Presentation follows receipt of FDA Fast Track designation for BH-30643 for the treatment of advanced or metastatic EGFR C797S-positive NSCLC (Form 8-K)

Presentation follows receipt of FDA Fast Track designation for BH-30643 for the treatment of

advanced or metastatic EGFR C797S-positive NSCLC

SAN DIEGO, September 15, 2026 - BlossomHill Therapeutics, Inc. (Nasdaq: BLSM), a clinical-stage biopharmaceutical company applying an intentional, chemistry-based approach to design and develop innovative small molecule medicines for the treatment of cancer, today announced updated data from the ongoing Phase 1/2 SOLARA trial of BH-30643 in non-small cell lung cancer (NSCLC) patients with secondary epidermal growth factor receptor (EGFR) resistance mutations such as EGFR C797S. The data were highlighted in a mini-oral presentation at the International Association for the Study of Lung Cancer (IASLC) 2026 World Conference on Lung Cancer in Seoul, South Korea.

BH-30643 is an investigational, novel, orally bioavailable, non-covalent, macrocyclic, brain active, mutant-selective, OMNI-EGFR^™ inhibitor designed to overcome the limitations of currently approved EGFR inhibitors for the treatment of EGFR-mutant NSCLC. In patients with EGFR C797S-positive resistance to prior EGFR inhibitor treatment, with or without concurrent T790M, BH-30643 demonstrated a 45% objective response rate (ORR; 18/40) and an 88% disease control rate (DCR; 35/40). At the time of efficacy follow-up, 63% (25/40) of patients remained on treatment with a median follow-up of 6.9 months. BH-30643 also demonstrated a favorable safety profile with low rates of dose reduction or discontinuation due to treatment-related adverse events.

"C797S-driven resistance to third-generation EGFR inhibitors was first described over 10 years ago, yet patients whose tumors develop this mutation currently have no approved targeted treatment options," said Hidehito Horinouchi, M.D., Ph.D., National Cancer Center Hospital, Tokyo, and presenting author of the study. "The responses observed with BH-30643 in this heavily pretreated population, together with encouraging early evidence of durability, support the potential of BH-30643 to directly target this resistance mechanism. These results are particularly encouraging given the need for new precision treatment options that can extend the benefits of targeted therapy for patients with EGFR-mutant lung cancer."

"These updated results provide important clinical validation of the approach we took in intentionally designing BH-30643 to address on-target EGFR resistance, including C797S," said Geoff Oxnard, M.D., Chief Medical Officer of BlossomHill Therapeutics. "We are encouraged to see meaningful anti-tumor activity across a molecularly diverse group of patients with C797S-positive disease, including patients with concurrent T790M and those who have received multiple prior therapies, with a favorable safety profile. Together with the recent FDA Fast Track designation, these data strengthen our conviction in the potential of BH-30643 and support our plans to advance into a Phase 2 trial in patients with C797S-positive NSCLC in 2027."

Presentation highlights:

As of the May 12, 2026 data cutoff, with efficacy follow-up through August 10, 2026:

The presentation is available on the company's Posters & Presentations page here:

https://bhtherapeutics.com/pipeline/#posters-and-presentations.

About BH-30643

BH-30643 is an investigational, novel, orally bioavailable, non-covalent, macrocyclic, brain active, mutant-selective, OMNI-EGFR^™ inhibitor for the treatment of EGFR-mutant NSCLC. BH-30643 was designed to overcome the limitations of currently approved EGFR inhibitors, which were discovered over a decade ago without the current, modern understanding of the structure and protein dynamics of mutant EGFRs. In preclinical studies, BH-30643 demonstrated potent inhibitory activity across diverse EGFR mutation categories - classical mutations, on-target resistance mutations such as C797S with or without T790M, atypical mutations and exon 20 insertions - while maintaining marked selectivity over wild-type EGFR. BH-30643 has received Fast Track designation and is being evaluated in SOLARA, a global Phase 1/2, first-in-human clinical trial spanning more than 40 sites in 10 countries. Ongoing dose expansion cohorts are enrolling in both TKI-pretreated and TKI-naive settings, including a C797S resistance cohort. For additional information on SOLARA, including a list of study sites and how to enroll, please visit clinicaltrials.gov (NCT06706076).

About the SOLARA Trial

The Phase 1/2 SOLARA clinical trial (NCT06706076) is a global, open label, multicenter study assessing the safety, tolerability, pharmacokinetics, pharmacodynamics and preliminary anti-tumor activity of BH-30643 in patients with epidermal growth factor receptor (EGFR) mutant non-small cell lung cancer (NSCLC). Phase 1 will determine the recommended Phase 2 dose (RP2D) of BH-30643 as a monotherapy and in combination with chemotherapy. Phase 2 is designed to evaluate the antitumor efficacy and safety in specified cohorts determined by mutation subtypes and/or treatment history at the RP2D, as well as the population pharmacokinetics.

About BlossomHill Therapeutics

BlossomHill Therapeutics, Inc. is a clinical-stage biopharmaceutical company applying an intentional, chemistry-based approach to design and develop innovative small molecule medicines that address significant unmet medical needs in cancer treatment. Founded and led by industry veteran J. Jean Cui, Ph.D., with her proven track record in oncology drug design and development - including three FDA-approved drugs - BlossomHill Therapeutics applies cutting-edge science with a goal to address key oncogenic drivers and improve patient outcomes in difficult-to-treat cancers. The company's lead clinical program is BH-30643, an investigational, non-covalent, macrocyclic, brain active, mutant-selective OMNI-EGFR^TM inhibitor for the treatment of EGFR-mutant non-small cell lung cancer (NSCLC), which has received Fast Track designation for the C797S resistance population after 3rd generation EGFR TKI treatment. The company is also conducting clinical development of BH-30236, an investigational macrocyclic CDC-like kinase (CLK) inhibitor initially being studied in a clinical trial for the treatment of relapsed or refractory acute myeloid leukemia (R/R AML) and higher-risk myelodysplastic syndromes (HR-MDS). The company's pipeline also includes BH-501284, a preclinical, non-covalent, selective, pan-KRAS Switch-II inhibitor for potential future development in diverse KRAS-mutant tumors.

BlossomHill Therapeutics is headquartered in San Diego, California. For more information, visit bhtherapeutics.com and follow us on LinkedIn and X.