Oct 25, 2021

Donnelly Centre investigators create mirror-image peptides capable of neutralizing Sars-CoV-2

Protein Engineering, Research
left handed and mirror image peptides on left and right, respectively
Dr. Pedro Valiente
Mirror-image peptides (D-peptides on the right) engineered by Donnelly Centre investigators neutralised Sars-CoV-2 and prevented infection of cultured human cells.
By Jovana Drinjakovic

U of T researchers have created first in class chemical compounds which can neutralize Sars-CoV-2 and several of its variants. Known as mirror-image peptides, the compounds have chemical properties that make them suitable for the development of low-cost antiviral therapeutics.

“A big advantage of mirror-image peptides is their long stability and that they are relatively cheap to produce,” says Philip Kim, senior author of the study and a professor of molecular genetics and computer science at the Donnelly Centre for Cellular and Biomolecular Research at U of T’s Temerty Faculty of Medicine. “You could imagine them being formulated as a nasal spray to take prophylactically to prevent infection from occurring.”

In a paper published recently in the Journal of Medicinal Chemistry, the researchers report the creation of D-peptides that neutralize the virus and stop infection of cultured human cells.

Peptides are similar to proteins in that they are composed of the same amino-acids building blocks although they are smaller than protein molecules. They can be designed to bind virtually any molecular target and with a greater specificity than small molecule drugs which reduces risk of side effects. In this regard, peptides are similar to antibodies but are at least a hundred times cheaper to produce thanks to their small size. The low cost and easy scaling of manufacturing makes peptides attractive options, in particular for low-income countries.

But there’s a caveat. Peptides get rapidly degraded in the body by the enzymes evolved to stamp out harmful peptides produced by bacteria and other pathogens. But science has found a solution in mirror-image peptides that are resistant to degradation.

A big advantage of mirror-image peptides is their long stability and that they are relatively cheap to produce
Philip Kim, Professor, Donnelly Centre
researchers head shots
Images courtesy of Pedroi Valiente and Philip Kim.
Postdoctoral research fellow Pedro Valiente and Professor Philip Kim, both of the Donnelly Centre.

For reasons that remain unclear, all naturally occurring amino-acids exist in a left-handed configuration, as defined by the direction in which they rotate plane polarized light. Consequently, all proteins and peptides are also left-handed — and known as L-peptides.

A few years ago, Kim’s team developed a computational tool for the design of so-called D-peptides that have inverse geometry. These mirror-image molecules are manufactured from synthetic D-amino-acids strung together in the same way as their left-handed counterparts. And using Kim’s design method, they can be engineered to bind the same targets with undiminished specificity. The main difference is that their unusual geometry makes them resistant to enzymes in the bloodstream that break down normal L-peptides.

The prospect on working with D-peptides is what enticed postdoctoral researcher Pedro Valiente to join Kim’s lab in the first place. When the pandemic hit, he realized they could apply their tool to try to make antivirals for COVID-19. By May 2020, Valiente had already created the compounds that would prove to be potent inhibitors of the virus, although it took another year to verify that they work as expected in human cells. The delay was caused by an overwhelmed capacity of high security labs for study of dangerous pathogens as scientists around the globe rushed to study the novel coronavirus.

Valiente designed several D-peptides that mimic the region of the virus spike that binds the ACE2 receptor on the surface of cells. He reasoned that the peptides will bind to the receptor before the virus makes contact with it therefore preventing infection. This was later confirmed by the experiments with cultured human cells that were carried out by collaborators at two high security labs in the Republic of Korea.

What’s more, the peptides worked just as well against several variants, Alpha, Beta and Gamma, which wreaked havoc over the past year after first appearing in the UK, South Africa and Brazil, respectively. Although the researchers did not investigate the Delta variant, other evidence suggests that it too would be susceptible to the peptide drugs.

“While we focused on the variants that were circulating at the time when we were doing this work, the peptides should work on Delta as well based on the similarity with its receptor binding domain,” says Kim.

Valiente, who joined Kim’s lab a year before the pandemic, said that the experience was especially gratifying as he was able to create a potential therapeutic at record speed during the first lockdown when most of the world was at a standstill.

By the time the researchers published their findings however, several treatments have become available, including antiviral medications, antibody cocktails and vaccines. Prompted by these global advances, the team has shifted focus from COVID-19 to trying to create compounds that target all coronaviruses, including SARS and MERS in a bid to design a universal therapeutic as safeguard from future pandemics. To this end, Kim has partnered up with a Boston biotech company Decoy Therapeutics to commercialize the research.

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