Successes in antiviral drug discovery: a tribute to Nick Meanwell.

Drug discovery is a difficult task, and is even more challenging when the target evolves during therapy. Antiviral drug therapy is an excellent example, exemplified by the evolution of therapeutic approaches for treatment of hepatitis C and HIV-1. Nick Meanwell and his colleagues made important contributions leading to molecules for treatment of hepatitis C and HIV-1, each with distinct mechanisms of action. This review summarizes the discovery and impact of these drugs, and will highlight, where applicable, the broader contributions of these discoveries to medicinal chemistry and drug discovery.

Characterization of Competitive Inhibitors of Plasmodium falciparum cGMP-Dependent Protein Kinase.

Plasmodium falciparum cGMP-dependent protein kinase (PfPKG) is an enticing antimalarial drug target. Novel chemotypes are needed because existing inhibitors have safety issues that may prevent further development. This work demonstrates isoxazole-based compounds are potent ATP competitive inhibitors of PfPKG and discloses a new analogue in this series. Isoxazoles 3 and 5 had Ki values that are comparable to a known standard, 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H pyrrol-3-yl] pyridine. They also exhibited excellent selectivity for PfPKG over the human orthologue and the gatekeeper mutant T618Q PfPKG, which mimics the less accessible binding site of the human orthologue. The human orthologue's larger binding site volume is predicted to explain the selectivity of the inhibitors for the P. falciparum enzyme.

Mechanism of inhibition of botulinum neurotoxin type A light chain by two quinolinol compounds.

Quinolinol-based compounds are a promising starting point for discovery of effective inhibitors of the clostridial neurotoxin, botulinum neurotoxin type A light chain (BoNT/A LC). Insights into the mechanism of inhibition by quinolinol compounds facilitate interpretation of docking data and inhibitor optimization. In this study, a fluorogenic substrate of BoNT/A, SNAPtide, was used to study the mechanism by which two new quinolinol compounds, MSU58 and MSU84, with IC50 values of 3.3 µM and 5.8 µM, respectively, inhibit BoNT/A LC. Kinetic studies and model discrimination analysis showed both compounds to be competitive inhibitors of BoNT/A LC with inhibition constants (KI) 3.2 µM and 6.2 µM for MSU58 and MSU84, respectively. These data indicate that the inhibitors bind in the BoNT/A LC active site and that inhibitor binding is mutually exclusive with the binding of the substrate. This is the first study to report the competitive inhibition of BoNT/A LC by quinolinol compounds. These data help define the inhibitor binding pocket and, along with structure activity relationship studies, provide immediate direction for further compound synthesis.

Structure-activity studies of (-)-epigallocatechin gallate derivatives as HCV entry inhibitors.

Preventing viral entry into cells is a recognized approach for HIV therapy and has attracted attention for use against the hepatitis C virus (HCV). Recent reports described the activity of (-)-epigallocatechin gallate (EGCG) as an inhibitor of HCV entry with modest potency. EGCG is a polyphenolic natural product with a wide range of biological activity and unfavorable pharmaceutical properties. In an attempt to identify more drug-like EGCG derivatives with improved efficacy as HCV entry inhibitors, we initiated structure-activity investigations using semi-synthetic and synthetic EGCG analogs. The data show that there are multiple regions in the EGCG structure that contribute to activity. The gallate ester portion of the molecule appears to be of particular importance as a 3,4-difluoro analog of EGCG enhanced potency. This derivative and other active compounds were shown not to be cytotoxic in Huh-7 cell culture. These data suggest that more potent, non-cytotoxic EGCG analogs can be prepared in an attempt to identify more drug-like candidates to treat HCV infection by this mechanism.

Towards the discovery of drug-like epigallocatechin gallate analogs as Hsp90 inhibitors.

(-)-Epigallocatechin gallate (EGCG) is the major flavonoid of green tea and has been widely explored for a range of biological activities including anti-infective, anti-inflammatory, anti-cancer, and neuroprotection. Existing structure-activity data for EGCG has been largely limited to exploration of simple ethers and hydroxyl deletion. EGCG has poor drug-like properties because of multiple phenolic hydroxyl moieties and a metabolically labile ester. This work reports a substantial expansion of structure-activity understanding by exploring a range of semi-synthetic and synthetic derivatives with ester replacements and variously substituted aromatic and alicyclic groups containing more drug-like substituents. Structure-activity relationships for these molecules were obtained for Hsp90 inhibition. The results indicate that amide and sulfonamide linkers are suitable ester replacements. Hydroxylated aromatic rings and the cis-stereochemistry in EGCG are not essential for Hsp90 inhibition. Selected analogs in this series are more potent than EGCG in a luciferase refolding assay for Hsp90 activity.

Structure-Activity Relationship of a Pyrrole Based Series of PfPKG Inhibitors as Anti-Malarials.

Controlling malaria requires new drugs against Plasmodium falciparum. The P. falciparum cGMP-dependent protein kinase (PfPKG) is a validated target whose inhibitors could block multiple steps of the parasite's life cycle. We defined the structure-activity relationship (SAR) of a pyrrole series for PfPKG inhibition. Key pharmacophores were modified to enable full exploration of chemical diversity and to gain knowledge about an ideal core scaffold. In vitro potency against recombinant PfPKG and human PKG were used to determine compound selectivity for the parasite enzyme. P. berghei sporozoites and P. falciparum asexual blood stages were used to assay multistage antiparasitic activity. Cellular specificity of compounds was evaluated using transgenic parasites expressing PfPKG carrying a substituted "gatekeeper" residue. The structure of PfPKG bound to an inhibitor was solved, and modeling using this structure together with computational tools was utilized to understand SAR and establish a rational strategy for subsequent lead optimization.

First Optimization of Novel, Potent, Selective PDE11A4 Inhibitors for Age-Related Cognitive Decline.

Phosphodiesterase 11A4 (PDE11A4) is a dual-acting cyclic nucleotide hydrolase expressed in neurons in the CA1, subiculum, amygdalostriatal transition area and amygdalohippocampal area of the extended hippocampal formation. PDE11A4 is the only PDE enzyme to emanate solely from hippocampal formation, a key brain region for the formation of long-term memory. PDE11A4 expression increases in the hippocampal formation of both humans and rodents as they age. Interestingly, PDE11A knockout mice do not show age-related deficits in associative memory and show no gross histopathology. This suggests that inhibition of PDE11A4 might serve as a therapeutic option for age-related cognitive decline. A novel, yeast-based high throughput screen previously identified moderately potent, selective PDE11A4 inhibitors, and this work describes initial efforts that improved potency more than 10-fold and improved some pharmaceutical properties of one of these scaffolds, leading to selective, cell-penetrant PDE11A4 inhibitors, one of which is 10-fold more potent compared to tadalafil in cell-based activity.

Recent results in protein kinase inhibition for tropical diseases.

Protein kinases are becoming widely investigated targets for treatment of protozoal parasitic tropical diseases such as malaria and leishmaniasis. The search for potent, selective inhibitors of these parasitic enzymes has been aided by the extensive variety of structures prepared for human diseases. Genomic approaches to target identification and validation have aided the search. Substantial progress has been made and research is continuing to expand in an effort to find safe, effective drug candidates for these difficult to treat and widespread diseases.

Discovery of Imidazole-Based Inhibitors of Plasmodium falciparum cGMP-Dependent Protein Kinase.

The discovery of new targets for the treatment of malaria, in particular those aimed at the pre-erythrocytic stage in the life cycle, advanced with the demonstration that orally administered inhibitors of Plasmodium falciparum cGMP-dependent protein kinase (PfPKG) could clear infection in a murine model. This enthusiasm was tempered by unsatisfactory safety and/or pharmacokinetic issues found with these chemotypes. To address the urgent need for new scaffolds, this paper presents initial structure-activity relationships in an imidazole scaffold at four positions, representative in vitro ADME, hERG characterization, and cell-based antiparasitic activity. This series of PfPKG inhibitors has good in vitro PfPKG potency, low hERG activity, and cell-based antiparasitic activity against multiple Plasmodium species that appears to be correlated with the in vitro potency.

Potent dihydroquinolinone dopamine D2 partial agonist/serotonin reuptake inhibitors for the treatment of schizophrenia.

A dihydroquinolinone moiety was found to be a potent serotonin reuptake inhibitor pharmacophore when combined with certain amines. This fragment was coupled with selected D(2) ligands to prepare a series of dual acting compounds with attractive in vitro profiles as dopamine D(2) partial agonists and serotonin reuptake inhibitors. Structure-activity studies revealed that the linker plays a key role in contributing to D(2) affinity, function, and SRI activity.

Discovery of isoxazolyl-based inhibitors of Plasmodium falciparum cGMP-dependent protein kinase.

The cGMP-dependent protein kinase in Plasmodium falciparum (PfPKG) plays multiple roles in the life cycle of the parasite. As a result, this enzyme is a potential target for new antimalarial agents. Existing inhbitors, while potent and active in malaria models are not optimal. This communication describes initial optimization of a structurally distinct class of PfPKG inhibitors.

Tetrahydrocarbazole-based serotonin reuptake inhibitor/dopamine D2 partial agonists for the potential treatment of schizophrenia.

A 5-fluoro-tetrahydrocarbazole serotonin reuptake inhibitor (SRI) building block was combined with a variety of linkers and dopamine D2 receptor ligands in an attempt to identify potent D2 partial agonist/SRI molecules for treatment of schizophrenia. This approach has the potential to treat a broader range of symptoms compared to existing therapies. Selected compounds in this series demonstrate high affinity for both targets and D2 partial agonism in cell-based and in vivo assays.

Discovery of a Stress-Activated Protein Kinase Inhibitor for Lymphatic Filariasis.

Lymphatic filariasis infects over 120 million people worldwide and can lead to significant disfigurement and disease. Resistance is emerging with current treatments, and these therapies have dose limiting adverse events; consequently new targets are needed. One approach to achieve this goal is inhibition of parasitic protein kinases involved in circumventing host defense mechanisms. This report describes structure-activity relationships leading to the identification of a potent, orally bioavailable stress activated protein kinase inhibitor that may be used to investigate this hypothesis.

Expression, purification, and inhibition profile of dihydrofolate reductase from the filarial nematode Wuchereria bancrofti.

Filariasis is a tropical disease caused by the parasitic nematodes Wuchereria bancrofti and Brugia malayi. Known inhibitors of dihydrofolate reductase (DHFR) have been previously shown to kill Brugia malayi nematodes and to inhibit Brugia malayi DHFR (BmDHFR) at nanomolar concentrations. These data suggest that BmDHFR is a potential target for the treatment of filariasis. Here, protocols for cloning, expression and purification of Wuchereria bancrofti DHFR (WbDHFR) were developed. The Uniprot entry J9F199-1 predicts a 172 amino acid protein for WbDHFR but alignment of this sequence to the previously described BmDHFR shows that this WbDHFR sequence lacks a crucial, conserved 13 amino acid loop. The presence of the loop in WbDHFR is supported by a noncanonical splicing event and the loop sequence was therefore included in the gene design. Subsequently, the KM for dihydrofolate (3.7 ± 2 µM), kcat (7.4 ± 0.6 s-1), and pH dependence of activity were determined. IC50 values of methotrexate, trimethoprim, pyrimethamine, raltitrexed, aminopterin, (-)-epicatechin gallate, (-)-epicatechin, and vitexin were measured for WbDHFR and BmDHFR. Methotrexate and structurally related aminopterin were found to be effective inhibitors of WbDHFR, with an KI of 1.2 ± 0.2 nM and 2.1 ± 0.5 nM, respectively, suggesting that repurposing of known antifolate compound may be an effective strategy to treating filariasis. Most compounds showed similar inhibition profiles toward both enzymes, suggesting that the two enzymes have important similarities in their active site environments and can be targeted with the same compound, once a successful inhibitor is identified.

Potent non-nitrile dipeptidic dipeptidyl peptidase IV inhibitors.

The synthesis and structure-activity relationships of novel dipeptidyl peptidase IV inhibitors replacing the classical cyanopyrrolidine P1 group with other small nitrogen heterocycles are described. A unique potency enhancement was achieved with beta-branched natural and unnatural amino acids, particularly adamantylglycines, linked to a (2S,3R)-2,3-methanopyrrolidine based scaffold.

Phosphodiesterase 5 inhibitors: current status and potential applications.

Phosphodiesterase enzymes convert cyclic GMP and cyclic AMP to the corresponding nucleotide monophosphates. Phosphodiesterase 5 (PDE5) inhibition is now a widely accepted and efficacious therapeutic option for the treatment of erectile dysfunction in men, as a result of extensive clinical experience with sildenafil and other new PDE5 inhibitors. Research in the field continues at a substantial level to identify new, selective PDE5 inhibitors and to investigate their usefulness and activity in other areas. This review summarizes recent clinical trials with PDE5 inhibitors, advances in medicinal chemistry, and other activities and potential applications of this class of compounds.

The Critical Role of Organic Chemistry in Drug Discovery.

Small molecules remain the backbone for modern drug discovery. They are conceived and synthesized by medicinal chemists, many of whom were originally trained as organic chemists. Support from government and industry to provide training and personnel for continued development of this critical skill set has been declining for many years. This Viewpoint highlights the value of organic chemistry and organic medicinal chemists in the complex journey of drug discovery as a reminder that basic science support must be restored.