🧾 What This Document Is 📑

This document is a Preliminary Prospectus Supplement, filed on April 22, 2026. Essentially, it is a detailed legal roadmap informing investors about the company and the specific securities Altimmune, Inc. plans to sell. Since it is a "preliminary" document, it means the information is not final and could change before the actual sale takes place.

👉 What to Expect: This document focuses heavily on the terms of the upcoming offering—selling shares, warrants, and pre-funded warrants—and uses this opportunity to provide an extensive update on the company's lead drug candidate, pemvidutide, across multiple liver diseases.

🏢 What Altimmune Does 🧬

Altimmune, Inc. is a late-stage biopharmaceutical company based in Gaithersburg, Maryland. In simple terms, the company is developing novel therapies for serious liver diseases. Their lead product candidate is a drug called pemvidutide.

• Mechanism: Pemvidutide is a balanced 1:1 glucagon/GLP-1 dual receptor agonist. (GLP-1 and glucagon are hormones that naturally help regulate blood sugar and metabolism, suggesting this drug targets metabolic issues in the liver).
• Focus Indications: Altimmune initially developed this drug for MASH (Metabolic Dysfunction-Associated Steatohepatitis), but they are actively expanding its use into treating Alcohol Use Disorder (AUD) and Alcohol-Associated Liver Disease (ALD).
• Corporate History: Altimmune was incorporated in Delaware in April 2005 and changed its name to Altimmune, Inc. in May 2017 following a merger with PharmAthene, Inc.

🧪 MASH Clinical Results (The Biggest Win) 🏆

The most significant part of this filing is the extensive clinical data demonstrating the effectiveness of pemvidutide in treating MASH. These results, from the IMPACT Phase 2b trial, suggest strong evidence of liver healing and anti-fibrotic activity.

• MASH Resolution (24 Weeks): In the IMPACT Phase 2b trial (enrolling 212 subjects), treatment with pemvidutide showed significant MASH resolution at 24 weeks. The proportion of subjects achieving resolution was 58.2% for the 1.2 mg dose and 52.1% for the 1.8 mg dose, compared to 19.9% for the placebo group (a statistically significant difference, p< 0.0001).
• Anti-Fibrotic Activity: The drug also demonstrated improvement in reducing fibrosis (scarring) without worsening MASH. For instance, the 1.8 mg dose showed an anti-fibrotic effect of 35.7%, versus 27.9% for placebo.
• Advanced Readouts (48 Weeks): In a separate analysis of topline 48-week data (which did not include histological readouts), the drug maintained positive trends. Key anti-fibrosis markers, including Enhanced Liver Fibrosis (ELF) score and Liver Stiffness Measurement (LSM), showed continued, statistically significant improvements at 48 weeks compared to placebo.
• Tolerability: The drug maintained a favorable safety profile across time points, with discontinuation rates due to adverse events being low (less than 1% to 1.2% for the active doses vs. 2.4% for placebo).

📈 Expansion into Alcohol-Related Liver Disease (AUD & ALD) 🍺

Altimmune is aggressively pursuing pemvidutide for two other indications associated with heavy drinking: AUD and ALD. The company has successfully secured key regulatory designations, accelerating development.

• For Alcohol Use Disorder (AUD):
  • The FDA granted Fast Track designation for pemvidutide for AUD on August 19, 2025. This designation speeds up the review process for drugs tackling serious, unmet medical needs.
  • The RECLAIM Phase 2 trial, evaluating efficacy in AUD, enrolled the first subject on May 19, 2025. It is randomized, placebo-controlled, and has a primary endpoint measuring changes in alcohol consumption at 24 weeks.
  • Enrollment for RECLAIM was completed ahead of schedule as of November 3, 2025.
• For Alcohol-Associated Liver Disease (ALD):
  • The RESTORE Phase 2 trial, evaluating ALD, began enrolling its first patient on July 9, 2025. This 48-week randomized trial uses the same drug, investigating its efficacy and safety.
  • The primary endpoint for ALD is the change from baseline in LSM by VCTE (Vibration-Controlled Transient Elastography) at Week 24.

👑 Regulatory Milestones and Designations 🛡️

Securing government recognition is crucial in the biopharma world. The filing highlights several major regulatory victories that enhance confidence in the drug candidate.

• MASH Designation: On December 19, 2025, the U.S. Food and Drug Administration (FDA) granted Breakthrough Therapy Designation for pemvidutide in MASH. This was awarded based on the 24-week data from the IMPACT Phase 2b trial, which showed MASH resolution, substantial improvements in liver fat, and anti-fibrotic activity.

💲 Mechanics of the Securities Offering 💼

This section details exactly what Altimmune is selling to raise capital. The company is offering three related securities: common stock, common stock warrants, and pre-funded warrants.

• The Offering Structure: Altimmune is offering shares of common stock, common stock warrants, and pre-funded warrants. These are sold together but are immediately separable (meaning they can be bought or sold separately later).
• Warrants: The common stock warrants are key because they grant the holder the right to buy shares later. They are exercisable until the date that is the earlier of:
  1. The fifth anniversary of the original issue date.
  2. 45 days following a public announcement of a successful data readout of the Phase 3 trial for pemvidutide in MASH.
• Pre-Funded Warrants: These are sold as an alternative to common stock for certain investors. While they appear similar, they are structured to purchase shares of common stock and do not expire, though they are subject to an ownership limitation.
• Market Data: As of April 21, 2026, the last reported sale price of Altimmune’s common stock on Nasdaq was $3.41 per share.

⚙️ Capital Structure Changes 🏛️

To facilitate this large offering, the company had to adjust its foundational corporate documents.

• Charter Amendment: On April 16, 2026, Altimmune filed an amendment to increase the total number of authorized shares of common stock from 200,000,000 to 400,000,000.
• Financial Reporting: The company's consolidated financial statements for the year ended December 31, 2024, were audited by Ernst & Young LLP (EY).

🎯 How the Money Will Be Used 💸

The filing explains the general purpose of the capital raised from this sale. Altimmune intends to use the net proceeds for broad, strategic purposes.

• General Corporate Purposes: Funds are earmarked for general corporate purposes, including:
  • Research and clinical development of current or additional pipeline candidates.
  • Funding working capital and capital expenditures.
• Management Discretion: Management is retaining broad discretion over the allocation of the net proceeds, meaning they haven't determined the specific amount to be used for any single purpose.

⚠️ Key Risks and Disclaimers 🛑

As with all securities offerings, the document contains significant risk warnings and legal disclaimers. It is critical to review these sections fully.

• High Degree of Risk: Investing in Altimmune’s securities involves a high degree of risk.
• Forward-Looking Statements: Any statements about future events (like successful trials or future earnings) are forward-looking statements and are subject to change.
• Reliance on Documents: Investors are warned to rely only on information contained within the prospectus supplement and the accompanying prospectuses, as they may include future filings that modify or supersede current statements.

📞 Contact and Filing Information 🗺️

This section details where investors and analysts can find more information about the company and the filing itself.

• Company Website: www.altimmune.com
• Investor Relations: Information is available under the "Investors" tab on the company website.
• Principal Executive Office: 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878.
• Main Phone Number: (240) 654-1450.
• Contact for Filings: The Chief Financial Officer is listed as the contact for requests regarding the SEC filings.

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🧠 The Analogy 🔬

Think of Altimmune like a scientist with a miraculous, potential vaccine (pemvidutide). The company has done great groundwork, proving the vaccine works in several tests (MASH, AUD, ALD). But to turn that vaccine into a blockbuster drug that can be manufactured and sold globally, they need billions of dollars. This preliminary prospectus supplement is the fundraising brochure and legal agreement: it's not just showing the results; it's asking for money, explaining the mechanics of the sale (stock, warrants, pre-funded warrants), and managing expectations about the risks and the massive effort required to get the drug to market.

🧩 Final Takeaway 💡

Altimmune is making a major capital raise to fund its pipeline, with the primary goal being advancing the drug pemvidutide. The company's most compelling signal is the positive, statistically significant data showing the drug's efficacy in MASH, coupled with its success in getting Fast Track and Breakthrough Therapy designations for multiple liver conditions.