Arcturus calls its OTC deficiency data encouraging; the Phase 2 had seven evaluable patients and no placebo

Arcturus Therapeutics reported interim Phase 2 results on Wednesday for ARCT-810, its mRNA treatment for ornithine transcarbamylase (OTC) deficiency, a rare inherited liver disorder that lets toxic ammonia build up in the blood. The company described the data as encouraging, and Seeking Alpha headlined it as positive. Arcturus also introduced a new delivery technology, LUNAR 2.0, and said it will buy AI drug-design company myNEO, according to its press release.

The filing was accepted by the SEC at 4:11 p.m. ET, after the market closed. Arcturus shares had fallen 7.5% during the regular session to $13.30, before the results were public, and were little changed after hours at $13.27, according to Nasdaq data. The company is valued at about $378 million.

What the trial actually showed

The investor presentation filed with the release gives the detail the headline leaves out:

• Size and design: an open-label, dose-ascending U.S. study with no placebo group. Eight people enrolled, four at each dose. One stopped after a single dose because of an infusion-site reaction, leaving seven in the efficacy results.
• Ammonia: only 2 of the 7 had elevated ammonia at the start. Average first-morning ammonia at day 60 was 19% below baseline at the 0.3 mg/kg dose and 42% below at 0.5 mg/kg.
• Glutamine: all 7 started with elevated glutamine. Averages fell 17% at the lower dose and 25% at the higher dose.
• Safety: no serious adverse events. There were two grade 3 liver-enzyme elevations, which the company says resolved after the study drug was stopped.
• Who was in it: all seven evaluable participants were female.

What the "positive" headline misses

These are group averages from three or four people per dose, in a trial where both patients and doctors knew the treatment. That supports the company's claim of a signal, but it cannot show how well the drug works. The patient mix matters too. OTC deficiency is an X-linked disorder, and Arcturus's own release notes that it usually hits males harder, but no male patients were among those evaluated. The presentation lists relative urea function, a direct measure of the enzyme's activity, as an efficacy measure; we did not find results for it in the slides.

The bigger signal may be what Arcturus plans next. It says LUNAR 2.0 produced 38 times more OTC protein than the lipid used in ARCT-810 in monkey studies. It plans to begin dosing a new candidate built on that system, ARCT-2601, near year-end, inside the same Phase 2 study, with three to six patients, after what it called favorable FDA feedback. On our reading, a company moving this quickly to a more potent successor is treating ARCT-810 as a bridge to the next drug rather than the one it expects to sell.

For shareholders, that means the next results that matter are ARCT-2601's, and dosing only starts near year-end.

Sources: Arcturus Therapeutics Form 8-K, press release and presentation, Sept. 23, 2026; Seeking Alpha; Nasdaq. This is market information, not investment advice.