Liquid oral solutions pharmaceutical properties

The pharmacist should have anticipated the bio-pharmaceutical consequences of the physico-chemical properties of oxcarbazepine. The drug is classified as a Class II substance for oral application. Logically, lack of adequate solubility is even more evident for the rectal administration as the volume of rectal fluid is limited (see Table 17.1). With an aqueous solubility of approximately 300 mg/L, the solubUity of the substance in the lipophilic base of the suppositories would certainly not be higher than 9.5 mg/mL (being a direct consequence of the value of the log P = 1.5 of oxcarbamazepine). This means that oxcarbazepine is not dissolved in the lipid but dispersed as crystals, which settle from the molten suppository once introduced in the rectal cavity. The amount of rectal liquid is limited and therefore a saturated solution will exist which involves only less than 1 mg dissolved oxcarbamazepine. Low solubility yields a low concentration and hence a low driving force for diffusion to occur. As a consequence, the rate of absorption is relatively low. This slow release may lead to hardly any uptake, due to defecation within several hours after insertion. 

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