A) Amount of 99mTc-labeled PEGylated liposomes remaining in circulation after i.v. administration in rats quantified by scintigraphic image analysis. (●) indicates the first dose, and (○) indicates the second dose given 7 days later. B) Tissue biodistribution of 99mTc-labeled PEGylated liposomes in rats for the initial injection (control) and for second doses given after 7, 14, 21, or 28 days. *p < 0.05, **p < 0.01. C) PEG-specific antibodies responses after an initial injection of PEGylated liposomes (0.001 μmol/kg) in rats, as determined using ELISA. *p < 0.05, ***p < 0.005. D) PEG-specific antibodies responses after an initial injection of PEGylated liposomes (100 μg/animal) in mice, as determined using ELISA. Panels A and B were reprinted from Ref 7; panel C was reprinted from Ref 39 with permission from Elsevier; and panel D was reprinted from Ref 41 with permission from Elsevier.