In patients who completed the phase III studies on 8mg and had an actual ( last ) dose of 12mg during weeks 1-13 of the extension Maintenance Period ( n = 181 ) , median percent change in seizure frequency per 28days improved from -34.1 % ( phase III Maintenance Period ) to -46.0 % ( weeks 1-13 extension Maintenance Period ) ; 50 % responder rates were 39.2 % and 46.4 % .