ADHD is a chronic condition characterized by hyperactivity, impulsive behavior and a difficulty in sustaining focus.

There are currently two types of approved medications used to manage symptoms: stimulants and non-stimulants.

NLS' drug, mazindol, is a non-stimulant, but trial data showed it could be as effective as a stimulant.

Stimulants are typically more effective than other drugs and are believed to elevate levels of dopamine — a neurotransmitter associated with motivation and attention. But they carry a risk of abuse, may be poorly tolerated by patients, and even be fatal in rare cases.

Shire Plc's Adderall and Vyvanse are examples of stimulants that belong to a class of drugs called amphetamines — the same class that includes crystal meth and ecstasy. Other stimulants include Concerta and Ritalin.

Non-stimulants, including Eli Lilly's Strattera, work slower than stimulants, but have a cleaner side-effect profile.

The "holy grail" of ADHD treatments is to develop a non-stimulant medicine that works as well as a stimulant, Gregory Mattingly, an NLS study investigator said.

Mazindol does not have the same magnitude of effect on dopamine as stimulants do. As a result, it is categorized as a less-restrictive, Schedule IV drug under U.S. regulations, Mattingly said.

Standard stimulants are considered Schedule II drugs, which invite more scrutiny due to their potential for abuse. Patient access can be inconvenient as these drugs require monthly refills and written prescriptions.

In NLS' 85-patient trial, more than half the patients who received mazindol experienced an over 50 percent reduction in ADHD symptoms, compared with 15.8 percent of patients on a placebo.

Many studies, especially those testing non-stimulant ADHD drugs, have used a 25-30 percent threshold for reduction in symptoms, Mattingly said.

Mazindol was previously approved for the short-term treatment of obesity, but has since been withdrawn for reasons unrelated to safety or efficacy.

Up to 11 percent of American children have ADHD, and more than half continue to suffer from it as adults, according to recent figures.

Two-thirds of children diagnosed are receiving treatment, but up to 80 percent of adults are not, making that market ripe for growth, Mattingly said.

Other ADHD drug developers include Neos Therapeutics Inc, Alcobra Ltd and Aevi Genomic Medicine Inc.

(Reporting by Natalie Grover in Bengaluru; Editing by Sai Sachin Ravikumar)