Assessment of Liposome Biodistribution by Non-Invasive Optical Imaging: A Feasibility Study in Tumour-Bearing Mice

Liposomal encapsulation of cytostatics improves drug delivery to tumour tissue and reduces dose-limiting systemic toxicities. Development and evaluation of new liposome formulations is time consuming and costly with high demands for experimental animals. A faster and less demanding means of comparing several product candidates may be provided by use of non-invasive methods for assessing pharmacokinetics and biodistribution. In this study we have evaluated the feasibility of using small animal fluorescence optical imaging as a strategy to study liposome accumulation in tumours. Liposomal doxorubicin (Caelyx^®) was labelled with a lipophilic carbocyanine tracer and administered to tumour-bearing mice. Subsequently, the in vivo distribution of the labelled liposomes was followed over time by fluorescent optical imaging. The results revealed a gradual increase in tumour fluorescence, indicating accumulation of the liposomes reaching plateau levels at 48 h post injection. However, due to loss of dye from liposomes during circulation combined with substantial scattering and absorption of in vivo fluorescent signal, reliable quantitative correlation between the biodistribution profile of the labelled liposomes and doxorubicin could not be obtained.