Investigations of a (99m)Tc-labeled bacteriophage as a potential infection-specific imaging agent.
Because bacteriophages (phages) have a natural specificity for bacteria, it may be possible to develop radiolabeled phages as infection-specific agents.


METHODS
The M13 phage was radiolabeled with (99m)Tc via mercaptoacetyltriglycine and purified by polyethylene glycol precipitation. After radiolabeling, the phage was tested for binding at 1, 5, and 10 min to Escherichia coli strain 2537, E. coli strain 25922, and Staphylococcus aureus strain 29213. The radiolabeled phage was also tested for specificity in mouse models that had received a subcutaneous injection of either live (infection/inflammation model) or heat-inactivated (inflammation model) cultures in a thigh. The labeled phage (10(9) plaque-forming units, 1-3.7 MBq) was administered either within 20 min (to minimize the contribution from inflammation) or 3 h after induction. The animals were killed 3 h later.


RESULTS
The radiochemical purity of the labeled phage exceeded 95% by strip chromatography using instant thin-layer chromatography/acetone and paper/saline. Binding of the labeled phage to each of the 3 bacterial strains in vitro was immediate, reaching a maximum at 1 min. However, the percentage bound was significantly higher (P = 0.0008) for E. coli 2537 than for either of the other 2 bacteria (84% vs. 41% and 48%). Furthermore, binding to E. coli 2537 was unchanged at 10 min, whereas binding to both E. coli 25922 and S. aureus decreased to 33%. At 3 h in vivo, the ratio of target thigh to normal thigh was significantly higher (P < or = 0.017) in the infection/inflammation model (2 to 2.5 fold) than in the inflammation model (1.5 to 1.8) and therefore suggestive of increased accumulation specific to infection. The difference was slightly more pronounced in animals that received labeled phage at 20 min after inoculation, showing a ratio of 2.3 for infected thigh to normal thigh and a ratio of 1.6 for inflamed thigh to normal thigh. Although absolute uptake was lowest in the infection/inflammation thigh of mice infected with E. coli 2537, this finding was presumably due to the therapeutic effect of the phage on this strain.


CONCLUSIONS
Radiolabeled bacteriophages should be further investigated as potential agents for specific imaging of infection.
Rusckowski
Mary
M
Division of Nuclear Medicine, Department of Radiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA. mary.rusckowski@umassmed.edu
Gupta
Suresh
S
Liu
Guozheng
G
Dou
Shuping
S
Hnatowich
Donald J
DJ
D000818:Animals; D017104:Bacteriophage M13 / Q000302:isolation & purification* / Q000502:physiology; D004927:Escherichia coli Infections / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000821:virology*; D007553:Isotope Labeling / Q000379:methods; D008657:Metabolic Clearance Rate; D051379:Mice; D009220:Myositis / Q000000981:diagnostic imaging* / Q000821:virology*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D017263:Technetium Tc 99m Mertiatide / Q000493:pharmacokinetics; D014018:Tissue Distribution
D019275:Radiopharmaceuticals; D017263:Technetium Tc 99m Mertiatide