At trial termination on Oct 25 , 2011 , cladribine was associated with a risk reduction versus placebo for time to conversion to clinically definite MS ( hazard ratio [ HR ] for 5.25 mg/kg = 0.38 , 95 % CI 0.25-0 .58 , p < 0.0001 ; HR for 3.5 mg/kg = 0.33 , 0.21-0 .51 , p < 0.0001 ) .