On May 18, 2026 Biotheryx, Inc., a biopharmaceutical company focused on the discovery and development of first-in-class protein degraders for cancer and inflammatory diseases, reported the data from its first-in-human Phase 1 Dose Escalation study evaluating BTX-9341, a novel CDK4/6 bifunctional degrader, as monotherapy and in combination with fulvestrant in participants with advanced/metastatic HR+/HER2- breast cancer who received prior CDK4/6 inhibitor therapy. The Phase 1 Dose Expansion portion of the study is currently ongoing.

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"BTX-9341 has a highly favorable safety profile, with no clear evidence of key class related toxicities of other CDK4/6 inhibitors including gastrointestinal toxicities (diarrhea, nausea, vomiting), hepatotoxicity and prolongation of the QTc interval," said Dr. Rachel M Layman, MD (Department of Breast Medical Oncology, The University of Texas MD Anderson Cancer Center). "The drug shows encouraging clinical activity in a heavily pretreated patient population in the post-CDK4/6 inhibitor setting and shows compelling evidence of the importance of targeting CDK2/4/6 to address multiple resistance pathways."

Data from the Phase 1 Dose Escalation study demonstrate evidence of clinical activity, including prolonged partial response and stable disease in a heavily pretreated patient population. Pharmacokinetic analyses showed BTX-9341 exposures consistent with target engagement concentrations. Importantly, BTX-9341 was well tolerated, with participants on treatment for over 12 cycles (48 weeks), including on monotherapy and in combination with fulvestrant. Adverse events were mild to moderate and easily manageable.

Key Highlights from Dose Escalation Portion of the Phase 1 Study

Favorable safety and tolerability profile with no serious adverse events or treatment discontinuations due to adverse events in all participants (N=28). Hematologic adverse events were transient and reversible. There were no ≥ Grade 3 non-hematologic events.
Approximate dose-proportional pharmacokinetics supporting once-daily dosing.
Clinical activity was observed in a heavily pretreated patient population post-CDK4/6 inhibitors with up to 6 prior lines of therapy in the metastatic setting; approximately 40% and 36% of participants had received chemotherapy and PI3K/AKT/mTOR or other targeted therapies, respectively. Clinical Benefit Rates1 (CBRs) of 41.7% and 80% were observed in treatment-evaluable participants2 (n=24), and participants representative of the dose expansion criteria (n=10), respectively.
Target engagement consistent with CDK4/6 degradation mechanism based on a significant reduction in serum Thymidine Kinase activity, and reductions in the levels of CDK4, CDK6, and CDK2 in peripheral blood mononuclear cells (PBMCs). This is consistent with BTX-9341’s unique ability to address multiple convergent resistance mechanisms to CDK4/6 inhibitor therapies by degrading CDK4/6 and inhibiting downstream transcription of CDK2.
The study is still ongoing with multiple participants remaining on treatment for 12 to >18 cycles. The data are as of May 6, 2026.
"These encouraging clinical results mark an important milestone for our Phase 1 study evaluating BTX-9341, a first-in-class CDK4/6 bifunctional degrader," said Dr. Leah Fung, CEO of Biotheryx. "BTX-9341 has demonstrated a promising safety and tolerability profile, along with early signs of clinical activity in participants whose disease progressed following prior CDK4/6 inhibitor therapy. Given that CDK6 upregulation and Cyclin E amplification are established mechanisms of resistance to CDK4/6 inhibitors, these findings highlight the importance of degrading CDK4/6 and inhibiting downstream transcription of CDK2 to address convergent resistance mechanisms and may redefine the post-CDK4/6 inhibitor setting in advanced/metastatic HR+/HER2- breast cancer."

Advancing to Dose Expansion

Based on these findings, the company is continuing with its Phase 1 Dose Expansion portion of the trial. The expansion cohorts have begun enrolling participants and will further evaluate the efficacy and safety of BTX-9341 in combination with fulvestrant and will build on the safety, tolerability, pharmacokinetics and pharmacodynamic activity of BTX-9341 established in the Dose Escalation phase. The trial is being conducted at multiple sites in the United States and is expected to enroll up to 78 participants across two treatment arms based on the recommended dose identified in the earlier Dose Escalation phase. The primary endpoint of this study is the Overall Response Rate (ORR), with key secondary endpoints including the measurement of investigator-assessed Clinical Benefit Rate (CBR) and Progression Free Survival (PFS).

About BTX-9341

BTX-9341 is a first-in-class, oral degrader of CDK4/6, important targets for a range of cancers and clinically validated in HR+/HER2- breast cancer. In preclinical breast cancer models, BTX-9341 demonstrated enhanced activity compared with CDK4-only and CDK4/6 inhibitors through potent and highly selective catalytic degradation of CDK4 and CDK6, robust inhibition of CDK2 transcription, cell cycle arrest, and ultimately superior in vivo efficacy in breast cancer xenografts. Beyond this increased efficacy potential, BTX-9341 is differentiated from CDK4-only and CDK4/6 inhibitor approaches through the ability to overcome key resistance mechanisms that can limit the impact of inhibitors in second line HR+/HER2- metastatic breast cancer.

(Press release, BioTheryX, MAY 18, 2026, View Source;breast-cancer-302774140.html [SID1234665838])