Georgia B. McGaughey

Discovery of the dual orexin receptor antagonist [(7R)-4-(5-chloro-1,3-benzoxazol-2-yl)-7-methyl-1,4-diazepan-1-yl][5-methyl-2-(2H-1,2,3-triazol-2-yl)phenyl]methanone (MK-4305) for the treatment of insomnia.

Despite increased understanding of the biological basis for sleep control in the brain, few novel mechanisms for the treatment of insomnia have been identified in recent years. One notable exception is inhibition of the excitatory neuropeptides orexins A and B by design of orexin receptor antagonists. Herein, we describe how efforts to understand the origin of poor oral pharmacokinetics in a leading HTS-derived diazepane orexin receptor antagonist led to the identification of compound 10 with a 7-methyl substitution on the diazepane core. Though 10 displayed good potency, improved pharmacokinetics, and excellent in vivo efficacy, it formed reactive metabolites in microsomal incubations. A mechanistic hypothesis coupled with an in vitro assay to assess bioactivation led to replacement of the fluoroquinazoline ring of 10 with a chlorobenzoxazole to provide 3 (MK-4305), a potent dual orexin receptor antagonist that is currently being tested in phase III clinical trials for the treatment of primary insomnia.

First Demonstration of Cerebrospinal Fluid and Plasma Aβ Lowering with Oral Administration of a β-Site Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitor in Nonhuman Primates

β-Site amyloid precursor protein (APP)-cleaving enzyme (BACE) 1 cleavage of amyloid precursor protein is an essential step in the generation of the potentially neurotoxic and amyloidogenic Aβ42 peptides in Alzheimers disease. Although previous mouse studies have shown brain Aβ lowering after BACE1 inhibition, extension of such studies to nonhuman primates or man was precluded by poor potency, brain penetration, and pharmacokinetics of available inhibitors. In this study, a novel tertiary carbinamine BACE1 inhibitor, tertiary carbinamine (TC)-1, was assessed in a unique cisterna magna ported rhesus monkey model, where the temporal dynamics of Aβ in cerebrospinal fluid (CSF) and plasma could be evaluated. TC-1, a potent inhibitor (IC50 ∼ 0.4 nM), has excellent passive membrane permeability, low susceptibility to P-glycoprotein transport, and lowered brain Aβ levels in a mouse model. Intravenous infusion of TC-1 led to a significant but transient lowering of CSF and plasma Aβ levels in conscious rhesus monkeys because it underwent CYP3A4-mediated metabolism. Oral codosing of TC-1 with ritonavir, a potent CYP3A4 inhibitor, twice daily over 3.5 days in rhesus monkeys led to sustained plasma TC-1 exposure and a significant and sustained reduction in CSF sAPPβ, Aβ40, Aβ42, and plasma Aβ40 levels. CSF Aβ42 lowering showed an EC50 of ∼20 nM with respect to the CSF [TC-1] levels, demonstrating excellent concordance with its potency in a cell-based assay. These results demonstrate the first in vivo proof of concept of CSF Aβ lowering after oral administration of a BACE1 inhibitor in a nonhuman primate.

Discovery of 3-{5-[(6-amino-1H-pyrazolo[3,4-b]pyridine-3-yl)methoxy]-2-chlorophenoxy}-5-chlorobenzonitrile (MK-4965): a potent, orally bioavailable HIV-1 non-nucleoside reverse transcriptase inhibitor with improved potency against key mutant viruses.

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) have been shown to be a key component of highly active antiretroviral therapy (HAART). The use of NNRTIs has become part of standard combination antiviral therapies producing clinical outcomes with efficacy comparable to other antiviral regimens. There is, however, a critical issue with the emergence of clinical resistance, and a need has arisen for novel NNRTIs with a broad spectrum of activity against key HIV-1 RT mutations. Using a combination of traditional medicinal chemistry/SAR analyses, crystallography, and molecular modeling, we have designed and synthesized a series of novel, highly potent NNRTIs that possess broad spectrum antiviral activity and good pharmacokinetic profiles. Further refinement of key compounds in this series to optimize physical properties and pharmacokinetics has resulted in the identification of 8e (MK-4965), which has high levels of potency against wild-type and key mutant viruses, excellent oral bioavailability and overall pharmacokinetics, and a clean ancillary profile.

Discovery of [(2R,5R)-5-{[(5-fluoropyridin-2-yl)oxy]methyl}-2-methylpiperidin-1-yl][5-methyl-2-(pyrimidin-2-yl)phenyl]methanone (MK-6096): a dual orexin receptor antagonist with potent sleep-promoting properties.

Insomnia is a common disorder that can be comorbid with other physical and psychological illnesses. Traditional management of insomnia relies on general central nervous system (CNS) suppression using GABA modulators. Many of these agents fail to meet patient needs with respect to sleep onset, maintenance, and next‐day residual effects and have issues related to tolerance, memory disturbances, and balance. Orexin neuropeptides are central regulators of wakefulness, and orexin antagonism has been identified as a novel mechanism for treating insomnia with clinical proof of concept. Herein we describe the discovery of a series of α‐methylpiperidine carboxamide dual orexin 1 and orexin 2 receptor (OX1R/OX2R) antagonists (DORAs). The design of these molecules was inspired by earlier work from this laboratory in understanding preferred conformational properties for potent orexin receptor binding. Minimization of 1,3‐allylic strain interactions was used as a design principle to synthesize 2,5‐disubstituted piperidine carboxamides with axially oriented substituents including DORA 28. DORA 28 (MK‐6096) has exceptional in vivo activity in preclinical sleep models, and has advanced into phase II clinical trials for the treatment of insomnia.

Discovery and X-ray Crystallographic Analysis of a Spiropiperidine Iminohydantoin Inhibitor of β-Secretase‡

A high-throughput screen at 100 microM inhibitor concentration for the BACE-1 enzyme revealed a novel spiropiperidine iminohydantoin aspartyl protease inhibitor template. An X-ray cocrystal structure with BACE-1 revealed a novel mode of binding whereby the inhibitor interacts with the catalytic aspartates via bridging water molecules. Using the crystal structure as a guide, potent compounds with good brain penetration were designed.

Progress Towards the Development of a HIV-1 gp41-Directed Vaccine

The HIV-1 gp41 envelope glycoprotein mediates fusion of the viral and cellular membranes. The core of the gp41 ectodomain undergoes a receptor-triggered conformational transition forming a trimeric, alpha-helical coiled-coil structure. This trimer-of-hairpins species facilitates insertion of the viral envelope protein into the host cell membrane promoting viral entry. The prefusogenic conformation of gp41 is capable of stimulating a neutralizing antibody immune response and is therefore an attractive therapeutic target. Several broadly neutralizing HIV-1 monoclonal antibodies which bind to gp41 have been characterized and include 4E10, Z13 and 2F5. A conserved segment of gp41 (residues 661-684) has been identified as the epitope for the HIV-1 neutralizing antibody 2F5 (MAb 2F5). MAb 2F5 has attracted considerable attention because of the highly conserved recognition epitope and the ability to neutralize both laboratory-adapted and primary viral isolates. Antibodies which recognize the immunodominant regions of gp41 may provide protection against HIV infection if elicited at appropriate concentrations. Here we review the rational design, structure-activity relationships and conformational features of both linear and constrained peptide immunogens incorporating variants of both the 2F5 epitope and the gp41 ectodomain. This review describes a rational design approach combining structural characterization with traditional SAR to optimize MAb 2F5 antibody affinities of gp41-based peptide immunogens. The immunogens are shown to stimulate a high titer, peptide-specific immune response; however, the resulting antisera were incapable of viral neutralization. The implication of these findings with regard to structural and immunological considerations is discussed.

Conformational analysis of N,N-disubstituted-1,4-diazepane orexin receptor antagonists and implications for receptor binding

NMR spectroscopy, X-ray crystallography, and molecular modeling studies indicate that N,N-disubstituted-1,4-diazepane orexin receptor antagonists exist in an unexpected low-energy conformation that is characterized by an intramolecular pi-stacking interaction and a twist-boat ring conformation. Synthesis and evaluation of a macrocycle that enforces a similar conformation suggest that this geometry mimics the bioactive conformation.

The design and synthesis of diaryl ether second generation HIV-1 non-nucleoside reverse transcriptase inhibitors (NNRTIs) with enhanced potency versus key clinical mutations.

Using a combination of traditional Medicinal Chemistry/SAR analysis, crystallography, and molecular modeling, we have designed and synthesized a series of novel, highly potent NNRTIs that possess broad antiviral activity against a number of key clinical mutations.

Discovery of tetrahydropyridopyrimidine phosphodiesterase 10A inhibitors for the treatment of schizophrenia

We describe the discovery of potent and orally bioavailable tetrahydropyridopyrimidine inhibitors of phosphodiesterase 10A by systematic optimization of a novel HTS lead. Lead compound THPP-1 exhibits nanomolar potencies, excellent pharmacokinetic properties, and a clean off-target profile. It displays in vivo target engagement as measured by increased rat striatal cGMP levels upon oral dosing. It shows dose-dependent efficacy in a key pharmacodynamic assay predictive of antipsychotic activity, the psychostimulant-induced rat hyperlocomotion assay. Further, THPP-1 displays significantly fewer preclinical adverse events in assays measuring prolactin secretion, catalepsy, and weight gain, in contrast to the typical and atypical antipsychotics haloperidol and olanzapine.

Design and Synthesis of 2,3,5-Substituted Imidazolidin-4-one Inhibitors of BACE-1

Alzheimer’s disease is a slowly progressing neurodegenerative condition that is increasing in prevalence because of the ageing population and is a significant healthcare burden. Although the pathophysiology of the disease has not been completely elucidated, abnormal production and/or clearance of a small peptide called Ab has been implicated from genetic and other studies. The Ab peptide arises from proteolytic processing of the APP protein, first by b-secretase followed by g-secretase. Based on these observations, b-secretase (BACE-1) has been identified as a promising drug target for disease-modifying therapy and has attracted significant attention from the medicinal chemistry community. BACE-1 is an aspartyl protease with a site of action inside the CNS and thus represents a challenging target. Hydroxyethylamine (HEA) transition state isosteres are well-known inhibitory motifs for aspartyl proteases, and prior work from these laboratories identified 1 (Figure 1) as a potent inhibitor of BACE-1. Whereas this compound has good enzyme potency (IC50=11 nm) and cell activity (sAPPb_NF= 29 nm), molecular weight is still high (578 Daltons) and the compound is a P-glycoprotein transporter (PGP) substrate with poor brain penetration. To improve brain-penetration properties, truncation of the structure and removal of hydrogen bond donors and acceptors was deemed necessary. One approach is to further optimize the P3-P1 portion and eliminate the hydroxyethyl amine. The work described in this report instead focuses on prime side modifications with the goal of improving potency sufficiently to allow removal of the P2P3 portion of the inhibitor. Closer examination of the X-ray crystal structure of 1 in the BACE-1 active site shows that the basic amine makes hydrogen-bond contacts to both Asp228 and the Gly34 carbonyl (Figure 2), and SAR studies have shown that primary and secondary amines are preferred over tertiary amines in this posi-