Compounds with poor absorption can be categorized into three main categories.
In cases of dissolution rate-limited absorption, the permeation rate is much higher than the dissolution rate. The amount of absorbed drug is dose proportional and decreases in particle size will improve oral absorption. This is typical of a BCS Class II compound (see Sect. 3.5 for definition of BCS Classes).
In cases of permeability rate-limited absorption, the dissolution rate is much higher than the permeation rate. The amount of absorbed drug is dose proportional. Any changes in particle size will not influence the amount of drug absorbed. This is typical of a BCS Class III compound.
In cases of solubility rate-limited absorption, solubility of the drug in the intestinal fluid is rate-limiting to the oral absorption process. The amount of drug absorbed is not dose proportional and any changes in particle size will not influence the amount of drug absorbed. BCS class II or IV compounds are likely to exhibit solubility rate-limited absorption.
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