ARCT Ends $4.3B CSL Vaccine Deal, Shifts to Cystic Fibrosis

Arcturus Therapeutics (ARCT) terminated its $4.3 billion self-amplifying mRNA vaccine collaboration with CSL Seqirus on August 6, disclosed alongside second-quarter earnings. The settlement returns global rights to KOSTAIVE, the world’s first approved sa-mRNA COVID-19 vaccine, to Arcturus, along with seasonal flu, pandemic flu, RSV, and EBV vaccine programs. CSL will pay $12 million in cash and release Arcturus from roughly $16 million in R&D credit liabilities, an effective $28 million settlement on a deal that was signed in November 2022 with $200 million upfront and up to $4.3 billion in milestones.

Arcturus stock closed at $7.41 on August 7, giving the company a market capitalization of roughly $160 million. The stock has traded in a narrow range since the announcement, reflecting the market’s view that the original deal’s milestone structure had been largely written off as COVID vaccine demand collapsed.

Revenue for the second quarter fell to $3.0 million from $28.3 million a year earlier, driven by the reduction in collaboration revenue as the CSL partnership unwound. Cash and cash equivalents stood at $191.5 million as of June 30. Management guided to a cash runway beyond year-end 2028, more than two and a half years at current spend.

The investment thesis has shifted entirely from vaccines to rare disease. ARCT-032, an inhaled mRNA therapy for cystic fibrosis, is now the lead asset. The drug uses the same LUNAR lipid nanoparticle delivery platform that KOSTAIVE validated. An open-label Phase 2 study evaluating 10 mg daily dosing over 12 weeks is enrolling patients with Class I CF mutations, the population that produces no CFTR protein and cannot benefit from Vertex’s modulator drugs. Enrollment is active in the US, Israel, and Turkey, and the decision to advance to Phase 3 is expected in the fourth quarter of 2026.

Arcturus secured a manufacturing and clinical services deal with Thermo Fisher in July 2026 that provides up to $40 million in clinical manufacturing and $40 million in CRO support for a potential Phase 3 program, with exclusive commercial manufacturing rights if the drug reaches market.

KOSTAIVE remains an approved product in Japan (since November 2023), the European Union (February 2025), and the United Kingdom (January 2026). Meiji Seika Pharma retains commercialization rights in Japan for the 2026-2027 season under a pre-existing arrangement that survives the CSL termination. Arcturus now faces the choice of finding a new commercialization partner or licensing the vaccine portfolio to a third party. The platform technology also underpins an H5N1 pandemic flu program that received FDA Fast Track designation in April 2025.

The near-term risk is ARCT-032 Phase 2 failure. Inhaled mRNA for CF has no efficacy data in humans. The Phase 2 is open-label with no placebo control. A negative go/no-go decision in Q4 2026 eliminates the rare disease thesis and leaves Arcturus with $191.5 million in cash and a vaccine platform without a partner. A green light means a Phase 3 program the company cannot fully fund without dilution or a new partner.

The broader mRNA platform story has been covered in this mRNA platform analysis. The Q2 2026 results confirmed the CSL termination terms and provided updated pipeline timelines. The lesson from the Arcturus-CSL unwinding is that platform validation through regulatory approval does not guarantee commercial viability when the underlying market has shrunk. For investors, the near-term catalyst is the ARCT-032 Phase 2 data and Phase 3 go/no-go decision in Q4 2026.