R1 Therapeutics is developing AP306 for people with chronic kidney disease who are on dialysis and have hyperphosphatemia, or high phosphate levels. The experimental drug is designed to reduce phosphate absorption in the gut. On September 30, the company said a new investment from Samsara BioCapital expanded its Series A to $80.5 million.
Phosphate binders have been the standard treatment for decades, yet R1 says more than 70% of dialysis patients in the United States don’t reach normal phosphate levels. AP306 targets three key active phosphate transporters in the gut. R1 describes it as the only agent that blocks active phosphate transport and says the approach could lower patients’ treatment burden.
The drug didn’t originate at R1. Chugai Pharmaceutical discovered AP306 and later licensed it to Alebund Pharmaceuticals. R1 holds exclusive development and commercial rights outside Greater China and is working with Alebund on the global clinical program.
AP306 is now being evaluated as a monotherapy in a randomized, double-blind, placebo-controlled Phase 2b study. The trial is running at sites in the United States and China, with topline data expected in the first half of 2027.
This isn’t a new $80.5 million check. The investment increased the Series A by $3 million from the $77.5 million total announced when R1 launched in March 2026. The company said the proceeds will support AP306’s continued development.
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