Prelude and Lilly Partner to Test PRT13722 in Breast Cancer

WILMINGTON, Del. — October 13, 2026

Phase 1 Trial to Evaluate Monotherapy and Combination Regimens

The U.S. Food and Drug Administration (FDA) has cleared Prelude to proceed with the Phase 1 study under its Investigational New Drug (IND) application for PRT13722. The company plans to begin with monotherapy dose escalation before advancing to multiple combination cohorts, including regimens involving abemaciclib and imlunestrant. The study is designed to evaluate the investigational candidate in patients with HR+/HER2− breast cancer, a disease subtype commonly treated with endocrine therapy and targeted medicines. The collaboration provides access to Lilly’s established breast cancer therapies for clinical evaluation alongside Prelude’s experimental KAT6A degrader. The companies have not disclosed the planned enrollment numbers, specific dose levels, detailed study timelines beyond the expected fourth-quarter 2026 enrollment start, or criteria for advancing individual combination cohorts. Prelude retains full global development and commercial rights to PRT13722 as a wholly owned program.

Preclinical Findings Support Combination Strategy

The collaboration is supported by preclinical findings presented by Prelude at the American Association for Cancer Research (AACR) 2026 Annual Meeting in April. According to the company, PRT13722 selectively degrades KAT6A and produces more complete disruption of KAT6A-regulated pathways than dual KAT6A/B inhibitors. These effects were associated with stronger antitumor activity across multiple HR+/HER2− breast cancer models. As a monotherapy, PRT13722 produced durable complete tumor regressions in cell-line-derived xenograft (CDX) and patient-derived xenograft (PDX) models, including models representing cancers that were both endocrine therapy-naive and previously exposed to endocrine treatment. The company reported these responses at doses described as well tolerated. In additional preclinical experiments, PRT13722 showed synergistic activity when combined with endocrine therapies, CDK4/6 inhibitors and PI3K inhibitors. Activity was also observed in models featuring ESR1 mutations and acquired treatment resistance, findings that could be relevant to cancers that become less responsive to existing therapies. These results remain preclinical and do not establish clinical benefit or safety in patients.

Selective KAT6A Degradation May Support Improved Tolerability

Prelude is developing PRT13722 as an orally bioavailable, highly potent and selective degrader of KAT6A, a member of the KAT6 protein family involved in cellular gene regulation. The company’s strategy is to target KAT6A while sparing KAT6B, with the aim of achieving antitumor activity while potentially reducing toxicity associated with inhibiting both proteins. Prelude reported that PRT13722 demonstrated an improved preclinical hematological safety profile compared with prifetrastat, a dual KAT6A/B inhibitor. The company believes this distinction could make the candidate more suitable for combination regimens with standard-of-care medicines used in earlier treatment settings for metastatic and early-stage HR+/HER2− breast cancer. However, whether the selective degradation approach delivers a meaningful tolerability advantage will need to be established through clinical testing. The Phase 1 program is expected to provide initial human evidence on the candidate’s safety, tolerability and dosing, while subsequent development will determine whether its biological activity translates into meaningful outcomes for patients. The agreement with Lilly represents a step toward evaluating PRT13722 alongside established treatment approaches, while Prelude continues to lead the program’s global development.

Source::Prelude Therapeutics, press release