It is generally accepted that for a drug to be absorbed from the human gastrointestinal (GI) tract, it should be in solution form which is established based on solubility/dissolution characteristics of the drug. This in vivo dissolution is determined using in vitro drug dissolution tests.
It is also generally accepted that higher the solubility of the drug, the higher the dissolution and absorption, and their corresponding rates, will be. In addition, it is also a well established fact that absorption preferentially occurs from the non-polar or undissociated form of a drug. On the other hand, the undissociated, or non-polar moiety, of a drug often shows lesser aqueous solubility compared to its polar version.
For example, propranolol is a drug which is basic in nature with a pKa value of 9.42 and its aqueous solubility is of 61.7 mg/L or 1 part in ~16,000 (link). Therefore, propranolol should be considered to be a low solubility drug. However, its products are usually manufactured using the drug in its hydrochloride salt form i.e. propranolol·HCl which is freely or highly soluble in water. In water it would exist in its ionic/protonated form, which would be less absorbable than the native propranolol. On the other hand, propranolol is known to be highly absorbable/permeable (bioavailability higher than 90%), which suggests that in reality the body sees propranolol as non-polar/undissociated moiety. Therefore, for in vivo dissolution/absorption purposes the solubility of native propranolol should be considered, not of its salt form. This means that in reality propranolol (and other similar drugs) is a BCS class II drug and not the class I drug, as commonly considered.
In conclusion, for drug dissolution and absorption evaluation purposes, one should consider solubility characteristics of a drug in its native form and not that of its salt form. For further discussion on the topic, the following links would be useful (1, 2, 3, and 4).
