IDEAYA/Hengrui Report Positive Phase 1 Data on Novel DLL3-Targeting ADC (IDE849/SHR-4849) in SCLC at WCLC 2025

• 80.0% (8/10) Overall Response Rate (ORR) and 70.0% (7/10) confirmed ORR in 2nd-Line (2L) Small Cell Lung Cancer (SCLC); a 73.7% (14/19) ORR and 57.9% (11/19) confirmed ORR (1 pending confirmation) were observed across all lines of SCLC at the 2.4 mg/kg expansion dose of IDE849.
• 77.1% (27/35) ORR and 60.0% (21/35) confirmed ORR (4 pending confirmation) in 2L SCLC; a 73.2% (52/71) ORR and 47.9% (34/71) confirmed ORR (10 pending confirmation) were observed across all lines of SCLC at all expansion doses of IDE849.
• In patients with baseline brain metastases, a 83.3% (5/6) confirmed ORR at the 2.4 mg/kg dose was observed; across all doses ≥2.4 mg/kg (n=18), a 66.7% (12/18) confirmed ORR (1 pending confirmation) was observed.
• 14.1% (10/71) of patients across all doses ≥2.4 mg/kg are still pending confirmation, and some patients have had limited follow-up, suggesting that the study has not yet achieved a fully mature confirmed ORR%.
• Median Progression-Free Survival (PFS) of 6.7 months across all lines of SCLC at all dose levels (≥2.4 mg/kg); median PFS not reached in 2L SCLC patients.
• Manageable safety profile observed across multiple expansion cohorts, including the 2.4 mg/kg, 3.0 mg/kg, and 3.5 mg/kg dose levels with a once every 3-week dosing interval.

SHANGHAI and SOUTH SAN FRANCISCO, Calif., Sept. 7, 2025 — IDEAYA Biosciences, Inc. (Nasdaq: IDYA), a precision medicine oncology company, and Hengrui Pharma, a global pharmaceutical company presented initial data from Hengrui’s Phase 1 clinical trial of IDE849 (SHR-4849) at the IASLC 2025 World Conference on Lung Cancer (WCLC) in Barcelona, Spain. IDE849 is a potential first-in-class delta-like ligand 3 (DLL3)-targeting Topoisomerase-1 (TOP1) antibody-drug conjugate (ADC). The presentation covered data from 100 patients who received IDE849 at doses ranging from 0.8 mg/kg to 4.2 mg/kg, administered every three weeks.

“The Overall Response Rate (ORR), median Progression-Free Survival (PFS), and overall safety data presented at WCLC 2025, derived from over 70 efficacy-evaluable SCLC patients, indicates IDE849 possesses a potential best-in-class DLL3 TOP1 ADC profile,” stated Yujiro S. Hata, President and Chief Executive Officer, IDEAYA Biosciences. “We intend to accelerate the global clinical development of IDE849 in SCLC, Neuroendocrine Tumors (NETs), and other DLL3 upregulated solid tumors. IDE849 has the potential to address critical unmet medical needs in cancer.”

The data presented, which had a cut-off date of June 20, 2025, encompassed 87 patients with small-cell lung cancer (SCLC) and 13 patients with other neuroendocrine carcinomas (NEC). All patients had progressed after first-line therapy, with 33% having progressed after two prior lines and 15% after three or more prior lines of therapy. Of the 87 SCLC patients, 72.4% (63/87) had received prior immunotherapy. A total of 71 patients with refractory SCLC (2L+) were evaluated for initial efficacy at doses of 2.4 mg/kg (n=19), 3.0 mg/kg (n=18), and 3.5 mg/kg (n=31) in the trial’s expansion phase. Patients in the 4.2 mg/kg cohort (n=3) of the dose escalation phase were also included in the analysis. All efficacy-evaluable patients had received at least one post-baseline tumor assessment per RECIST v1.1.

Key Data Highlights from the Presentation

Efficacy (n=71 evaluable SCLC patients treated with IDE849)

• Robust Overall Response Rate (ORR) and Disease Control Rate (DCR) were consistently observed across all expansion doses and across lines of therapy; however a reduction in ORR/DCR was associated with later-line patients, consistent with their more advanced stage of disease.
• 14.1% (10/71) of patients across all doses ≥2.4 mg/kg are still pending confirmation, and several patients have had only limited follow-up, suggesting that the study’s confirmed ORR% has not yet fully matured.
• In patients with baseline brain metastasis, a confirmed ORR of 83.3% (5/6) and a DCR of 100% (6/6) was observed at the 2.4 mg/kg dose. Across all doses ≥2.4 mg/kg (n=18) with baseline brain metastasis, the confirmed ORR was 66.7% (12/18) with a DCR of 100% (18/18). A confirmation scan for one unconfirmed partial response is pending, which, if confirmed, would increase the confirmed ORR to 72.2% (13/18).
• Median PFS was 6.7 months across all lines of treatment at doses of IDE849 ≥2.4 mg/kg (n=86); median PFS was not yet reached (NR) in 2L patients (n=42).
• As of the cut-off date of June 20, 2025, the median length of follow-up was 3.5 months. This relatively short follow-up period necessitates further observation to fully assess the long-term durability of responses and overall survival.

Safety (n=100 SCLC and NEC patients treated with IDE849)

• Across all patients and all dose levels (n=100), Grade 3 or higher (Gr≥3) treatment-related adverse events (TRAEs) occurred in 48% (48/100), and serious TRAEs in 16% (16/100) of patients. The most common TRAEs were white blood cell reduction (27% Gr≥3), neutropenia (33% Gr≥3), anemia (6% Gr≥3), and nausea (0% Gr≥3). These hematologic toxicities are common with Topoisomerase-1 inhibitors, and underscore the necessity for careful patient monitoring and potential dose adjustments.
• TRAEs led to dose reduction in 15% (15/100) of patients; the treatment-related discontinuation rate was 2% (2/100), with no treatment-related deaths reported. This suggests that IDE849 is generally well-tolerated, and the low discontinuation rate indicates patients are able to remain on treatment to realize its potential clinical benefits.

IDEAYA will review the data that was presented by Hengrui at their 10-Year Anniversary R&D Day on September 8^th in New York.

In December 2024, Hengrui Pharma granted IDEAYA an exclusive worldwide license to develop and commercialize SHR-4849 (IDE849) outside of Greater China. The partners will collaborate to accelerate the global development of this innovative therapy for patients worldwide.

About IDEAYA Biosciences

IDEAYA Biosciences is a precision medicine oncology company focused on discovering, developing, and commercializing transformative cancer therapies. The company integrates small-molecule drug discovery, structural biology, and bioinformatics expertise with robust internal capabilities in identifying and validating translational biomarkers. IDEAYA’s approach is designed to develop tailored, potentially first-in-class targeted therapies aligned to the genetic drivers of disease. The company has built a pipeline of product candidates focused on synthetic lethality and antibody-drug conjugates (ADCs) for molecularly defined solid tumor indications. The mission is to deliver next-generation precision oncology therapies that are more selective, effective, and deeply personalized with the goal of altering the course of disease and improving clinical outcomes for patients with cancer.

Forward-Looking Statements

This press release contains forward-looking statements regarding the potential therapeutic benefits of IDE849, including combination therapies; and the timing of and potential of clinical trials to evaluate TOP1-payload based ADCs in SCLC, NETs, and other DLL3-upregulated solid tumors. These statements involve risks and uncertainties that could cause IDEAYA’s preclinical and clinical development programs, future results, performance, or achievements to differ significantly from those expressed or implied by the forward-looking statements. These risks and uncertainties include drug development, clinical trials, regulatory approval, manufacturing drug products, IDEAYA’s ability to protect its intellectual property, and the sufficiency of existing cash to fund operations. IDEAYA undertakes no obligation to update or revise any forward-looking statements.