[(125)I]FIAU imaging in a preclinical model of lung infection: quantification of bacterial load

Abstract

2'-Fluoro-2'-deoxy-1β-D-arabinofuranosyl-5-[(125)I]iodouracil ([(125)I]FIAU), a substrate for the thymidine kinase (TK) present in most bacteria, has been used as an imaging agent for single photon emission computed tomography (SPECT) in an experimental model of lung infection. Using SPECT-CT we show that [(125)I]FIAU is specific for bacterial infection rather than sterile inflammation. We report [(125)I]FIAU lung uptake values of 1.26 ± 0.20 percent injected dose per gram (%ID/g) in normal controls, 1.69 ± 0.32 %ID/g in lung inflammation and up to 7.14 ± 1.09 %ID/g in lung infection in ex vivo biodistribution studies at 24 h after intranasal administration of bacteria. Images of [(125)I]FIAU signal within lung can be used to estimate the number of bacteria present, with a limit of detection of 10(9) colony forming units per mL on the X-SPECT scanner. [(125)I]FIAU-Based bacterial imaging may be useful in preclinical models to facilitate the development of new antibiotics, particularly in cases where a corresponding human trial is planned.

Keywords: Inflammation; PET; SPECT; molecular imaging; nucleoside; thymidine kinase.

Figure 1

[¹²⁵I]FIAU cell uptake specificity. E. coli RS218 and E. coli KY895 were incubated with 1 μCi/mL (37 MBq/mL) [¹²⁵I]FIAU in an incubator at 37ºC. Accumulated radioactivity was measured by withdrawing equal aliquots of E. coli RS218 (black bars) and E. coli KY895 (gray bars) at different time points (2 h – 24 h) following incubation with radiotracer. Values depicted are the average from three aliquots of experiments performed in triplicate. Data are represented as accumulated activity in counts per minute (cpm) ± SEM.

Figure 2

Ex vivo biodistribution data in lung inflammation and infection. Control mice, mice treated with lipopolysaccharide (LPS, an inducer of inflammation) and mice that were infected with different bacterial concentrations (10⁵ to 10⁷ CFU/mL) were injected with 2 μCi (74 KBq) [¹²⁵I]FIAU via the tail vein. At 2 h following injection of radiotracer, and coinciding with 18 h, 24 h or 48 h following administration of phosphate-buffered saline (PBS), LPS or bacteria, mice were sacrificed and their organs were harvested, weighed and radioactivity was counted in a gamma spectrometer. Values are represented as percentage of injected dose (%ID) per gram of tissue. Data are means ± SEM of four animals.

Figure 3

Ex vivo biodistribution data for [¹²⁵I]FIAU uptake in infected and antibiotic treated animals. Mice infected with 10⁵ CFU/mL E. coli RS218 at 48 h after infection were used as the infected group. Mice that were infected with 10⁵ CFU/mL E. coli RS218 and subsequently administered antibiotics beginning at 48 h after infection represent the treatment group. Both groups received 2 μCi (74 KBq) [¹²⁵I]FIAU via the tail vein and were sacrificed at 2 h following radiotracer injection. Organs were harvested, weighed and radioactivity was counted in a gamma spectrometer. Values are represented as percentage of injected dose (%ID) per gram of tissue. Data are means ± SEM of five animals.

Figure 4

SPECT-CT imaging of lung inflammation and infection. Control (PBS) and LPS-treated (inflammation) mice were injected with 1 mCi (37 MBq) of [¹²⁵I]FIAU via the tail vein and SPECT-CT images were acquired at (A) 24 h after PBS administration (control), (B) 24 h after LPS administration. Four mice were imaged per group. All scans were performed two hours after injection of [¹²⁵I]FIAU. All images were adjusted to the same maximum signal threshold. Mice harboring lung infection were similarly injected with 1 mCi (37 MBq) of [¹²⁵I]FIAU with SPECT-CT images acquired at 18 h after infection with (C)107 CFU/mL, (D) 10⁶ CFU/mL, and at 24 h after infection with (E) 10⁵ CFU/mL of E. coli RS218. SPECT-CT images of mice imaged at 24 h after infection with (F) 107 CFU/mL, (G) 10⁶ CFU/mL and at 48 h after infection with (H) 10⁵ CFU/mL of E. coli RS218. Four mice were imaged for each bacterial infecting dose. For monitoring antibiotic therapy, SPECT-CT images (I) of a mouse inoculated with 10⁵ CFU/mL E. coli RS218 and with doxycycline for 10 days (5 mg/kg/day) were obtained. Four mice were imaged per group. A representative image is depicted for each group. G – gallbladder, S- stomach, L- Lung.

Figure 5

Average Value (AV) of [¹²⁵I]FIAU SPECT signal intensity within the region-of-interest corresponding to the lungs versus colony forming units per mL (CFU/mL) in log scale together with the line of regression (Pearson R = 0.78).

Figure 6

Mouse lung histology. Histological staining with hematoxylin and eosin (H & E) of mouse lungs. (A) PBS-treated (control); (B) LPS-treated (inflamed); (C) bacterial infection at 48 h after inoculation with 10⁵ CFU/mL bacteria; (D) bacterial infection at 18 h after inoculation with 10⁷ CFU/mL bacteria. Arrows depict the presence of neutrophils (A, B) and bacterial colonies (C, D) inside the alveoli.