Circle Pharma develops oral macrocyclic medicines designed to enter cells and block protein interactions that conventional drug formats have struggled to reach. The South San Francisco biotechnology company has raised a $92.5 million Series E to advance CID-165, an experimental treatment intended for patients with estrogen receptor-positive breast cancer, through early clinical development. The drug remains preclinical, with a first clinical study planned for the first quarter of 2027.
The Column Group led the oversubscribed financing. Nextech Invest, RA Capital Management, Euclidean Capital and Eli Lilly also participated. Circle said the proceeds will primarily support early clinical development of CID-165, while also funding its broader pipeline of medicines directed at cyclins, proteins that regulate cell division.
Circle’s MXMO discovery platform combines computer-guided structural design with synthetic chemistry. The company screens virtual and physical libraries for ring-shaped molecules predisposed to pass through cell membranes, uses simulations and machine learning to compare possible designs, and then synthesizes variations in the laboratory to optimize a potential drug. The goal is to combine the selectivity associated with larger biologic medicines with oral dosing, cell penetration and scalable manufacturing.
CID-165 applies that process to cyclin D1, which can promote uncontrolled cell division in estrogen receptor-positive breast cancer by disabling the tumor-suppressing retinoblastoma protein. The candidate is designed to interrupt that interaction so the suppressor remains active. Circle reports that CID-165 inhibited tumors in preclinical models when tested alone and alongside endocrine treatments and drugs that inhibit CDK4 or CDK4/6, but those findings do not establish safety or efficacy in people.
The company already has a different molecule, CID-078, in a Phase 1 trial for patients with advanced solid tumors. That study is evaluating safety and dosing before expanding to examine anti-tumor activity. CID-078 targets cyclins A and B, while CID-165 gives Circle a second, separately owned program aimed at another member of the same protein family. Circle also identifies an undisclosed discovery program as partnered with Boehringer Ingelheim.
The financing moves CID-165 from laboratory testing toward the point where Circle can evaluate its platform in a second human program. The consequential test will be whether selectively interrupting the cyclin D1 interaction is tolerable and produces useful activity in patients, including when paired with existing breast-cancer treatments.