Replicate Bioscience, a San Diego startup developing a second-generation mRNA vaccine technology that could spur stronger immunity with smaller doses, said its experimental rabies shots performed better than expected in an 84-person clinical trial.

In an exclusive interview with Endpoints News, CEO Nathaniel Wang said his company plans to raise a Series B round based on the rabies data. But without a pharma or government partner, Replicate is unlikely to advance the rabies shot into a larger study, he added.

Instead, the startup is shifting its focus to a previously undisclosed program, an mRNA shot for Epstein-Barr virus, which Replicate will test in a clinical trial next year, Wang said. There’s no approved vaccine to protect against EBV, which causes infectious mononucleosis in adolescents. Interest in efforts to stop the virus has increased since scientists linked EBV to the development of multiple sclerosis in 2022.

Replicate’s vaccines encode instructions for making viral proteins along with molecular photocopy machines that churn out more protein blueprints, mimicking a natural viral infection. The startup hopes its self-replicating RNA approach will lead to stronger and more durable immune responses with lower doses and fewer side effects.

The preliminary results, announced Wednesday via press release, indicate that one or two injections of the shot spurred potentially protective immune responses with doses as low as one-tenth of a microgram — between 300 and 1,000 times lower than what’s used in the Moderna and Pfizer-BioNTech primary vaccines and boosters for Covid-19. Replicate said there were no severe adverse events.

Other companies are developing similar vaccine technologies, including Arcturus Therapeutics, which won approval from Japanese regulators in November for a Covid vaccine, and Gritstone bio, which is also developing a next-gen Covid shot as part of the US government’s follow-up to Operation Warp Speed.

Replicate decided to test its vaccine technology in rabies first because it gives the company a clean read of how good the shots are at spurring an immune response.

“If you do a flu or Covid-19 trial, everyone has already been vaccinated,” Wang said. “With rabies, it’s easy to find people who haven’t been vaccinated, or bitten by a bat or raccoon.”

The study measured whether levels of neutralizing antibodies in the blood passed the World Health Organization’s threshold for protection against rabies.

“We’re seeing the substantive majority of subjects protected after two doses,” Wang said. Until now, no one has crossed the threshold of at least 50% protection with that low of a dose, he added.

But Replicate didn’t disclose the exact proportion of vaccine recipients who passed that threshold at each dose, or detail the frequencies and kinds of side effects that the study volunteers experienced. Wang said these details will be presented at a medical meeting this year.

Replicate also plans to begin a clinical trial of an off-the-shelf vaccine for breast cancer in 2025, Wang said. BioNTech, Gritstone bio and Moderna are also developing their own mRNA cancer vaccines customized to match the unique genetic makeup of a patient’s tumor.

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