An investment perspective by Doug Sylvester, General Partner, Xcellerant Ventures, and Fund Manager, Jetstream Venture Fund
Jetstream recently invested in Rejuvenate Bio, a San Diego gene therapy company focused on treating chronic, age-related disease with innovative therapies. It is one of the investments I am most excited about this year.
Three things drew us in. their science came out of one of the most important genetics labs in the world. The company is going after a very large market with a smart first step. And it has put together the kind of money and partners that most companies at its stage don’t have. So far, most of its patients are dogs. That turns out to be part of the appeal.
The lab behind the company
If you follow genetics at all, you know George Church’s name. In 1984, working with Walter Gilbert, he developed the first direct method for sequencing genomic DNA, and that same year he helped get the Human Genome Project started. Versions of the next-generation sequencing methods his lab developed are now used commercially by companies such as Illumina. In 2013, his lab and Feng Zhang’s lab at the Broad Institute published papers in the same issue of Science showing that CRISPR could edit human cells. He is a member of both the National Academy of Sciences and the National Academy of Engineering, and his work has been the basis for a number of companies, including the gene editing company Editas Medicine.
Rejuvenate Bio comes straight out of that lab. Its co-founder and Chief Scientific Officer, Noah Davidsohn, PhD, developed the core idea as a postdoctoral researcher there, and the lab published the work in the Proceedings of the National Academy of Sciences in 2019. Harvard granted the company an exclusive worldwide license to the technology. Church is a co-founder and sits on the scientific advisory board.
With an early-stage biotech company, the hardest question is whether the science underneath it is real. Church’s lab has spent decades building tools that other scientists rely on, and work that gets used and widely tested. Rejuvenate’s core science was peer reviewed and published, not just described in a pitch deck. A famous lab is not a guarantee, but it gave us real confidence in the foundation.
A look at the science
Rejuvenate uses an adeno-associated virus (AAV), a widely used gene therapy delivery vehicle, to carry a gene to the liver. The liver then produces a therapeutic protein and releases it into the bloodstream. The company’s lead protein is FGF21, a hormone the body uses to regulate metabolism. The approach does not edit the patient’s DNA. The goal is a single treatment that keeps working, rather than a drug taken for years.
The 2019 study tested three longevity-associated genes in mice with four age-related conditions: obesity, type 2 diabetes, heart failure and kidney failure. A single dose combining FGF21 with a second gene treated all four. Mice are not people, but the work gave the platform a clear direction.
The size of the opportunity
Let’s look at spending on pets — just within the U.S. Americans spent $158 billion on their pets in 2025, according to the American Pet Products Association, and $41 billion of that went to veterinary care. About 71 million U.S. households own a dog. People treat their dogs like family, and it’s clear they pay for their care that way.
Unlike pharmaceuticals intended for human use, new kinds of pet medicine can grow quickly when they work. For example, Zoetis, the largest animal health company, recently brought monoclonal antibodies for arthritis pain to dogs and cats, a kind of drug that until recently was used mostly in people. Those two products brought in about $581 million in 2024. Rejuvenate Bio is aiming at the same kinds of common, chronic problems of older dogs. More than 7 million U.S. dogs have mitral valve disease, according to the Wyss Institute, and Zoetis estimates that nearly 40% of dogs may be affected by osteoarthritis pain. Rejuvenate Bio has treatments for both.
Now let’s look at the human side. FGF21, the protein Rejuvenate Bio’s therapy makes, has become one of the most sought-after targets in drug development. In 2025, Novo Nordisk agreed to buy Akero Therapeutics for up to $5.2 billion, Roche agreed to buy 89bio for up to $3.5 billion, and GSK agreed to pay up to $2 billion for an FGF21 drug from Boston Pharmaceuticals. That is more than $10 billion committed to this one protein in a single year, for a common liver disease. Those drugs are injected on a regular schedule. Rejuvenate is trying to get the body to make FGF21 itself after one treatment.
None of these numbers is a forecast for Rejuvenate. Large markets attract strong competitors, and a product has to work, win approval, and sell before any of this matters. But when I ask whether a company is chasing something big enough to be worth the risk, this one clearly is.
Why start with dogs?
There is a lot of money and a lot of noise in longevity right now. Much of it runs into a basic regulatory fact: the FDA does not treat aging as a disease. No company can win approval for a drug that slows aging. Drugs are approved for specific conditions, based on trials designed around those conditions.
As a longtime advisor and former law school dean, I tend to read a company’s regulatory path before I read its pitch deck. So the question I ask any longevity company is simple. What is the first disease you will treat, and how will you get it approved? Rejuvenate’s answer is a common heart disease in dogs.
Dogs develop many of the same age-related diseases people do, on a shorter timeline. Mitral valve disease is a degenerative heart condition that is often fatal in its advanced stages. In a pilot study, 17 dogs received the company’s gene therapy. The efficacy results come from 12 Cavalier King Charles Spaniels, a breed prone to the disease, that received the gene therapy on top of the standard heart drug. The company reported that their disease progressed more than a year and a half later than published results for dogs on the standard drug alone. That is a comparison to historical data, not to a control group in the same study. The company also reported that the gene was still active nearly three years after treatment, and that four dogs had been on the therapy for more than two years with no safety concerns reported.
It’s important to consider that these results were presented in 2023, the study was small, the data are the company’s own, and a historical comparison is weaker evidence than a controlled trial. But they are also the kind of early results that justify running a controlled trial.
In clinical trials, starting with animals helps in a few ways. The development path is generally shorter and less costly than the human one, so the platform can reach the market sooner. A veterinary product can earn revenue while the human programs are still years away. And every treated dog adds safety and durability data in a large animal, which will matter when the company goes to human regulators.
The money and the partners
Rejuvenate Bio has been unusually good at attracting both capital and partners. According to CEO Daniel Oliver, the company has raised $36 million in equity since 2017, plus close to $15 million from government grants and animal health deals. In June it closed a $6 million financing led by VCapital, with Merck Animal Health, Kendall Capital Partners, Connecticut Innovations and Digitalis participating. Jetstream took its position in Rejuvenate the same month.
The partner list is the part I find most telling. Merck Animal Health did more than invest: it signed a research and development collaboration to advance a new gene therapy program in animal health. Phibro Animal Health signed on in 2022 to develop and commercialize the mitral valve therapy. In 2024 the company announced a partnership with another leading animal health company, and has since reannounced it was Merck Animal Health, for a gene therapy for canine osteoarthritis. Oliver says Rejuvenate Bio now has partnerships with about two-thirds of the leading animal health companies.
Outside of animal health, the California Institute for Regenerative Medicine awarded the company $4 million in 2024 for its first human program, and the company has received grant funding from the U.S. Department of Defense. Its dog heart study was run with Tufts University’s Cummings School of Veterinary Medicine.
This matters for two reasons. Grants and corporate partners do their own diligence, so each one is an outside vote on the science. And animal health partners bring something a small company can’t build quickly: the sales forces and relationships with veterinarians that it takes to get a product into clinics. Strategic money tells you something about the science. It does not tell you how the story ends.
The human program
The long-term opportunity is human disease. Rejuvenate Bio’s first human candidate, RJB-0402, uses the same FGF21 approach for arrhythmogenic cardiomyopathy caused by variants in the desmoplakin gene, a rare inherited heart disease marked by dangerous heart rhythms and progressive damage to the heart muscle. The CIRM award supports preclinical work on that program.
The human path is longer, more expensive and less certain than the veterinary one. We view RJB-0402 as a longer-term program that builds on the animal health work.
The team
CEO Daniel Oliver, who previously co-founded the 3D printing company Voxel8, started Rejuvenate with Davidsohn. Deborah Ascheim, MD, a heart failure cardiologist with long experience running gene and cell therapy trials, is Chief Medical Officer. Katherine High, MD, who co-founded Spark Therapeutics and led the team behind the first FDA-approved gene therapy for an inherited disease, sits on the scientific advisory board.
What I like about this group is its discipline. With a co-founder of Church’s stature, the team could have chased the biggest headline in longevity. Instead it picked a specific disease in a specific species, with a regulatory path it could see.
The risks
Every investor should consider the risks involved when looking at any early stage company. Our dilligence team considered a few before investing. Gene therapy is hard, and early-stage investing in it is risky. Rejuvenate Bio has no approved product. A small pilot study measured against historical data is a starting point, not proof. Results in dogs may not carry over to people. Regulatory timelines slip, manufacturing at scale is difficult, and even successful pet drugs can lose ground after launch if safety questions come up. The company will need more capital to reach its goals. Any of these could reduce or eliminate the value of the investment.
That is why Jetstream holds a diverse portfolio. We do not expect every company we back to succeed. We invest in companies like Rejuvenate Bio because, if the approach works, it could matter a great deal for animals first and, later, for people. Which could translate into a great opportunity for our investors
Why we invested
Rejuvenate’s science was built in one of the most productive genetics labs in the world. It is going after markets measured in the billions, starting with a practical first step. It has raised real money, and it has partners in Merck Animal Health, Phibro and others who know how to get animal medicines to market. And the team has chosen a disciplined path toward an ambitious goal.
Longevity is a big idea. Rejuvenate’s plan starts with a smaller, specific one: help older dogs with failing hearts. We think that is the right place to start.
