Dianthus Therapeutics’ claseprubart has met its primary and secondary endpoints during a Phase II trial in generalised myasthenia gravis (gMG).
During the MaGic study (NCT06282159), the active complement component 1s (C1s) inhibitor offered significant improvements to Myasthenia Gravis Activities of Daily Living (MG-ADL) scores, with the 300mg and 600mg doses triggering a 1.8 point and 2.6 point placebo-adjusted reduction, respectively. MG-ADL is a key marker of gMG disease severity used in clinical trials.
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Meanwhile, biweekly administration of the drug exhibited statistically significant improvements to Quantitative Myasthenia Gravis (QMG) scores, exhibiting placebo-adjusted scores by -2.4 and -2.5, respectively, for the low and high dose.
Claseprubart was also proven safe and tolerable, with no patients discontinuing treatment before the data cutoff and no reported meningococcal bacterial infections associated with the monoclonal antibody (mAb).
The US biotech will now liaise with the US Food and Drug Administration (FDA) to design claseprubart’s Phase III trial, which will likely begin in 2026.
Combatting safety concerns
While no autoimmune reactions were observed after treatment with claseprubart, there has been growing investor concern around the capacity for complement inhibitors to increase the risk of developing comorbid autoimmune conditions like systemic lupus erythematosus (SLE).
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By GlobalDataInvestor apprehension was further driven by the prevalence of patients in the MaGic study who were anti-nuclear antibody (ANA)-positive, a biomarker associated with an increased risk of developing SLE.
This concern seemed to be a significant roadblock for Dianthus’ stock growth, as the biotech’s value dropped by 5% upon the result debut from $26.50 at market close on 5 September 2025 to $25.03 at market open on 8 September.
However, analysts at William Blair believe this concern is “overblown”, with biotech equity research specialist Myles R Minter noting: “While we acknowledge that ANA positivity observed in the MaGic study add data points supportive of the theoretical risk for developing SLE, the risk of developing the condition with C1s or classic complement pathway inhibition remains theoretical and based on reported associations”.
By market close on 8 September, investors seemed to have a change of heart, with Dianthus’ stock value shooting up 27% to $31.80.
Fending off gMG competition
According to Minter, the totality of the efficacy data from the low-dose cohort is “competitive with other complement inhibitors approved in gMG, but with clear dosing advantages and potentially superior safety”.
However, if the drug were to make it to market, it would still have to fend off tough competition from other therapies on the market, with six targeted drugs now being available to US patients for this indication.
This includes Johnson & Johnson’s neonatal Fc receptor (FcRn) blocker Imaavy (nipocalimab), which was granted approval by the FDA in May 2025.
Meanwhile, AstraZeneca’s gefurulimab and Regeneron’s cemdisiran are racing to market after both drugs met their primary endpoints in recent Phase III trials, potentially further bolstering gMG competition for Dianthus.
Dianthus is looking to enter a market that was worth $6.1bn in 2024, according to analysts at Clinical Trials Arena’s parent company, GlobalData. The indication’s value is also growing rapidly, with analysts forecasting that it will reach $10.5bn in 2034.
