SEATTLE and SINGAPORE, May 27, 2026
Callio Therapeutics has announced the presentation of its Phase 1 clinical trial design for CLIO-8221, a novel dual-payload antibody-drug conjugate (ADC) targeting HER2-expressing solid tumors, at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago. The company’s lead oncology candidate represents a first-in-class approach designed to address resistance mechanisms that limit the effectiveness of current HER2-targeted therapies. The presentation highlights ongoing clinical development efforts and underscores growing interest in next-generation ADC technologies aimed at improving outcomes for patients with advanced cancers.
First-in-Class Dual-Payload ADC Targets Treatment Resistance
CLIO-8221 is engineered to deliver two complementary anti-cancer agents directly to HER2-positive tumors: a topoisomerase 1 inhibitor (exatecan) and an ATR inhibitor (berzosertib). This dual-payload design seeks to overcome one of the major challenges in oncology treatment—tumor resistance to existing HER2-targeted ADCs such as trastuzumab deruxtecan.
According to Callio Therapeutics, the investigational therapy combines targeted delivery with a multi-mechanism approach that simultaneously damages tumor DNA and blocks cellular repair pathways. This strategy is intended to enhance anti-tumor activity while minimizing systemic toxicity. The company noted that the Phase 1 trial is currently enrolling patients with advanced HER2-expressing solid tumors across sites in the United States and Australia, with plans to expand into China.
Preclinical Data Demonstrate Strong Efficacy and Safety Profile
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The ASCO presentation will also include previously disclosed preclinical findings supporting the development of CLIO-8221. Research demonstrated that the ADC achieved superior anti-tumor activity compared to single-payload ADCs, delivering robust tumor cell killing while maintaining a strong bystander effect. Investigators reported significant tumor regression after a single dose in both trastuzumab deruxtecan-sensitive and resistant models, indicating potential utility in patients who have exhausted existing HER2-directed treatment options.
Additionally, CLIO-8221 showed activity across varying levels of HER2 expression, broadening its potential applicability across multiple tumor types. Safety studies in non-human primates revealed favorable tolerability with no significant adverse effects observed at the highest tested dose, establishing a safety margin that compares favorably with currently available HER2-targeted ADC therapies.
Phase 1 Study Aims to Expand Options for Advanced Cancer Patients
The ongoing Phase 1 clinical trial is structured as a dose-escalation study with optional expansion cohorts, allowing researchers to evaluate safety, tolerability, pharmacokinetics, and preliminary anti-tumor activity across a range of HER2-expressing cancers. Importantly, the study includes patients whose tumors have relapsed after or failed to respond to prior HER2-targeted treatments.
Clinical investigators believe the dual-payload mechanism may provide a meaningful opportunity to address resistance pathways that have historically limited long-term treatment success. By integrating two synergistic therapeutic mechanisms into a single targeted ADC, Callio Therapeutics aims to establish a differentiated treatment option capable of delivering deeper and more durable responses. The ASCO presentation marks a significant milestone for the company as it advances its oncology-focused ADC platform and seeks to generate clinical data that could shape the future of targeted cancer therapy.
Source: Callio Therapeutics press release
