Liposomal drug delivery to the brain via monocytes

Delivery of drugs to the brain is limited due to poor penetrability of many drugs via the blood brain barrier (BBB). The brain is under immune-surveillance, and circulating phagocytic cells, such as neutrophils and monocytes, are crossing the BBB. It has been demonstrated in our group that negatively-charged liposomes, are transported to the brain following their phagocytosis by circulating monocytes, bypassing the BBB. In this work, we aimed to deliver a proteinaceous drug to the brain following IV administration of positively-charged liposomes.
Lysozyme, a 14.3 kDa protein was successfully encapsulated in positively-charged, fluorescently-labeled liposomes (PE-Rhodamine). Free PE-Rhodamine and negatively-charged liposomes were used for comparison. Desired properties of the liposomal formulations were achieved including size, polydispersity index (PDI), high drug concentration and stability. Liposomes were phagocytized by RAW264.7 cells in a time- and dose-dependent manner, with no marked cytotoxicity. Circulating WBC, mainly monocytes, exhibited 20-fold higher levels of liposomal lysozyme, compared to free PE-Rhodamine treated rats. Screening of rat brain sections by means of confocal microscopy revealed that the transport of liposomes into the brain was 30-fold higher, in comparison to the free fluorophore. The model drug, lysozyme, was found intact in the brain. Quantification of lysozyme in the brain, determined by western blot analysis, showed 5 times higher levels of lysozyme in the brain of rats treated with the formulation, in comparison to free lysozyme. Anti-CD68 immunofluorescent staining of brain sections demonstrated co-localization of liposomes and monocytes in different sections of the brain, confirming that the designed liposomes were transported to the brain following their phagocytosis by circulating monocytes.
It can be concluded that specifically-designed liposomes, containing high MW drugs, can be transported to the brain following their phagocytosis by circulating monocytes.