Direct ^99mTc Labeling of Pegylated Liposomal Doxorubicin (Doxil) for Pharmacokinetic and Non-Invasive Imaging Studies

Pharmacokinetic and organ distribution studies of liposomal drugs in humans are a challenge. A direct labeling method using ^99mTc-N,N-bis(2-mercaptoethyl)-N′,N′ -diethyl-ethylenediamine (BMEDA) complex to label the commercially available pegylated liposomal doxorubicin, Doxil, has been introduced. Biodistributions of ^99mTc-Doxil in normal rats were performed to evaluate the feasibility of using it for monitoring the pharmacokinetics of liposomes encapsulating drugs. Labeling efficiency of ^99mTc-Doxil was 70.6 ± 0.8% (n = 3). In vitro incubation of ^99mTc-Doxil in 50% fetal bovine serum or 50% human serum at 37°C showed good labeling stability with 72.3 ± 3.6% or 78.6 ± 1.8% of activity associated with Doxil at 24 h, respectively (n = 3). There was a two-phase blood clearance with half-clearance times of 2.2 and 26.2 h after bolus intravenous injection in normal rats. Distribution of ^99mTc-Doxil at 44 h after injection had 19.8 ± 1.3% of injected dose in blood, 14.1 ± 1.7% in liver, 2.6 ± 0.3% in spleen, 9.0 ± 0.8% in bone with marrow, 6.0 ± 0.5% in skin, and 15.3 ± 4.3% in bowel (n = 5). Unencapsulated ^99mTc-BMEDA had a very rapid blood clearance with a half-clearance time of only 0.12 h (n = 4). By using this ^99mTc labeling method, biodistribution and pharmacokinetics of ammonium gradient liposomes encapsulating drugs can be determined by noninvasive scintigraphic imaging. This labeling method may be extended to ¹⁸⁶Re and ¹⁸⁸Re labeling to combine chemotherapy and radionuclide therapy for tumor treatment.