Parabilis filed for an initial public offering one day after signing an up-to-$2.3 billion-plus strategic research collaboration with Regeneron. The deal centers on discovery and development of antibody–Helicon conjugates, a format intended to broaden ADC-style targeting to previously “undruggable” biology. Regeneron has agreed to purchase about $75 million of Parabilis common stock in a concurrent private placement at 90% of the IPO price. Parabilis’ S-1 includes a placeholder $100 million figure and highlights continued clinical development of zolucatetide (FOG-001) across desmoid tumors and planned registrational steps, including a Phase III trial topline pathway. The filing underscores renewed appetite for single-asset and platform-driven biotechs in the window for first-time IPO capital.