Estuary launches with $103m led by RA Capital to take kidney gene therapies into trials

Estuary Biotherapeutics launched on 16 October 2026 with $103 million, made up of a $93 million Series A led by RA Capital Management and an earlier $10 million seed round, to take two kidney gene therapies into the clinic¹². The California-based company targets autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disease¹².

The round and the backers

Deep Track Capital, Biogen New Ventures and UPMC Enterprises joined the Series A. 4BIO Capital and GV, which co-led the seed, also took part¹. The cap table pairs a specialist crossover investor with a large biotech's venture arm, and the management team carries the same Biogen link: at least three senior staff came from Biogen, including Jeremy Duffield, a former vice president of R&D, and Uptal Patel, who ran its west coast hub². Co-founder Xavier Anguela is vice president for gene therapy at Eli Lilly¹. RA Capital managing director Emily Minkow and 4BIO partner Owen Smith sit on the board².

The science comes from two Yale School of Medicine labs. Stefan Somlo's team showed in mice that switching polycystin proteins back on could shrink cysts and reverse kidney inflammation and scarring, and Michael Caplan's lab found that only a small fragment of the polycystin protein is needed to suppress the disease².

What the money buys

The funds are meant to produce clinical proof-of-concept data for both programmes¹. EST-201 delivers a full-length PKD2 gene in a modified adeno-associated virus and is due in the clinic next year. EST-101 uses the same vector to deliver a functional region of polycystin-1, the protein made by the PKD1 gene, with a clinical start planned for 2028¹. Both are intended as one-time treatments given locally to the kidney¹². Chief executive Wayne Sackett said the company is now focused on solving delivery, which he sees as the main barrier left for kidney gene therapy².

What it means for the market

ADPKD is a large and thinly served indication. Around 90% of patients carry a mutation in PKD1 or PKD2¹, estimates put prevalence at about 1 in every 400 to 1,000 people, and only one drug, Otsuka's tolvaptan, is approved specifically for it¹². Tolvaptan works by lowering cyclic AMP to slow cyst growth¹. Estuary is pitching a different product: Sackett told Fierce Biotech the aim is to move treatment beyond slowing the disease towards halting or reversing it¹.

The space is getting crowded with capital. Novartis bought Regulus Therapeutics last year for its phase 3-ready oligonucleotide farabursen in ADPKD¹². Renasant Bio emerged a few months later with $55 million in seed funding from 5AM Ventures, Atlas Venture and OrbiMed, and a Sanofi candidate failed in a key study². For investors, Estuary adds a gene therapy bet to a field where big pharma has so far paid for RNA-targeting drugs. Its advisers include Frank Czerwiec, who led tolvaptan's clinical development, and Jeff Marrazzo, former chief executive of Spark Therapeutics, which sold to Roche for almost $5 billion in 2019².

Caveats

Neither programme has been tested in people. The supporting evidence named in the sources is mouse work and laboratory findings from the founders' labs². Local delivery of AAV to the kidney is, on the company's own account, the main unsolved problem². The next marker is EST-201's planned entry into human testing in 2027².