Delineating the Ligand-Receptor Interactions That Lead to Biased Signaling at the µ-Opioid Receptor.

Biased agonists, which selectively stimulate certain signaling pathways controlled by a G protein-coupled receptor (GPCR), hold great promise as drugs that maximize efficacy while minimizing dangerous side effects. Biased agonists of the µ-opioid receptor (µOR) are of particular interest as a means to achieve analgesia through G protein signaling without dose-limiting side effects such as respiratory depression and constipation. Rational structure-based design of biased agonists remains highly challenging, however, because the ligand-mediated interactions that are key to activation of each signaling pathway remain unclear. We identify several compounds for which the R- and S-enantiomers have distinct bias profiles at the µOR. These compounds serve as excellent comparative tools to study bias because the identical physicochemical properties of enantiomer pairs ensure that differences in bias profiles are due to differences in interactions with the µOR binding pocket. Atomic-level simulations of compounds at µOR indicate that R- and S-enantiomers adopt different poses that form distinct interactions with the binding pocket. A handful of specific interactions with highly conserved binding pocket residues appear to be responsible for substantial differences in arrestin recruitment between enantiomers. Our results offer guidance for rational design of biased agonists at µOR and possibly at related GPCRs.

Impact of GPCR Structures on Drug Discovery.

Structures of 70 unique G protein-coupled receptors (GPCRs) have been determined, with over 370 structures in total bound to different ligands and the receptors in various conformational states. Structure-based drug design has been applied to an increasing number of GPCR targets over the past decade and now a few of these drug candidates have entered clinical trials. Given the length of time required for a drug to reach the market, there are no documented examples of licensed drugs being developed with the aid of a structure, but this is likely to change as current efforts come to fruition.

The role of organic anion-transporting polypeptides and formulation in the clearance and distribution of a novel Nav 1.7 channel blocker.

PF-06456384 is an extremely potent and selective blocker of the Nav 1.7 sodium channel designed as a potential intravenous (i.v.) analgesic targeting high potency and rapid clearance to minimize the potential for residual effects following the end of infusion. In our previous experience targeting oral molecules, the requirement to obtain potent, Nav 1.7 selective molecules led to a focus on acidic, amphipilic compounds cleared primarily by organic anion-transporting polypeptide mediated hepatic uptake and subsequent biliary excretion. However, the physicochemical properties of the i.v. lead matter were substantially different, moving from acidic, amphiphilic chemical space to zwitterions as well as substantially increasing molecular weight. This report describes the continued relevance of organic anion-transporting polypeptide driven hepatic uptake in this physicochemical space and highlights an apparent impact of the formulation excipient Solutol on the clearance and distribution of PF-06456384.

Highly potent and selective NaV1.7 inhibitors for use as intravenous agents and chemical probes.

The discovery and selection of a highly potent and selective NaV1.7 inhibitor PF-06456384, designed specifically for intravenous infusion, is disclosed. Extensive in vitro pharmacology and ADME profiling followed by in vivo preclinical PK and efficacy model data are discussed. A proposed protein-ligand binding mode for this compound is also provided to rationalise the high levels of potency and selectivity over inhibition of related sodium channels. To further support the proposed binding mode, potent conjugates are described which illustrate the potential for development of chemical probes to enable further target evaluation.

Discovery of Clinical Candidate 4-[2-(5-Amino-1H-pyrazol-4-yl)-4-chlorophenoxy]-5-chloro-2-fluoro-N-1,3-thiazol-4-ylbenzenesulfonamide (PF-05089771): Design and Optimization of Diaryl Ether Aryl Sulfonamides as Selective Inhibitors of NaV1.7.

A series of acidic diaryl ether heterocyclic sulfonamides that are potent and subtype selective NaV1.7 inhibitors is described. Optimization of early lead matter focused on removal of structural alerts, improving metabolic stability and reducing cytochrome P450 inhibition driven drug-drug interaction concerns to deliver the desired balance of preclinical in vitro properties. Concerns over nonmetabolic routes of clearance, variable clearance in preclinical species, and subsequent low confidence human pharmacokinetic predictions led to the decision to conduct a human microdose study to determine clinical pharmacokinetics. The design strategies and results from preclinical PK and clinical human microdose PK data are described leading to the discovery of the first subtype selective NaV1.7 inhibitor clinical candidate PF-05089771 (34) which binds to a site in the voltage sensing domain.

Discovery of a Series of Indazole TRPA1 Antagonists.

A series of TRPA1 antagonists is described which has as its core structure an indazole moiety. The physical properties and in vitro DMPK profiles are discussed. Good in vivo exposure was obtained with several analogs, allowing efficacy to be assessed in rodent models of inflammatory pain. Two compounds showed significant activity in these models when administered either systemically or topically. Protein chimeras were constructed to indicate compounds from the series bound in the S5 region of the channel, and a computational docking model was used to propose a binding mode for example compounds.

Subtype-Selective Small Molecule Inhibitors Reveal a Fundamental Role for Nav1.7 in Nociceptor Electrogenesis, Axonal Conduction and Presynaptic Release.

Human genetic studies show that the voltage gated sodium channel 1.7 (Nav1.7) is a key molecular determinant of pain sensation. However, defining the Nav1.7 contribution to nociceptive signalling has been hampered by a lack of selective inhibitors. Here we report two potent and selective arylsulfonamide Nav1.7 inhibitors; PF-05198007 and PF-05089771, which we have used to directly interrogate Nav1.7's role in nociceptor physiology. We report that Nav1.7 is the predominant functional TTX-sensitive Nav in mouse and human nociceptors and contributes to the initiation and the upstroke phase of the nociceptor action potential. Moreover, we confirm a role for Nav1.7 in influencing synaptic transmission in the dorsal horn of the spinal cord as well as peripheral neuropeptide release in the skin. These findings demonstrate multiple contributions of Nav1.7 to nociceptor signalling and shed new light on the relative functional contribution of this channel to peripheral and central noxious signal transmission.

Voltage gated sodium channels as drug discovery targets.

Voltage-gated sodium (NaV) channels are a family of transmembrane ion channel proteins. They function by forming a gated, water-filled pore to help establish and control cell membrane potential via control of the flow of ions between the intracellular and the extracellular environments. Blockade of NaVs has been successfully accomplished in the clinic to enable control of pathological firing patterns that occur in a diverse range of conditions such as chronic pain, epilepsy, and cardiac arrhythmias. First generation sodium channel modulator drugs, despite low inherent subtype selectivity, preferentially act on over-excited cells which reduces undesirable side effects in the clinic. However, the limited therapeutic indices observed with the first generation demanded a new generation of sodium channel inhibitors. The structure, function and the state of the art in sodium channel modulator drug discovery are discussed in this chapter.

Recent progress in sodium channel modulators for pain.

Voltage-gated sodium channels (Navs) are an important family of transmembrane ion channel proteins and Nav drug discovery is an exciting field. Pharmaceutical investment in Navs for pain therapeutics has expanded exponentially due to genetic data such as SCN10A mutations and an improved ability to establish an effective screen sequence for example IonWorks Barracuda®, Synchropatch® and Qube®. Moreover, emerging clinical data (AZD-3161, XEN402, CNV1014802, PF-05089771, PF-04531083) combined with recent breakthroughs in Nav structural biology pave the way for a future of fruitful prospective Nav drug discovery.

Voltage sensor interaction site for selective small molecule inhibitors of voltage-gated sodium channels.

Voltage-gated sodium (Nav) channels play a fundamental role in the generation and propagation of electrical impulses in excitable cells. Here we describe two unique structurally related nanomolar potent small molecule Nav channel inhibitors that exhibit up to 1,000-fold selectivity for human Nav1.3/Nav1.1 (ICA-121431, IC50, 19 nM) or Nav1.7 (PF-04856264, IC50, 28 nM) vs. other TTX-sensitive or resistant (i.e., Nav1.5) sodium channels. Using both chimeras and single point mutations, we demonstrate that this unique class of sodium channel inhibitor interacts with the S1-S4 voltage sensor segment of homologous Domain 4. Amino acid residues in the "extracellular" facing regions of the S2 and S3 transmembrane segments of Nav1.3 and Nav1.7 seem to be major determinants of Nav subtype selectivity and to confer differences in species sensitivity to these inhibitors. The unique interaction region on the Domain 4 voltage sensor segment is distinct from the structural domains forming the channel pore, as well as previously characterized interaction sites for other small molecule inhibitors, including local anesthetics and TTX. However, this interaction region does include at least one amino acid residue [E1559 (Nav1.3)/D1586 (Nav1.7)] that is important for Site 3 α-scorpion and anemone polypeptide toxin modulators of Nav channel inactivation. The present study provides a potential framework for identifying subtype selective small molecule sodium channel inhibitors targeting interaction sites away from the pore region.

Ion channels as therapeutic targets: a drug discovery perspective.

Ion channels are membrane proteins expressed in almost all living cells. The sequencing of the human genome has identified more than 400 putative ion channels, but only a fraction of these have been cloned and functionally tested. The widespread tissue distribution of ion channels, coupled with the plethora of physiological consequences of their opening and closing, makes ion-channel-targeted drug discovery highly compelling. However, despite some important drugs in clinical use today, as a class, ion channels remain underexploited in drug discovery and many existing drugs are poorly selective with significant toxicities or suboptimal efficacy. This Perspective seeks to review the ion channel family, its structural and functional features, and the diseases that are known to be modulated by members of the family. In particular, we will explore the structure and properties of known ligands and consider the future prospects for drug discovery in this challenging but high potential area.

Synthesis of novel histamine H4 receptor antagonists.

This letter describes the discovery and synthesis of a series of octahydropyrrolo[3,4-c]pyrrole based selective histamine hH4 receptor antagonists. The amidine compound 20 was found to be a potent and selective histamine H4 receptor antagonist with moderate clearance and a high volume of distribution.

Challenges of drug discovery in novel target space. The discovery and evaluation of PF-3893787: a novel histamine H4 receptor antagonist.

We describe the development of novel benzimidazoles as small molecule histamine H4 receptor (H4R) antagonists and their profiling in rat early toxicity studies. The discovery and optimisation of a second series of pyrimidine based antagonists is then described culminating in the identification of the clinical development candidate 13 (PF-3893787). The pre-clinical profile of 13 (PF-3893787) is presented including the development of a translatable biomarker. Our pragmatic approach to target selection, safety assessment, and testing for efficacy faced numerous challenges and we share a number of lessons which the team learned and which will assist us and others in future drug discovery projects.

Novel indazole non-nucleoside reverse transcriptase inhibitors using molecular hybridization based on crystallographic overlays.

A major problem associated with non-nucleoside reverse transcriptase inhibitors (NNRTIs) for the treatment of HIV is their lack of resilience to mutations in the reverse transcriptase (RT) enzyme. Using structural overlays of the known inhibitors efavirenz and capravirine complexed in RT as a starting point, and structure-based drug design techniques, we have created a novel series of indazole NNRTIs that possess excellent metabolic stability and mutant resilience.

Intramolecular C-H insertions adjacent to sulfur for the diastereoselective synthesis of thienofuranones.

A new approach to the diastereoselective synthesis of thienofuranones is described in which an intramolecular 1,5-carbenoid C-H insertion adjacent to sulfur features as a key step.

A versatile stereoselective synthesis of endo,exo-furofuranones: application to the enantioselective synthesis of furofuran lignans.

A new stereoselective route to endo,exo-2,6-diarylfurofuranones has been developed using Mn(III)-mediated intramolecular cyclopropanation and C-H insertion reactions as key C-C bond-forming steps. Mn(III)-mediated oxidative cyclization of acetoacetate derivative 11 afforded 1-acetyl-4-aryl-3-oxabicyclo[3.1.0]hexan-2-one (12) with excellent diastereocontrol (d.r. 22:1). Subsequent Lewis acid-catalyzed opening of the activated cyclopropane ring present in 12 with benzylic alcohols then gave alpha-acetyl-gamma-butyrolactones 16 and 18-20, which reacted efficiently with in situ-generated TfN(3) to secure the key alpha-diazo-gamma-butyrolactones 22-25. Highly stereoselective rhodium-catalyzed C-H insertion reactions of diazolactones 22-25 completed the synthesis of endo,exo-2,6-diarylfurofuranones 26-29 in overall yields ranging from 41 to 48% from 1-phenylallyl alcohol (+/-)-10. The approach developed for the furofuranones 26-29 was then applied to the asymmetric syntheses of four furofuran lignans, (+)-xanthoxylol (1), (+)-methylxanthoxylol (2), (+)-epipinoresinol (3), and (+)-epieudesmin (4), starting from enantiomerically enriched 1-arylallyl alcohol (S)-31.

An efficient synthesis of endo,exo-furofuranone derivatives.

The ring openings of 1-acetyl-4-phenyl-3-oxabicyclo[3.1.0]hexane afforded alpha-acetyl-gamma-butyrolactone that underwent a novel diazo-transfer reaction, followed by C-H insertion, to provide a series of endo,exo-furofuranone analogues.