title: Evaluation of a Novel Tc-99m Labelled Vitamin B12 Derivative for Targeting Escherichia coli and Staphylococcus aureus In Vitro and in an Experimental Foreign-Body Infection Model.
abstract: OBJECTIVE
  Vitamin B12 (cyanocobalamin, Cbl) is accumulated by rapidly replicating prokaryotic and eukaryotic cells. We investigated the potential of a Tc-99m labelled Cbl derivative ([(99m)Tc]PAMA(4)-Cbl) for targeting infections caused by Escherichia coli and Staphylococcus aureus. In vitro binding assays were followed by biodistribution studies in a mouse model of foreign body infection.
  
  
  METHODS
  E. coli (ATCC 25922) and S. aureus (ATCC 43335) were used as test strains. [(57)Co]Cbl, [(67)Ga]citrate and [(99m)Tc]DTPA served as reference compounds. The in vitro competitive binding of [(57)Co]Cbl or [(99m)Tc]PAMA(4)-Cbl, and unlabeled Cbl, to viable or killed bacteria, was evaluated at 37 and 4 °C. A cage mouse model of infection was used for biodistribution of intravenous [(57)Co]Cbl and [(99m)Tc]PAMA(4)-Cbl in cage and dissected tissues of infected and non-infected mice.
  
  
  RESULTS
  Maximum binding (mean ± SD) of [(57)Co]Cbl to viable E. coli was 81.7 ± 2.6 % and to S. aureus 34.0 ± 6.7 %, at 37 °C; no binding occurred to heat-killed bacteria. Binding to both test strains was displaced by 100- to 1000-fold excess of unlabeled Cbl. The in vitro binding of [(99m)Tc]PAMA(4)-Cbl was 100-fold and 3-fold lower than the one of [(57)Co]Cbl for E. coli and S. aureus, respectively. In vivo, [(99m)Tc]PAMA(4)-Cbl showed peak percentage of injected dose (% ID) values between 1.33 and 2.3, at 30 min post-injection (p.i.). Significantly higher retention occurred in cage fluids infected with S. aureus at 4 h and with E. coli at 8 h p.i. than in non-infected animals. Accumulation into infected cages was also higher than the one of [(99m)Tc]DTPA, which showed similar biodistribution in infected and sterile mice. [(57)Co]Cbl gradually accumulated in cages with peaks % ID between 3.58 and 4.83 % achieved from 24 to 48 h. Discrimination for infection occurred only in E. coli-infected mice, at 72 h p.i. [(67)Ga]citrate, which showed a gradual accumulation into cage fluids during 12 h, was discriminative for infection from 48 to 72 h p.i. (P < 0.05).
  
  
  CONCLUSIONS
  Cbl displayed rapid and specific in vitro binding to test strains. [(99m)Tc]PAMA(4)-Cbl was rapidly cleared from most tissues and discriminated between sterile and infected cages, being a promising candidate for imaging infections in humans.
authors: Infectious Diseases Research Laboratory, Department of Biomedicine, University Hospital, Basel, Switzerland.
authors: Daniela
authors: D
authors: Baldoni
authors: Center for Radiopharmaceutical Science, Paul Scherrer Institute, Villigen PSI, Switzerland.
authors: Robert
authors: R
authors: Waibel
authors: Center for Radiopharmaceutical Science, Paul Scherrer Institute, Villigen PSI, Switzerland.
authors: Peter
authors: P
authors: Bläuenstein
authors: Nuclear Medicine, Department of Medical-Surgical Sciences and of Translational Medicine, Faculty of Medicine and Psychology, "Sapienza" University, Ospedale S. Andrea, via di Grottarossa 1035, 00189, Rome, Italy.
authors: Filippo
authors: F
authors: Galli
authors: Nuclear Medicine, Department of Medical-Surgical Sciences and of Translational Medicine, Faculty of Medicine and Psychology, "Sapienza" University, Ospedale S. Andrea, via di Grottarossa 1035, 00189, Rome, Italy.
authors: Violetta
authors: V
authors: Iodice
authors: Nuclear Medicine, Department of Medical-Surgical Sciences and of Translational Medicine, Faculty of Medicine and Psychology, "Sapienza" University, Ospedale S. Andrea, via di Grottarossa 1035, 00189, Rome, Italy. alberto.signore@uniroma1.it.
authors: Alberto
authors: A
authors: Signore
authors: Center for Radiopharmaceutical Science, Paul Scherrer Institute, Villigen PSI, Switzerland.
authors: Roger
authors: R
authors: Schibli
authors: Septic Surgery Unit, Center for Musculoskeletal Surgery, Charité - University of Medicine, Berlin, Germany.
authors: Andrej
authors: A
authors: Trampuz
mesh_terms: D000818:Animals; D015169:Colony Count, Microbial; D016503:Drug Delivery Systems; D004926:Escherichia coli / Q000187:drug effects*; D005549:Foreign-Body Reaction / Q000382:microbiology*; D066298:In Vitro Techniques; D051379:Mice; D008810:Mice, Inbred C57BL; D013211:Staphylococcus aureus / Q000187:drug effects*; D013667:Technetium / Q000737:chemistry* / Q000493:pharmacokinetics; D014805:Vitamin B 12 / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology*
chemical_list: D013667:Technetium; D014805:Vitamin B 12
keywords: 99mTc; Escherichia coli; Infection imaging; Staphylococcus aureus; Vitamin B12