective, orally bioavailable positive modulators of mGluR2

The optimisation of an HTS hit series (1) leading to the identification of structurally novel, selective, orally bioavailable mGluR2 positive modulators GSK1331258 and GSK1331268 is described. Structure-activity relationships, attenuation of dopaminergic activity, and potentiation of mGluR2 responses in rat hippocampal MPP-DG synapses are also reported.

Dopamine D4 receptor involvement in the discriminative stimulus effects in rats of LSD, but not the phenethylamine hallucinogen DOI

RATIONALE

Lysergic acid diethylamide (LSD) differs from other types of hallucinogens in that it possesses direct dopaminergic effects. The exact nature of this component has not been elucidated.

OBJECTIVE

The present study sought to characterize the effects of several dopamine D(4) agonists and antagonists on the discriminative stimulus effect of LSD at two pretreatment times and 2,5-dimethoxy-4-iodoamphetamine (DOI), a selective 5-HT(2A/2C) agonist.

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in a two-lever, fixed ratio (FR) 50, food-reinforced task with LSD-30 (0.08 mg/kg, i.p., 30-min pretreatment time), LSD-90 (0.16 mg/kg, i.p., 90-min pretreatment time), and DOI (0.4 mg/kg, i.p., 30-min pretreatment time) as discriminative stimuli. Substitution and combination tests with the dopamine D(4) agonists, ABT-724 and WAY 100635, were performed in all groups. Combination tests were run using the dopamine D(4) antagonists A-381393 and L-745,870 and two antipsychotic drugs, clozapine and olanzapine.

RESULTS

WAY 100635 produced full substitution in LSD-90 rats, partial substitution in LSD-30 rats, and saline appropriate responding in DOI-trained rats. ABT-724 partially mimicked the LSD-90 and LSD-30 cues, but produced no substitution in DOI-trained rats. In combination tests, both agonists shifted the dose-response curve of LSD leftward, most potently for the LSD-90 cue. The D(4) antagonists significantly attenuated both the LSD-90 and LSD-30 cue, but had no effect on the DOI cue.

CONCLUSION

Dopamine D(4) receptor activation plays a significant modulatory role in the discriminative stimulus effects in LSD-90-trained rats, most markedly for the later temporal phase of LSD, but has no effect on the cue produced by DOI.

Synthesis and evaluation of ligands for D2-like receptors: the role of common pharmacophoric groups

Arylcycloalkylamines, such as phenyl piperidines and piperazines and their arylalkyl substituents, constitute pharmacophoric groups exemplified in several antipsychotic agents. A review of previous reports indicates that arylalkyl substituents can improve the potency and selectivity of the binding affinity at D(2)-like receptors. In this paper, we explored the contributions of two key pharmacophoric groups, that is, 4'-fluorobutyrophenones and 3-methyl-7-azaindoles, to the potency and selectivity of synthesized agents at D(2)-like receptors. Preliminary observation of binding affinities indicates that there is little predictability of specific effects of the arylalkyl moieties but the composite structure is responsible for selectivity and potency at these receptors.

Enantioselective synthesis of SNAP-7941: chiral dihydropyrimidone inhibitor of MCH1-R

An enantioselective synthesis of SNAP-7941, a potent melanin concentrating hormone receptor antagonist, was achieved by using two organocatalytic methods. The first method utilized to synthesize the enantioenriched dihydropyrimidone core was the Cinchona alkaloid-catalyzed Mannich reaction of beta-keto esters to acylimines and the second was the chiral phosphoric acid-catalyzed Biginelli reaction. Completion of the synthesis was accomplished via selective urea formation at the N3 position of the dihydropyrimidone with the 3-(4-phenylpiperidin-1-yl)propylamine side chain fragment. The synthesis of SNAP-7921 highlights the utility of asymmetric organocatalytic methods in the construction of an important class of chiral heterocycles.

Possible participation of D3 and D4 dopaminergic receptors on genital reflexes induced by cocaine in paradoxical sleep deprived male rats

Previous studies have demonstrated that paradoxical sleep deprivation (PSD) potentiates cocaine-induced genital reflexes in male rats and both D1 and D2 receptors may play a role in those effects, and to examine the possibility that such might involve other dopaminergic receptors, we investigated the effects of D3 and D4 receptor subtype antagonists on cocaine-induced reflexes in sleep-deprived rats. Separate groups of PSD rats received saline, D3 (U9919A; 0.75, 1.5 and 3 mg/kg) or D4 (L745870; 0.75, 1.5 and 3 mg/kg) antagonists prior to acute cocaine challenge. Results demonstrated that U9919A induced significant reduction in the number of animals that displayed erection and the frequency of erection at two smaller doses, while no significant difference was reported for the D4 receptor antagonist. Although our studies indicate that there is a relevant participation of D3 receptors in male sexual function, D4 receptors seem not to exert an essential role in this model.

The future of the oral pharmacotherapy of male erectile dysfunction: things to come

The convincing clinical data on the use of the orally active phosphodiesterase inhibitors sildenafil, vardenafil and tadalafil for the treatment of male erectile dysfunction have boosted research activities on the physiology of the male erectile mechanism. This included both peripheral intracellular signal transduction in the corpus cavernosum as well as central brain and spinal cord pathways controlling penile erection. This work provided the basis for the development and introduction of several new therapeutic modalities into the management of erectile dysfunction, some of which are already offered to the patients. As the concept of 'taking a pill' as a cure for an illness or the relief of symptoms of a disease has become widely accepted by the consumers, the pharmacologic treatment of erectile dysfunction has primarily focussed on selective, orally available drugs acting by influencing intracellular or central regulatory mechanisms, combining a high response rate and the advantage of an on-demand intake. These agents are regarded as more efficacious, and have a faster onset of drug action in the target tissue and an improved effect to side-effect ratio. The purpose of this review is to describe the major novel and evolving pharmacologic advances in the field of oral pharmacotherapy for the treatment of male erectile dysfunction.

Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.

The supraspinal network in the control of erection

Penile erection is a complex event controlled by vascular, hormonal and neuronal systems. The neuronal system involved in erection is often divided into spinal and supraspinal networks. It is generally accepted that the spinal system directly controls erection and that the supraspinal network modulates this control mechanism through different ascending and descending pathways. In contrast to the spinal control of erection, relatively little is known about the supraspinal network. In the present review, the authors outline the supraspinal network involved in the control of penile erection. Firstly, the brain regions reported to be involved in erection are described and the brain circuit of erection is outlined. Subsequently, the neuromediators involved in erection are summarised. Finally, these data are discussed in the light of therapeutic possibilities in the management of erectile dysfunction by targeting the supraspinal system.

PIP3EA and PD-168077, two selective dopamine D4 receptor agonists, induce penile erection in male rats: site and mechanism of action in the brain

PIP3EA (2-[4-(2-methoxyphenyl)piperazin-1-yl-methyl]imidazo[1,2-a]pyridine) and PD-168077 (N-[4-2-cyanophenylpiperazin-1-ylmethyl]-3-methylbenzamide maleate), two selective dopamine D4 agonists, administered systemically, intracerebroventricularly or into the paraventricular nucleus of the hypothalamus induce penile erection in male Sprague-Dawley rats. A U-inverted dose-response curve was found with either compound when given subcutaneously (1-100 microg/kg) or intracerebroventricularly (0.1-20 microg/rat), but not into the paraventricular nucleus (10-200 ng/rat). The pro-erectile effect of PIP3EA and of PD-168077 occurs concomitantly with an increased nitric oxide (NO) production in the paraventricular nucleus, as measured by the increased concentration of nitrites and nitrates found in the dialysate obtained from the paraventricular nucleus by intracerebral microdialysis. These effects of PIP3EA and PD-168077 were reduced by L-745,870 (3-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride), a selective dopamine D4 receptors antagonist, by omega-conotoxin, a blocker of voltage-dependent Ca2+ channels of the N-type, by S-methyl-thiocitrulline, a neuronal nitric oxide synthase inhibitor, and by d(CH2)5Tyr(Me)2-Orn8-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles, but not into the paraventricular nucleus. Comparison of the dose-response curves of PIP3EA and PD-168077 revealed that PIP3EA is as potent as PD-168077 when given into the paraventricular nucleus, but more potent when given systemically. However, both compounds are less efficacious (e.g. induce a lower number of penile erection episodes) than apomorphine, a classical mixed dopamine receptor agonist, irrespective of the route of administration. These results confirm previous findings showing that central D4 receptors mediate penile erection and show that dopamine D4 receptor agonists act in the paraventricular nucleus to facilitate penile erection by increasing central oxytocinergic neurotransmission.

Gram Scale Synthesis of the Glucuronide Metabolite of ABT-724

A gram scale synthesis of the glucuronide metabolite of ABT-724 is reported. Glycosidic coupling between a trichloroacetimidate glucuronyl donor and a Cbz-protected hydroxypyridylpiperazine glycosyl acceptor is the key step in the synthesis, since attempts to directly glucuronidate the aglycon, aglycon derivatives, and other truncated glycosyl acceptors were unsuccessful. The route was used to produce 2.1 g of metabolite in eight steps from 2-chloro-5-hydroxypyridine in 21% overall yield.

1-Aryl-3-(4-pyridine-2-ylpiperazin-1-yl)propan-1-one Oximes as Potent Dopamine D4 Receptor Agonists for the Treatment of Erectile Dysfunction

A new series of dopamine D4 receptor agonists, 1-aryl-3-(4-pyridinepiperazin-1-yl)propanone oximes, was designed through the modification of known dopamine D4 receptor agonist PD 168077. Replacement of the amide group with a methylene-oxime moiety produced compounds with improved stability and efficacy. Structure-activity relationsips (SAR) of the aromatic ring linked to the N-4-piperazine ring confirmed the superiority of 2-pyridine as a core for D4 agonist activity. A two-methylene linker between the oxime group and the N-1-piperazine ring displayed the best profile. New dopamine D4 receptor agonists, exemplified by (E)-1-(4-chlorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (59a) and (E)-1-(3-chloro-4-fluorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (64a), exhibited favorable pharmacokinetic profiles and showed oral bioavailability in rat and dog. Subsequent evaluation of 59a in the rat penile erection model revealed in vivo activity, comparable in efficacy to apomorphine. Our results suggest that the oximes provide a novel structural linker for 4-arylpiperazine-based D4 agonists, possessing leadlike quality and with potential to develop a new class of potent and selective dopamine D4 receptor agonists.

Synthesis and biological investigations of dopaminergic partial agonists preferentially recognizing the D4 receptor subtype

Aminomethyl-substituted biaryls bearing a pyrazole or triazole moiety were synthesized and investigated for dopamine and serotonin receptor binding. The N-arylpyrazoles 3b,f,g and 4 revealed Ki values in the subnanomolar range (0.28-0.70 nM) for the dopamine D4 receptor subtype. Employing both mitogenesis and GTPgammaS assays, ligand efficacy was evaluated indicating partial agonist properties. Interestingly, the tetrahydropyrimidine 4 (FAUC 2020) displayed significant intrinsic selectivity for D2(long) over D2(short).

A-412997 is a selective dopamine D4 receptor agonist in rats

A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide) is a highly selective dopamine D4 receptor agonist that binds with high affinity to rat dopamine D4 and human dopamine D4.4 receptors (Ki=12.1 and 7.9 nM, respectively). In contrast to the dopamine D4 receptor agonists PD168077 and CP226269, A-412997 showed a better selectivity profile and no affinity <1000 nM for other dopamine receptors or any other proteins in a panel of seventy different receptors and channels. In functional assays using calcium flux, A-412997 was a potent full agonist at rat dopamine D4 receptors (28.4 nM, intrinsic activity=0.83) and did not activate rat dopamine D2L receptors, unlike CP226269. Dopamine D4 receptor selective agonists have been shown to induce penile erection in rats by central mechanisms. A-412997 induces penile erection in a conscious rat model (effective dose=0.1 micromol/kg, s.c.) with comparable efficacy as the nonselective D2-like agonist, apomorphine. When dosed systemically, A-412997 crossed the blood brain barrier rapidly and achieved significantly higher levels than PD168077. A-412997 is a highly selective dopamine D4 receptor agonist and a useful tool to understand the role of dopamine D4 receptors in rat models of central nervous system processes and disease.

Synthesis and evaluation of 3-aryl piperidine analogs as potent and efficacious dopamine D4 receptor agonists

A series of 3-aryl piperidine analogs with 2-piperidinoalkylamino or 2-piperidinoalkyloxy fused bicyclic rings were prepared and found to be potent and efficacious human dopamine D4 agonists. The synthesis and structure-activity relationship (SAR) studies that led to the identification of these compounds are discussed.