Novel PET and Near Infrared Imaging Probes for the Specific Detection of Bacterial Infections Associated With Cardiac Devices

Abstract

Objectives: The aim of this study was to develop imaging agents to detect early stage infections in implantable cardiac devices.

Background: Bacteria ingest maltodextrins through the specific maltodextrin transporter. We developed probes conjugated with either a fluorescent dye (maltohexaose fluorescent dye probe [MDP]) or a F-18 (F18 fluoromaltohexaose) and determined their usefulness in a model of infections associated with implanted cardiac devices.

Methods: Stainless steel mock-ups of medical devices were implanted subcutaneously in rats. On post-operative day 4, animals were injected with either Staphylococcus aureus around the mock-ups to induce a relatively mild infection or oil of turpentine to induce noninfectious inflammation. Animals with a sterile implant were used as control subjects. On post-operative day 6, either the MDP or F18 fluoromaltohexaose was injected intravenously, and the animals were scanned with the appropriate imaging device. Additional positron emission tomography imaging studies were performed with F18-fluorodeoxyglucose as a comparison of the specificity of our probes (n = 5 to 9 per group).

Results: The accumulation of the MDP in the infected rats was significantly increased at 1 h after injection when compared with the control and noninfectious inflammation groups (intensity ratio 1.54 ± 0.07 vs. 1.26 ± 0.04 and 1.20 ± 0.05, respectively; p < 0.05) and persisted for more than 24 h. In positron emission tomography imaging, both F18 fluoromaltohexaose and F18 fluorodeoxyglucose significantly accumulated in the infected area 30 min after the injection (maximum standard uptake value ratio 4.43 ± 0.30 and 4.87 ± 0.28, respectively). In control rats, there was no accumulation of imaging probes near the device. In the noninfectious inflammation rats, no significant accumulation was observed with F18 fluoromaltohexaose, but F18 fluorodeoxyglucose accumulated in the mock-up area (maximum standard uptake value 2.53 ± 0.39 vs. 4.74 ± 0.46, respectively; p < 0.05).

Conclusions: Our results indicate that maltohexaose-based imaging probes are potentially useful for the specific and sensitive diagnosis of infections associated with implantable cardiac devices.

Keywords: PET; bacterial imaging; medical device infections; optical imaging.

Figure 1. Structures of Imaging Agents

A) Structural formula of maltohexaose-based near infrared fluorescent dye probe and B) F18 fluoro-maltohexaose.

Figure 2. Specific uptake of maltohexaose fluorescent dye probe by bacteria

1 × 10⁸/CFU/ml of MSSA or Lam B mutant E. coli were cultured with 20μM of maltohexaose fluorescent dye probe for 1 hour. (A) MSSA exhibited a clear signal whereas Lam B mutant E. coli had very little uptake of the dye probe. Note that there is some autofluorescence from the plastic culture dish. B) Optical imaging demonstrating similar bacterial density. C) 1 × 10⁸/CFU/ml of MSSA was cultured with 20 mol/l of maltohexaose fluorescent dye probe for 1 hour and washed. The bacterial culture was incubated LB broth without maltohexaose fluorescent dye probe, washed again, and the remaining maltohexaose fluorescent dye probe in bacteria quantified. 1 × 10⁸/CFU of MSSA internalized 1.52 ± 0.54 × 10² pmol of maltohexaose fluorescent dye probe in 1 hour, and the concentrations of fluorescent dye in MSSA showed no significant changes for up to 6 hours. n = 3 to 4/time point.

Figure 3. Fluorescent imaging with maltohexaose fluorescent dye probe in vivo

A) Robust accumulation of maltohexaose fluorescent dye probe was seen around the mock-up in the infection group as soon as 1 hour after the injection of the dye probe, while no significant accumulation was observed in the non-infectious inflammation group and in the control group. B) The corrected intensity ratios in the infection group were significantly increased compared to those in the non-infectious inflammation group and in the control group at all time points. n = 6 to 9 per group.

Figure 4. Demonstration of the presence of bacteria in the infected mock-ups

Skin samples around the mock-ups demonstrated an inflammatory response was seen in all groups but was most intense in the inflammation and infection groups. Note that, as expected, gram positive cocci were seen only in the infection group (C). Magnification 20×, bars 20 μm. Magnification in the inset 100×, bars 5μm.

Figure 5. FMH and FDG PET imaging in vivo

(A) F18 fluoro-maltohexaose accumulation was clearly observed in the infection group with very low accumulation in the control group and in the non-infectious inflammation group. Conversely, with FDG PET imaging (B), the accumulation of radioactivity was observed in the infection group as well as in the noninfectious inflammation group. Yellow arrows indicate the locations of the mock-ups. For F18 fluoro-maltohexaose PET imaging, the infection group had a significant increase in both SUVRmax and SUVRmean when compared to both the control group and the non-infectious inflammation group. With FDG PET imaging, the infection group and the non-infectious inflammation group had a similar, significant increase in both SUVRmax and SUVRmean when compared to the control group demonstrating a lack of specificity (C). N = 5 to 6 per group.

Figure 6. Detection of bacteria in a biofilm on implanted mock-up devices

Rats were implanted with the mock-up devices, and a small amount of biofilm forming MSSA was inoculated on the implanted mock-up devices as a biofilm model. Two days after inoculation, the rats were injected with the maltohexaose fluorescent dye probe or F18 fluoro-maltohexaose via the tail vein. Both the maltohexaose fluorescent dye probe (A) and F18 fluoro-maltohexaose (B) accumulated at the site of the infected mock-up device. Arrows indicate locations of the mock-ups.