Feeling a bit under the weather? There’s a decent chance you’re suffering from an infection with an enterovirus. Enteroviruses are a commonly encountered virus, especially in the summer and fall. They can cause a variety of symptoms, from cold-like symptoms such as runny nose or fever to more serious sequelae such as meningitis or encephalitis. In 2014, an outbreak of enterovirus D68 correlated with nearly 100 cases of acute flaccid paralysis, though no specific cause has been identified (and there are additional viruses also under investigation). Most people infected with enteroviruses experience only mild, if any, disease, but those who suffer severe cases illustrate the importance of developing treatments for this disease.
Enteroviruses have been studied for a number of years – decades, really – yet despite the large volume of knowledge scientists have gathered on these viruses, there is little outside of palliative care to treat enterovirus-infected patients. A recent study published in Antimicrobial Agents and Chemotherapy covers the discovery of several compounds that may someday be used to treat enterovirus infections.
To identify compounds that might work as enterovirus therapy, Paul Krogstad’s lab at UCLA performed a high-throughput screen of over 85,000 compounds. They investigated the activity of these compounds against the common isolate coxsackievirus B3 (CVB3), looking for compounds that would inhibit viral replication. They identified 69 compounds that were able to inhibit viral replication after one replication cycle, and 11 of these were selected for further study based on inhibitory activity, rather than direct virucidal activity (see figure from paper, right).
The virus life cycle has many points where drugs can act. All 11 compounds were found to act after infection had occurred, meaning that the virus can enter the cell, but can’t make more virus. A timecourse study suggested these compounds acted at an early life cycle stage, inhibiting viral protein and RNA synthesis.
Having identified potential compounds, the researchers tested the compound on additional pathogenic enteroviruses. They were hoping that several shared viral characteristics would allow the drugs to be active against multiple enterovirus types, including several other coxsackieviruses, echoviruses, enterovirus, and polioviruses. Indeed, they found broad anti-enterovirus activity by one compound, with the others inhibiting all but the polioviruses.
How are these viruses inhibited by the various compounds? One way to identify drug targets is to select for drug-resistant virus mutations, and identify where those mutations have occurred. If most mutations occur in a particular gene (especially in a common region of that gene), it becomes a good candidate for the drug target, and hints at the drug’s mechanism of action. Using CVB3, the virus was passaged in gradually increasing drug concentrations of one of the compounds. When resistant viruses were sequenced, most mutations were found at two residues in the viral protein 2C. When this mutated 2C allele was inserted into a sensitive CVB3 clone, the resulting virus was resistant to all the tested compounds. This suggests a common mechanism against viral protein 2C is used by all the compounds tested.
Enteroviruses are a type of picornavirus, viruses that have single-stranded (+)-RNA genomes. After internalization, the viral RNA is translated into a single polyprotein, which is then cleaved into smaller structural and enzymatic proteins (see schematic, right, from this review). The 2C region of the genome is highly conserved, which may explain why drugs acting against a CVB3 virus would exhibit broad activity against other enteroviruses.
These studies identified a number of potential candidate drugs through in vitro cell culture studies, and the ability of these compounds to treat in vivo infections is an obvious next step. However, with a number of candidates, the research seems very promising.
-- Julie Wolf