Circle Pharma has closed an oversubscribed $92.5 million Series E financing to advance CID-165, an oral macrocycle the company says is the first to directly block the cyclin D1-Rb protein interaction that drives a common form of breast cancer, toward its first clinical trial. The South San Francisco biotech announced the round on 15 September 2026, led by The Column Group, with Eli Lilly and Company among the participating investors.
Five rounds, one macrocycle bet
Circle Pharma's road to a second cancer program
- 2013
Circle Pharma founded
David J. Earp joins as President and CEO to build a drug-discovery platform around synthetic macrocycles — large, ring-shaped molecules able to reach protein targets smaller drugs can't.
- April 2017
Series A
Circle Pharma's first institutional financing round.
- March 2020
Series B, $45 million
A second round backs continued build-out of the company's macrocycle platform, later named MXMO.
- 2021
Series C, $66 million
Funding advances Circle's first clinical candidate, CID-078, a dual cyclin A/B inhibitor for solid tumors.
- September 2024
Series D, $90 million
Led by The Column Group; CID-078 continues into Phase 1 dose escalation (NCT06577987).
- December 2025
CID-165 nominated
Circle names CID-165, a cyclin D1-Rb inhibitor, as a development candidate and its second oncology program.
- 15 September 2026
Series E, $92.5 million
Led again by The Column Group, with Eli Lilly participating, to fund CID-165 into the clinic.
- Ahead
IND and Phase 1
An IND submission is targeted for late 2026 and a Phase 1 start for the first quarter of 2027, both subject to change.
A platform's second shot at cancer
CID-165 is not Circle Pharma's first attempt to turn its macrocycle chemistry into a clinical program. That distinction belongs to CID-078, a dual cyclin A/B inhibitor already dosing patients in an open-label Phase 1 trial across solid tumors including small-cell and non-small-cell lung cancer. CID-165 applies the same underlying MXMO platform — which combines structure-based and computational drug design with synthetic macrocycle chemistry and machine learning — to a different cell-cycle target, cyclin D1, and a different tumor type, ER-positive breast cancer.
The company describes preclinical data presented at the AACR-NCI-EORTC conference showing CID-165 disrupts the cyclin D1-Rb interaction with more than 2,000-fold selectivity over the closely related cyclin D3-Rb interaction, suppressing phospho-Rb and arresting cyclin D1-dependent tumor cells in the G1 phase of the cell cycle. Regulatory News did not find independent, non-company replication of these figures; they are presented here as the company's own preclinical claim, consistent with how such early-stage data is customarily reported before a drug reaches human trials.
“This financing marks a significant milestone as we execute upon our vision of harnessing the power of macrocycles to transform the treatment of cancer.” David J. Earp, President and CEO, Circle Pharma — per the company's 15 September 2026 announcement
The Series E in four numbers
Figures from Circle Pharma's 15 September 2026 announcement and its AACR-NCI-EORTC preclinical data presentation.
Blocking a protein handshake, not a kinase
Every CDK4/6 inhibitor on the market today — Circle Pharma's stated point of contrast — works by inhibiting the kinase activity of the CDK4 and CDK6 enzymes broadly. CID-165 takes a narrower approach: it's designed to block the specific protein-protein interaction between cyclin D1 and the retinoblastoma protein (Rb) — the physical binding event, sometimes called an RxL interaction, that the kinase inhibitors act on indirectly. The company frames this as a potential backbone therapy that could combine with, rather than simply replace, existing CDK4/6 drugs and endocrine therapies, though that combination claim has not yet been tested in patients.
What backs Circle's second bet
A protein interaction, not a kinase
CID-165 targets the cyclin D1-Rb binding event directly rather than CDK4/6 kinase activity broadly — a mechanistic distinction from every approved drug in the class.
In-house clinical experience already
CID-078, Circle's first macrocycle to reach the clinic, is already dosing patients in Phase 1 — giving the company operational experience with its own platform before CID-165 follows.
An investor with a seat at the AI table
Eli Lilly joined the Series E as an equity investor, alongside a standing September 2025 collaboration giving Circle access to Lilly's TuneLab AI/ML drug-discovery platform — notable given Lilly also markets Verzenio, an approved CDK4/6 inhibitor CID-165 would eventually compete against.
Three approved rivals, one unproven newcomer
CID-165 will not reach patients, if it reaches them at all, for several years — and it will arrive into a category three approved drugs already share. Pfizer's Ibrance, Novartis's Kisqali and Eli Lilly's own Verzenio have treated ER-positive, HER2-negative breast cancer for close to a decade combined, with real-world safety and efficacy data CID-165 does not yet have. Circle's bet is that a different mechanism — and the ability to combine with, rather than replace, those existing drugs — is worth the years of clinical development still ahead.
The category CID-165 would enter, in dollars
$17.59B → $35.71B
Global CDK4/6 inhibitor drugs market, 2026 estimate to 2030 forecast, as estimated by Research and Markets.
Sources: Research and Markets, Cyclin-Dependent Kinase (CDK) 4/6 Inhibitor Drugs Market Report 2026
Regulatory News outlook
If CID-165 clears trials, we'd expect it to compete for a low-single-digit share of that market within its first years on the market, well behind the three incumbents' combined position, rather than displace any of them outright.
How we got here: New entrants into large, incumbent-held oncology drug classes have typically captured single-digit percentage shares in their first several years on the market; applied to Research and Markets' $35.71 billion 2030 CDK4/6 forecast, that implies a plausible opening addressable slice in the hundreds of millions of dollars, years after any approval.
CID-165 against the drugs already treating ER+ breast cancer
| Product | Maker | Mechanism | Stage |
|---|---|---|---|
| CID-165 (Circle Pharma) | Circle Pharma | Cyclin D1-Rb protein-protein inhibitor (oral macrocycle) | IND-enabling; Phase 1 targeted Q1 2027 |
| Ibrance (palbociclib) | Pfizer | CDK4/6 kinase inhibitor | FDA-approved, first-line ER+/HER2- metastatic breast cancer |
| Kisqali (ribociclib) | Novartis | CDK4/6 kinase inhibitor | FDA-approved, early and metastatic ER+/HER2- breast cancer |
| Verzenio (abemaciclib) | Eli Lilly | CDK4/6 kinase inhibitor | FDA-approved, early and metastatic ER+/HER2- breast cancer |
From public FDA approval records and company announcements; not a clinical comparison.
- IND submission: targeted for late 2026, per reporting on the financing; not yet filed as of this announcement.
- Phase 1 start: targeted for the first quarter of 2027, subject to IND clearance.
- Independent trade coverage: Regulatory News found no independent reporting on this specific financing beyond restatements of Circle Pharma's own release — noted here in the interest of transparency about sourcing depth.
- Total funding: Circle Pharma's own release framing puts cumulative funding at roughly $212 million; PitchBook's third-party estimate is $305 million. Regulatory News could not reconcile the discrepancy and presents both rather than choosing one.
Sources & further reading
- Circle Pharma, “Circle Pharma Announces $92.5 Million Series E Financing to Advance First-in-class Cyclin D1 Program into Clinical Development,” press release, 15 September 2026. businesswire.com
- Fierce Biotech, Fierce Biotech Fundraising Tracker '26. fiercebiotech.com
- Circle Pharma, “Circle Pharma Announces Nomination of CID-165, an Oral First-in-Class Cyclin D1 RxL Inhibitor, as Development Candidate for its Second Oncology Program,” 17 December 2025. circlepharma.com
- Circle Pharma, “Circle Pharma Closes $90 Million Series D Financing to Advance Innovative Oral Macrocycle Therapies,” 3 September 2024. businesswire.com
- ClinicalTrials.gov, Phase 1 study of CID-078 in advanced solid tumors, NCT06577987. clinicaltrials.gov
- Research and Markets, Cyclin-Dependent Kinase (CDK) 4/6 Inhibitor Drugs Market Report 2026. researchandmarkets.com
Regulatory News reports on public regulatory documents. It is not legal advice, and the primary sources above govern. If we have made an error, we will say so in public: see corrections.
Frequently asked questions
What is CID-165?
An oral, first-in-class macrocyclic inhibitor that Circle Pharma says disrupts the cyclin D1-Rb protein-protein interaction directly, rather than inhibiting CDK4/6 kinase activity the way approved drugs like palbociclib, ribociclib and abemaciclib do. It targets ER-positive breast cancer and is in IND-enabling development, not yet in clinical trials.
How big was Circle Pharma's Series E, and who led it?
$92.5 million, described by the company as oversubscribed, led by The Column Group. Named participants include Nextech Invest, RA Capital Management, Euclidean Capital and Eli Lilly and Company. Circle Pharma has raised at least $212 million across its Series A through E rounds by the company's own framing; a third-party aggregator (PitchBook) puts total funding at $305 million, a figure Regulatory News could not independently reconcile.
Is Eli Lilly acquiring or licensing CID-165?
No announcement says so. Lilly's Series E participation is an equity investment, occurring alongside a separate, pre-existing September 2025 collaboration that gave Circle Pharma access to Lilly's TuneLab AI drug-discovery platform. Notably, Lilly also markets Verzenio (abemaciclib), an approved CDK4/6 inhibitor CID-165 would compete with if it reaches the market.
When will CID-165 enter clinical trials?
Circle Pharma has not filed an IND or registered a clinical trial for CID-165 as of this announcement. Reporting describes an IND submission targeted for late 2026 and a Phase 1 start targeted for the first quarter of 2027, both subject to change.
