Excellence of Next-Generation ADC Platform and Design of CS5007

CStone Pharmaceuticals (HKEX: 2616) presented preclinical data for three novel pipeline drugs based on its proprietary Antibody-Drug Conjugate (ADC) platform at the American Association for Cancer Research (AACR) 2026. The lead drug, CS5007, is a dual-target ADC that simultaneously targets Epidermal Growth Factor Receptor (EGFR) and Human Epidermal Growth Factor Receptor 3 (HER3. It is designed with a Drug-to-Antibody Ratio (DAR) of 4, combining Daiichi Sankyo's Exatecan payload with the hydrophilic linker CSL20. The aim is to overcome the drug resistance limitations of existing single-target EGFR inhibitors by employing a novel mechanism of simultaneously blocking both EGFR and HER3. Non-clinical plasma stability assessment showed that CS5007, compared to Enhertu (DS-8201), a Herceptin-based ADC, exhibited a favorable payload release rate of less than 0.5%, demonstrating a significant improvement in reducing potential toxicity.

Superior Preclinical Data Compared to Competitor Drugs

The most notable aspect of this presentation is the direct comparison data with BL-B01D1 (EGFR/HER3 dual ADC), a competitor drug for which Bristol Myers Squibb (BMS) conducted a licensing agreement worth a total of $8.4 billion. In a colorectal cancer model (SW620) with high expression of both EGFR and HER3, CS5007 induced complete tumor regression, while the control group, BL-B01D1, did not show significant anti-cancer activity. Furthermore, in an animal model (FaDu CDX) evaluation, CS5007 showed a half-life of approximately 20 hours at the same dose, which is about twice as long as the control group's 10 hours, and demonstrated statistically significant superior tumor inhibitory efficacy (p < 0.05). This suggests that it may offer a more attractive commercial option in terms of dosing frequency and duration of efficacy compared to competing products when it enters clinical trials.

Innovation and Potential of First-in-Class Drug CS5006

The second pipeline drug, CS5006, is a first-in-class ADC targeting Integrin beta-4 (ITGB4), which is overexpressed in various solid tumors but has very low expression in normal tissues. Integrin beta-4 is known as a key factor in the MEK/ERK signaling pathway that induces cancer cell metastasis and resistance to Anti-PD-1 immune checkpoint inhibitors. In preclinical models, CS5006 demonstrated the ability to rapidly and deeply internalize into ITGB4-overexpressing cancer cells, inducing cell death, and also exhibited a bystander effect, killing surrounding cancer cells. In non-human primate toxicity studies, the Highest Non-Severe Toxic Dose (HNSTD) was observed at 45 mg/kg, demonstrating a high safety profile, and CStone plans to submit an Investigational New Drug (IND) application for this drug in the second half of 2026.

CS5008 to Overcome Small Cell Lung Cancer Subtype Switching Resistance

Finally, CS5008, which was also presented, is a dual-target ADC targeting DLL3 and SSTR2 for the treatment of Small Cell Lung Cancer (SCLC) and Neuroendocrine Tumors (NET). Up to 50% of SCLC patients experience subtype switching during standard treatment, acquiring resistance to anti-cancer drugs. CS5008 is designed to perfectly target this evasion mechanism. During treatment, when SCLC-A subtype transforms into SCLC-N subtype, SSTR2 is overexpressed by NEUROD1 instead of DLL3. By targeting both antigens simultaneously, it aims to fundamentally block the resistance that occurs during treatment. In a non-human primate model, it achieved an excellent half-life of approximately 14 days and a non-severe toxic dose (HNSTD) of 60 mg/kg, demonstrating high safety, and this drug is also scheduled to submit an IND application and enter clinical trials in the second half of 2026.