Biotheryx reported Phase I dose escalation data for BTX-9341, a first-in-class oral CDK4/6 bifunctional degrader. The molecule produced a clinical benefit rate of 80% in a subset of heavily pretreated patients with advanced or metastatic HR+/HER2- breast cancer who had progressed on prior CDK4/6 inhibitor therapy, with no serious adverse events or treatment discontinuations observed across all 28 participants.
The BTX-9341 Phase I dose escalation study is an open-label, first-in-human dose escalation and expansion trial evaluating BTX-9341 as monotherapy and in combination with fulvestrant in patients with advanced or metastatic HR+/HER2- breast cancer who had received prior CDK4/6 inhibitor therapy. The dose escalation portion, which is now complete, enrolled 28 participants at multiple sites in the US.
The population was heavily pretreated. Patients had received up to six prior lines of therapy in the metastatic setting, with approximately 40% having received prior chemotherapy and approximately 36% having received prior PI3K/AKT/mTOR or other targeted therapy. This context matters: the post-CDK4/6 inhibitor setting in HR+/HER2- metastatic breast cancer remains an area where treatment options are expanding but resistance mechanisms are multiple and overlapping, and where durable responses in later lines are difficult to achieve.
Efficacy and target engagement
Among the 24 treatment-evaluable participants — defined as those who completed at least one cycle with one post-baseline scan — the clinical benefit rate was 41.7%. In the ten participants who met criteria representative of the planned dose expansion population, the CBR rose to 80%. The company also reported prolonged partial responses and stable disease, with multiple participants remaining on treatment for 12 to more than 18 cycles, equivalent to 48 weeks or longer, across both monotherapy and combination arms.
Pharmacokinetic analyses showed approximately dose-proportional exposures consistent with once-daily oral dosing, and pharmacodynamic data provided direct evidence of target engagement. Reductions in CDK4, CDK6, and CDK2 protein levels were observed in peripheral blood mononuclear cells, and serum thymidine kinase activity — a marker of cell proliferation — declined materially, consistent with the proposed degrader mechanism.
That last point carries some weight scientifically. BTX-9341 is not simply a CDK4/6 kinase inhibitor. As a bifunctional degrader, it recruits an E3 ubiquitin ligase to promote proteasomal elimination of CDK4 and CDK6 protein, and the company says it also suppresses downstream transcription of CDK2. The pharmacodynamic data from this dose escalation study provide early clinical evidence that this mechanism is operating in patients, not just in preclinical breast cancer xenograft models where BTX-9341 previously demonstrated activity.
The safety data from this Biotheryx clinical trial results readout are notable in the context of the CDK4/6 inhibitor class. Across all 28 participants, there were no serious adverse events and no treatment discontinuations due to adverse events. There were no grade 3 or higher non-hematologic events. Hematologic adverse events occurred but were described as transient and reversible. Critically, the company and its clinical investigators reported no clear evidence of the class-related toxicities associated with approved CDK4/6 inhibitors — specifically gastrointestinal events including diarrhea, nausea, and vomiting, hepatotoxicity, and QTc interval prolongation.