Carrageenans and the Carrageenan-Echinochrome Complex as Anti-SARS-CoV-2 Agents.

The diversity of structural types of carrageenans (CRGs)-sulfated polysaccharides of red algae-determines their different biological activities. The different types of CRGs (kappa, lambda, kappa/beta-CRGs) were isolated from the red algae of the Pacific coast. Molecular docking was performed to determine potential interactions of CRGs with the receptor-binding domain (RBD) of SARS-CoV-2 and its cellular receptor-angiotensin-converting enzyme type 2 (ACE2). CRGs interacted with ACE2 and RBD via hydrogen bonding and ionic interactions. The strongest binding affinity of CRGs and ACE2 was observed for kappa-CRG. Molecular docking was confirmed by results studying the effects of CRGs against SARS-CoV-2 in vitro. The ability of CRGs, as well as the complex CRG with sea urchin echinochrome (Ech), to inhibit SARS-CoV-2 replication in Vero E6 cells was studied using cytopathic effect (CPE) inhibition and RT-PCR assays. The simultaneous treatment of cells with CRGs and the virus revealed that kappa-CRG exhibited the most significant antiviral effect among all the polysaccharides, with a selective index (SI) of 33. The kappa-CRG/Ech complex exhibited the highest virucidal effect on SARS-CoV-2 particles with an SI above 70 (more than two times higher than that of CRG and Ech) and reduced viral RNA levels by 45% (IC = 45%). Our results illustrate that CRGs and kappa-CRG/Ech complex can act as protective agents against SARS-CoV-2.