Studies on the absorption of practically water-insoluble drugs following injection. IV. An approach for predicting relative intramuscular absorption rates of a drug in oily solution,aqueous suspension and aqueous surfactant solution in rats.

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Abstract
The differences were studied of i.m. absorption rates of practically water-insoluble drugs in rats among the 3 basic dosage forms: oily solutions (including oily suspensions), aqueous suspensions and aqueous solutions containing nonionic surfactants as solubilizers. A review of previous studies indicated that the rank order of magnitude of the absorption rates among such dosage forms was not fixed, but varied remarkably depending on the drug, the oily solvent, the surfactant and its concentration and colloidal properties, i.e., particle size, etc., in addition to the initial drug concentration and injection volume. A method for predicting the relative absorption rates from the 3 dosage forms empirically was sought. The feasibility of the proposed approach was confirmed experimentally with testosterone for the 3 dosage forms injected i.m. into intact rats. This approach is expected to provide a novel and useful guide for predicting the relative absorption rates from various dosage forms and for selecting optimal preparations for more detailed screening and preclinical testing in laboratory animals of new drugs under development.