However, the researchers said clinical trials will need to run their course before anyone can be sure that the drug is both safe and effective for treating Covid-19 in humans.
In cats at least, GC376 works by interfering with a virus' ability to replicate, thus ending an infection, they said.
Derivatives of this drug were first studied following the 2003 outbreak of severe acute respiratory syndrome (SARS), and it was further developed by veterinary researchers who showed it cures fatal feline affliction.
Lemieux and colleagues first tested two variants of the feline drug against SARS-CoV-2 protein in test tubes and with the live virus in human cell lines.
They then crystallised the drug variants in conjunction with virus proteins.
The researchers determined the orientation of the cat drug as it bound to an active site on a SARS-CoV-2 protein, revealing how it inhibits viral replication.
"This will allow us to develop even more effective drugs," Lemieux said, adding the team will continue to test modifications of the inhibitor to make it an even better fit inside the virus.
Aina Cohen, from the U.S. Department of Energy's SLAC National Accelerator Laboratory, said she was excited by the drug's effectiveness.
"Until an effective vaccine can be developed and deployed, drugs like these add to our arsenal of COVID-19 treatments," Cohen said.
"We are thrilled to learn of these important results and look forward to learning the outcome of clinical trials," she said.