Circle Pharma raises $92.5 million to advance oral cyclin-targeting cancer drug

Circle Pharma announced a $92.5 million Series E financing on September 16 to advance its experimental oral cancer drug CID-165 toward human testing. The investment supports a program aimed at estrogen receptor-positive breast cancer and offers a concrete next step for a drug-development approach that targets interactions between proteins controlling cell division.

The Column Group led the round, with participation from Nextech Invest, RA Capital Management, Euclidean Capital and Eli Lilly. Fierce Biotech independently reported the financing and its intended use in early clinical development. Financial backing, however, provides resources to test a therapeutic hypothesis; it does not establish that the drug will help patients.

### A different way to target cell division

Circle describes CID-165 as an oral macrocyclic inhibitor of cyclin D1, a protein involved in regulating the cell cycle. The company expects clinical development to begin in the first quarter of 2027. That date is a development target, and the September announcement does not report that human testing has begun.

According to the company, CID-165 is designed to disrupt the interaction between cyclin D1 and retinoblastoma protein, or Rb. Rb helps restrain cell proliferation. The proposed approach aims to preserve that restraint by preventing a particular protein interaction involved in its inactivation.

Circle reports activity in preclinical cancer models, including experiments combining CID-165 with endocrine therapies and inhibitors of cyclin-dependent kinases. Those observations remain company-reported laboratory evidence. The financing announcement provides no patient response rate, survival result or human safety findings for CID-165.

### What published research contributes

A related peer-reviewed paper offers context for the company's macrocyclic peptide strategy, although it examines cyclins A and B rather than CID-165's cyclin D1 target. Published online on February 21 in the Journal of Medicinal Chemistry, the study described chemical optimization intended to improve oral availability and other drug properties.

Researchers tested compounds in cancer-cell systems and cell-line-derived xenograft models of small-cell lung cancer. In these animal models, human cancer cells are grown as tumors. The authors reported tumor regression after oral administration of an optimized compound. These experiments evaluated antitumor activity in a controlled preclinical setting, without establishing clinical benefit in people.

The distinction between the programs matters. Evidence that a macrocyclic peptide can reach an intracellular target and affect experimental tumors supports further research into the approach. It cannot establish that a different molecule targeting another cyclin will have the same activity, acceptable toxicity or useful exposure in patients. The paper's authors were affiliated with Circle Pharma, another relevant consideration when assessing the evidence.

### The clinical questions remain open

For CID-165, the next consequential evidence would come from a clearly described human study: who is enrolled, how safety is assessed, whether the drug reaches its intended target and which measures are used to evaluate tumor response. The financing release does not supply a trial population size, clinical endpoints or a human results dataset.

Any eventual assessment will also need to distinguish activity from durable benefit. Tumor changes, treatment tolerability, time before disease progression and effects on daily functioning answer different questions. A development timetable cannot substitute for those observations.

The immediate significance is therefore the financing of a specific experimental program. Circle now has additional backing to investigate a cyclin-directed treatment strategy, while the evidence for CID-165 remains preclinical and its practical value for breast-cancer care remains unresolved.