Estuary Launches With $103 Million to Bring Gene Therapy to Polycystic Kidney Disease
On October 16, 2026, Estuary Biotherapeutics, a California startup, launched with $103 million to develop one-time gene therapies for autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disorder. The money comes from a $93 million Series A led by RA Capital Management, on top of a $10 million seed round co-led by 4BIO Capital and GV.
Other investors include Deep Track Capital, Biogen's early-stage investment arm, the National Kidney Foundation Innovation Fund, and a family office connected to a person living with ADPKD. Several of Estuary's leaders previously worked at Biogen.
What goes wrong in ADPKD
ADPKD affects an estimated 1 in 400 to 1 in 1,000 people. It is autosomal dominant, which means one mutated copy of the gene, inherited from one parent, is enough to cause the disease. Each child of an affected parent has a 50% chance of inheriting it.
More than 90% of cases are caused by mutations in one of two genes: PKD1 or PKD2. These genes make two proteins, polycystin-1 and polycystin-2, which work together in the cells that line the kidney's tubules. When polycystin levels fall too low, sections of those tubules swell into fluid-filled cysts.
Over decades, thousands of cysts grow, enlarge the kidneys, and crowd out healthy tissue. About half of people with ADPKD reach kidney failure by around age 60, and then need dialysis or a kidney transplant.
The current treatment only slows the disease
Tolvaptan (Jynarque) is the only drug approved by the FDA specifically for ADPKD. It blocks the vasopressin V2 receptor, a hormone receptor that drives cyst growth, and slows the decline in kidney function. It acts downstream of the genetic problem, so it does not restore the missing polycystin proteins.
The science behind Estuary
Estuary is built on research from two Yale School of Medicine professors, Stefan Somlo and Michael Caplan.
Somlo's lab showed in mice that turning the polycystin genes back on after cysts had formed could shrink the cysts and reverse the inflammation and scarring around them.
Caplan's lab found that only a small fragment of the polycystin-1 protein is needed to suppress the disease.
Together, these findings suggest that ADPKD may not be a one-way street: restoring polycystin activity in adult kidneys could stop, and possibly reverse, damage that is already there.
Two gene therapies, one for each gene
Both of Estuary's lead programs use adeno-associated virus (AAV) vectors to deliver genetic instructions into kidney cells.
EST-201 delivers a full-length copy of the PKD2 gene. Estuary expects to start human testing in 2027.
EST-101 delivers the most useful portion of the protein made by PKD1. Human testing is expected in 2028.
The difference comes down to size. An AAV vector can carry only about 4.7 kilobases of DNA. The PKD2 coding sequence fits, but PKD1 is one of the largest genes in the body, with a coding sequence of roughly 13 kilobases. Delivering a shortened, functional fragment is a way around that limit.
Delivery is the hard part
Most AAV gene therapies given into the bloodstream end up largely in the liver. The kidney has been much harder to reach. Estuary says its treatments are designed for local, one-time administration that targets the kidney directly.
"The most crucial remaining barrier to kidney gene therapy" is delivery, said Wayne Sackett, Estuary's co-founder and CEO. RA Capital's Emily Minkow described the approach as "focal, kidney-directed delivery designed to reach the nephron directly." The company has not yet published details of the route or of its kidney-targeting vector.
What to watch
Estuary joins a growing field. Novartis agreed last year to acquire Regulus Therapeutics, which is developing an RNA-based ADPKD drug, for $800 million, and Renasant Bio launched with $55 million in seed funding. Sanofi's earlier ADPKD candidate failed a key study.
Key questions for Estuary's programs include how many kidney cells the vectors can reach, how long gene expression lasts, and whether the reversal seen in mice carries over to people. The challenges of developing gene therapies, from delivery to immune responses, apply here too.
The bottom line
Estuary has $103 million to test whether a one-time, kidney-directed AAV gene therapy can restore the missing polycystin proteins in ADPKD. If the mouse findings hold up in people, it could become the first treatment that targets the cause of the disease instead of slowing it. The first human trial is planned for 2027.
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