The FDA sent a Complete Response Letter to Azurity Pharmaceuticals, rejecting the Woburn, Mass.-based company’s New Drug Application (NDA) for an oral suspension formulation of lamotrigine.

Eton Pharmaceuticals licensed the investigational formulation to Azurity as part of a package involving three neurology drugs in February 2021, so Eton stands to gain up to $45 million in potential milestone payments.

In February 2020, the FDA asked Eton to conduct a validation study to show the oral suspension drug’s safety and effectiveness, because the agency was concerned about the ability of patients to use the oral suspension formulation.

The companies said they are now “assessing the FDA’s comments” in the Complete Response Letter and considering what they will need to gain approval for the indication.

Branded as GSK’s Lamictal, lamotrigine is already FDA-approved for treatment of certain forms of epilepsy and as maintenance treatment of bipolar I disorder.