Arcturus ARCT-810: Safe Phase 2 OTC Deficiency Data

Arcturus Therapeutics (ARCT) reported interim Phase 2 results for ARCT-810, its mRNA therapy for ornithine transcarbamylase (OTC) deficiency, on September 23, showing the drug was generally safe and improved key urea-cycle biomarkers. The company simultaneously unveiled LUNAR 2.0, a next-generation delivery platform, and a no-terms deal to acquire AI drug-discovery firm myNEO.

OTC deficiency is the most common urea-cycle disorder, an inherited condition in which the liver cannot convert toxic ammonia to urea. Roughly 10,000 people in the US and Europe have it, and apart from a liver transplant there is no cure. High ammonia causes seizures, cognitive decline, and can be fatal. ARCT-810 is an intravenous mRNA therapeutic that delivers the instructions for a working OTC enzyme to liver cells, addressing the missing enzyme directly rather than just mopping up ammonia. The drug carries FDA Orphan Drug, Fast Track, and Rare Pediatric Disease designations, the last of which could yield a transferable priority review voucher, which biotechs have sold for more than $100 million.

In the ongoing study, ARCT-810 reduced or held first-morning fasting ammonia within the normal range, including during increased protein intake, and lowered glutamine, with patients on the 0.5 mg/kg dose reaching mean glutamine in the normal range. Weight gain was observed across participants, a meaningful signal in a disease where children often fail to thrive.

The platform news is the bigger swing factor. LUNAR 2.0 showed a 40-fold improvement over the prior LUNAR 1.0 in expressing human erythropoietin in primates, and 38-fold more potency than the lipid used in ARCT-810 for the OTC enzyme. Arcturus plans to fold ARCT-2601, an OTC candidate built on LUNAR 2.0, into the current Phase 2 study near the end of 2026 after an FDA Type C meeting gave the path clarity. The company flagged phenylketonuria and gout as new targets the platform can reach.

The myNEO acquisition, expected to close in October with terms undisclosed, adds an AI engine Arcturus has worked with since 2024 to improve mRNA design. Arcturus shares closed at $13.85. The company has spent years defined by its COVID and flu vaccine work, and this readout is an attempt to recast it as a rare-disease mRNA play, a pivot complicated by its CSL termination of the Kostaive vaccine partnership.

Two risks sit behind the headline. The ammonia control shown so far is an interim read, and durability is unproven. Whether it holds over months of protein challenge and illness, and whether better biomarkers translate into fewer ammonia crises, will decide if ARCT-810 is a real answer or a biomarker headline. The platform gains are primate data, not human data, and ARCT-2601 has yet to clear the jump into patients.

What to watch next: the ARCT-2601 integration into the Phase 2 trial, which determines whether the LUNAR 2.0 improvement translates from primates to patients, and whether the myNEO AI tools sharpen the next generation of candidates.

Source: Arcturus ARCT-810 interim Phase 2 results and LUNAR 2.0 press release

What It Means for the Trade

Arcturus at $13.85 is a platform-credibility trade, and this readout is the company’s attempt to reset its multiple from COVID-vaccine also-ran to rare-disease mRNA play. The OTC deficiency opportunity itself is modest — roughly 10,000 patients across the US and Europe — but rare-disease economics run on pricing power and chronic dosing. Near-term catalysts are procedural: the ARCT-2601 fold-in near the end of 2026 and the myNEO close in October.

The platform news is what changes the multiple if it holds. LUNAR 2.0’s 40-fold expression improvement over LUNAR 1.0 in primates, and 38-fold more potency than the lipid currently used in ARCT-810, implies meaningfully lower doses and better margins if it translates to humans — and it widens the target list to phenylketonuria and gout, the latter a large-market disease. The CSL Kostaive termination (covered previously) already demonstrated how quickly partnership confidence can evaporate.

Mechanism and Pipeline Context

OTC deficiency is the most common urea-cycle disorder: a missing liver enzyme lets toxic ammonia accumulate, causing seizures, cognitive decline, and death, with liver transplant as the only cure. ARCT-810 is an intravenous mRNA therapeutic — lipid-delivered instructions that make liver cells produce working OTC enzyme, replacing the missing function at its source. The interim Phase 2 biomarkers point the right way: first-morning fasting ammonia held in the normal range even under increased protein intake, glutamine falling into the normal range at the 0.5 mg/kg dose, and weight gain across participants — meaningful in a disease where children often fail to thrive.

LUNAR is Arcturus’s lipid nanoparticle delivery platform, and 2.0 is its generational upgrade. The myNEO acquisition adds an AI design engine for mRNA sequence optimization — the company has worked with the firm since 2024 — which is the kind of tooling that compounds quietly across every future candidate.

Risks and What Could Go Wrong

1. Biomarkers do not become outcomes — ammonia control in an interim read is not fewer hyperammonemic crises; the label fight will be over clinical events, and durability over months of protein challenge and illness is unproven.
2. The primate-to-human gap — LUNAR 2.0’s 40-fold claim is animal data; ARCT-2601 has yet to clear the jump into patients.
3. Integration and distraction — an mRNA company acquiring an AI firm mid-pivot, with no disclosed terms, invites execution questions.
4. The pivot’s shadow — CSL’s Kostaive termination means the vaccine partnership revenue story is gone; if the rare-disease data stalls, there is no fallback narrative left.