Objectives of the study are to evaluate the safety, tolerability, and efficacy of two different doses of AVP-923 (capsules containing either 30 mg of dextromethorphan hydrobromide and 10 mg of quinidine sulfate [AVP-923-30] or 20 mg of dextromethorphan hydrobromide and 10 mg of quinidine sulfate [AVP-923-20]) when compared to placebo, for the treatment of PBA in a population of patients with amyotrophic lateral sclerosis (ALS) or multiple sclerosis (MS) over a 12-week period. An additional objective is to determine the pharmacokinetic parameters of the two different doses of AVP-923 in a subset of the study population.

Pseudobulbar Affect (PBA) is a condition characterized by involuntary, sudden and frequent episodes of laughing and/or crying out of proportion or incongruous to the underlying emotion of happiness or sadness Other terms used to describe this condition include emotional lability, emotionalism, emotional incontinence, emotional discontrol, excessive emotionalism, and pathological laughing and crying. The outbursts can occur spontaneously or in response to provocative stimuli such as questions or events.

A body of evidence suggests that PBA can be modulated through pharmacologic intervention.

Dextromethorphan (DM) is a low-affinity uncompetitive antagonist of the NMDA receptor, reducing the level of excitatory activity. DM also acts at the phencyclidine-binding site, which is part of the NMDA receptor complex. DM is a sigma receptor agonist, suppressing the release of excitatory neurotransmitters.

Quinidine is a known potent inhibitor of cytochrome CYP2D6, that decreases the metabolism of dextromethorphan and helps to achieve sustained and therapeutic levels of this drug.