Bispecific repurposed medicines targeting the viral and immunological arms of COVID-19.

Effective agents to treat coronavirus infection are urgently required, not only to treat COVID-19, but to prepare for future outbreaks. Repurposed anti-virals such as remdesivir and human anti-inflammatories such as barcitinib have received emergency approval but their overall benefits remain unclear. Vaccines are the most promising prospect for COVID-19, but will need to be redeveloped for any future coronavirus outbreak. Protecting against future outbreaks requires the identification of targets that are conserved between coronavirus strains and amenable to drug discovery. Two such targets are the main protease (Mpro) and the papain-like protease (PLpro) which are essential for the coronavirus replication cycle. We describe the discovery of two non-antiviral therapeutic agents, the caspase-1 inhibitor SDZ 224015 and Tarloxotinib that target Mpro and PLpro, respectively. These were identified through extensive experimental screens of the drug repurposing ReFRAME library of 12,000 therapeutic agents. The caspase-1 inhibitor SDZ 224015, was found to be a potent irreversible inhibitor of Mpro (IC50 30 nM) while Tarloxotinib, a clinical stage epidermal growth factor receptor inhibitor, is a sub micromolar inhibitor of PLpro (IC50 300 nM, Ki 200 nM) and is the first reported PLpro inhibitor with drug-like properties. SDZ 224015 and Tarloxotinib have both undergone safety evaluation in humans and hence are candidates for COVID-19 clinical evaluation.

Remote stereocontrol in reactions between 4- and 5-alkoxyalk-2-enylstannanes and 1-alkoxycarbonylimines and analogues: stereoselective approaches to novel α-amino acids.

Reactions of the allyltin trichloride 45 generated from (4S)-4-benzyloxypent-2-enyl(tributyl)stannane 1 with imines prepared from glyoxylates proceed with useful levels of 1,5-stereocontrol in favour of (4E)-2,6-anti-2-(alkylamino)-6-benzyloxyhept-4-enoates 49. This stereoselectivity, controlled by the chirality of the stannane, dominates over any intrinsic stereochemical bias of the imine although a small amount of matching and mis-matching was observed. The allyltin trichloride 77 prepared from (4S)-4-(tert-butyldimethylsilyloxy)pent-2-enyl(tributyl)stannane 52 reacts with 1-alkoxycarbonylimines with the opposite 1,5-stereoselectivity to give the (4E)-2,6-syn-diastereoisomers 79. Matching and mismatching was more pronounced for tin(IV) chloride mediated reactions of (4R)-5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane 80 with chiral 1-alkoxycarbonylimines but useful stereoselectivity in favour of (4E)-2,6-syn-2-alkyl- and arylthio-amino-7-benzyloxy-6-methylhept-4-enoates 177 was observed for reactions with achiral imines and similar, but reduced, stereoselectivity was observed for the 5-tert-butyldimethylsilyloxypentenylstannane 82. However, excellent 1,5-stereocontrol in favour of the (4E)-2,6-anti-isomers 179 was found using the 4,5-bis-alkoxypent-2-enylstannane 106. Modest (4E)-2,7-anti-stereoselectivity was observed in the analogous tin(IV) bromide mediated reactions of (S)-5-methoxy- and (S)-5-hydroxyhex-2-enyl(tributyl)stannanes (S)-123 and (S)-122 with achiral 1-alkoxycarbonylimines but in this series the intrinsic stereochemical bias of the imine controls the facial selectivity of reactions of chiral 1-alkoxycarbonylimines. Useful (4E)-2,6-anti-stereoselectivity was also observed in the tin(IV) chloride promoted reaction of the 4-benzyloxypent-2-enylstannane 1 with an oxime O-benzyl ether.

Fragment-based discovery and optimization of BACE1 inhibitors.

A novel series of 2-aminobenzimidazole inhibitors of BACE1 has been discovered using fragment-based drug discovery (FBDD) techniques. The rapid optimization of these inhibitors using structure-guided medicinal chemistry is discussed.

Discovery of substituted benzyl tetrazoles as histamine H3 receptor antagonists.

A series of potent and subtype selective H3 receptor antagonists containing a novel tetrazole core and diamine motif is reported. A one-pot multi-component Ugi reaction was utilised to rapidly develop the structure-activity relationships (SAR) of these compounds. Optimisation for liver microsome stability (t(1/2)>60 min), minimal CYP inhibition (IC(50)>50 microM) and high cell permeability (Caco-2 P(app) >20x10(-6) cm/s) identified several compounds with drug-like properties.

Differential contribution of GABA(A) receptor subtypes to the anticonvulsant efficacy of benzodiazepine site ligands.

Non-selective benzodiazepines, such as diazepam, interact with equivalent affinity and agonist efficacy at GABA(A) receptors containing either an alpha1, alpha2, alpha3 or alpha5 subunit. However, which of these particular subtypes are responsible for the anticonvulsant effects of diazepam remains uncertain. In the present study, we examined the ability of diazepam to reduce pentylenetetrazoLe (PTZ)-induced and maximal electroshock (MES)-induced seizures in mice containing point mutations in single (alpha1H101R, alpha2H101R or alpha5H105R) or multiple (alpha125H-->R) alpha subunits that render the resulting GABA(A) receptors diazepam-insensitive. Furthermore, the anticonvulsant properties of diazepam, the alpha1- and alpha3-selective compounds zolpidem and TP003, respectively, and the alpha2/alpha3 preferring compound TP13 were studied against PTZ-induced seizures. In the transgenic mice, no single subtype was responsible for the anticonvulsant effects of diazepam in either the PTZ or MES assay and neither the alpha3 nor alpha5 subtypes appeared to confer anticonvulsant activity. Moreover, whereas the alpha1 and alpha2 subtypes played a modest role with respect to the PTZ assay, they had a negligible role in the MES assay. With respect to subtype-selective compounds, zolpidem and TP003 had much reduced anticonvulsant efficacy relative to diazepam in both the PTZ and MES assays whereas TP13 had high anticonvulsant efficacy in the PTZ but not the MES assay. Taken together, these data not only indicate a role for alpha2-containing GABA(A) receptors in mediating PTZ and MES anticonvulsant activity but also suggest that efficacy at more than one subtype is required and that these subtypes act synergistically.

The role of GABAbeta2 subunit-containing receptors in mediating the anticonvulsant and sedative effects of loreclezole.

The majority of inhibitory neurotransmission in the brain is mediated by the gamma-aminobutyric acid (GABA) type A (GABA(A)) receptor. The anticonvulsant loreclezole largely acts by potentiating GABA(A) receptors containing beta2 and beta3 subunits. We used a genetically modified mouse containing a loreclezole-insensitive beta2 subunit (beta2N265S) to determine the role of this subunit in mediating the sedative and anticonvulsive effects of loreclezole. Sedation was assessed by measuring spontaneous locomotor activity and beam walking performance, and anticonvulsant efficacy was determined by pentylenetetrazole (PTZ) and amygdala kindling-induced seizures. The beta2N265S mice did not exhibit loreclezole-mediated sedation as shown by normal locomotor activity and beam walking performance. However, loreclezole also failed to provide significant protection against PTZ-induced seizures in the beta2N265S mice. Reduced efficacy against amygdala-kindled seizures, both acutely and over a 13-day chronic dosing study, was also observed in beta2N265S mice. These results suggest that the majority of the sedative effects and a significant proportion of the anticonvulsant efficacy of loreclezole are mediated via beta2-containing GABA(A) receptors.

A pyridazine series of alpha2/alpha3 subtype selective GABA A agonists for the treatment of anxiety.

The development of a series of GABA(A) alpha2/alpha3 subtype selective pyridazine based benzodiazepine site agonists as anxiolytic agents with reduced sedative/ataxic potential is described, including the discovery of 16, a remarkably alpha3-selective compound ideal for in vivo study. These ligands are antagonists at the alpha1 subtype, with good CNS penetration and receptor occupancy, and excellent oral bioavailability.

Discovery of imidazo[1,2-b][1,2,4]triazines as GABA(A) alpha2/3 subtype selective agonists for the treatment of anxiety.

The identification of a series of imidazo[1,2-b][1,2,4]triazines with high affinity and functional selectivity for the GABA(A) alpha3-containing receptor subtype is described, leading to the identification of a clinical candidate, 11. Compound 11 shows good bioavailability and half-life in preclinical species, and it is a nonsedating anxiolytic in both rat and squirrel monkey behavioral models.

Imidazo[1,2-b][1,2,4]triazines as alpha2/alpha3 subtype selective GABA A agonists for the treatment of anxiety.

Imidazo[1,2-a]pyrimidines and imidazo[1,2-b][1,2,4]triazines are ligands for the benzodiazepine binding site of GABA(A) receptors that are functionally selective for the alpha2/alpha3 subtypes over the alpha1 subtype. SAR studies to optimise this functional selectivity, pharmacokinetic and behavioural data are described.

Imidazo[1,2-a]pyrimidines as functionally selective and orally bioavailable GABA(A)alpha2/alpha3 binding site agonists for the treatment of anxiety disorders.

A series of high-affinity GABA(A) agonists with good oral bioavailability in rat and dog and functional selectivity for the GABA(A)alpha2 and -alpha3 subtypes is reported. The 7-trifluoromethylimidazopyrimidine 14g and the 7-propan-2-olimidazopyrimidine 14k are anxiolytic in both conditioned and unconditioned animal models of anxiety with minimal sedation observed at full BZ binding site occupancy.

Imidazo[1,2-a]pyrimidines as functionally selective GABA(A) ligands.

Imidazo[1,2-a]pyrimidines are GABA(A) receptor benzodiazepine binding site ligands which can exhibit functional selectivity for the alpha(3) subtype over the alpha(1) subtype. SAR studies to optimize this functional selectivity are described.

Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and imidazo[2,1-f][1,2,4]triazin-8-ones as alpha2/alpha3 subtype selective GABA A agonists for the treatment of anxiety.

Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and imidazo[2,1-f][1,2,4]triazin-8-ones are high affinity GABA(A) agonists. Compound 16d has good oral bioavailability in rat, functional selectivity for the GABA(A)alpha2 and alpha3-subtypes and is anxiolytic in a conditioned animal model of anxiety with minimal sedation observed at full BZ binding site occupancy.