RNA viruses pose a persistent global threat due to their high mutation rates, zoonotic potential, and rapid adaptability. Emergence events have risen steadily, as demonstrated by major outbreaks caused by Influenza A, Ebola, Zika, and Chikungunya viruses, followed by the coronavirus epidemics of Severe Acute Respiratory Syndrome coronavirus (SARS-CoV-1) and Middle East Respiratory Syndrome Coronavirus (MERS-CoV) and culminating in the COVID-19 pandemic. These characteristics frequently compromise the durability of existing vaccines and antiviral therapies, highlighting the urgent need for new antiviral agents. Alkaloids, a structurally diverse class of nitrogen-containing natural compounds, have gained attention for their ability to interfere with multiple stages of the viral life cycle, including entry, replication, protein synthesis, and host immune modulation. To our knowledge, this review compiles all currently reported alkaloids with antiviral activity against RNA viruses and summarizes their proposed mechanisms of action, distinguishing evidence from in vitro, in vivo, and in silico studies. Quaternary alkaloids are discussed separately because their permanent ionic charge enables distinctive interactions with membranes and host pathways. Although many findings are promising, clinical translation remains limited by incomplete mechanistic validation, scarce in vivo data, suboptimal bioavailability, narrow therapeutic windows, and inconsistent experimental methodologies. To advance the field, future research should prioritize RT-qPCR–based antiviral evaluation to accurately quantify viral replication, incorporate mechanistic assays to clarify modes of action, apply structure–activity relationship (SAR) approaches for rational optimization, and expand in vivo pharmacokinetic and efficacy studies to assess therapeutic feasibility. Overall, alkaloids represent a promising yet underdeveloped reservoir for next-generation antiviral discovery against rapidly evolving RNA viruses.

Emerging RNA viruses continue to pose major global health threats, underscoring the urgent need for broad-spectrum antiviral agents. Usambarensine, a bis-indole alkaloid isolated from Strychnos usambarensis, has historically been investigated for its antimalarial and antihelminthic properties, but its antiviral potential has remained unexplored. In this study, usambarensine was isolated through bioguided fractionation and structurally confirmed by HPTLC, HPLC-UV, FT-IR, MS/MS, and NMR. The compound was first evaluated against representatives of three major RNA virus families: Coronaviridae (SARS-CoV-2), Flaviviridae (West Nile virus), and Orthomyxoviridae (Influenza A) using Vero E6 cells. Antiviral activity was quantified by RT-qPCR at 24, 48, and 72 h postinfection. Usambarensine showed strong, dose-dependent inhibition of SARS-CoV-2, achieving ∼4log₁₀ reduction at 72 h, transient inhibition of West Nile virus and no significant activity against Influenza A. Based on the robust activity observed against SARS-CoV-2, screening was expanded within the Coronaviridae family to include SARS-CoV-1 and MERS-CoV, both of which yielded similar results. This selective antiviral profile, as well as the observation that viral RNA levels remain similar to the initial viral inoculum, suggests that usambarensine interferes with a coronavirus-specific early entry or fusion process, potentially involving a shared entry pathway in Vero E6 cells. These findings identify usambarensine as a promising natural product with selective antiviral activity against coronaviruses. Further mechanistic studies, including determining the specific stage of the viral replication cycle affected, elucidating the molecular target, and performing structure–activity relationship optimization, are needed to assess its potential as a lead scaffold for coronavirus entry inhibitors.