Phenytoin is a class 1B antidysrythmIc as well as an anticonvulsant that is used to treat partial seizures, tonic-clonic seizures, bipolar disorder and ventricular dysrhythmias. It has a very narrow therapeutic index despite having a very high protein binding affinity which makes dosing difficult in patients. Phenytoin mainly inhibits sodium channels protein type 5 subunit alpha but also inhibits the potassium voltage gated channel subfamily H member 2 and L type calcium channels. The main antidysrythmIc effect is mediated through the sodium channel blockage though. Phenytoin slows the rate of rise in the pacemaker potential and shortens the plateau phase of atrial and ventricular myocytes as well as purkinje fibre cells as they have 'fast' action potential. This converts a one way block into a two block effectively stopping the circus rhythm irregularity. In seizures, blockages of sodium and potassium channels to prolong the inactivation of the cells making repetitive firing more difficult. It can be used to help treat partial and tonic-clonic seizures. It also inhibits calcium influx which important in the treatment of partial and tonic-clonic seizures when they are secondary to other types of seizures. Phenytoin works through use-dependent blockage meaning that it preferentially binds to the inactivate state of the sodium channel. The more active the channel the more chances phenytoin can bind to the channel and block it. Phenytoin can be administered through either oral or intravenous routes with the oral route having varying half-life of anywhere from 7 to 42 hours while intravenous is from 10-15 hours.

Phenytoin Mechanism of Action References