Phenytoin is the classic Michaelis-Menten drug and various dosage schemes have been devised based on this concept. This is also referred to as Michaelis-Menten kinetics. Ed Barnes TRI 3202 Tower Oaks Blvd. The clinical pharmacokinetics of ethanol. A combination of a mixed order and first order elimination pathway is required to describe ethanol PK over a wide range of concentrations and this combined model should probably always be considered for any drug which appears to have capacity limited kinetics.
Clearance approaches zero.
At least in teaching PK to undergraduates I have found this approach an effective way of structuring those concepts. The first order approximation on the other hand can work very well, and has obviously done so for countless drugs, because it is not uncommon for the highest concentrations to be well under the Km. The finding of organ clearance being apparently independent of blood flow is therefore not unusual. I can attest that zero order elimination does occur clinically.