Antiviral activity of retrocyclin RC-101, a candidate microbicide against cell-associated HIV-1

Baseline and time-updated factors in preclinical development of anionic dendrimers as successful anti-HIV-1 vaginal microbicides

Although a wide variety of topical microbicides provide promising in vitro and in vivo efficacy, most of them failed to prevent sexual transmission of human immunodeficiency virus type 1 (HIV‐1) in human clinical trials. In vitro, ex vivo, and in vivo models must be optimized, considering the knowledge acquired from unsuccessful and successful clinical trials to improve the current gaps and the preclinical development protocols. To date, dendrimers are the only nanotool that has advanced to human clinical trials as topical microbicides to prevent HIV‐1 transmission. This fact demonstrates the importance and the potential of these molecules as microbicides. Polyanionic dendrimers are highly branched nanocompounds with potent activity against HIV‐1 that disturb HIV‐1 entry. Herein, the most significant advancements in topical microbicide development, trying to mimic the real‐life conditions as closely as possible, are discussed. This review also provides the preclinical assays that anionic dendrimers have passed as microbicides because they can improve current antiviral treatments' efficacy.

This article is categorized under:

Nanotechnology Approaches to Biology > Nanoscale Systems in Biology

Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease

Toxicology and Regulatory Issues in Nanomedicine > Regulatory and Policy Issues in Nanomedicine

Inhibition of SARS-CoV-2 Infection by Human Defensin HNP1 and Retrocyclin RC-101

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 is an enveloped virus responsible for the COVID-19 pandemic. The emergence of new potentially more transmissible and vaccine-resistant variants of SARS-CoV-2 is an ever-present threat. Thus, it remains essential to better understand innate immune mechanisms that can inhibit the virus. One component of the innate immune system with broad antipathogen, including antiviral, activity is a group of cationic immune peptides termed defensins. The ability of defensins to neutralize enveloped and non-enveloped viruses and to inactivate numerous bacterial toxins correlate with their ability to promote the unfolding of proteins with high conformational plasticity. We found that human neutrophil α-defensin HNP1 binds to SARS-CoV-2 Spike protein with submicromolar affinity that is more than 20 fold stronger than its binding to serum albumin. As such, HNP1, as well as a θ-defensin retrocyclin RC-101, both interfere with Spike-mediated membrane fusion, Spike-pseudotyped lentivirus infection, and authentic SARS-CoV-2 infection in cell culture. These effects correlate with the abilities of the defensins to destabilize and precipitate Spike protein and inhibit the interaction of Spike with the ACE2 receptor. Serum reduces the anti-SARS-CoV-2 activity of HNP1, though at high concentrations, HNP1 was able to inactivate the virus even in the presence of serum. Overall, our results suggest that defensins can negatively affect the native conformation of SARS-CoV-2 Spike, and that α- and θ-defensins may be valuable tools in developing SARS-CoV-2 infection prevention strategies.

Design, synthesis, and antiviral activity of a series of CD4-mimetic small-molecule HIV-1 entry inhibitors

We presented our continuing stride to optimize the second-generation NBD entry antagonist targeted to the Phe43 cavity of HIV-1 gp120. We have synthesized thirty-eight new and novel analogs of NBD-14136, earlier designed based on a CH₂OH "positional switch" hypothesis, and derived a comprehensive SAR. The antiviral data confirmed that the linear alcohol towards the "N" (C4) of the thiazole ring yielded more active inhibitors than those towards the "S" (C5) of the thiazole ring. The best inhibitor, NBD-14273 (compound 13), showed both improved antiviral activity and selectivity index (SI) against HIV-1_HXB2 compared to NBD-14136. We also tested NBD-14273 against a large panel of 50 HIV-1 Env-pseudotyped viruses representing clinical isolates of diverse subtypes. The overall mean data indicate that antiviral potency against these isolates improved by ~3-fold, and SI also improved ~3-fold compared to NBD-14136. This new and novel inhibitor is expected to pave the way for further optimization to a more potent and clinically relevant inhibitor against HIV-1.

Preclinical Optimization of gp120 Entry Antagonists as anti-HIV-1 Agents with Improved Cytotoxicity and ADME Properties through Rational Design, Synthesis, and Antiviral Evaluation

We previously reported a milestone in the optimization of NBD-11021, an HIV-1 gp120 antagonist, by developing a new and novel analogue, NBD-14189 (Ref1), which showed antiviral activity against HIV-1_HXB2, with a half maximal inhibitory concentration of 89 nM. However, cytotoxicity remained high, and the absorption, distribution, metabolism, and excretion (ADME) data showed relatively poor aqueous solubility. To optimize these properties, we replaced the phenyl ring in the compound with a pyridine ring and synthesized a set of 48 novel compounds. One of the new analogues, NBD-14270 (8), showed a marked improvement in cytotoxicity, with 3-fold and 58-fold improvements in selectivity index value compared with that of Ref1 and NBD-11021, respectively. Furthermore, the in vitro ADME data clearly showed improvements in aqueous solubility and other properties compared with those for Ref1. The data for 8 indicated that the pyridine scaffold is a good bioisostere for phenyl, allowing the further optimization of this molecule.

Structure-based lead optimization to improve antiviral potency and ADMET properties of phenyl-1H-pyrrole-carboxamide entry inhibitors targeted to HIV-1 gp120

We are continuing our concerted effort to optimize our first lead entry antagonist, NBD-11021, which targets the Phe43 cavity of the HIV-1 envelope glycoprotein gp120, to improve antiviral potency and ADMET properties. In this report, we present a structure-based approach that helped us to generate working hypotheses to modify further a recently reported advanced lead entry antagonist, NBD-14107, which showed significant improvement in antiviral potency when tested in a single-cycle assay against a large panel of Env-pseudotyped viruses. We report here the synthesis of twenty-nine new compounds and evaluation of their antiviral activity in a single-cycle and multi-cycle assay to derive a comprehensive structure-activity relationship (SAR). We have selected three inhibitors with the high selectivity index for testing against a large panel of 55 Env-pseudotyped viruses representing a diverse set of clinical isolates of different subtypes. The antiviral activity of one of these potent inhibitors, 55 (NBD-14189), against some clinical isolates was as low as 63 nM. We determined the sensitivity of CD4-binding site mutated-pseudoviruses to these inhibitors to confirm that they target HIV-1 gp120. Furthermore, we assessed their ADMET properties and compared them to the clinical candidate attachment inhibitor, BMS-626529. The ADMET data indicate that some of these new inhibitors have comparable ADMET properties to BMS-626529 and can be optimized further to potential clinical candidates.

Pharmacokinetics of the Protein Microbicide 5P12-RANTES in Sheep following Single-Dose Vaginal Gel Administration

5P12-RANTES, a chemokine analogue that potently blocks the HIV CCR5 coreceptor, is being developed as both a vaginal and rectal microbicide for prevention of sexual transmission of HIV. Here, we report the first pharmacokinetic data for 5P12-RANTES following single-dose vaginal gel administration in sheep. Aqueous gel formulations containing low (1.24-mg/ml), intermediate (6.18-mg/ml), and high (32.0-mg/ml; suspension-type gel) concentrations of 5P12-RANTES were assessed via rheology, syringeability, and in vitro release testing. Following vaginal gel administration to sheep, 5P12-RANTES concentrations were measured in vaginal fluid, vaginal tissue, and serum over a 96-h period. All gels showed non-Newtonian pseudoplastic behavior, with the high-concentration gels exhibiting a greater viscosity and cohesive structure than the intermediate- and low-concentration gels. In in vitro release testing, >90% 5P12-RANTES was released from the low- and intermediate-concentration gels after 72 h. For the high-concentration gel, ∼50% 5P12-RANTES was detected, attributed to protein denaturation during lyophilization and/or subsequent solvation of the protein within the gel matrix. In sheep, 5P12-RANTES concentrations in vaginal fluid, vaginal tissue, and serum increased in a dose-dependent manner. The highest concentrations were measured in vaginal fluid (10⁵ to 10⁷ ng/ml), followed by vaginal tissue (10⁴ to 10⁶ ng/ml). Both of these concentration ranges are several orders of magnitude above the reported half-maximal inhibitory concentrations. The lowest concentration was measured in serum (<10² ng/ml). The 5P12-RANTES pharmacokinetic data are similar to those reported previously for other candidate microbicides. These data, coupled with 5P12-RANTES's potency at picomolar concentrations, its strong barrier to resistance, and the full protection that it was observed to provide in a rhesus macaque vaginal challenge model, support the continued development of 5P12-RANTES as a microbicide.

HIV-Enhancing and HIV-Inhibiting Properties of Cationic Peptides and Proteins

Cationic antimicrobial peptides and proteins have historically been ascribed roles in innate immunity that infer killing of microbial and viral pathogens and protection of the host. In the context of sexually transmitted HIV-1, we take an unconventional approach that questions this paradigm. It is becoming increasingly apparent that many of the cationic polypeptides present in the human genital or anorectal mucosa, or human semen, are capable of enhancing HIV-1 infection, often in addition to other reported roles as viral inhibitors. We explore how the in vivo environment may select for or against the HIV-enhancing aspects of these cationic polypeptides by focusing on biological relevance. We stress that the distinction between enhancing and inhibiting HIV-1 infection is not mutually exclusive to specific classes of cationic polypeptides. Understanding how virally enhancing peptides and proteins act to promote sexual transmission of HIV-1 would be important for the design of topical microbicides, mucosal vaccines, and other preventative measures.

A synthetic antimicrobial peptide BTD-S expressed in Arabidopsis thaliana confers enhanced resistance to Verticillium dahliae

BTD-S is a synthetic non-cyclic θ-defensin derivative which was previously designed in our laboratory based on baboon θ-defensins (BTDs). It shows robust antimicrobial activity against economically important phytopathogen, Verticillium dahliae. Here, we deduced the coding nucleotide sequence of BTD-S and introduced the gene into wild-type (ecotype Columbia-0) Arabidopsis thaliana plants. Results demonstrated that BTD-S-transgenic lines displayed in bioassays inhibitory effects on the growth of V. dahliae in vivo and in vitro. Based on symptom severity, enhanced resistance was found in a survey of BTD-S-transgenic lines. Besides, crude protein extracts from root tissues of BTD-S-transformed plants significantly restricted the growth of fungal hyphae and the germination of conidia. Also, fungal biomass over time determined by real-time PCR demonstrated the overgrowth of V. dahliae in wild-type plants 2-3 weeks after inoculation, while almost no fungal DNA was detected in aerial tissues of their transgenic progenitors. The result suggested that fungus failed to invade and progress acropetally up to establish a systemic infection in BTD-S-transgenic plants. Moreover, the assessment of basal defense responses was performed in the leaves of WT and BTD-S-transgenic plants. The mitigated oxidative stress and low antioxidase level in BTD-S-transgenic plants revealed that BTD-S acts via permeabilizing target microbial membranes, which is in a category different from hypersensitive response-dependent defense. Taken together, our results demonstrate that BTD-S is a promising gene to be explored for transgenic engineering for plant protection against Verticillium wilt.

Development of water-soluble polyanionic carbosilane dendrimers as novel and highly potent topical anti-HIV-2 microbicides

The development of topical microbicide formulations for vaginal delivery to prevent HIV-2 sexual transmission is urgently needed. Second- and third-generation polyanionic carbosilane dendrimers with a silicon atom core and 16 sulfonate (G2-S16), napthylsulfonate (G2-NS16) and sulphate (G3-Sh16) end-groups have shown potent and broad-spectrum anti-HIV-1 activity. However, their antiviral activity against HIV-2 and mode of action have not been probed. Cytotoxicity, anti-HIV-2, anti-sperm and antimicrobial activities of dendrimers were determined. Analysis of combined effects of triple combinations with tenofovir and raltegravir was performed by using CalcuSyn software. We also assessed the mode of antiviral action on the inhibition of HIV-2 infection through a panel of different in vitro antiviral assays: attachment, internalization in PBMCs, inactivation and cell-based fusion. Vaginal irritation and histological analysis in female BALB/c mice were evaluated. Our results suggest that G2-S16, G2-NS16 and G3-Sh16 exert anti-HIV-2 activity at an early stage of viral replication inactivating the virus, inhibiting cell-to-cell HIV-2 transmission, and blocking the binding of gp120 to CD4, and the HIV-2 entry. Triple combinations with tenofovir and raltegravir increased the anti-HIV-2 activity, consistent with synergistic interactions (CIwt: 0.33-0.66). No vaginal irritation was detected in BALB/c mice after two consecutive applications for 2 days with 3% G2-S16. Our results have clearly shown that G2-S16, G2-NS16 and G3-Sh16 have high potency against HIV-2 infection. The modes of action confirm their multifactorial and non-specific ability, suggesting that these dendrimers deserve further studies as potential candidate microbicides to prevent vaginal/rectal HIV-1/HIV-2 transmission in humans.

Structure-Based Design of a Small Molecule CD4-Antagonist with Broad Spectrum Anti-HIV-1 Activity

Earlier we reported the discovery and design of NBD-556 and their analogs which demonstrated their potential as HIV-1 entry inhibitors. However, progress in developing these inhibitors has been stymied by their CD4-agonist properties, an unfavorable trait for use as drug. Here, we demonstrate the successful conversion of a full CD4-agonist (NBD-556) through a partial CD4-agonist (NBD-09027), to a full CD4-antagonist (NBD-11021) by structure-based modification of the critical oxalamide midregion, previously thought to be intolerant of modification. NBD-11021 showed unprecedented neutralization breath for this class of inhibitors, with pan-neutralization against a panel of 56 Env-pseudotyped HIV-1 representing diverse subtypes of clinical isolates (IC50 as low as 270 nM). The cocrystal structure of NBD-11021 complexed to a monomeric HIV-1 gp120 core revealed its detail binding characteristics. The study is expected to provide a framework for further development of NBD series as HIV-1 entry inhibitors for clinical application against AIDS.

MIV-150/zinc acetate gel inhibits cell-associated simian-human immunodeficiency virus reverse transcriptase infection in a macaque vaginal explant model

The transmission of both cell-free and cell-associated immunodeficiency viruses has been demonstrated directly in multiple animal species and possibly occurs in humans, as suggested by genotyping of the infecting human immunodeficiency virus (HIV) in acutely infected women and in semen from their partners. Therefore, a microbicide may need to block both mechanisms of HIV transmission to achieve maximum efficacy. To date, most of the preclinical evaluation of candidate microbicides has been performed using cell-free HIV. New models of mucosal transmission of cell-associated HIV are needed to evaluate candidate microbicide performance. The MIV-150/zinc acetate/carrageenan (MZC) gel protects Depo-Provera-treated macaques against cell-free simian-human immunodeficiency virus reverse transcriptase (SHIV-RT) infection when applied vaginally up to 8 h before challenge. We recently demonstrated the potent activity of MZC gel against cell-free SHIV-RT in macaque vaginal explants. In the current study, we established a cell-associated SHIV-RT infection model of macaque vaginal tissues and tested the activity of MZC gel in this model. MZC gel protected tissues against cell-associated SHIV-RT infection when present at the time of viral exposure or when applied up to 4 days prior to viral challenge. These data support clinical testing of the MZC gel. Overall, our ex vivo model of cell-associated SHIV-RT infection in macaque vaginal mucosa complements the cell-free infection models, providing tools for prioritization of products that block both modes of HIV transmission.

Modeling mucosal cell-associated HIV type 1 transmission in vitro

Human immunodeficiency virus type 1 (HIV-1) can efficiently spread by direct cell-to-cell contact, a mechanism termed cell-associated HIV transmission. By some estimates, cell-associated HIV transmission is 10-1000-fold more effective than cell-free HIV infection. Mucosal cell-associated HIV transmission may occur when HIV-bearing cells in mucosal secretions from an HIV-infected donor transfer virus directly to recipient target cells in or below the mucosal epithelium, or through HIV transcytosis across the mucosal epithelium of a noninfected host. This mechanism may play an important role in the sexual and vertical transmission of HIV-1, yet most in vitro tests of vaccine and microbicide efficacy assess cell-free virus transmission. This article reviews in vitro assays that have been used to model mucosal cell-associated transmission, including microscopy, immune cell cocultures, use of HIV-infected cells in epithelial cell transcytosis assays, and cell-associated infection of mucosal tissue explants. Assays that authentically simulate mucosal cell-associated HIV transmission could provide valuable insight into mechanisms and molecules that can potentially be targeted for HIV prevention, as well as critical models for testing novel HIV prevention strategies for efficacy against cell-associated HIV transmission.

Nanotech-derived topical microbicides for HIV prevention: the road to clinical development

More than three decades since its discovery, HIV infection remains one of the most aggressive epidemics worldwide, with more than 35 million people infected. In sub-Saharan Africa, heterosexual transmissions represent nearly 80% of new infections, with 50% of these occurring in women. In an effort to stop the dramatic spread of the HIV epidemic, new preventive treatments, such as microbicides, have been developed. Nanotechnology has revolutionized this field by designing and engineering novel highly effective nano-sized materials as microbicide candidates. This review illustrates the most recent advances in nanotech-derived HIV prevention strategies, as well as the main steps required to translate promising in vitro results into clinical trials.

Defensins in innate immunity

PURPOSE OF REVIEW

Defensins are a major family of antimicrobial peptides expressed predominantly in neutrophils and epithelial cells, and play important roles in innate immune defense against infectious pathogens. Their biological functions in and beyond innate immunity, structure and activity relationships, mechanisms of action, and therapeutic potential continue to be interesting research topics. This review examines recent progress in our understanding of alpha and theta-defensins - the two structural classes composed of members of myeloid origin.

RECENT FINDINGS

A novel mode of antibacterial action is described for human enteric alpha-defensin 6, which forms structured nanonets to entrap bacterial pathogens and protect against bacterial invasion of the intestinal epithelium. The functional multiplicity and mechanistic complexity of defensins under different experimental conditions contribute to a debate over the role of enteric alpha-defensins in mucosal immunity against HIV-1 infection. Contrary to common belief, hydrophobicity rather than cationicity plays a dominant functional role in the action of human alpha-defensins; hydrophobicity-mediated high-order assembly endows human alpha-defensins with an extraordinary ability to acquire structural diversity and functional versatility. Growing evidence suggests that theta-defensins offer the best opportunity for therapeutic development as a novel class of broadly active anti-infective and anti-inflammatory agents.

SUMMARY

Defensins are the 'Swiss army knife' in innate immunity against microbial pathogens. Their modes of action are often reminiscent of the story of 'The Blind Men and the Elephant'. The functional diversity and mechanistic complexity, as well as therapeutic potential of defensins, will continue to attract attention to this important family of antimicrobial peptides.

The anti-HIV microbicide candidate RC-101 inhibits pathogenic vaginal bacteria without harming endogenous flora or mucosa

PROBLEM

Vaginal microbicides represent a promising approach for preventing heterosexual HIV transmission. However, preclinical evaluation should be conducted to ensure that microbicides will be safe for human cells and healthy microflora of the female reproductive tract. One microbicide candidate, RC-101, has been effective and well tolerated in preliminary cell culture and macaque models. However, the effect of RC-101 on primary vaginal tissues and resident vaginal microflora requires further evaluation.

METHOD OF STUDY

We treated primary vaginal tissues and vaginal bacteria, both pathogenic and commensal, with RC-101 to investigate effects of this microbicide.

RESULTS

RC-101 was well tolerated by host tissues, and also by commensal vaginal bacteria. Simultaneously, pathogenic vaginal bacteria, which are known to increase susceptibility to HIV acquisition, were inhibited by RC-101.

CONCLUSIONS

By establishing vaginal microflora, the specific antibacterial activity of RC-101 may provide a dual mechanism of HIV protection. These findings support advancement of RC-101 to clinical trials.