In Vitro – In Vivo Correlation of Injectable Lipid Formulations

Abstract: In order to develop new pharmaceuticals, to optimize pharmacotherapy and secure patient safety, documentation of the properties of pharmaceutical formulations is a most important process. A significant practical problem associated with pre-clinical evaluation of lipophilic substances with pharmacological properties is their low aqueous solubility. The overall aim of this thesis was to develop techniques which can be used in vitro in order to predict relevant in vivo activity of injectable lipid formulations. In the “classic” dissolution test, a dialysis membrane separates the pharmaceuticals from the release media. When the formulation is based on lipids, the impact on the final result of such a membrane is difficult to predict, and some studies in the literature clearly indicate a decrease in release rate compared to non-membrane techniques. Only few in vitro-in vivo correlations have been reported for injectable oily lipid formulations. However, the two new membrane-free techniques presented in this thesis showed similar results and arterial blood concentrations and effect of duration could be predicted approximately for several lipid-based preparations. Peripheral nerve blockade with a local anaesthetic provides excellent pain relief but its clinical utility is limited by short duration. In cancer pain, a more or less irreversible effect may even be desirable. In this thesis, the base forms of local anaesthetics were incorporated into lipid which thereafter could be injected outside the surgically exposed perineurium of experimental rats. Some of the high concentration formulations showed the longest duration of action yet reported for experimental sciatic nerve block, making them possible candidates for neurolytic agents. Theoretically, after degradation of glycerides, possible harmful effects caused by breakdown products from a lipid vehicle may be accumulated at the site of injection in a local tissue. In addition, increased concentrations of free fatty acids in human plasma are associated with several severe conditions. Using erythrocytes suspended in autologous human plasma, a new miniaturized in vitro method has been developed with the intention of studying the harmful effects of various lipophilic substances on erythrocytes in an environment as close as possible to in vivo conditions. Structure-activity relationships, EC-50 % and degree of cooperativity were mapped. Provided that the technique is thoughtfully designed and the experiments are carefully performed, in vitro studies can predict relevant effects in vivo.