On October 23, 2025 HUTCHMED (China) Limited ("HUTCHMED") (Nasdaq/AIM:​HCM; HKEX:​13) reported preclinical data for HMPL-A251 at the AACR (Free AACR Whitepaper)‑NCI‑EORTC AACR-NCI-EORTC (Free AACR-NCI-EORTC Whitepaper) International Conference on Molecular Targets and Cancer Therapeutics (EORTC-NCI-AACR) (Free ASGCT Whitepaper) (Free EORTC-NCI-AACR Whitepaper), held October 22–26, 2025, in Boston, USA. HMPL-A251 is a first-in-class PI3K/AKT/mTOR ("PAM")-HER2 Antibody-Targeted Therapy Conjugate ("ATTC") comprising of a highly selective and potent PI3K/PIKK inhibitor payload linked to a humanized anti-HER2 IgG1 antibody, via a cleavable linker.

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HER2 is a well-established therapeutic target. HER2 overexpression is found in a variety of cancer types and often associated with poor prognosis. As a key downstream signaling pathway of HER2, the PAM pathway contributes significantly to the resistance against HER2-targeting treatments when altered. HMPL-A251 is innovatively designed to leverage the synergy between HER2 targeting and PAM pathway inhibition to address limitations of traditional toxin-based antibody-drug conjugates ("ADCs") and standalone PAM inhibitors.

In vitro, the PI3K/PIKK inhibitor payload exhibited high potency, selectivity, and broad anti-tumor activity across a panel of 130 tumor cell lines. By conjugating this potent payload with an anti-HER2 antibody via a hydrophilic linker, the ATTC compound HMPL-A251, upon binding to the HER2-positive target cells, undergoes rapid internalization, lysosomal trafficking, payload release, and inhibition of PAM and PIKK signaling, inducing tumor cell apoptosis. HMPL-A251 demonstrated HER2-dependent antitumor activity in vitro, potently inhibiting HER2-positive tumor cell growth regardless of PAM pathway alterations, with moderately reduced activity in HER2-low, PAM-altered cell lines. HMPL-A251 also demonstrated a bystander effect on HER2-null cells when co-cultured with HER2-positive cells.

Unlike toxin-based ADCs, which often face challenges with toxicity related to their cytotoxic payloads, ATTCs are designed to prioritize tumor-specific delivery of a pathway-modulating payload, enhancing safety for long‑term use and enabling potential frontline combinations with chemotherapy. In vivo, HMPL-A251 demonstrated superior anti-tumor efficacy and tolerability as compared to the naked antibody and payload administered together. A single intravenous dose of HMPL-A251 induced tumor regression across multiple models including HER2-positive and HER2-low models with or without PAM alteration. Efficacy correlated strongly with payload concentration and target inhibition in tumor tissue. Notably, when benchmarked against T-DXd (trastuzumab deruxtecan, a HER2‑directed ADC), HMPL-A251 achieved superior or comparable efficacy at equivalent doses in most tested models. Moreover, payload-based toxicities are expected to be low, as the plasma exposure of free payload was much lower than for HMPL-251, with a mass ratio of less than 1:500,000.

"We are excited to share the progress of HMPL-A251, the first candidate from our ATTC platform. It represents a potentially significant leap forward in addressing the limitations of toxin-based ADCs and narrow therapeutic window of systemic PAM inhibitors. By combining selective PI3K/PIKK inhibition with precise HER2 targeting, HMPL-A251 achieves potent antitumor effects while maintaining a favorable safety profile," said Dr Michael Shi, Head of R&D and Chief Medical Officer of HUTCHMED. "The compelling preclinical data presented underscore its potential to redefine treatment for a wide spectrum of cancers, and we are excited to advance HMPL-A251 as well as more ATTC drug candidates toward clinical trials."

HUTCHMED plans to initiate global clinical trials for HMPL-A251 around the end of 2025, followed by multiple global Investigational New Drug (IND) filings for more ATTC candidates in 2026.

About the ATTC platform
HUTCHMED’s Antibody-Targeted Therapy Conjugate platform represents a next-generation approach to precision oncology, combining monoclonal antibodies with proprietary small-molecule inhibitor payloads to deliver dual mechanisms of action. Unlike traditional cytotoxin-based ADCs, ATTCs combine targeted therapies to achieve synergistic anti-tumor activity and durable responses in preclinical models, outperforming standalone antibody or small-molecule inhibitor components in efficacy and safety.

Built on over 20 years of targeted therapy expertise, the platform enables development of drug candidates for diverse cancer types. By leveraging antibody-guided delivery and tumor-specific payload release, ATTCs improve the accessibility to tumors and reduce off-tumor toxicity. This overcomes challenges of traditional small-molecule inhibitors, ensures safer long-term use, and supports combinations with chemotherapy and immunotherapy, unlocking potential for early-line treatments.

(Press release, Hutchison China MediTech, OCT 23, 2025, View Source [SID1234656897])