[11C]Para-Aminobenzoic Acid: A Positron Emission Tomography Tracer Targeting Bacteria-Specific Metabolism - PubMed (original) (raw)

. 2018 Jul 13;4(7):1067-1072.

doi: 10.1021/acsinfecdis.8b00061. Epub 2018 May 8.

Alvaro A Ordonez 2, Hecong Qin 1, Matthew Parker 1, Lauren E Bambarger 2, Javier E Villanueva-Meyer 1, Joseph Blecha 1, Valerie Carroll 1, Celine Taglang 1, Robert Flavell 1, Renuka Sriram 1, Henry VanBrocklin 1, Oren Rosenberg 3, Michael A Ohliger 1 4, Sanjay K Jain 2, Kiel D Neumann 5, David M Wilson 1

Affiliations

[11C]Para-Aminobenzoic Acid: A Positron Emission Tomography Tracer Targeting Bacteria-Specific Metabolism

Christopher A Mutch et al. ACS Infect Dis. 2018.

Abstract

Imaging studies are frequently used to support the clinical diagnosis of infection. These techniques include computed tomography (CT) and magnetic resonance imaging (MRI) for structural information and single photon emission computed tomography (SPECT) or positron emission tomography (PET) for metabolic data. However, frequently, there is significant overlap in the imaging appearance of infectious and noninfectious entities using these tools. To address this concern, recent approaches have targeted bacteria-specific metabolic pathways. For example, radiolabeled sugars derived from sorbitol and maltose have been investigated as PET radiotracers, since these are efficiently incorporated into bacteria but are poor substrates for mammalian cells. We have previously shown that para-aminobenzoic acid (PABA) is an excellent candidate for development as a bacteria-specific imaging tracer as it is rapidly accumulated by a wide range of pathogenic bacteria, including metabolically quiescent bacteria and clinical strains, but not by mammalian cells. Therefore, in this study, we developed an efficient radiosynthesis for [11C]PABA, investigated its accumulation into Escherichia coli and Staphylococcus aureus laboratory strains in vitro, and showed that it can distinguish between infection and sterile inflammation in a murine model of acute bacterial infection.

Keywords: bacteria; folate; infection; metabolism; positron emission tomography.

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Conflict of interest statement

Notes:

The authors declare no competing financial or other interests.

Figures

Figure 1

Figure 1. Folate biosynthesis pathway and possible incorporation sites of [11C]PABA

The chemical structure of [11C]PABA is shown with the 11C radionucleus highlighted in red. Pathways inhibited by common sulfonamide antibiotics and trimethoprim are also shown. The first step, incorporation of PABA into dihydropteroate via dihydropteroate synthase (EC 2.5.1.15) via condensation with 2-amino-4-hydroxy-7,8-dihydropterin-6-yl)methyl diphosphate is the site of action of sulfonamide antibiotics. Following incorporation of glutamate to form dihydrofolate, trimethoprim inhibits reduction to tethrahydrofolate. DHPPP = 2-amino-4-hydroxy-7,8-dihydropterin-6-yl)methyl diphosphate, H2Pte = dihydropteroate, Glu = glutamate, H2PteGlu = dihydrofolate.

Figure 2

Figure 2. Radiosynthesis of [11C]PABA

(a) Radiosynthesis of [11C]PABA via [11C]CO2 and a commercially available Grignard precursor followed by quenching in aqueous HCl.

Figure 3

Figure 3. In vitro uptake of [11C]PABA

1 μCi of [11C]PABA was incubated in 1 mL culture of E. coli (white bars) or S. aureus (gray bars) for 20, 40 or 60 minutes. Accumulation of the tracer relative to colony-forming units (CFUs) was determined by serial dilution and plating. As the incubation time increases, both live E. coli and S. aureus exhibited increased radiotracer uptake. At 60 minutes, there was significantly higher uptake in live E. coli and S. aureus than heat-killed E. coli and S. aureus, respectively(****p<0.0001 for both bacteria). Uptake of [11C]PABA was effectively blocked in both live E. coli and S. aureus by an excess of 2mM cold PABA, shown by significantly lower radiotracer signal at 60 minutes than regular incubation (****p<0.0001 for both bacteria).

Figure 4

Figure 4. [11C]PABA PET can differentiate active bacterial infection from sterile inflammation in vivo

(a) Representative microPET/CT image acquired in a murine myositis model using [11C]PABA (n=4 animals described in this study). The left side of the image corresponds to deltoid muscle inoculated with live E. coli, while the right side corresponds to inoculation with 10-fold greater (10X) heat-killed bacteria. This image highlights the specificity of [11C]PABA for living bacteria, its primary renal excretion, and the low background in most tissues. (b) ROI analysis of PET images showed a significantly (*p = 0.0286) higher radiotracer uptake (expressed in percent injected dose per cubic centimeter (%ID/cc) in infection sites induced by live E. coli than sterile inflammation sites induced by heat-killed E. coli. (c) Biodistribution analysis using a gamma counter was performed on harvested tissues to corroborate ROI findings. This study showed a significant (*p = 0.0178) difference in [11C]PABA retention in live versus heat-killed inoculation sites, which showed tracer retention similar to normal muscle (p > 0.9999). (d) Histologic analysis of harvested deltoid muscles. The left panel corresponds to a Gram-stain of muscle infected with living E. coli, with arrows highlighting morphologically intact bacteria infiltrating muscle fibers. The right panel shows deltoid muscle stained with hematoxylin and eosin (H&E) corresponding to inoculation with heat-killed bacteria. Intact bacteria are not seen, but numerous inflammatory cells are present (highlighted by *).

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