Imaging Active Infection in vivo Using D-Amino Acid Derived PET Radiotracers - PubMed (original) (raw)
Imaging Active Infection in vivo Using D-Amino Acid Derived PET Radiotracers
Kiel D Neumann et al. Sci Rep. 2017.
Abstract
Occult bacterial infections represent a worldwide health problem. Differentiating active bacterial infection from sterile inflammation can be difficult using current imaging tools. Present clinically viable methodologies either detect morphologic changes (CT/ MR), recruitment of immune cells (111In-WBC SPECT), or enhanced glycolytic flux seen in inflammatory cells (18F-FDG PET). However, these strategies are often inadequate to detect bacterial infection and are not specific for living bacteria. Recent approaches have taken advantage of key metabolic differences between prokaryotic and eukaryotic organisms, allowing easier distinction between bacteria and their host. In this report, we exploited one key difference, bacterial cell wall biosynthesis, to detect living bacteria using a positron-labeled D-amino acid. After screening several 14C D-amino acids for their incorporation into E. coli in culture, we identified D-methionine as a probe with outstanding radiopharmaceutical potential. Based on an analogous procedure to that used for L-[methyl-11C]methionine ([11C] L-Met), we developed an enhanced asymmetric synthesis of D-[methyl-11C]methionine ([11C] D-Met), and showed that it can rapidly and selectively differentiate both E. coli and S. aureus infections from sterile inflammation in vivo. We believe that the ease of [11C] D-Met radiosynthesis, coupled with its rapid and specific in vivo bacterial accumulation, make it an attractive radiotracer for infection imaging in clinical practice.
Conflict of interest statement
The authors declare that they have no competing interests.
Figures
Figure 1
Incorporation of D-amino acids into peptidoglycan via racemase-dependent and racemase-independent pathways. The intracellular processing of D-alanine is contrasted with the perpiplasmic addition of D-methionine, which is “swapped” at the C-terminus of peptidoglycan, mediated by the transpeptidase domains of penicillin binding proteins (PBPs). D-Met may be incorporated into peptidoglycan muropeptides by exogenous administration or by physiologic production, with the latter associated with transition into the stationary phase and downregulation of peptidoglycan synthesis. The putative pathway of 11C retention following [11C] D-Met administration to infected animals is highlighted in red. GlcNAc = N-acetylglucosamine, MurNAc = N-acetyl- muramic acid, m-DAP = meso-diaminopimelic acid.
Figure 2
In vitro accumulation of 14C D-amino acids in E. coli and S. aureus. (A) Panel of 14C amino acids studied for relative uptake. (B) Dedicated 14C D-Met study in E. coli and S. aureus including incubation with heat-killed organisms and in the presence of 1 mM unlabeled D-methionine.
Figure 3
Radiosynthesis of [11C] D-Met from D-homocystinethiolactone precursor. (A) Synthetic scheme. (B) Chiral stationary-phase HPLC showing enhanced enantiomeric excess synthesis of [11C] D-Met.
Figure 4
In vivo studies using [11C] D-Met and [11C] L-Met a murine myositis model (n = 4 for each condition studied). In all cases the site of live bacterial inoculation is denoted by a red arrow, while the site of 10X heat-killed bacterial inoculation is denoted by a white arrow. (A) [11C] D-Met studies in E. coli and _S. aureus_-infected animals. Representative images show marked uptake in areas corresponding to live bacterial injection (left deltoid), in contrast to sterile inflammation (right deltoid) and normal muscle. ROI analysis of E. Coli and S. aureus [11C] D-Met cohorts, corrected using normal muscle uptake. (B) [11C] L-Met studies in E. coli and S. aureus. There is no observable difference in signals from the left (infected) and right (sterile inflammation) deltoid muscles. ROI analysis of E. Coli and S. aureus [11C] L-Met cohorts, showing no significant difference in accumulation between infection and sterile inflammation.
Figure 5
Ex vivo analysis of both infected and normal mice, obtained via tissue harvesting and gamma-counting. In all cases mice injected intravenously with [11C] D-Met were sacrificed at 70 minutes. (A) Analysis of harvested deltoid muscles, performed immediately following the imaging studies described in this manuscript. The deltoid muscles corresponding to inoculation with live bacteria were compared with the contralateral side (heat-killed). (B) The biodistribution of [11C] D-Met was also studied in a separate cohort of normal CBA/J female mice. The highest accumulation was observed in the kidneys and liver, similar to previously reported findings for [11C] L-Met.
Figure 6
Representative histology for an S. aureus infected mouse sampled from deltoid muscle, using Gram-staining (left image) and hematoxylin & eosin (H&E, center and right image). In live inoculations, scattered inflammatory cells and intact bacteria are seen, best identified by Gram-staining and denoted by arrowheads. In heat-killed inoculations (right), several inflammatory cells are present on H&E staining without discernible bacteria. Images are representative of four animals.
References
- Stacy, G. S. & Kapur, A. Mimics of bone and soft tissue neoplasms. Radiologic clinics of North America49, 1261–1286, vii, doi:10.1016/j.rcl.2011.07.009 (2011). -PubMed
- Gafter-Gvili, A. et al. [18F]FDG-PET/CT for the diagnosis of patients with fever of unknown origin. QJM: monthly journal of the Association of Physicians, doi:10.1093/qjmed/hcu193 (2014). -PubMed
Publication types
MeSH terms
Substances
Grants and funding
- P41 EB013598/EB/NIBIB NIH HHS/United States
- R01 CA166766/CA/NCI NIH HHS/United States
- R01 EB024014/EB/NIBIB NIH HHS/United States
- T32 EB001631/EB/NIBIB NIH HHS/United States
LinkOut - more resources
Full Text Sources
Other Literature Sources
Medical
Molecular Biology Databases
Miscellaneous