The speed at which the various processes within ADME occur are described by a series of rate constants. The usual assumption is that the rate of ADME processes can be described as ‘First order’. This means that there is a simple proportionality between the rate at which the process goes ahead and the mass of drug waiting to undergo that process. Putting this into a more formal equation:
Rate of process = Mass of drug available x Constant
Based on this relationship, if there is twice as much drug available, the process will occur twice as fast.
It is generally assumed that pharmacokinetic processes take place at a rate that is directly proportional to the mass of drug available to undergo that process. i.e. They are considered first order.
Where the specific process in question is drug absorption, the relevant constant is the Absorption rate constant (Ka) and the relationship is:
Rate of absorption = Mass of drug available for absorption x Ka
If a drug is being given orally, then it follows that the rate at which drug is absorbed will depend on the mass of drug in the gut available for absorption and upon Ka.
The value of Ka is not simply a fixed value for each drug entity; it depends upon the dosage form. Theophylline contained in a simple tablet will be absorbed more rapidly than the same drug in a slow release formulation.
Ka is a parameter that would be of interest in industrial pharmacokinetic studies undertaken as part of drug development; it is unlikely that the value of Ka would ever be used in the practical calculation of a dosage regimen within clinical practice.