Inhibitors of HIV-1 attachment. Part 7: Indole-7-carboxamides as potent and orally bioavailable antiviral agents

Development of Azaindole-Based Frameworks as Potential Antiviral Agents and Their Future Perspectives

The azaindole (AI) framework continues to play a significant role in the design of new antiviral agents. Modulating the position and isosteric replacement of the nitrogen atom of AI analogs notably influences the intrinsic physicochemical properties of lead compounds. The intra- and intermolecular interactions of AI derivatives with host receptors or viral proteins can also be fine tuned by carefully placing the nitrogen atom in the heterocyclic core. This wide-ranging perspective article focuses on AIs that have considerable utility in drug discovery programs against RNA viruses. The inhibition of influenza A, human immunodeficiency, respiratory syncytial, neurotropic alpha, dengue, ebola, and hepatitis C viruses by AI analogs is extensively reviewed to assess their plausible future potential in antiviral drug discovery. The binding interaction of AIs with the target protein is examined to derive a structural basis for designing new antiviral agents.

Therapeutic potential of indole derivatives as anti-HIV agents: A mini-review

Acquired immunodeficiency syndrome (AIDS), caused by human immunodeficiency virus (HIV), is one of the leading causes of human deaths. The advent of different anti-HIV drugs over different disease progress has made AIDS/HIV from a deadly infection to chronic and manageable disease. However, the development of multidrug-resistant viruses, together with the severe side effects of anti-HIV agents, compromised their efficacy and limited the treatment options. Indoles, the most common frameworks in the bioactive molecules, represent attractive scaffolds for the design and development of novel drugs. Indole derivatives are potential inhibitors of HIV enzymes such as reverse transcriptase, integrase and protease, and some indole-based agents like Delavirdine have already been applied in clinics or under clinical evaluations for the treatment of AIDS/HIV, revealing that indole moiety is a useful template for the development of anti-HIV agents. This review focuses on the recent advancement of indole derivatives including indole alkaloids, hybrids, and dimers with anti-HIV potential, covering articles published between 2010 and 2020. The chemical structures, structure-activity relationship and mechanisms of action are also discussed.

Synthesis of Medicinally Important Indole Derivatives: A Review

Indoles constitute a widely occurring functional group in nature and are present in an extensive number of bioactive natural products and medicinally important compounds. As a result, exponential increases in the development of novel methods for the formation of indole core along with site-specific indoles have been established. Conventional methods for the synthesis of indoles are getting replaced with green methods involving ionic liquids, water as a solvent, solid acid catalyst, microwave irradiation and the use of nanoparticles under solvent-free conditions. In addition, there are immense applications of the substituted indoles in diverse fields.

Innovation in the discovery of the HIV-1 attachment inhibitor temsavir and its phosphonooxymethyl prodrug fostemsavir

The discovery and development of fostemsavir (2), the tromethamine salt of the phosphonooxymethyl prodrug of temsavir (1), encountered significant challenges at many points in the preclinical and clinical development program that, in many cases, stimulated the implementation of innovative solutions in order to enable further progression. In the preclinical program, a range of novel chemistry methodologies were developed during the course of the discovery effort that enabled a thorough examination and definition of the HIV-1 attachment inhibitor (AI) pharmacophore. These discoveries helped to address the challenges associated with realizing a molecule with all of the properties necessary to successfully advance through development and this aspect of the program is the major focus of this retrospective. Although challenges and innovation are not unusual in drug discovery and development programs, the HIV-1 AI program is noteworthy not only because of the serial nature of the challenges encountered along the development path, but also because it resulted in a compound that remains the first and only example of a mechanistically novel class of HIV-1 inhibitor that is proving to be very beneficial for controlling virus levels in highly treatment-experienced HIV-1 infected patients.

External oxidant-compatible phosphorus(III)-directed site-selective C-H carbonylation

The first development of an external oxidant-compatible system involving a phosphorus(III)-directed C-H functionalization has been uncovered. An efficient C-H esterification of indoles with CO and alcohols has been reported in which the high reactivity and the exclusive C7-selectivity derives from the selection of a P(III)-directing group and the utilization of benzoquinone as an external oxidant with palladium catalysis. This strategy shows many advantages, involving an easily accessible and removable directing group, the use of cheap carbonylation sources, a broad substrate scope, and excellent positional selectivity. Two cyclopalladated intermediates were confirmed by x-ray analysis, uncovering key mechanistic features of this P(III)-directed C-H metalation event.

C7-Indole Amidations and Alkenylations by Ruthenium(II) Catalysis

C7-H-functionalized indoles are ubiquitous structural units of biological and pharmaceutical compounds for numerous antiviral agents against SARS-CoV or HIV-1. Thus, achieving site-selective functionalizations of the C7-H position of indoles, while discriminating among other bonds, is in high demand. Herein, we disclose site-selective C7-H activations of indoles by ruthenium(II) biscarboxylate catalysis under mild conditions. Base-assisted internal electrophilic-type substitution C-H ruthenation by weak O-coordination enabled the C7-H functionalization of indoles and offered a broad scope, including C-N and C-C bond formation. The versatile ruthenium-catalyzed C7-H activations were characterized by gram-scale syntheses and the traceless removal of the directing group, thus providing easy access to pharmaceutically relevant scaffolds. Detailed mechanistic studies through spectroscopic and spectrometric analyses shed light on the unique nature of the robust ruthenium catalysis for the functionalization of the C7-H position of indoles.

Computational design, synthesis and evaluation of new sulphonamide derivatives targeting HIV-1 gp120

Attachment of envelope glycoprotein gp120 to the host cell receptor CD4 is the first step during the human immunodeficiency virus-1 (HIV-1) entry into the host cells that makes it a promising target for drug design. To elucidate the crucial three dimensional (3D) structural features of reported HIV-1 gp120 CD4 binding inhibitors, 3D pharmacophores were generated and receptor based approach was employed to quantify these structural features. A four-partial least square factor model with good statistics and predictive ability was generated for the dataset of 100 molecules. To further ascertain the structural requirement for gp120-CD4 binding inhibition, molecular interaction studies of inhibitors with gp120 was carried out by performing molecular docking using Glide 5.6. Based on these studies, structural requirements were drawn and new molecules were designed accordingly to yield new sulphonamides derivatives. A water based green synthetic approach was adopted to obtain these compounds which were evaluated for their HIV-1 gp120 CD4 binding inhibition. The newly synthesized compounds exhibited remarkable activity (10-fold increase) when compared with the standard BMS 806. Further the stability of newly synthesized derivatives with HIV-1 gp120 was also investigated through molecular dynamics simulation studies. This provides a proof of concept for molecular modeling based design of new inhibitors for inhibition of HIV-1 gp120 CD4 interaction.

An insight into the medicinal perspective of synthetic analogs of indole: A review

Heterocycles occupy a salient place in chemistry due to their wide range of activity in the fields of drug design, photochemistry, agrochemicals, dyes, and so on. Amongst all, indole scaffold is considered as one of the most promising heterocycles found in natural and synthetic sources and has been shown to possess various biological activity, including anti-inflammatory, anti-HIV, antitubercular, antimalarial, anticonvulsant, antidiabetic, antihypertensive, analgesics, antidepressant, anticancer, antioxidant, antifungal, and antimicrobial, etc. All the reported indole molecules bind to multiple receptors with high affinity, thus expedite the research on the development of novel biologically active compounds through the various approach. In this review, we aimed to highlight synthetic and medicinal perspective on the development of indole-based analogs. In addition, structural activity relationship (SAR) study to correlate for their biological activity also discussed.

Medicinal chemistry of indole derivatives: Current to future therapeutic prospectives

Indole is a versatile pharmacophore, a privileged scaffold and an outstanding heterocyclic compound with wide ranges of pharmacological activities due to different mechanisms of action. It is an superlative moiety in drug discovery with the sole property of resembling different structures of the protein. Plenty of research has been taking place in recent years to synthesize and explore the various therapeutic prospectives of this moiety. This review summarizes some of the recent effective chemical synthesis (2014-2018) for indole ring. This review also emphasized on the structure-activity relationship (SAR) to reveal the active pharmacophores of various indole analogues accountable for anticancer, anticonvulsant, antimicrobial, antitubercular, antimalarial, antiviral, antidiabetic and other miscellaneous activities which have been investigated in the last five years. The precise features with motives and framework of each research topic is introduced for helping the medicinal chemists to understand the perspective of the context in a better way. This review will definitely offer the platform for researchers to strategically design diverse novel indole derivatives having different promising pharmacological activities with reduced toxicity and side effects.

Discovery of the Human Immunodeficiency Virus Type 1 (HIV-1) Attachment Inhibitor Temsavir and Its Phosphonooxymethyl Prodrug Fostemsavir

The optimization of the 4-methoxy-6-azaindole series of HIV-1 attachment inhibitors (AIs) that originated with 1 to deliver temsavir (3, BMS-626529) is described. The most beneficial increases in potency and pharmacokinetic (PK) properties were attained by incorporating N-linked, sp²-hybridized heteroaryl rings at the 7-position of the heterocyclic nucleus. Compounds that adhered to a coplanarity model afforded targeted antiviral potency, leading to the identification of 3 with characteristics that provided for targeted exposure and PK properties in three preclinical species. However, the physical properties of 3 limited plasma exposure at higher doses, both in preclinical studies and in clinical trials as the result of dissolution- and/or solubility-limited absorption, a deficiency addressed by the preparation of the phosphonooxymethyl prodrug 4 (BMS-663068, fostemsavir). An extended-release formulation of 4 is currently in phase III clinical trials where it has shown promise as part of a drug combination therapy in highly treatment-experienced HIV-1 infected patients.

Inhibitors of HIV-1 Attachment: The Discovery and Development of Temsavir and its Prodrug Fostemsavir

Human immunodeficiency virus-1 (HIV-1) infection currently requires lifelong therapy with drugs that are used in combination to control viremia. The indole-3-glyoxamide 6 was discovered as an inhibitor of HIV-1 infectivity using a phenotypic screen and derivatives of this compound were found to interfere with the HIV-1 entry process by stabilizing a conformation of the virus gp120 protein not recognized by the host cell CD4 receptor. An extensive optimization program led to the identification of temsavir (31), which exhibited an improved antiviral and pharmacokinetic profile compared to 6 and was explored in phase 3 clinical trials as the phosphonooxymethyl derivative fostemsavir (35), a prodrug designed to address dissolution- and solubility-limited absorption issues. In this drug annotation, we summarize the structure-activity and structure-liability studies leading to the discovery of 31 and the clinical studies conducted with 35 that entailed the development of an extended release formulation suitable for phase 3 clinical trials.

Iridium(iii)-catalyzed regioselective C7-sulfonamidation of indoles

Iridium(iii)-catalyzed direct C7-sulfonamidation of indoles with sulfonyl azides is described.

Inhibitors of HIV-1 attachment: The discovery and structure-activity relationships of tetrahydroisoquinolines as replacements for the piperazine benzamide in the 3-glyoxylyl 6-azaindole pharmacophore

6,6-Fused ring systems including tetrahydroisoquinolines and tetrahydropyrido[3,4-d]pyrimidines have been explored as possible replacements for the piperazine benzamide portion of the HIV-1 attachment inhibitor BMS-663068. In initial studies, the tetrahydroisoquinoline compounds demonstrate sub-nanomolar activity in a HIV-1 pseudotype viral infection assay used as the initial screen for inhibitory activity. Analysis of SARs and approaches to optimization for an improved drug-like profile are examined herein.

Profiling the interaction mechanism of indole-based derivatives targeting the HIV-1 gp120 receptor

A glycoprotein exposed on a viral surface, human immunodeficiency virus type 1 (HIV-1) gp120 is essential for virus entry into cells as it plays a vital role in seeking out specific cell surface receptors for entry.

Homology models of the HIV-1 attachment inhibitor BMS-626529 bound to gp120 suggest a unique mechanism of action

HIV-1 gp120 undergoes multiple conformational changes both before and after binding to the host CD4 receptor. BMS-626529 is an attachment inhibitor (AI) in clinical development (administered as prodrug BMS-663068) that binds to HIV-1 gp120. To investigate the mechanism of action of this new class of antiretroviral compounds, we constructed homology models of unliganded HIV-1 gp120 (UNLIG), a pre-CD4 binding-intermediate conformation (pCD4), a CD4 bound-intermediate conformation (bCD4), and a CD4/co-receptor-bound gp120 (LIG) from a series of partial structures. We also describe a simple pathway illustrating the transition between these four states. Guided by the positions of BMS-626529 resistance substitutions and structure-activity relationship data for the AI series, putative binding sites for BMS-626529 were identified, supported by biochemical and biophysical data. BMS-626529 was docked into the UNLIG model and molecular dynamics simulations were used to demonstrate the thermodynamic stability of the different gp120 UNLIG/BMS-626529 models. We propose that BMS-626529 binds to the UNLIG conformation of gp120 within the structurally conserved outer domain, under the antiparallel β20-β21 sheet, and adjacent to the CD4 binding loop. Through this binding mode, BMS-626529 can inhibit both CD4-induced and CD4-independent formation of the "open state" four-stranded gp120 bridging sheet, and the subsequent formation and exposure of the chemokine co-receptor binding site. This unique mechanism of action prevents the initial interaction of HIV-1 with the host CD4+ T cell, and subsequent HIV-1 binding and entry. Our findings clarify the novel mechanism of BMS-626529, supporting its ongoing clinical development.

C7-derivatization of C3-alkylindoles including tryptophans and tryptamines

A versatile strategy for C7-selective boronation of tryptophans, tryptamines, and 3-alkylindoles by way of a single-pot C2/C7-diboronation-C2-protodeboronation sequence is described. The combination of a mild iridium-catalyzed C2/C7-diboronation followed by an in situ palladium-catalyzed C2-protodeboronation allows efficient entry to valuable C7-boroindoles that enable further C7-derivatization. The versatility of the chemistry is highlighted by the gram-scale synthesis of C7-boronated N-Boc-L-tryptophan methyl ester and the rapid synthesis of C7-halo, C7-hydroxy, and C7-aryl tryptophan derivatives.

A review on recent developments of indole-containing antiviral agents

Indole represents one of the most important privileged scaffolds in drug discovery. Indole derivatives have the unique property of mimicking the structure of peptides and to bind reversibly to enzymes, which provide tremendous opportunities to discover novel drugs with different modes of action. There are seven indole-containing commercial drugs in the Top-200 Best Selling Drugs by US Retail Sales in 2012. There are also an amazing number of approved indole-containing drugs in the market as well as compounds currently going through different clinical phases or registration statuses. This review focused on the recent development of indole derivatives as antiviral agents with the following objectives: 1) To present one of the most comprehensive listings of indole antiviral agents, drugs on market or compounds in clinical trials; 2) To focus on recent developments of indole compounds (including natural products) and their antiviral activities, summarize the structure property, hoping to inspire new and even more creative approaches; 3) To offer perspectives on how indole scaffolds as a privileged structure might be exploited in the future.

Discovery of small molecular inhibitors targeting HIV-1 gp120-CD4 interaction drived from BMS-378806

The HIV-1 entry into host cells is a complex, multi-factors involved, and multi-step process. Especially, the attachment of HIV-1 envelope glycoprotein gp120 to the host cell receptor CD4 is the first key step during entry process, representing a promising antiviral therapeutic target. Among the HIV-1 attachment inhibitors blocking the interaction between gp120 and CD4 cells, BMS-378806 and NBD-556 are two representative small molecular chemical entities. Particularly, BMS-378806 and its derivatives are newly identified class of orally bioavailable HIV-1 inhibitors that interfere gp120-CD4 interaction. In this review, we focused on describing the structure-activity relationships (SARs), structural modifications, in vitro or even in vivo pharmacodynamics and pharmacokinetics of BMS-378806 and its analogues as HIV-1 gp120 attachment inhibitors. In addition, the brief SARs, structural modifications of NBD-556 and its derivatives targeting the "Phe-43 cavity" as CD4 mimics were also described.

Inhibitors of human immunodeficiency virus type 1 (HIV-1) attachment. 12. Structure-activity relationships associated with 4-fluoro-6-azaindole derivatives leading to the identification of 1-(4-benzoylpiperazin-1-yl)-2-(4-fluoro-7-[1,2,3]triazol-1-yl-1h-pyrrolo[2,3-c]pyridin-3-yl)ethane-1,2-dione (BMS-585248)

A series of highly potent HIV-1 attachment inhibitors with 4-fluoro-6-azaindole core heterocycles that target the viral envelope protein gp120 has been prepared. Substitution in the 7-position of the azaindole core with amides (12a,b), C-linked heterocycles (12c-l), and N-linked heterocycles (12m-u) provided compounds with subnanomolar potency in a pseudotype infectivity assay and good pharmacokinetic profiles in vivo. A predictive model was developed from the initial SAR in which the potency of the analogues correlated with the ability of the substituent in the 7-position of the azaindole to adopt a coplanar conformation by either forming internal hydrogen bonds or avoiding repulsive substitution patterns. 1-(4-Benzoylpiperazin-1-yl)-2-(4-fluoro-7-[1,2,3]triazol-1-yl-1H-pyrrolo[2,3-c]pyridin-3-yl)ethane-1,2-dione (BMS-585248, 12m) exhibited much improved in vitro potency and pharmacokinetic properties than the previous clinical candidate BMS-488043 (1). The predicted low clearance in humans, modest protein binding, and good potency in the presence of 40% human serum for 12m led to its selection for human clinical studies.

Inhibitors of HIV-1 attachment. Part 11: the discovery and structure-activity relationships associated with 4,6-diazaindole cores

A series of HIV-1 attachment inhibitors containing a 4,6-diazaindole core were examined in an effort to identify a compound which improved upon the potency and oral exposure of BMS-488043 (2). BMS-488043 (2) is a 6-azaindole-based HIV-1 attachment inhibitor which established proof-of-concept for this mechanism in human clinical studies but required high doses and concomitant administration of a high fat meal to achieve efficacious exposures. Based on previous studies in indole and azaindole scaffolds, SAR investigation was concentrated around the key 7-position in the 4,6-diazaindole series and led to the discovery of molecules with 5- to 20-fold increases in potency and three- to seven-fold increases in exposure over 2 in a rat PK studies.