Protein binding and CSF penetration of phenytoin following acute oral dosing in man

Prophylactic phenytoin (DPH) has been evaluated in 20 patients undergoing diagnostic myelography. DPH (0.75 g) was ingested at 20.00 h the night before and 0.5 g at 08.00 h on the morning of the procedure. Total DPH concentrations at myelography (mean +/- s.d.: 12.7 +/- 4.3 mg l-1; range 6.3-21.5 mg l-1) correlated with CSF values (1.3 +/- 0.46 mg l-1; range 0.7-2.2 mg l-1; r = 0.83, P less than 0.001). DPH protein binding at that time varied two-fold (9.2-18.5%) and free drug levels (1.7 +/- 0.6 mg l-1) correlated with CSF (r = 0.83, P less than 0.001) and total (r = 0.89, P less than 0.001) plasma DPH concentrations. There were significant negative correlations between patient weight (n = 17) and total (r = 0.57, P less than 0.05) and CSF (r = -0.55, P less than 0.05) DPH concentrations at myelography. Total plasma DPH levels 8 h (14.5 +/- 3.9 mg l-1; range 7.3-20.6 mg l-1) and 24 h (12.3 +/- 3.8 mg l-1; range 5.0-19.8 mg l-1) after myelography were largely within the 'therapeutic range' of 10-20 mg l-1 for the drug. No patient suffered a seizure although, in two, spike discharges were seen on a post-myelography electroencephalogram. A simple regime involving two doses of DPH would provide acceptable plasma CSF concentrations as a basis for controlled studies in seizure prophylaxis following neuroradiological investigations involving intrathecal contrast.