Spatial distribution of oil depots monitored in human muscle using MRI

Oil depots are parenteral drug formulations meant for sustained release of lipophilic compounds. According to mass transport models, the drug-release rate from these injections is determined by the surface area of the oil depot. Until now, the size of the surface area of injected depots has not been assessed, however. MRI provides an excellent possibility to distinguish between water and adipose tissue. The aim of this study was to investigate whether MRI can be used to determine the shape and hence the surface area of oil depots in muscle tissue. The developed MRI-scan protocol is demonstrated to be suitable for visualising oil depots. It was applied to determine the surface area of 0.5mL oil, i.m. injected in healthy volunteers. The mean (±RSD) surface area and volume of the depots recovered after injection was 755.4mm(2) (±26.5) and 520.1mm(3) (±24.6). It is shown that the depot disappearance from the injection site is very variable between volunteers. It is suggested that the oil is first solubilized and subsequently distributed. In all cases, the oil was not detectable after 14days. These factors are relevant for the understanding of the mechanism by which compounds are released out of oil depots.