Omeros saw its shares drop more than 12% in the immediate aftermath of announcing that it has suspended a mid-stage trial of experimental Huntington’s disease drug OMS824.

The US group said its four-week human trial was on hold while evidence from a parallel animal study is investigated, after it emerged that several rats receiving the maximum dose exhibited blood concentrations of the drug much higher than those measured in patients.   

So US regulators have asked that Omeros further evaluate data from the 13-week nonclinical rat study as well as others that did not yield the observation to better understand the observation, before potentially reinitiating the clinical trial, sparking fears over the safety of the drug.

The firm was quick to stress, however, that non-human primates exposed to plasma concentrations equal to those in the rat demonstrated no findings similar to the observation reported from the rat study, and that OMS824 has been well tolerated in all human clinical trials.  

OMS824, a phosphodiesterase 10 (PDE10) inhibitor, has been awarded Orphan Drug designation and a Fast Track review in the US for Huntington’s disease, and is also being developed by the firm for schizophrenia.