AIDS-related mortality has declined considerably because of the widespread application of combination antiretroviral therapy. However, pretreatment resistance to antiretroviral drugs has been reported in many countries1. Therefore, development of anti-HIV drugs against new targets and with high genetic barriers to resistance is urgently needed. Xiao et al.2 now describe a new class of small-molecule HIV-1 inhibitors targeting the highly conserved membrane-proximal external region (MPER) in the HIV-1 enveloped glycoprotein Env (Fig. 1), which contains the epitopes recognized by several broadly neutralizing antibodies (bNAbs). MPER is a region of glycoprotein 41 (gp41) adjacent to the viral membrane and is required for viral infectivity3. Even though the exact role of MPER in the mechanism of viral fusion is still unknown, the bNAbs targeting MPER appear to block HIV-1 infection by binding to the prehairpin intermediate state of gp41. Therefore, the authors reasoned that small molecules targeting MPER could act similarly to the bNAbs. So far, the US Food and Drug Administration (FDA)-approved HIV small-molecule drugs mainly fall into four categories: nucleoside/nonnucleoside reverse-transcriptase inhibitors (NRTIs/NNRTIs), protease inhibitors (PIs), integrase inhibitors (IIs), and entry inhibitors (EIs, including a fusion inhibitor and a co-receptor antagonist). Most drugs target either an enzyme active site (such as those of NRTIs/NNRTIs, PIs, and IIs) or a ligand-binding site (such as those of EIs). Also, several bNAbs have been isolated from chronically HIV-infected patients. Among them, the MPER-specific bNAbs, including 2F5 and 10E8, are highly potent and broadly effective against divergent HIV-1 strains4.

Fig. 1: The functional domains in the HIV-1 Env, the paradigm of HIV entry and the inhibition mechanism of dequalinium and its analogs on HIV infection.
figure 1

a, Schematic diagram of HIV-1 Env. The positions of the gp120, the fusion peptide (FP), the heptad repeats 1 and 2 (HR1 and HR2, respectively), the membrane-proximal external region (MPER), the transmembrane domain (TMD) and the cytoplasmic region (CP) are shown. b, Dequalinium and its analogs binds to the MPER region at the prefusion stage, and this binding halts the conformational change of HIV Env into the prehairpin intermediate and postfusion conformations, thus inhibiting HIV-1 infection.

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To investigate whether small-molecule compounds can mimic these bNAbs to bind the MPER, the authors performed an in vitro high-throughput screening assay seeking compounds that compete with 2F5 for binding MPER. After screening of a library of 162,106 compounds, dequalinium, an FDA-approved antimicrobial drug emerged. Dequalinium could bind MPER and effectively inhibit HIV-1 Env-mediated membrane fusion and HIV-1 infection in vitro. Structure–activity relationship analysis of dequalinium derivatives identified S2C3, which has improved potency against diverse HIV-1 strains. Mechanistic and NMR structural studies showed that S2C3 bound to the conserved hydrophobic pockets formed by two adjacent MPERs at the prefusion stage of the viral life cycle, and S2C3 binding halted the conformational change of HIV-1 Env into the prehairpin intermediate and postfusion conformation, thus explaining its ability to inhibit HIV-1 infection (Fig. 1).

Use of a small-molecule drug can help overcome several of the disadvantages of an antibody-based drug, such as high cost of production, cold-chain transportation, and requirement for delivery by injection. Several series of small-molecule EIs have been identified previously on the basis of their ability to bind the corresponding target sites in the HIV-1 Env5. As a small-molecule EI, S2C3 targets a site in the HIV-1 Env distinct from those targeted by small-molecule EIs that have been identified so far. The new approach for the identification of S2C3 based on its competitive inhibition of the MPER binding by the antibodies that have been proven to be effective in inhibiting HIV-1 infection is more straightforward and time saving, as it exploits the immune system, which has used evolution to find a feasible target.

Because anti-MPER bNAbs are effective against a broad range of HIV-1 strains, MPER remains one of the most desirable targets in the HIV-1 vaccine research field. As S2C3 did not interfere with binding of Env to the anti-gp120 antibody VRC01, it can be expected to have a synergistic effect with other EIs. Besides the potential as a direct drug candidate, S2C3 could stabilize HIV-1 Env in the prefusion conformation, making it potentially useful in a vaccine preparation to reduce the conformational flexibility of Env.

Finally, this antibody-based screening strategy for drug discovery could be applicable to many other human diseases, especially those caused by infectious agents. For example, Ebola virus is still epidemic in Africa, especially those poor tropic or subtropic areas. However, most of the clinical trial of Ebola virus disease therapeutics are based on antibodies, such as MAb114 and ZMapp6. This antibody-based approach will possibly facilitate the discovery of small-molecule drugs for treatment of Ebola virus disease or COVID-19 caused by 2019-nCoV (also known as HCoV-19 or SARS-CoV-2)7, whereas small-molecule drugs are generally orally available and cost-effective compared to antibody drugs.