Close-up: HIV/SIV intasome structures shed new light on integrase inhibitor binding and viral escape mechanisms

N-Substituted Bicyclic Carbamoyl Pyridones: Integrase Strand Transfer Inhibitors that Potently Inhibit Drug-Resistant HIV-1 Integrase Mutants

HIV-1 integrase (IN) is an important molecular target for the development of anti-AIDS drugs. A recently FDA-approved second-generation integrase strand transfer inhibitor (INSTI) cabotegravir (CAB, 2021) is being marketed for use in long-duration antiviral formulations. However, missed doses during extended therapy can potentially result in persistent low levels of CAB that could select for resistant mutant forms of IN, leading to virological failure. We report a series of N-substituted bicyclic carbamoyl pyridones (BiCAPs) that are simplified analogs of CAB. Several of these potently inhibit wild-type HIV-1 in single-round infection assays in cultured cells and retain high inhibitory potencies against a panel of viral constructs carrying resistant mutant forms of IN. Our lead compound, 7c, proved to be more potent than CAB against the therapeutically important resistant double mutants E138K/Q148K (>12-fold relative to CAB) and G140S/Q148R (>36-fold relative to CAB). A significant number of the BiCAPs also potently inhibit the drug-resistant IN mutant R263K, which has proven to be problematic for the FDA-approved second-generation INSTIs.

Biktarvy for the treatment of HIV infection: Progress and prospects

Bictegravir (BIC), a second-generation integrase strand-transfer inhibitor (INSTI) with high resilience to INSTI-resistance mutations, is integrated as a key component of Biktarvy® - a fixed-dose once-daily triple-drug regimen of bictegravir (BIC), emtricitabine (FTC) plus tenofovir alafenamide (TAF). Based on the accumulated evidence from HIV clinical trials and real-world studies, the clinical effectiveness of BIC + FTC + TAF has been proven non-inferior to other fixed-dose once-daily combinations such as dolutegravir + FTC + TAF and dolutegravir + abacavir + lamivudine. Biktarvy also shows limited drug-drug interactions and a high barrier to drug resistance. According to recent HIV guidelines, BIC + FTC + TAF is recommended as initial and long-term therapy for the treatment of HIV infection. For the pre-exposure prophylaxis, tenofovir disoproxil fumarate (TDF) or tenofovir alafenamide (TAF) remains advisable, but BIC may be possibly added to TDF or TAF. In the development of a long-acting once-monthly regimen, the novel nano-formulation of BIC + FTC + TAF could be possibly developed in the future.

5-Nitro-3-(2-(4-phenylthiazol-2-yl)hydrazineylidene)indolin-2-one derivatives inhibit HIV-1 replication by a multitarget mechanism of action

In the effort to identify and develop new HIV-1 inhibitors endowed with innovative mechanisms, we focused our attention on the possibility to target more than one viral encoded enzymatic function with a single molecule. In this respect, we have previously identified by virtual screening a new indolinone-based scaffold for dual allosteric inhibitors targeting both reverse transcriptase-associated functions: polymerase and RNase H. Pursuing with the structural optimization of these dual inhibitors, we synthesized a series of 35 new 3-[2-(4-aryl-1,3-thiazol-2-ylidene)hydrazin-1-ylidene]1-indol-2-one and 3-[3-methyl-4-arylthiazol-2-ylidene)hydrazine-1-ylidene)indolin-2-one derivatives, which maintain their dual inhibitory activity in the low micromolar range. Interestingly, compounds 1a, 3a, 10a, and 9b are able to block HIV-1 replication with EC₅₀ < 20 µM. Mechanism of action studies showed that such compounds could block HIV-1 integrase. In particular, compound 10a is the most promising for further multitarget compound development.

Retroviral integrase: Structure, mechanism, and inhibition

The retroviral protein Integrase (IN) catalyzes concerted integration of viral DNA into host chromatin to establish a permanent infection in the target cell. We learned a great deal about the mechanism of catalytic integration through structure/function studies over the previous four decades of IN research. As one of three essential retroviral enzymes, IN has also been targeted by antiretroviral drugs to treat HIV-infected individuals. Inhibitors blocking the catalytic integration reaction are now state-of-the-art drugs within the antiretroviral therapy toolkit. HIV-1 IN also performs intriguing non-catalytic functions that are relevant to the late stages of the viral replication cycle, yet this aspect remains poorly understood. There are also novel allosteric inhibitors targeting non-enzymatic functions of IN that induce a block in the late stages of the viral replication cycle. In this chapter, we will discuss the function, structure, and inhibition of retroviral IN proteins, highlighting remaining challenges and outstanding questions.

Long-Acting Cabotegravir for HIV/AIDS Prophylaxis

The retrovirus HIV-1 is the etiological agent of the decades-long AIDS pandemic. Although vaccination is the most common preexposure route to prevent acquisition of viral disease, scalable efficacious vaccination strategies have yet to be developed for HIV-1. By contrast, small molecule inhibitors of the HIV-1 enzymes reverse transcriptase, integrase, and protease have been developed that effectively block virus replication. Three different drug compounds are commonly prescribed for people living with HIV as once-daily oral tablets. Once-daily pills composed of two different reverse transcriptase inhibitors are moreover approved as preexposure prophylaxis (PrEP) treatment for virus naïve individuals who may partake in behaviors associated with increased risk of HIV-1 acquisition such as unprotected sex or injection drug use. Long-acting (LA) injectable HIV-1 enzyme inhibitors are at the same time being developed to sidestep adherence noncompliance issues that can arise from self-administered once-daily oral dosing regimens. Cabotegravir (CAB)-LA, which inhibits integrase strand transfer activity, has in recent clinical trials been shown to prevent HIV-1 acquisition more effectively than once-daily oral dosed reverse transcriptase inhibitors. In this Perspective, we examine bench to bedside aspects of CAB-LA treatment and development, starting from the biochemical basis of HIV-1 integration and pharmacological inhibition of integrase catalysis. We also review the results of recent clinical trials that evaluated CAB-LA, as well as the promises and challenges that surround its use for HIV/AIDS PrEP.

The active form of the influenza cap-snatching endonuclease inhibitor baloxavir marboxil is a tight binding inhibitor

Baloxavir marboxil (BXM) is an FDA-approved antiviral prodrug for the treatment of influenza A and B infection and postexposure prophylaxis. The active form, baloxavir acid (BXA), targets the cap-snatching endonuclease (PA) of the influenza virus polymerase complex. The nuclease activity delivers the primer for transcription, and previous reports have shown that BXA blocks the nuclease activity with high potency. However, biochemical studies on the mechanism of action are lacking. Structural data have shown that BXA chelates the two divalent metal ions at the active site, like inhibitors of the human immunodeficiency virus type 1 (HIV-1) integrase or ribonuclease (RNase) H. Here we studied the mechanisms underlying the high potency of BXA and how the I38T mutation confers resistance to the drug. Enzyme kinetics with the recombinant heterotrimeric enzyme (FluB-ht) revealed characteristics of a tight binding inhibitor. The apparent inhibitor constant (K_i^app) is 12 nM, while the I38T mutation increased K_i^app by ∼18-fold. Order-of-addition experiments show that a preformed complex of FluB-ht, Mg²⁺ ions and BXA is required to observe inhibition, which is consistent with active site binding. Conversely, a preformed complex of FluB-ht and RNA substrate prevents BXA from accessing the active site. Unlike integrase inhibitors that interact with the DNA substrate, BXA behaves like RNase H inhibitors that compete with the nucleic acid at the active site. The collective data support the conclusion that BXA is a tight binding inhibitor and the I38T mutation diminishes these properties.

Integrase Strand Transfer Inhibitors Are Effective Anti-HIV Drugs

Integrase strand transfer inhibitors (INSTIs) are currently recommended for the first line treatment of human immunodeficiency virus type one (HIV-1) infection. The first-generation INSTIs are effective but can select for resistant viruses. Recent advances have led to several potent second-generation INSTIs that are effective against both wild-type (WT) HIV-1 integrase and many of the first-generation INSTI-resistant mutants. The emergence of resistance to these new second-generation INSTIs has been minimal, which has resulted in alternative treatment strategies for HIV-1 patients. Moreover, because of their high antiviral potencies and, in some cases, their bioavailability profiles, INSTIs will probably have prominent roles in pre-exposure prophylaxis (PrEP). Herein, we review the current state of the clinically relevant INSTIs and discuss the future outlook for this class of antiretrovirals.