Antiviral drug development against respiratory syncytial virus (RSV) is urgently needed due to the public health significance of the viral infection. Here, we report the anti‐RSV activity of a small molecule, (1S,3R,4R,5R)‐3,4‐ bis {[(E)‐3‐(3,4‐dihydroxyphenyl)prop‐2‐enoyl]oxy}‐l,5‐dihydroxycyclohexane‐1‐ca methyl ester (3,4‐DCQAME) or 3,4‐ O ‐Dicaffeoylquinic acid methyl ester, which can be isolated from several plants of traditional Chinese medicine. We showed for the first time that compound 3,4‐DCQAME potently inhibits RSV entry and infection. In vitro , 3,4‐DCQAME can interact with F(ecto), the ectodomain of RSV fusion (F) protein. In cultured cells, the compound can block the interaction of F(ecto) protein with the cellular membrane and inhibit viral fusion during RSV entry, leading to inhibition of viral gene expression and infection. In RSV‐infected mice that were treated with 3,4‐DCQAME, we observed a reduction of RSV‐induced pathologic changes and substantial inhibition of viral infection and growth in the lung tissues. Our results provide the first direct evidence of the anti‐RSV activity of 3,4‐DCQAME. Furthermore, these results suggest that 3,4‐DCQAME represents a promising lead compound for anti‐RSV therapeutic development.—Tang, W., Li, M., Liu, Y., Liang, N., Yang, Z., Zhao, Y., Wu, S., Lu, S., Li, Y., Liu, F. Small molecule inhibits respiratory syncytial virus entry and infection by blocking the interaction of the viral fusion protein with the cell membrane. FASEB J. 33, 4287–4299 (2019). www.fasebj.org

Respiratory syncytial virus (RSV) causes serious respiratory tract disease in children and the elderly. Therapeutics against RSV infection are urgently needed.

The building blocks-based molecular network (BBMN) strategy was applied to the phytochemical investigation of Cleistocalyx operculatus, leading to the targeted isolation of eighteen novel cinnamoylphloroglucinol-terpene adducts (CPTAs) with diverse skeleton types (cleistoperones A–R, 1–18). Their structures including absolute configurations were determined by extensive spectroscopic methods, quantum chemical calculations, and single-crystal X-ray crystallographic experiments. Cleistoperone A (1), consisting of a cinnamoylphloroglucinol motif and two linear monoterpene moieties, represents an unprecedented macrocyclic CPTA, whose densely functionalized tricyclo[15.3.1.0^3,8]heneicosane bridge ring skeleton contains an enolizable β,β′-triketone system and two different kinds of stereogenic elements (including five point and three planar chiralities). Cleistoperones B and C (2 and 3) are two new skeletal CPTAs with an unusual coupling pattern between the (nor)monoterpene moiety and the cinnamoyl chain of the cinnamoylphloroglucinol unit. Cleistoperone D (4) possesses an unprecedented cage-like 6/6/6/4/6-fused heteropentacyclic scaffold. The plausible biosynthetic pathways for 1–18 were also proposed. Notably, compounds 1, 4, 7, 8, and 18 exhibited significant antiviral activity against respiratory syncytial virus (RSV). The most potent one, cleistoperone A (1) with IC₅₀ value of 1.71 ± 0.61 μmol/L, could effectively inhibit virus replication via affecting the Akt/mTOR/p70S6K signaling pathway.

Infection with respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract disease in young children and older people. Despite intensive efforts over the past few decades, no direct-acting small-molecule agents against RSV are available. Most small-molecule candidates targeting the RSV fusion (F) protein pose a considerable risk of inducing drug-resistant mutations. Here, we explored the in vitro and in vivo virological properties of the K394R variant, a cross-resistant mutant capable of evading multiple RSV fusion inhibitors. Our results demonstrated that the K394R variant is highly fusogenic in vitro and more pathogenic than the parental strain in vivo . The small molecule (2 E ,2′ E )- N , N ′-((1 R ,2 S ,3 S )−3-hydroxycyclohexane-1,2-diyl)bis(3-(2-bromo-4-fluorophenyl) acrylamide) (CL-A3-7), a structurally optimized compound derived from a natural caffeoylquinic acid derivative, substantially reduced in vitro and in vivo infections of both wild-type RSV and the K394R variant. Mechanistically, CL-A3-7 significantly inhibited virus–cell fusion during RSV entry by blocking the interaction between the viral F protein and the cellular insulin-like growth factor 1 receptor (IGF1R). Collectively, these results indicate severe disease risks caused by the K394R variant and reveal a new anti-RSV mechanism to overcome K394R-associated resistance. IMPORTANCE: Respiratory syncytial virus (RSV) infection is a major public health concern, and many small-molecule candidates targeting the viral fusion (F) protein are associated with a considerable risk of inducing drug-resistant mutations. This study investigated virological features of the K394R variant, a mutant strain conferring resistance to multiple RSV fusion inhibitors. Our results demonstrated that the K394R variant is highly fusogenic in cell cultures and more pathogenic than the parental strain in mice. The small-molecule inhibitor CL-A3-7 substantially reduced in vitro and in vivo infections of both wild-type RSV and the K394R variant by blocking the interaction of viral F protein with its cellular receptor, showing a new mechanism of action for small-molecules to inhibit RSV infection and overcome K394R-associated resistance.