Annexon shares dip 30% as patient dropouts dog midphase Huntington's trial

Annexon found out the hard way that investors do read the fine print. Despite the small biotech reporting “promising” interim results from a midphase Huntington’s disease trial, the company saw its shares plummet 30%.

Although the company found that ANX005 engaged its target and half of patients experienced some clinical improvement in disease progression, five patients discontinued treatment in the 28-subject study. This is despite Annexon’s claim that “ANX005 has been generally well-tolerated in the study.”

That little tidbit in the after-market Tuesday release sent the small biotech’s shares tumbling 30% to $7.68 around noon Wednesday.

Three of the patients who stopped treatment did so because of a drug-related adverse event. Annexon reported that one patient had a case of systemic lupus. Another experienced idiopathic pneumonitis, a type of noninfectious pneumonia. No serious cases of infection or death were reported, Annexon said.

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Of the remaining two who discontinued treatment, one was due to a case of COVID and the other withdrew consent.

Executives told investors on a call that they had been monitoring for lupus, while the idiopathic pneumonitis was not as easy to link to the study treatment. But in both cases, the adverse events resolved after treatment was stopped.

The main goal of the trial was to assess treatment-emergent adverse events. Annexon reported that all patients experienced some sort of treatment-emergent adverse event, with the majority reporting reactions on the first day after infusion. Skin rashes were the most commonly reported event. These results are consistent with a previous trial of ANX005 conducted in patients with Guillain Barré syndrome, or GBS.

As for efficacy, Annexon found that the evaluable patients at Week 24 maintained clinical function according to a measure of Huntington’s disease progression.

Improvement was seen in more than half of the patients, with the rate climbing to 75% of those who had excess complement activity when the trial started. This measure has been associated with disease progression, and Annexon said the findings suggest ANX005 could work best in patients with this biomarker. That information could inform future trial design.

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Annexon will continue on with the phase 2 trial, and full data are expected in the second quarter. Once those data are available, Annexon will consider a phase 3 study.

ANX005 is also being studied in amyotrophic lateral sclerosis, GBS and warm autoimmune hemolytic anemia.