Single sample estimate of ethosuximide clearance.

Ethosuximide was administered in single oral doses (ca. 15 mg/kg) to each of 12 healthy, male subjects. Serial blood and salivary ethosuximide concentrations were measured by a polarization immunofluorescence technique. The following parameters were estimated: CL/F, V/F, and plasma disappearance t1/2. Mean (+/- SD) values were 0.0091 (+/- 0.0023) l X h-1 X kg-1, 0.063 (+/- 0.12) l X kg-1, and 48.8 (+/- 9.3) h, respectively. Estimates of CL/F (CL/F) were calculated from single plasma or single salivary measurements. When V/F was set at 0.63 l X kg-1 and the 120-h plasma concentration was used, CL/F was 0.0091 (+/- .0018) l X h-1 X kg-1. CL/F was 0.0089 (+/- .0020) when the 120-h salivary concentration was used. These data demonstrate that under specified conditions ethosuximide clearance (CL/F) can be estimated from a single measurement of ethosuximide in either plasma or saliva, thereby permitting ethosuximide to be used safely and noninvasively as a probe of hepatic mixed function oxidase activity in humans.