<img src="https://ml.globenewswire.com/Resource/Download/3511fd62-eaba-4921-a3da-4e0b00d976e6?size=2" nborder="0" align="left" hspace="5" vspace="5" title="Callio Logo.Stacked.Default.png" alt="Callio Therapeutics"/>Veteran biotech executive to lead company growth as it advances multiple dual-payload ADCs through clinical development for people living with cancer Piers Ingram, PhD, Co-Founder, transitions to Senior Advisor and continues to serve on the Board of Directors; Jerome Boyd-Kirkup, PhD, founding Chief Scientific Officer, transitions to Senior Scientific Advisor SAN DIEGO and SEATTLE and SINGAPORE, Sept. 09, 2026 <a href="https://www.globenewswire.com" target="_blank">(GLOBE NEWSWIRE)</a> -- Callio Therapeutics (“Callio” or “the Company”), a 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 Zachary (“Zach”) Hornby as Chief Executive Officer. Hornby brings significant experience leading innovative biotechnology companies through clinical development, private and public financings, and strategic partnerships. Hornby has served in executive and director roles across multiple public and private biotechnology companies. He was most recently CEO, President, and a Director at Boundless Bio (Nasdaq: BOLD), a precision oncology company that raised $350 million in private and public capital and advanced three internally discovered molecules into clinical development. Previously, Hornby held Chief Financial Officer and Chief Operating Officer roles at Ignyta, where he helped lead the company's IPO, oversaw its development portfolio, was the team leader for cancer drug Rozlytrek™, and led the business development process behind its $2 billion acquisition by Roche. He currently serves as a Director at Radionetics Oncology and Telperian, Inc. Hornby holds BS and MS degrees in biology from Stanford University and an MBA from Harvard Business School. "Resistance is the defining challenge facing ADCs today, and Callio's leading approach in the emerging dual-payload space tackles resistance head-on with rational combination payloads delivered in a tumor-specific manner," said Hornby. "With CLIO-8221 in the clinic and additional programs advancing toward the clinic in the near future, I look forward to building on the strong scientific foundation to deliver meaningful impact for patients and shareholders." As part of the leadership transition, Piers Ingram, PhD, Callio’s founding CEO, will transition to a Senior Advisor role and continue to serve on Callio’s Board of Directors, ensuring continuity of the company's foundational scientific and strategic leadership. Jerome Boyd-Kirkup, PhD, Callio’s founding Chief Scientific Officer, will transition to a Senior Scientific Advisor role. The leadership transition reflects Callio’s corporate evolution as it advances its pipeline, including its lead clinical program, CLIO-8221, and expands the global impact of its leading dual-payload ADC platform. Ingram will continue to lead Hummingbird Bioscience (“Hummingbird”), from which Callio exclusively licensed its multi-payload ADC platform in oncology and associated intellectual property and pipeline assets in 2025, as its CEO, and Boyd-Kirkup will continue to serve Hummingbird as its CSO. "Callio has entered an exciting stage as we advance CLIO-8221 through a Phase 1 clinical trial and broaden the reach of our dual-payload ADC platform across a growing pipeline," said Ingram. "Zach's track record of leading clinical-stage companies through drug development, global registrations, and capital raising makes him the right person to guide Callio forward in this next chapter and beyond. Through a collaborative development agreement, Hummingbird will continue to provide research and development support to Callio via Hummingbird’s expertise and AI-first approach to ADC discovery. I will continue to partner with the Callio team to advance dual-payload ADCs and deliver the next generation of ADC medicines for patients." 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 <a href="https://www.globenewswire.com/Tracker?data=Jj-IXaf9pff4SP1kgGjVzPmHQic2S96yQTOwZpWzBB-YPhOMs21QWhwsOjiKIbT7rg6O0-3ZEYxwZ-6DNdIWag1Ego8mxRJe7Wh2z9wvv5A=" rel="nofollow" target="_blank" title="www.calliotx.com">www.calliotx.com</a> and follow Callio Therapeutics on <a href="https://www.globenewswire.com/Tracker?data=iq-oB8ddF83JwOn12TfsMmtGC7RZjzjOiR2hNaeq9gUGtV6YA46wgMr6GZFLZD4DqQQuPTtl1uxk_nflOUMMANNJAukALKJMqMJlnp8MT08=" rel="nofollow" target="_blank" title="LinkedIn">LinkedIn</a>. 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 (<a href="https://www.globenewswire.com/Tracker?data=A5jN0YkW_30Q63Zcb1PDjVeQuEICiF3CbI9EWH65O1wG7G__LON15d4HYEFSO4ABxBPovZkHO9gzy75zVl4okRdXjj2hDDo3_qbD3uxOBQlY1uUNJO4FuB7RfbvCWQbq" rel="nofollow" target="_blank" title="NCT07300943">NCT07300943</a>). Contacts Media: Jeremy Sauer <a href="https://www.globenewswire.com/Tracker?data=EXKNift0cXLQd05WqHv-oa4J_EUEAejZsiARcxnPlJ4u7O-YtMAE2ftDiFFbFqkFebxofwJq0-wOQmpbRH_AWjJpg4_KvAeMcAjSm455du8=" rel="nofollow" target="_blank" title="media@calliotx.com">media@calliotx.com</a> Ally Stubin – ICR Healthcare <a href="https://www.globenewswire.com/Tracker?data=or7hLFTJbFW4sGheRahmZbJFYlD9AhkUq0IjqbHVyeExyRcB4f2dT_F-yI0QF9E47ZD8S5udnIi2pEaOSDBOJkih_36px9OaucrJjNEdb_yqavqNN8GkoKuXou2iebBE" rel="nofollow" target="_blank" title="Ally.Stubin@ICRHealthcare.com">Ally.Stubin@ICRHealthcare.com</a> (646) 667-1861 Investors: <a href="https://www.globenewswire.com/Tracker?data=5VUmdEAIt5IRnKvDfGM0HASCzYXEERhyJp5sRcmltrMkmE0ihOz79Y2lHq6hPMuC-GegsZu6swgljWQ10URxp2-5Rrid_PHe_2IIbrEeBPQ=" rel="nofollow" target="_blank" title="ir@calliotx.com">ir@calliotx.com</a> Stephanie Carrington – ICR Healthcare <a href="https://www.globenewswire.com/Tracker?data=dualtL_2T8p0wwuXrSTB4900lLxZx46gUFlxsM2kgsi5Je_7bWUf8aCE_dM94bNZvzHTowtmQK6xKEnFgyjjh7hA8ZLuvcO18k3HRlrzQp6tDv0ZkKCXpF2F-YDd18wLrWPCAX296-IeSKZ1G-Mb9g==" rel="nofollow" target="_blank" title="stephanie.carrington@ICRHealthcare.com">stephanie.carrington@ICRHealthcare.com</a> (646) 277-1282 <img alt="" src='https://ml.globenewswire.com/media/MWUyNDhjZTItZGM2Ni00NTJiLTkyM2UtNjNhMDhmZDc5OWI4LTEzMjA2MjItMjAyNi0wOS0wOS1lbg==/tiny/Callio-Therapeutics-Inc-.png' referrerpolicy='no-referrer-when-downgrade' />

Callio Therapeutics Appoints Zachary Hornby as Chief Executive Officer

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