In January 2025, CSL Seqirus’ partner Meiji announced that it received approval for a partial amendment to the manufacturing and marketing approval of KOSTAIVE to include manufacturing sites in Japan. With this approval, Meiji and ARCALIS, Inc., Arcturus’ manufacturing joint venture in Japan, have been added as manufacturing sites. As a result, KOSTAIVE, with active pharmaceutical ingredients manufactured at such sites, may be shipped for commercial use in Japan.
Approval of KOSTAIVE (ARCT-154) in Europe
In February 2025, the European Commission granted marketing authorization for KOSTAIVE (ARCT-154) for individuals 18 years of age and older. The European Commission approval follows a positive opinion adopted by the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMA) on December 12, 2024. The centralized marketing authorization of KOSTAIVE provided by the EC is valid in all 27 European Union (EU) member states and 3 additional European Economic Area (EEA) countries summarized here: Austria, Belgium, Bulgaria, Croatia, Republic of Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, The Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain and Sweden. The approval of KOSTAIVE in Europe, as well as the approval in Japan, are significant milestones, which further validate our LUNAR and STARR platforms, as well as sa-mRNA more generally as a meaningful modality.
Clinical Studies of KOSTAIVE (COVID-19 vaccine)
In connection with our collaboration with CSL Seqirus, we continue to collect data from various ongoing studies. See the 2024 Annual Report for a description of studies.
Recent KOSTAIVE Publications
In April 2025, we published a comprehensive analysis of safety data for KOSTAIVE®, with a 12-month follow-up from the pivotal clinical study in Vietnam (NCT05012943), which had 17,582 participants who received at least one dose of the study vaccine (https://doi.org/10.1080/14760584.2025.2487542). The study confirmed the favorable reactogenicity profile. Acceptable tolerability of ARCT-154 was also observed in older participants and in those liable to severe consequences of COVID-19 due to underlying medical conditions. ARCT-154 is the version of KOSTAIVE encoding the ancestral strain of the COVID-19 virus. Long-term follow-up has not revealed any safety concerns, with no reports of myocarditis or pericarditis. No serious consequences occurred in several pregnancies reported after vaccination. Long-term data from this large trial suggest that the sa-mRNA COVID-19 vaccine (ARCT-154; KOSTAIVE) is safe and well-tolerated.
In April 2025, our Japanese partner, Meiji, published an analysis characterizing the distribution and clearance of ARCT-154 encoded spike protein and non-structural proteins nsP1, nsP2, nsP3 and nsP4 in the lymph nodes and injection-site muscle in mice following a single vaccination. The study showed the encoded spike protein reached its highest level approximately three days after vaccination and quickly disappeared from the injection site muscle. The spike protein levels also peaked at an early time point in the lymph nodes, it remained detectable 28 days after the vaccination and disappeared by 44 days after the vaccination. Expression of nsP1, nsP2 and nsP4 was observed in the injected muscle and/or the lymph nodes for up to 15 days post-vaccination. The data indicates that the extended expression of spike proteins in lymph nodes may be responsible for the induction of higher and prolonged levels of neutralizing antibodies. The study also confirmed that the self-replication is limited over time.
Seasonal Influenza Program Updates
A Phase 1 dose-finding safety and immunogenicity study was initiated in January 2024 in Australia and completed recruitment of 132 young and older participants. The immunological testing and assessment of the study results are ongoing.
Pandemic Avian Influenza Program (H5N1 Influenza) Updates
Our LUNAR-H5N1 program, which is part of the CSL Collaboration Agreement, continues to progress under the award from BARDA that we obtained in 2022 to advance through Phase 1 a vaccine to protect against disease caused by H5N1 highly-pathogenic avian influenza.
The Phase 1 study of ARCT-2304, an sa-mRNA vaccine candidate, also known as LUNAR-H5N1, initiated dosing in December 2024 and continues to advance. We recently completed the recruitment of 212 adults (132 participants 18-59 years old; 80 participants 60+ years old) for the randomized placebo-controlled Phase 1 trial (NCT06602531), which is being conducted at multiple sites in the U.S. The primary objective of this initial clinical trial is to evaluate the safety and immune responses of three different dose levels and two different vaccination schedules of the ARCT-2304 vaccine. Immune responses are measured by hemagglutination inhibition (HAI), virus microneutralization (MN) and neuraminidase enzyme-linked lectin assays (ELLA). The recruitment in this study was completed in April 2025.
In April 2025, the U.S. Food and Drug Administration (“FDA”) granted Fast Track Designation for ARCT-2304. This designation recognizes the potential of ARCT-2304 as an innovative approach to address unmet medical needs for the prevention of disease caused by pandemic influenza A virus H5N1, a significant global health risk. Fast Track Designation from the FDA is granted to vaccines intended to prevent serious conditions caused by infectious diseases. The designation is designed to expedite the