Positron emission tomography imaging of the stability of Cu-64 labeled dipalmitoyl and distearoyl lipids in liposomes

Changes in lipid acyl chain length can result in desorption of lipid from the liposomal anchorage and interaction with blood components. PET studies of the stability of such lipids have not been performed previously although such studies can map the pharmacokinetics of unstable lipids non-invasively in vivo. The purpose of this study was to characterize the in vivo clearance of 64Cu-labeled distearoyl- and dipalmitoyl lipid included within long circulating liposomes. Distearoyl and dipalmitoyl maleimide lipids (1 mol%) in liposomes were labeled with a 64Cu-incorporated bifunctional chelator (TETA-PDP) after the activation of pyridine disulfide to thiol by TCEP. Long circulating liposomes containing HSPC:DSPE-PEG2k-OMe:cholesterol: x (55:5:39:1), where x was 64Cu-DSPE or 64Cu-DPPE, or HSPC:DSPE-PEG2k-OMe:cholesterol: 64Cu-DSPE:DPPC (54:5:39:1:1) were evaluated in serum ( in vitro) and via intravenous injection to FVB mice. The time–activity curves for the blood, liver, and kidney were measured from PET images and the biodistribution was performed at 48 h. In vitro assays showed that 64Cu-DPPE transferred from liposomes to serum with a 7.9 h half-life but 64Cu-DSPE remained associated with the liposomes. The half clearance of radioactivity from the blood pool was 18 and 5 h for 64Cu-DSPE- and 64Cu-DPPE liposome-injected mice, respectively. The clearance of radioactivity from the liver and kidney was significantly greater following the injection of 64Cu-DPPE-labeled liposomes than 64Cu-DSPE-labeled liposomes at 6, 18 and 28 h. Forty eight hours after injection, the whole body radioactivity was 57 and 17% ID/cc for 64Cu-DSPE and 64Cu-DPPE, respectively. These findings suggest that the acyl chain length of the radiolabel should be considered for liposomal PET studies and that PET is an effective tool for evaluating the stability of nanoformulations in vivo. [Display omitted]