Material Event (Form 8-K)

Korro Selects KRRO-111 as Development Candidate for the Potential Treatment of Alpha-1 Antitrypsin Deficiency (AATD)

CAMBRIDGE, Mass., May 19, 2026 - Korro Bio, Inc. (Korro) (Nasdaq: KRRO), a biopharmaceutical company leveraging a novel Oligonucleotide Promoted Editing of RNA (OPERA®) platform to develop a new class of genetic medicines for rare and highly prevalent diseases, today announced the addition of KRRO-111 for the potential treatment of AATD to its pipeline of therapeutic programs in development. KRRO-111 is a proprietary GalNAc-conjugated oligonucleotide that is delivered subcutaneously to the liver cells, where it is engineered to co-opt the hepatocytes' endogenous Adenosine Deaminase Acting on RNA (ADAR) enzyme and repair a pathogenic single nucleotide variant (SNV) on AAT mRNA to restore production of normal AAT protein. This therapeutic profile along with KRRO-111's rapid onset of action and titratable dosing to treat this disorder, position the compound as a potential best-in-class candidate.

"The addition of KRRO-111 for AATD to our pipeline of transformative therapeutic candidates is the culmination of extensive and robust pre-clinical optimization, achieving the highest level of SERPINA1 editing reported to date using RNA editing. It appears to be a level of highly specific protein correction that functionally approaches what would be expected from a permanent specific DNA modification, while preserving the inherent safety advantages of working at the RNA level," commented Ram Aiyar, Ph.D., Chief Executive Officer and President of Korro Bio. "For patients living with AATD, this represents a meaningful step toward a therapy designed to address the root cause of their disease. This is an important clinical and corporate development milestone for the Company as it highlights the power of the OPERA platform to generate potential best-in-class genetic medicines for patients suffering from debilitating diseases. As we advance KRRO-111 for the potential treatment of AATD into the clinic alongside KRRO-121 for the potential treatment of hyperammonemia in patients with urea cycle disorders, we are well-capitalized to report clinical data from both programs."

AATD is a genetic disorder most commonly caused by a single missense mutation (G-to-A) in the SERPINA1 gene. Affected adults experience pulmonary emphysema and/or hepatic cirrhosis, as well as end organ manifestations. Greater than 95% of severe clinical cases of AATD are homozygous for the PiZ mutation (known as the PiZZ genotype). There are an estimated 3.4 million individuals with deficiency allele combinations worldwide. The only current FDA-approved treatment for AATD is augmentation therapy, a once-weekly infusion of pooled human plasma-derived AAT protein, which does not adequately address the manifestations of AATD.

In preclinical studies, KRRO-111 demonstrated best-in-class RNA editing where >90% of the AAT transcripts in the liver cells have been corrected, translating into ~90% M-AAT protein. In a repeat-dose study, KRRO-111 (3 mg/kg) was administered to PiZZ mice with a loading phase followed by dosing once every two weeks. KRRO-111 nearly eliminated active Z protein production, reducing non-inclusion Z-AAT by ~95% versus vehicle at both days 28 and 56, reflecting near-complete cessation of Z protein synthesis across the hepatocyte population. Pre-existing aggregates were progressively cleared, with inclusion-associated Z-AAT significantly reduced by approximately 69% at day 28, consistent with autophagic clearance of accumulated protein following successful editing; the residual burden reflects pre-existing disease rather than incomplete editing. By simultaneously increasing circulating M-AAT protein and decreasing pathogenic Z-AAT aggregates in the lung, KRRO-111 has the potential to improve both lung and liver manifestations of the disease.

The Company plans to present additional data from its KRRO-111 program at a future scientific meeting or Company-hosted event.

About Korro

Korro is a biopharmaceutical company leveraging a novel oligonucleotide promoted editing of RNA (OPERA®) platform to develop a new class of genetic medicines for rare and highly prevalent diseases. OPERA provides precise, tissue-directed delivery of oligonucleotides that modify the targeted native mRNA transcript to repair or form a de-novo protein with enhanced functionality. The platform combines a suite of capabilities consisting of sophisticated knowledge of transcription biology through ADAR proteins (Adenosine Deaminases Acting on RNA), machine learning optimization of oligonucleotides, linker chemistry expertise, along with use of a highly targeted tissue-specific delivery methodology. As such, the OPERA platform has enabled Korro to generate and advance a portfolio of differentiated programs that are designed to harness the body's natural RNA editing process, providing precise yet transient single base edits to produce therapeutic proteins with augmented activity versus its endogenous counterpart. By editing RNA instead of DNA, Korro is expanding the reach of genetic medicines by delivering additional precision and tunability, which has the potential for increased specificity and improved long-term tolerability. Using an oligonucleotide-based approach, Korro expects to bring its medicines to patients by leveraging its proprietary OPERA platform with precedented delivery modalities, including N-acetylgalactosamine (GalNAc) conjugated for delivery for subcutaneous administration, manufacturing know-how, and established regulatory pathways of approved oligonucleotide medicines. Korro is based in Cambridge, Massachusetts. For more information, visit korrobio.com.

Korro intends to use its Investor Relations website, LinkedIn, and X (Twitter) as means of disclosing material nonpublic information and for complying with its disclosure obligations under Regulation FD. Accordingly, investors should monitor Korro's Investor Relations website and follow @KorroBio on LinkedIn, and X (Twitter), in addition to following Korro's press releases, SEC filings, public conference calls, presentations, and webcasts.