A bet on the RNA nobody wanted
Circular RNA was supposed to be a mistake. For years researchers found these loop-shaped molecules in cells, shrugged, and filed them under junk. Paul Sargeant built a company on them.
He runs Circular Genomics, a San Diego outfit of roughly seventeen people chasing two of the hardest targets in medicine: the brain in depression, and the brain in decline. The shared tool is a strand of RNA that closes into a ring. No loose ends means nothing for the body's enzymes to grab. That stubbornness is the whole point - a molecule that refuses to fall apart is exactly what you want surviving in a tube of blood long enough to be measured.
Most of the field stampeded toward the fashionable molecules. Sargeant has a habit of arriving early to the rooms others ignore. Before circular RNA, he was CEO of Rebus Biosystems, an Illumina Ventures-backed imaging venture in spatial biology - another field that was niche right up until it wasn't. The pattern is the bet: back the technology before the crowd, then do the unglamorous work of turning a lab curiosity into something a clinician will actually order.
What the company actually makes
The first product has a name with a quiet promise built in: MindLight. It is a blood test that reads circular RNA to predict whether a patient with major depressive disorder will respond to SSRI antidepressants - before they swallow the first pill. Anyone who has watched depression treatment up close knows the grind: try a drug, wait six weeks, adjust, wait again, repeat. MindLight's pitch is to replace that guesswork with a measurement. In 2024 the company published research on the test's accuracy at predicting SSRI response.
Then came the bigger swing. The same platform - circular RNA leaking out of the brain and floating in the bloodstream - turns out to be a candidate window into Alzheimer's. The disease writes its earliest chapters years before memory fails, and the brain is the one organ you cannot casually biopsy. So Circular Genomics reads it indirectly, from blood, looking for circular RNA signatures of disease biology at the earliest stage.
"This Series A represents a pivotal milestone in our journey to transform precision neurology, including Alzheimer's disease diagnosis and patient care."
- Paul Sargeant, December 2025
The year it got real
2025 was a year of moving and proving. The company relocated its headquarters from Albuquerque to San Diego and took up residence inside Lilly Gateway Labs - the rare arrangement where your landlord is also a strategic believer in your science. Then, in December 2025, Sargeant closed a $15 million Series A led by Mountain Group Partners, with Poplar Grove Investors, the HIP Fund, and the Alzheimer's Drug Discovery Foundation joining in. Having the ADDF at the table is its own signal: this is money that knows the graveyard of failed Alzheimer's ventures and chose to invest anyway.
Sargeant framed the round around access, not just accuracy. The urgent need, in his words, is for "accessible blood-based biomarkers that can detect Alzheimer's biology at the earliest stages." A test that requires a spinal tap or a PET scanner reaches a fraction of patients. A test that needs a blood draw reaches almost everyone.
Who he is when the slide deck closes
Sargeant trained as a physiologist, earning a PhD at Queens' College, Cambridge - a background in how living systems actually work, which is a useful lens for a diagnostics company. Across 25 years he has worked in the US, Europe and Asia Pacific, inside Fortune 100 corporations and venture-backed startups alike, living at the seam where science gets turned into a shippable product: development, regulatory approval, launch, commercialization, exit. He has raised more than $200 million and launched multiple FDA-regulated devices. The throughline is not invention for its own sake. It is getting a real thing into a real clinic.
When he joined Circular Genomics in April 2023, board chair Dave Blivin pointed to exactly that record - the ability to take early-stage companies past the milestones where most of them die. Sargeant's own first words were less about ego and more about the machine: "I am delighted to join the growing team at Circular Genomics and help drive their innovative circular RNA-AI data platform."
The thesis, stated plainly
Strip away the press releases and the bet is simple. The brain is hard to read. Blood is easy to draw. Circular RNA is the courier that survives the trip between them. If that holds, the same vial that today hints at which antidepressant will work could tomorrow flag Alzheimer's before a single symptom shows - and a molecule the field once discarded becomes the most useful thing in the room.
From the brain to a blood draw
The loop
Circular RNA closes into a ring with no free ends - so enzymes can't easily degrade it. It is unusually stable.
The leak
Brain-derived circular RNA escapes into the bloodstream, carrying signatures of what's happening in tissue you can't biopsy.
The read
A blood test measures those signatures - predicting SSRI response today, and aiming at early Alzheimer's biology next.
On the record
I am delighted to join the growing team at Circular Genomics and help drive their innovative circular RNA-AI data platform.
The strong support from our world-class investors validates the breakthrough potential of our circular RNA platform.
There is an urgent need for accessible blood-based biomarkers that can detect Alzheimer's biology at the earliest stages.
This Series A represents a pivotal milestone in our journey to transform precision neurology and patient care.