-- Updated Phase 1 dose escalation data show that BLU-222 in combination with ribociclib and fulvestrant was well-tolerated at clinically active dose levels with no dose-limiting toxicities --

-- Early signal of clinical activity includes compelling biomarker reductions correlated with BLU-222 exposure --

CAMBRIDGE, Mass., May 23, 2024 /PRNewswire/ -- Blueprint Medicines Corporation (Nasdaq: BPMC) today announced updated data from the ongoing Phase 1 dose escalation portion of the VELA clinical trial of BLU-222, an investigational, highly selective and potent CDK2 inhibitor, in combination with ribociclib and fulvestrant in patients with hormone-receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer. The data, which mark the first promising clinical results for a CDK2 inhibitor in combination with an approved CDK4/6 inhibitor, will be presented at the 2024 American Society of Clinical Oncology (ASCO) Annual Meeting on June 2, 2024.

"At ASCO, we will present highly encouraging clinical data showing that our selective CDK2 inhibitor BLU-222, in combination with ribociclib, a standard of care CDK4/6 inhibitor for HR+ breast cancer, is very well-tolerated and delivers early evidence of clinical activity. This represents a highly significant milestone and holds promise as an important new cornerstone for the treatment of breast cancer, including in the front-line metastatic setting," said Becker Hewes, M.D., Chief Medical Officer at Blueprint Medicines. "These data validate the potency and selectivity of BLU-222, its potential as a first- and best-in-class CDK2 inhibitor, and its use as the combination partner of choice in breast cancer. With these data in hand, we are advancing ongoing partnering discussions that aim to accelerate development of BLU-222 into registration-directed clinical trials."

Updated Phase 1/2 VELA Trial Results

Based on previously reported positive BLU-222 monotherapy clinical data, a combination dose escalation arm was initiated in the VELA trial to assess the safety and clinical activity of BLU-222 in combination with ribociclib, a CDK4/6 inhibitor approved by the U.S. Food and Drug Administration for advanced or metastatic HR+/HER2- breast cancer, and fulvestrant, a commonly used estrogen receptor antagonist. As of the data cutoff date, 19 patients with HR+/HER2- breast cancer who had progressed on prior CDK4/6 inhibitors were treated with 100 mg to 400 mg twice daily (BID) of BLU-222 plus 400 mg once daily (QD) of ribociclib and combined with fulvestrant.

The combination of BLU-222, ribociclib, and fulvestrant was well-tolerated at all BLU-222 dose levels tested. No dose-limiting toxicities, treatment-related severe adverse events (SAEs), or BLU-222-related treatment discontinuations were reported. Treatment-related hematologic and gastrointestinal AEs were generally mild. The maximum tolerated combination dose has not been identified, and combination dose escalation is ongoing.

Pharmacokinetic data showed dose-proportional exposures of BLU-222, with sustained coverage above the predicted efficacious concentration at the 400 mg BID dose level. In addition, the combination of BLU-222 with ribociclib and fulvestrant had no clinically meaningful impact on individual drug exposures.

Preliminary clinical activity showed compelling reductions in thymidine kinase 1 (TK1) and circulating tumor DNA (ctDNA), biomarkers that have been shown to be predictive of clinical benefit. TK1, a biomarker of tumor proliferation, had the deepest reduction among patients treated with BLU-222 400 mg BID, ribociclib 400 mg QD, and combined with fulvestrant, and was statistically significantly correlated with BLU-222 exposure. All patients with evaluable ctDNA, a biomarker of tumor burden, treated with the BLU-222 400 mg BID combination dose regimen showed ctDNA reductions. Early evidence of clinical benefit includes an unconfirmed partial response in a patient who had previously progressed following six lines of therapy in the metastatic setting, including prior palbociclib and trastuzumab deruxtecan. These data highlight the impact of CDK2 inhibition when BLU-222 is combined with other therapies.

Detailed data will be presented by Dr. Dejan Duric from the Henri and Belinda Termeer Center for Targeted Therapies at Massachusetts General Hospital on June 2, 2024, during the "Breast Cancer – Metastatic" poster session at 9:00 a.m. CT. At the time of presentation, a copy of the poster will be available in the "Science—Publications and Presentations" section of the company's website at .

About BLU-222

BLU-222 is a highly selective and potent investigational CDK2 inhibitor with first- and best-in-class potential, designed by scientists at Blueprint Medicines. CDK2 is a cell cycle regulator and an important cancer target, with relevance in HR+/HER2- breast cancer and other malignancies, such as subsets of ovarian and endometrial cancer. Across multiple cancer types, aberrant CCNE1 hyperactivates CDK2, resulting in cell cycle dysregulation and tumor proliferation. Aberrant CCNE1 has been observed as a primary driver of disease, as well as a mechanism of resistance to CDK4/6 inhibitors. In HR+/HER2- breast cancer, the advent of CDK4/6 inhibitors has improved treatment; however, disease progression is nearly universal, and new innovation is needed to improve outcomes and prolong clinical benefit. Historically, CDK2 inhibitor development by others has been challenged due to poor selectivity limiting tolerability and combination potential. Beyond BLU-222, Blueprint Medicines is advancing additional preclinical therapeutic candidates for cell cycle targets including BLU-956, a next-generation CDK2 inhibitor, a CDK2 targeted protein degradation program, and an additional undisclosed research program.