The characteristic ability of white blood cells (WBC) to accumulate at sites of infection and inflammation has been used as a tool for the detection of these pathological conditions. The leukocyte and monocyte components of the WBC have often been labeled under ex vivo conditions with radionuclides such as indium (111In), meta-stable technetium (99mTc), or radioactive compounds such as 99mTc-gluceptate, [67Ga]gallium citrate, and 99mTc-labeled antibodies, for the detection of infection or inflammation (1). Radiolabeling of leukocytes or monocytes involves the isolation of these cells from the WBC pool, a process that is slow and expensive, requires the handling of possibly infectious blood, and can lead to contamination of the samples. In addition, the labeled cells have a slow clearance from circulation and may be imaged up to 24 h after administration to allow time for accumulation at the site of infection or inflammation. Also, the labeled cells usually have a low signal-to-noise ratio, especially during the early time points, and show a high intestinal uptake that interferes with diagnostic imaging of the abdominal area (2).
Monoclonal antibodies labeled with radioactivity have also been used to image infections, but these radiopharmaceuticals have limited application because they also had a slow clearance from circulation and are not always sensitive enough to detect pulmonary or bone infections (3, 4). In addition, labeled antibodies have been shown to induce transient neutropenia and the formation of human anti-mouse antibodies (HAMA) in patients (4, 5). Therefore, repeat use of the antibodies can be limited by the HAMA response because it neutralizes and alters the biodistribution of the agent (6). The use of peptides to detect infection or inflammation is an attractive option because these compounds are cheap and easy to synthesize, can be modified to suit target requirements, and show rapid clearance from circulation (7, 8). Chemotactic peptides have been developed and evaluated for the imaging of infections and inflammation, but these compounds have limitations because the buffer components used to label the peptides may influence their biodistribution characteristics (8). Moyer et al. identified and developed a heparin-binding peptide, P483, for the imaging of infections (9). The sequence of this peptide was based on the platelet factor-4 heparin-binding region and was modified to contain a lysine-rich region to allow rapid renal clearance. It also contains a cys-gly-cys sequence to facilitate the formation of a coordination complex with 99mTc. The peptide was complexed with heparin to enhance binding to WBC and labeled with 99mTc to generate a labeled peptide (99mTc-P483H) that was shown to target leukocytes (9). The labeled peptide was evaluated for the imaging of infections in a rabbit model as well as in humans (9, 10). Although 99mTc-P483H was safe and could detect infections rapidly and accurately, it showed accumulation in the thyroid, salivary gland, and gastrointestinal tract (10).
In an effort to develop an improved imaging agent for infections compared to P483H, Krause et al. synthesized a peptide with a modified amino acid sequence (11). They substituted all the lysine residues of P483 with arginine to obtain Ac-arg-arg-arg-arg-arg-cys-gly-cys-gly-gly-pro-leu-tyr-arg-arg-ile-ile-arg-arg-leu-leu-glu-ser (P1827). This molecule was subsequently complexed with radioactive technetium (99mTc) and dermatan sulfate (DS), a more homogeneous molecule compared to heparin, to obtain 99mTc-P1827DS. The labeled peptide was then evaluated for the detection of infection in a rabbit model and compared to 99mTc-labeled interleukin 8 (IL-8), which has been shown to successfully detect infection and inflammation in a variety of models (12-15). Results obtained with 99mTc-P1827DS were also compared to those obtained with 99mTc-P483H (11).