Noninvasive pulmonary [18F]-2-fluoro-deoxy-D-glucose positron emission tomography correlates with bactericidal activity of tuberculosis drug treatment

Abstract

Tools for monitoring response to tuberculosis (TB) treatment are time-consuming and resource intensive. Noninvasive biomarkers have the potential to accelerate TB drug development, but to date, little progress has been made in utilizing imaging technologies. Therefore, in this study, we used noninvasive imaging to monitor response to TB treatment. BALB/c and C3HeB/FeJ mice were aerosol infected with Mycobacterium tuberculosis and administered bactericidal (standard and highly active) or bacteriostatic TB drug regimens. Serial pulmonary [(18)F]-2-fluoro-deoxy-D-glucose (FDG) positron emission tomography (PET) was compared with standard microbiologic methods to monitor the response to treatment. [(18)F]FDG-PET correctly identified the bactericidal activity of the drug regimens. Imaging required fewer animals; was available in real time, as opposed to having CFU counts 4 weeks later; and could also detect TB relapse in a time frame similar to that of the standard method. Lesion-specific [(18)F]FDG-PET activity also broadly correlated with TB treatment in C3HeB/FeJ mice that develop caseating lesions. These studies demonstrate the application of noninvasive imaging to monitor TB treatment response. By reducing animal numbers, these biomarkers will allow cost-effective studies of more expensive animal models of TB. Validated markers may also be useful as "point-of-care" methods to monitor TB treatment in humans.

FIG. 1.

[¹⁸F]FDG-PET activity correlates with the activity of TB treatment in BALB/c mice. Two weeks after aerosol infection, BALB/c mice were allocated to treatment groups to receive one of three TB drug regimens: the bactericidal first-line TB regimen (RHZ), a more bactericidal regimen (PMZ), and a bacteriostatic regimen of E alone. Three mice in each treatment group were allocated to the imaging cohort, and the same mice were serially imaged using [¹⁸F]FDG-PET/CT during treatment. (A) Mean lung [¹⁸F]FDG-PET activity normalized to the uninfected controls is shown. [¹⁸F]FDG-PET could distinguish between regimens of differing activity as early as 2 weeks after initiation of treatment. PET activities for the mice treated with the bactericidal regimens (PMZ and RHZ) were significantly lower than that for mice treated with the bacteriostatic regimen (E alone) at all time points measured during TB treatment (P < 0.033). Mean lung PET activity for the mice treated with the more bactericidal regimen, PMZ, was also significantly lower than that for RHZ starting at 8 weeks after initiation of TB treatment (P < 0.026). (B) Three additional mice were sacrificed at each time point to determine the lung CFU counts, which are shown for each corresponding time point. (C) Serial imaging also detected the development of relapse.

FIG. 2.

Mycobacterium tuberculosis-infected C3HeB/FeJ mice develop well-defined caseous granulomas. Unlike the BALB/c mouse model, C3HeB/FeJ mice display lung pathology with well-defined granulomas and central caseous necrosis after low-dose aerosol infection. (A) Formalin-fixed lungs 8 weeks after infection show caseous lesions. Yellow arrows point to the three large granulomas also visualized by [¹⁸F]FDG-PET imaging in the same animal at 6 weeks after infection (Fig. 3). (B) Lung histopathology from the same mice 8 weeks after infection, demonstrating well-defined granulomas with central necrosis and abundant acid-fast bacilli (AFB) (inset), is also shown.

FIG. 3.

[¹⁸F]FDG localizes to granulomas in C3HeB/FeJ mice. (A) Transverse, coronal, and sagittal sections (CT, PET, and fused PET+CT) from a Mycobacterium tuberculosis H37Rv-infected C3HeB/FeJ mouse 6 weeks after a low-dose aerosol infection. [¹⁸F]FDG-PET activity localized to TB granulomas (arrows). Mean lung [¹⁸F]FDG-PET activity in infected animals was significantly higher than that in the uninfected controls at 4, 6, and 8 weeks after infection (P < 0.031). (B) Three-dimensional tomography of the fused PET+CT (top) and isosurface representation of coregistered PET and CT images (bottom). The TB lesions are shown in pink (PET), while the bony rib cage is shown in yellow (CT). As expected, the heart (H) takes up FDG and is visible in all sections.

FIG. 4.

[¹⁸F]FDG-PET activity correlates with activity of TB treatment in C3HeB/FeJ mice. Six weeks after low-dose aerosol infection, C3HeB/FeJ mice were allocated to treatment groups to receive one of three TB drug regimens: the bactericidal first-line TB regimen (RHZ), a more bactericidal regimen (PMZ), and a bacteriostatic regimen of E alone. The same live animals were serially imaged using [¹⁸F]FDG-PET/CT after initiation of TB treatment. (A) Mean lung [¹⁸F]FDG-PET activity was useful in distinguishing the differential activities of the TB regimens. PET activities for the mice treated with both the bactericidal regimens (PMZ and RHZ) were significantly lower than that for those treated with the bacteriostatic regimen (E alone) starting at 4 weeks after initiation of TB treatment (P < 0.001). Mean lung PET activity for the mice treated with the more bactericidal regimen, PMZ, was also significantly lower than that for those treated with RHZ at 8 weeks after initiation of TB treatment (P = 0.045). (B) Three additional mice were sacrificed at each time point to determine the lung CFU counts, which are shown for each corresponding time point. Note that the differences between the bactericidal activities of RHZ and PMZ were more modest in this model.