Detecting infection and inflammation with technetium-99m-labeled Stealth liposomes.

Unlabelled: The performance of 99mTc Stealth liposomes was investigated in various rat models.

Methods: Preformed polyethyleneglycol-containing liposomes with encapsulated reduced glutathione, were radiolabeled using the lipophilic 99mTc-HMPAO. The labeled liposomes were intravenously administered to rats with focal S. aureus or E. coli infection, or turpentine-induced inflammation. For comparison, Tc-99m-nanocolloid- and 99mTc-labeled nonspecific IgG were tested. In rats with Pneumocystis carinii pneumonia (PCP), Tc-99m-liposomes were directly compared to In-111 labeled nonspecific IgG.

Results: Technetium-99m-liposomes accumulated in the infectious and inflammatory muscle foci over 24 hr (0.59% injected dose per gram tissue (%ID/g) for S. aureus; 1.18 %ID/g for turpentine). Abscess-to-muscle ratios increased to values as high as 24.0, 41.7 and 44.5 for the respective models at 24 hr postinjection. Technetium-99m-liposomes visualized the foci as early as 1 hr postinjection. Technetium-99m-IgG visualized S. aureus infection, but abscess-to-muscle ratios and abscess uptake at the later time points were significantly lower. Technetium-99m-nanocolloid failed to visualize any of the muscle foci. In PCP however, 99mTc-liposomes did not show preferential localization in the infection. The control agent 111In-IgG showed a significant, two-fold increase in lung uptake.

Conclusion: Technetium-99m-Stealth liposomes preferentially accumulated in abscesses, leading to very high target-to-nontarget ratios. This property appears to be related to a process based on uptake of long-circulating particles. In a specific type of infection, i.c. PCP, 99mTc-liposomes did not accumulate in diseased lung tissue, thus mimicking the in vivo behavior of labeled leukocytes.