Industry veteran brings more than two decades of oncology drug discovery leadership to advance Callio's dual-payload ADC pipeline

SAN DIEGO and SEATTLE and SINGAPORE, Sept. 28, 2026 (GLOBE NEWSWIRE) -- Callio Therapeutics (“Callio” or “the Company”), a global, clinical-stage biotechnology company advancing dual-payload antibody-drug conjugates (ADCs) as the next frontier of rationally designed targeted cancer therapy, today announced the appointment of Chris Hassig, PhD, as Chief Scientific Officer (CSO). Hassig will lead Callio's research and discovery organization, overseeing the advancement of the Company's dual-payload ADC platform and pipeline.

Hassig joins Callio from Boundless Bio, where he served as CSO, building the company's drug discovery organization from its inception through advancement of three internally discovered first-in-class molecules into clinical development. Prior to Boundless Bio, he was CSO of Sierra Oncology (acquired by GSK) and Vice President of Drug Discovery at Sanford Burnham Prebys Medical Discovery Institute. He previously held drug discovery and research roles at Kalypsys, Inc. and the Scripps Research Institute.

"Chris is a highly accomplished oncology drug discovery leader, and I had the privilege of seeing that firsthand," said Zachary Hornby, Chief Executive Officer of Callio Therapeutics. "Over seven years together at Boundless Bio, Chris helped build a discovery engine that took novel biology from concept to clinic. Callio's dual-payload ADC platform is at a similar inflection point, and Chris's experience translating cutting-edge science into differentiated therapeutics makes him the right leader to help us realize our technology's full potential for patients with cancer."

"Callio is pursuing one of the most compelling unmet needs in oncology drug development today by designing ADCs to overcome resistance mechanisms that limit current therapies," said Hassig. "The team has already advanced CLIO-8221 into the clinic and built an ADC platform that systematically tests rationally designed dual-payload combinations for ADCs, optimized for specific tumor targets. I look forward to working with the innovative and talented Callio team to help build and advance a pipeline that can change outcomes for patients who currently have limited options."

Hassig earned a BA in Chemistry and Biochemistry from the University of California, San Diego, and a PhD in Molecular and Cellular Biology from Harvard University. He completed a postdoctoral fellowship at the University of California, Berkeley.

About Callio Therapeutics

Callio Therapeutics is a clinical-stage biotechnology company that is developing dual-payload antibody-drug conjugates (ADCs) as the next frontier of targeted cancer therapy to address high incidence tumors where better therapies are needed. Its proprietary platform pairs modular, adaptable linker technology with rationally selected synergistic payload combinations designed to block key resistance mechanisms that enable cancer's escape. The Company's lead clinical program, CLIO-8221, is a HER2-targeted dual-payload ADC with potential to prevent and overcome treatment resistance that limits efficacy of current ADCs. Located in San Diego, Seattle, and Singapore, backed by top life science investors, and led by an experienced team with a track record of ADC development and oncology drug approvals, Callio Therapeutics is built to deliver the next generation of dual-payload ADCs and better outcomes for people living with cancer. For more information, visit www.calliotx.com and follow Callio Therapeutics on LinkedIn.

About CLIO-8221

HER2 is a clinically validated target for antibody-drug conjugates (ADCs), with multiple approved therapies demonstrating clinical benefit across tumor types. However, most patients eventually progress on treatment despite their tumors retaining HER2 expression. Mechanistic resistance to cytotoxic payloads has emerged as a key driver of treatment failure. CLIO-8221 is a novel, first-in-class, dual-payload ADC targeting HER2, designed to overcome emergent resistance.

CLIO-8221 delivers two mechanistically complementary payloads, a topoisomerase 1 (Topo1) inhibitor and an ATR inhibitor, directly to HER2-expressing tumors. While Topo1 inhibitors have shown strong clinical activity, activation of the DNA damage response following Topo1 inhibitor-induced replication stress represents a potential major driver of resistance. By simultaneously inhibiting Topo1 and blocking the DNA damage response through ATR inhibition, CLIO-8221 is engineered to overcome payload insensitivity and further sensitize tumors to Topo1 inhibition. Developed using proprietary linker and ADC platform technologies, CLIO-8221 aims to maximize anti-tumor activity while reducing systemic toxicity, offering the potential for deeper and more durable responses in patients with HER2-expressing tumors.

CLIO-8221 is currently being evaluated in an ongoing Phase 1/2, first-in-human, dose-escalation and expansion study in adult patients with metastatic or unresectable HER2-expressing solid tumors, including those who previously received trastuzumab deruxtecan (T-DXd). The first patient was dosed in March 2026, and enrollment is ongoing in Australia and the US, with plans to open the study in China (NCT07300943).

Contacts
Media:
Jeremy Sauer
media@calliotx.com

Ally Stubin – ICR Healthcare
ally.stubin@ICRHealthcare.com
(646) 667-1861

Investors:
ir@calliotx.com

Stephanie Carrington – ICR Healthcare
stephanie.carrington@ICRHealthcare.com
(646) 277-1282