The relationship between drug concentrations and RTG was described by a sigmoidal relationship with RTGmin ( minimum value of RTG ) of 37.5 ng/mL ( 95 % confidence interval ( CI ) : 34.3 , 40.8 ) and half-maximal effective concentration ( EC50 ) of 21 ng/mL ( 95 % CI : 18.3 , 23.8 ) .