Marine phlorotannins block SARS-CoV-2 entry via ACE2 and attenuate pulmonary inflammation in K18-hACE2 transgenic mice.

SARS-CoV-2 has transitioned from pandemic to endemic circulation, yet it continues to cause severe respiratory disease, particularly in vulnerable populations. Effective antivirals with well-defined mechanisms of action remain a critical unmet need. Marine phlorotannins are structurally complex polyphenols from edible brown seaweeds with established antiviral and anti-inflammatory properties, making them promising candidates for therapeutic development. Our previous work demonstrated that selected phlorotannins suppress SARS-CoV-2 replication through inhibition of the viral proteases 3C-like protease (3CLpro) and papain-like protease (PLpro). The present study extended this work by investigating whether these compounds could also block viral entry via the angiotensin-converting enzyme-2 (ACE2) receptor. Molecular docking of 16 phlorotannins against ACE2 revealed strong binding affinities for Ishophloroglucin A (IPA), diphlorethohydroxycarmalol (DPHC), and eckmaxol, isolated from Ishige okamurae and Ecklonia cava. In vitro assays confirmed that all three compounds dose-dependently inhibited ACE2 and spike receptor-binding domain (RBD) interaction and blocked SARS-CoV-2 pseudovirus entry into HEK293T-ACE2 cells. Based on its favorable pharmacokinetic profile, DPHC was selected for in vivo evaluation in K18-hACE2 transgenic mice infected with live SARS-CoV-2. DPHC administration at 25 and 50 mg/kg dose-dependently reduced pulmonary viral titers, suppressed nucleocapsid gene expression, and alleviated histopathological lung injury. It also selectively downregulated the pro-inflammatory mediators interferon-gamma (IFN-γ) and monocyte chemoattractant protein-1 (MCP-1), with high-dose efficacy comparable to remdesivir. These findings identify DPHC as a well-characterized phytochemical with integrated antiviral and immunomodulatory activity, targeting viral entry, replication, and inflammation through a multi-target mechanism.