Abstract
Tuberculosis (TB) remains a leading cause of death, but antibiotic treatments for tuberculous meningitis, the deadliest form of TB, are based on those developed for pulmonary TB and not optimized for brain penetration. Here, we perform first-in-human dynamic 18F-pretomanid positron emission tomography (PET) in eight human subjects to visualize 18F-pretomanid biodistribution as concentration-time exposures in multiple compartments (NCT05609552), demonstrating preferential brain versus lung tissue partitioning. Preferential, antibiotic-specific partitioning into brain or lung tissues of several antibiotics, active against multidrug resistant (MDR) Mycobacterium tuberculosis strains, are confirmed in experimentally-infected mice and rabbits, using dynamic PET with chemically identical antibiotic radioanalogs, and postmortem mass spectrometry measurements. PET-facilitated pharmacokinetic modeling predicts human dosing necessary to attain therapeutic brain exposures. These data are used to design optimized, pretomanid-based regimens which are evaluated at human equipotent dosing in a mouse model of TB meningitis, demonstrating excellent bactericidal activity without an increase in intracerebral inflammation or brain injury. Importantly, several antibiotic regimens demonstrate discordant activities in brain and lung tissues in the same animal, correlating with tissue antibiotic exposures. These data provide a mechanistic basis for the compartmentalized activities of antibiotic regimens, with important implications for developing treatments for meningitis and other infections in compartments with unique antibiotic penetration.
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Introduction
Achieving therapeutic antibiotic concentrations at infection sites is a prerequisite for effective treatments1. However, with few exceptions, current antibiotic dosing is based on plasma concentrations, without compartment-specific pharmacokinetic data at infection sites. Inappropriately low antibiotic tissue levels can select for antibiotic-resistant bacteria, leading to treatment failure. Therefore, a growing number of studies and the U.S. Food and Drug Administration (FDA) support measuring antibiotic concentrations in infected tissues2. Importantly, antibiotic treatments for infections in compartments traditionally thought to have restricted antibiotic penetration, such as tuberculous meningitis (TB meningitis), the deadliest form of tuberculosis (TB)3,4,5, are not optimized, and continue to be based on those developed for pulmonary TB3,4, without compartment-specific pharmacokinetic data. There are substantial challenges in sampling deep tissue from live human subjects, especially from the brain, due to the associated risks and costs of these procedures. Moreover, sampling is generally limited to the most accessible lesion at a single time-point, precluding multi-compartment measures in the same subject or determination of concentration-time profiles6.
To overcome these limitations, we have developed novel, clinically translatable tools for noninvasive, unbiased, and in situ multi-compartmental, three-dimensional visualization of antibiotic concentration-time profiles7,8. Here, we perform first-in-human, whole-body, dynamic 18F-pretomanid positron emission tomography (PET) and computed tomography (CT) in eight human subjects (NCT05609552)9 to simultaneously assess brain and lung tissue antibiotic exposures as the area under the concentration-time curve (AUC). 18F-Pretomanid is chemically identical to pretomanid, which is approved by the U.S. FDA for the treatment of multidrug-resistant (MDR) pulmonary TB in combination with bedaquiline and linezolid [BPaL - bedaquiline (B), pretomanid (Pa) and linezolid (L)]10. Further, a bidirectional process to integrate findings from human and animal studies is developed to optimize treatment regimens for TB meningitis (Fig. S1). We perform whole-body dynamic PET with radio analogs of antibiotics active against MDR strains (18F-pretomanid, 18F-sutezolid, 18F-linezolid, and 76Br-bedaquiline) in experimentally infected mice and rabbits to simultaneously assess brain and lung tissue AUCs. Direct measures of antibiotic levels in postmortem tissues from the animal studies are performed using mass spectrometry. PET-facilitated pharmacokinetic modeling and Monte Carlo simulations are used to predict tissue exposures and doses necessary to attain therapeutic brain exposures. These data are used to design optimized, pretomanid-based multidrug regimens which are tested in the mouse model of TB meningitis at human equipotent dosing. Bacterial burden in the brain and lung tissues is quantified as colony-forming units (CFU). Intracerebral inflammation is measured in live animals using 124I-DPA-713, a clinically translatable imaging biomarker of activated microglia and macrophages11,12,13,14, and complemented by postmortem analyses to assess neuroinflammation and markers of brain metabolism and injury.
Results
First-in-human 18F-pretomanid PET studies
Eight human subjects [six healthy volunteers7 and two newly diagnosed TB patients, median age 29, interquartile range (IQR), 27–32] (Supplementary Table S1) underwent, whole-body 18F-pretomanid PET/CT in accordance with the U.S. FDA guidelines for investigational drugs. Dynamic PET was performed for 50–60 min immediately after intravenous injection of 18F-pretomanid (Fig. 1 and Supplementary Fig. S2). Three-dimensional volumes of interest (VOI) were drawn in various compartments using the CT as a reference (Fig. 1b, e) to simultaneously measure time-concentration profiles in the brain and lung compartments (Fig. 1c, f). Tissue-to-plasma AUC ratios were calculated for each compartment as a measure of pretomanid exposures. Overall, 784 different measurements were made, of which 280 were from the brain and lung compartments and 18F-pretomanid distribution was consistent with its known metabolism. 18F-Pretomanid exposures were compartmentalized with median brain and lung AUC0-60min ratios of 2.25 (median; IQR, 2.08–2.54) and 0.97 (median; IQR, 0.67–1.47) respectively (P < 0.001) (Fig. 1g). 18F-pretomanid exposures were significantly lower in the cerebrospinal fluid (CSF) (ventricles) versus the brain parenchyma (Supplementary Fig. S2i; P = 0.027). There were no significant differences in brain or lung exposures between healthy or TB patients (P > 0.078).
Animal studies with antibiotics active against MDR strains
Brain and lung exposures of radio analogs of antibiotics active against MDR strains, 18F-pretomanid7, 18F-sutezolid, 18F-linezolid15, and 76Br-bedaquiline16 (all chemically identical to the parent antibiotic), were assessed using PET in experimentally-infected mice. 18F-Sutezolid was synthesized and validated as described in Supplementary Figs. S3–S5. Dynamic PET was acquired for 60 min for 18F-pretomanid, 18F-sutezolid, and, 18F-linezolid and 48 h for 76Br-bedaquiline (due to its much longer half-life). PET plasma values were derived from blood (left ventricles). Coronal, sagittal, and transverse brain and lung PET AUCtissue/plasma heatmaps, quantification of PET data, and the corresponding direct measures of antibiotic levels in postmortem tissues using mass spectrometry are shown (Fig. 2, Supplementary Figs. S6, S7, and SupplementaryTables S2–S7). All four antibiotics demonstrated compartmentalized exposures with significantly different brain and lung tissue exposures (P < 0.020). While 18F-pretomanid demonstrated a median AUC0-60min ratio > 1 in both brain (1.41; IQR 1.27–1.60) and lung compartments (1.07; IQR 0.96–1.23), sutezolid, linezolid and bedaquiline (AUC0-48h ratio) had substantially lower median brain AUC ratios of 0.29 (IQR, 0.28–0.31), 0.28 (IQR, 0.28–0.29), and 0.21 (IQR, 0.19–0.21) respectively. In contrast, sutezolid, linezolid, and bedaquiline had lung AUCtissue/plasma ratios ~ 1, with bedaquiline demonstrating the highest AUCtissue/plasma ratio of 2.19 (median; IQR, 1.98–2.90). Direct measures of antibiotic levels in postmortem tissues using mass spectrometry confirmed the antibiotic-specific, compartmentalized brain and lung exposures noted on PET imaging (P < 0.008). Mass spectrometry studies were also performed for moxifloxacin and pyrazinamide (Supplementary Fig. S8), which are also active against MDR strains. While pyrazinamide demonstrated excellent brain and lung penetration (median tissue/plasma ratio > 1), moxifloxacin demonstrated limited brain penetration (median tissue/plasma ratio 0.23; IQR 0.20–0.30) but excellent lung penetration (median tissue/plasma ratio 3.34; IQR 3.31–4.34). Similar experiments were performed in experimentally infected rabbits confirming the limited brain exposures for 18F-sutezolid (median AUC0-60min ratio 0.36; IQR 0.27–0.44) and 18F-linezolid (median AUC0-60min ratio 0.23; IQR 0.19–0.25) (Supplementary Fig. S9). However, both antibiotics had a median AUC0-60min ratio of ~ 1 or greater in the lung compartment of rabbits. Direct measures of bedaquiline levels in postmortem tissues using mass spectrometry were also performed in rabbits (Supplementary Fig. S10 and Supplementary Table S8), which confirmed the data from the mouse studies, demonstrating high lung, but much lower brain tissue and CSF levels.
Pharmacokinetic modeling to predict human brain and lung tissues exposures
We developed pharmacokinetic models for pretomanid, sutezolid, linezolid, and bedaquiline that correctly predict human PET and published pharmacokinetic data (Fig. 3 and Supplementary Fig. S11). Monte Carlo simulations (n = 1000 subjects for each antibiotic) were used to predict human brain and lung tissue exposures at various oral doses (Fig. 4). For pretomanid, the standard 200 mg/day oral dose achieved brain tissue exposures substantially higher than the lung tissue exposures. However, while the standard 600 mg/day dose for sutezolid and linezolid achieved therapeutic exposures in the lung tissues, it achieved subtherapeutic brain tissue exposures for both drugs. Several clinical trials are utilizing linezolid at 1200 mg/day for TB meningitis17,18. However, while better than with the 600 mg/day dosing, this higher dose still achieved subtherapeutic brain tissue exposures for sutezolid or linezolid. Finally, bedaquiline brain tissue exposures were substantially lower than in the lung tissues at the standard 400 mg/day dose. Importantly, brain tissue exposures would remain subtherapeutic even at a dose of up to 1600 mg/day.
Optimized pretomanid-based multidrug regimens for TB meningitis
Mice with experimentally-induced TB meningitis were randomly allocated to receive different multidrug regimens administered at human equipotent dosing via oral gavage (Fig. 5)7,14. All animals also received adjunctive dexamethasone, which is the standard of care for TB meningitis19. Bacterial burden was quantified in whole organs as CFU. Data at six weeks after initiation of multidrug regimens (Fig. 5b, c and Supplementary Table S9) show untreated animals representing the starting bacterial burden and two control regimens – first-line, standard TB treatment for drug-susceptible TB meningitis [standard-dose rifampin (human equipotent dose of 10 mg/kg/day, R10), isoniazid (H), and pyrazinamide (Z) – R10HZ] and the U.S. FDA approved treatment for MDR pulmonary TB (BPaL, here forth referred as BPa50L representing the human equipotent Pa dose used in mice equivalent to the standard human dose of 200 mg/day). Pyrazinamide substantially improved the bactericidal activities of all MDR-TB regimens (adjusted P < 0.001), and this effect was abrogated in infections with pyrazinamide-resistant Mycobacterium tuberculosis (pncA mutant)20 (Fig. 5b). Addition of bedaquiline to the Pa50L regimen did not improve its activity (adjusted P = 0.933) (Fig. 5c). Several pretomanid-based multidrug regimens (red regimens in Fig. 5) were found to have bactericidal activities similar (or substantially better) than the first-line standard TB treatment (R10HZ) and substantially better than the BPa50L regimen. The additive effects of sutezolid (S) and linezolid were not significantly different (P = 0.089) (Fig. 5 and Supplementary Fig. S12). Since bacteria disseminate to the lung after a brain infection14,21, we were able to assess the activities of several antibiotic regimens in the brain and lungs simultaneously in the same animal (Fig. 5d). Data are shown as the reduction in whole organ CFU two weeks after initiation of treatment, with larger reductions indicating increased bacterial killing. Consistent with prior data, the BPa50L regimen has excellent activity in the lung, but not in brain tissues. However, regimens optimized for TB meningitis (red regimens in Fig. 5) had better bactericidal activities in the brain compared to lung tissues. These data demonstrate discordant bactericidal activities in the brain versus lung tissues in the same animal.
We assessed intracerebral inflammation in live animals using 124I-DPA-713 two weeks after initiation of treatments (Fig. 6). While treatment with antibiotic regimens decreased intracerebral inflammation compared to untreated animals, pyrazinamide-containing regimens had significantly lower 124I-DPA-713 PET signal compared to the regimens without pyrazinamide (P = 0.016). Imaging studies in live animals were complemented by postmortem analyses to assess neuroinflammation and markers of brain metabolism and injury. Immunohistochemistry studies using Iba-1+ staining (a measure of microglia) of brain tissues were consistent with the imaging findings, with lower microglial density with antibiotic regimens (compared to untreated animals), and significantly lower microglial density in animals treated with pyrazinamide-containing regimens (P = 0.005; Supplementary Fig. S13). Brain and CSF cytokines (Supplementary Figs. S14, S15a), CSF and plasma tryptophan levels (Supplementary Figs. S15b, S16) and CSF and plasma levels of brain injury markers – glial fibrillary acidic protein (GFAP), neurofilament light chain (NEFL or Nfl), Tau and S100B (Supplementary Figs. S15c, S17) demonstrated a similar trend.
Discussion
TB remains a major threat to human health22. TB meningitis is a serious, life-threatening form of TB, and current treatments prevent death or disability in less than half5. Importantly, the central nervous system (CNS) has more than one compartment [e.g., brain parenchyma and CSF], which are separated from the circulation by the blood-brain barrier (BBB) that limits the penetration of many drugs. Despite the knowledge that many antibiotics do not penetrate into the brain adequately and that immunopathology is the critical pathologic process, current TB meningitis treatment is not optimized and continues to be based on those developed for pulmonary TB3,4. Importantly, the alarming rise of MDR strains of M. tuberculosis, poses further challenges in the management of TB meningitis, as tissue pharmacokinetics and activities of newer antibiotics effective against MDR strains are not known. In fact, TB meningitis due to MDR strains is associated with the highest mortality23,24, with drug resistance being an independent predictor of death25. In one report, mortality in TB meningitis patients with drug resistance using current regimens was significantly higher (67%) than in those with drug-susceptible disease (24%, P < 0.001)26. Therefore, effective treatments against TB meningitis due to MDR strains are urgently needed.
Using whole-body, noninvasive, and unbiased, dynamic PET imaging, we were able to obtain a rich dataset of concentration-time profiles in multiple compartments in three-dimensional space simultaneously, demonstrating compartmentalized brain and lung tissue exposures. Animal studies confirmed the compartmentalized pretomanid brain and lung tissue exposures noted in the human studies but also demonstrated antibiotic-specific compartmentalization, e.g., while pretomanid had higher brain versus lung tissue exposures, the opposite was noted for bedaquiline. Direct antibiotic measurements from postmortem animal tissues confirmed the findings from the PET studies. PET-facilitated pharmacokinetic modeling and Monte Carlo simulations were then used to predict antibiotic exposures in brain and lung tissues. Only pretomanid achieved therapeutic brain tissue exposures at the standard human oral dosing and bedaquiline brain tissue exposures remained subtherapeutic even at a dose four times the standard human oral dose. It should be noted that pretomanid up to 1200 mg/day is tolerated well in humans27. Given its concentration-time dependent activity as well as animal studies demonstrating better bactericidal activities at a human equivalent dose of 400–600 mg/day (versus 200 mg/day)28, higher pretomanid dosing could be considered29, especially when the exposures of other antibiotics in the regimen are predicted to be suboptimal. Pyrazinamide has excellent CSF30,31, and lung penetration and exposures correlate with treatment response in patients with MDR pulmonary TB with pyrazinamide-susceptible strains32. However, one study has reported increased mortality and neurological toxicity in TB meningitis with elevated CSF pyrazinamide33, but a mechanistic basis for this finding is not known, and therefore the significance of this association is unclear. Nonetheless, pyrazinamide continues to be used worldwide as the standard first-line regimen for drug-susceptible TB meningitis for the first two months of treatments. Pyrazinamide has strong sterilizing activity critical for shortening pulmonary TB treatments, and one study in children has demonstrated that regardless of disease stage at presentation, a 6-month pyrazinamide-containing regimen was more efficacious than 9 or 12 month regimens without pyrazinamide34. Although brain tissue exposures for pyrazinamide are unknown in humans, PET imaging in non-human primates with a chemically identical radio analog of pyrazinamide demonstrated excellent brain tissue exposures35, which were consistent with our data (mass spectrometry) from the mouse studies. Similarly, while moxifloxacin (Mx), active against several MDR strains of M. tuberculosis, has excellent CSF penetration36,37, brain tissue exposures are not known and our data from mouse studies demonstrated limited brain (tissue/plasma ratio 0.23) but excellent lung tissue penetration (tissue/plasma ratio 3.34). Overall, these data suggested that pretomanid and pyrazinamide based multidrug regimens could be highly effective in TB meningitis. It should be noted that concomitant use of pretomanid and pyrazinamide led to hepatotoxicity-related treatment discontinuations in 6-7% of participants in one study for the treatment of pulmonary TB38, with treatment-emergent elevations of alanine transaminase (ALT) greater than three times the normal limits in 10.8% of those treated with pretomanid-pyrazinamide regimens versus 8.6% and 5.6% in those treated with BPaL, or the first-line TB regimens. However, given the substantial benefits of pyrazinamide32, and the high mortality associated with TB meningitis, especially due to MDR strains, the risk-benefit comparison likely favors its use in TB meningitis. Finally, antibiotics such as sutezolid, linezolid, and moxifloxacin that are active against MDR strains and have moderate brain tissue exposures, could provide additive activity when combined with other highly effective antibiotics.
Data from the Monte Carlo simulations were used to design optimized, pretomanid-based multidrug regimens in the mouse model of TB meningitis7,14, administered at human equipotent doses. While the addition of pyrazinamide substantially improved the bactericidal activities of all MDR-TB regimens, as predicted, the addition of bedaquiline did not. A recent pharmacokinetic modeling study, validated using plasma pharmacokinetic time profiles following different dosing regimens and sparse CSF concentrations data from patients, also predicted bedaquiline and M2 brain levels to be significantly lower than in the lungs at clinically relevant doses39. The additive effects of sutezolid and linezolid were similar, but sutezolid has a better safety profile at higher dosing (up to 1600 mg/day)40, required to achieve better brain tissue exposures. Importantly, we developed several pretomanid-based multidrug regimens (BPa50LZ, Pa100LZ, and Pa50LMxZ) active against TB meningitis due to MDR strains, with bactericidal activities substantially better than R10HZ or BPa50L regimens. However, given the high rates of pyrazinamide resistance amongst MDR strains41, we also developed a pyrazinamide-sparing regimen (Pa100SMx), which while not as bactericidal as pyrazinamide-containing regimens, was still as effective as the first-line standard TB treatment, and substantially better than the BPa50L regimen. Finally, we were able to assess the activities of several antibiotic regimens in the brain and lung compartments in the same animal, which demonstrated discordant bactericidal activities, corresponding to the compartmentalized tissue exposures of the component antibiotics in the regimen. While bactericidal activity is an important endpoint for pulmonary TB treatments, clinical outcomes in TB meningitis may also be closely associated with intracerebral inflammatory responses42. This was assessed using live imaging as well as postmortem analyses demonstrating that the optimized, pretomanid-based multidrug regimens did not increase intracerebral inflammation or markers of brain metabolism and injury. Specifically, pyrazinamide-containing regimens had significantly lower intracerebral inflammation. High CSF tryptophan levels are associated with increased mortality in patients with TB meningitis43, and CSF and plasma tryptophan levels were lower in mice treated with the optimized regimens. Similarly, elevated levels of brain injury markers [GFAP, NEFL (or Nfl), Tau, and S100B] in the CSF or plasma, are associated with poor outcomes in patients with brain damage44, and in TB meningitis45, and brain injury marker levels in CSF and plasma were lower in mice treated with the optimized regimens.
Our studies have some limitations. Healthy volunteers and patients with pulmonary TB were imaged with 18F-pretomanid PET, and pretomanid brain exposures were excellent in these subjects with presumably an intact BBB and healthy brain tissues. It is anticipated that pretomanid brain exposures would remain higher than those plasma in infected tissues7, or in the setting of a leaky BBB in patients with TB meningitis. Due to the high risks of working with MDR M. tuberculosis strains in the laboratory, antibiotic regimens active against MDR strains were evaluated in animals infected with the drug-susceptible M. tuberculosis H37Rv strain. This is the standard and accepted approach for TB drug development, even for antibiotic regimens active against MDR strains46,47. In fact, the vast majority of MDR-TB regimens currently being evaluated in clinical trials or in clinical use (BPaL) were originally developed in mouse models utilizing infections with the drug-susceptible M. tuberculosis H37Rv strain10,20. While 18F-pretomanid is chemically identical to the parent antibiotic, for PET studies it was administered at a microdose (ng-µg per subject) rather than at a therapeutic dose. However, several studies support that microdosing is a reliable predictor of the drug biodistribution at therapeutic doses48,49. Further, 18F-pretomanid and the other radiolabeled antibiotics were administered intravenously with the injection time corresponding to the plasma Tmax, and brain uptake reaching Cmax (maximum concentration) within the first few minutes, enabling the first 60 min to adequately capture the pharmacokinetic profile. Of note, bedaquiline has a much longer half-life than the other antibiotics evaluated here, but given the much longer physical half-life (16 h) of Br-76 utilized as the radiolabel for bedaquiline (by replacing the endogenous Br with Br-76), imaging could be performed for 48 h after tracer injection16. Importantly, all radiolabels were introduced into the antibiotic to keep them chemically identical to the parent compound and retained even after their metabolism7,15,16.
In summary, we report first-in-human dynamic 18F-pretomanid PET/CT in eight human subjects to simultaneously measure brain and lung tissue exposures that demonstrated compartmentalized exposures. PET studies in live animals and direct antibiotic measurements in postmortem tissue samples validated these findings but also demonstrated preferential (AUCtissue/plasma > 1) antibiotic-specific partitioning into brain or lung tissues within the same subject, driven presumably by the physiochemical properties of each antibiotic. PET-facilitated pharmacokinetic modeling was used to design optimized, pretomanid-based multidrug regimens tested at human equipotent dosing in a mouse model of TB meningitis. We developed several multidrug regimens active against TB meningitis due to MDR strains, with bactericidal activities substantially better than R10HZ or BPa50L regimens, without an increase in intracerebral inflammation or markers of brain metabolism and injury. These optimized, pretomanid-based multidrug MDR regimens comprise antibiotics either already approved for human use or being assessed in clinical trials. Therefore, these regimens could be readily evaluated in clinical studies for TB meningitis. Importantly, several antibiotic regimens demonstrated discordant bactericidal activities in the brain versus lung tissues in the same animal, correlating with compartmentalized tissue exposures of the component antibiotics visualized with PET and confirmed by postmortem mass spectrometry. Our data provide a mechanistic basis for the discordant activities of antibiotic regimens for pulmonary TB and TB meningitis. Our approach is highly generalizable and has major implications for antimicrobial drug development and compartment-specific optimization of regimens, especially for meningitis and other infections in compartments with unique antibiotic penetration.
Methods
All protocols were approved by the Johns Hopkins University Biosafety, Radiation Safety, Animal Care and Use (MO19M382, RB22M351, RB19M24) and Institutional Review Board (IRB00303845) Committees. The clinical study was registered on clinicaltrials.gov (NCT05609552)9.
Human studies
18F-Pretomanid was synthesized as a sterile solution with high specific activity (45.54 ± 14.86 GBq/µmol) and high radiochemical purity by the Johns Hopkins PET Center. This observational study was performed in accordance with the U.S. FDA Radioactive Drug Research Committee guidelines50, with biodistribution of 18F-pretomanid, as concentration-time exposures in multiple compartments, as the primary outcome measure. Eight human subjects (six healthy volunteers7 and two newly diagnosed TB patients) were prospectively recruited from the Johns Hopkins Hospitals between May 2022 to December 2023 using the following inclusion criteria (Supplementary Table S10). Confirmation of the TB was performed using culture and / or nucleic acid amplification tests. Written informed consent was obtained from each subject, physical examination performed by a trained physician and screening laboratory tests performed and reviewed by the study principal investigator to confirm eligibility. On the day of imaging, a low-dose CT, selected to minimize the radiation exposure, was performed. An intravenous injection of 18F-pretomanid (358.90 ± 12.64 MBq) was administered. Immediately after tracer administration, a dynamic multi-bed PET acquisition was performed. This sequence involved data acquisition at multiple overlapping bed positions, forming a single “pass”. Multiple passes were sequentially acquired to form dynamic, time-series data over an extended scan range. For the healthy volunteers, each pass was acquired from vertex-of-skull to mid-thigh, a distance of 110 cm, corresponding to 8 bed positions. For TB patients, each pass was acquired from the vertex-of-skull to the abdomen, a distance of 60 cm, corresponding to 4 bed positions. In each case, data were acquired for 1 minute per bed position. Healthy volunteers had 7 passes, and TB patients had 14 passes, for a total scan duration of approximately 1 h. The reconstruction algorithm was ordered-subsets expectation-maximization plus time-of-flight (OSEM + TOF), 2 iterations, 21 subsets, Gaussian filter with 5 mm full-width-at-half-maximum. The matrix size was 200 × 200 and corresponded to 4x4x4 mm voxels. Adverse events were assessed immediately after imaging and 20–25 days after the imaging study via a telephone interview. Blood data was obtained by placing a VOI in the left ventricle of the heart and corrected to plasma using hematocrit and pretomanid red blood cell (RBC) partition coefficients.
Animal studies
Female C3HeB/FeJ mice (6–8 weeks old, Jackson Laboratory) were infected (titrated frozen stocks with ~ 6.5 log10 CFU of M. tuberculosis H37Rv or mutants) via a burr hole (Micro-Drill Kit, Braintree Scientific Inc.) using a Hamilton syringe (Hamilton, 88000) and stereotaxic instrument (David KOPF Instruments, model 900, coordinates 0.6 mm dorsal to bregma, 1.2 mm lateral to middle line, and 2 mm ventral). Male and female New Zealand White rabbits (5–7 days old, Robinson Services Inc.) were infected intraventricularly (titrated frozen stock with ~ 6.5 log10 of M. tuberculosis H37Rv) via the bregma using a 30-gauge insulin syringe. Prior to infection, rabbits were sedated with dexmedetomidine hydrochloride (0.2 µg/g; Zoetis, Florham Park, NJ), and topical anesthesia (lidocaine 4%; Ferndale IP Inc., Ferndale, MI) was applied to the bregma14. Another set of New Zealand White rabbits weighing 2.5–3.5 kg (Charles River, Wilmington, MA) were exposed five times to an M. tuberculosis H37Rv aerosol challenge in the Madison aerosol droplet generation chamber (University of Wisconsin, Madison) and noninvasively monitored by CT (CereTom, Neurologica, Danvers, MA) over 20 weeks for the development of pulmonary TB lesions8. All animals were housed in controlled light and temperature rooms without cross-ventilation in a biosafety level-3 (BSL-3) facility.
Imaging
Details of 18F-sutezolid synthesis, including the precursor and intermediates and in vivo characterization, are described in Supplementary Materials. 124I-DPA-713 was purchased from 3DImaging, LLC through a research contract51. Animal studies were performed within 10 days of starting multidrug regimens. The radiotracers were administered intravenously to M. tuberculosis-infected animals at doses outlined in Supplementary Table S11. Animals were imaged inside sealed biocontainment containers compliant with BSL-3 containment and capable of delivering an O2-anesthetic mixture to sustain live animals during imaging7,52,53. PET/CT acquisition was performed using the nanoScan PET/CT (Mediso, Arlington, VA). Blood data was obtained by placing a VOI in the left ventricle of the heart and corrected to plasma using hematocrit and RBC partitioning for each antibiotic7,21 (and Supplementary Tables S12, S13). For 76Br-bedaquiline mouse studies, raw imaging data from Ordonez et al.16 were reanalyzed and corrected to plasma using hematocrit and bedaquiline RBC partition coefficients (Supplementary Table S13). For 18F-linezolid rabbit studies, raw imaging data from Tucker et al.21 were reanalyzed to include lung tissue exposures.
Antimicrobial treatments
Drug stocks were prepared and administered five days a week via oral gavage at human equipotent dosing (Supplementary Table S14)7,54. Dexamethasone was administered intraperitoneally. Bacterial burden was quantified in whole organs as CFU at two and six weeks after initiation of treatment using 7H11 plates supplemented with activated charcoal. For aerosol infected rabbits, the BPaL regimen was administered five days a week via oral gavage at human equipotent dosing for two weeks.
Mass spectrometry
Tissues were collected at plasma Tmax for each antibiotic in animals that had received at least 10 days of multidrug regimens. Antibiotics and their metabolites were quantified using validated ultra-high-performance liquid chromatography (UPLC) and tandem mass spectrometry (LC–MS/MS) at the Infectious Diseases Pharmacokinetics Laboratory of the University of Florida. The lower limits of detection were 0.05, 0.10, 0.10, 0.30, 0.03, 0.12, 0.50 and 0.20 μg/mL for bedaquiline, M2, pretomanid, linezolid, sutezolid, PNU-101603, pyrazinamide and moxifloxacin, respectively
Cytokines, tryptophan, and brain injury markers
Samples were collected and stored at − 80 °C until analysis. Cytokines were analyzed using the Luminex Multiplex assays by the Johns Hopkins University Oncology Human Immunology Core. Brain injury markers [GFAP (ab233621), NEFL (ab288182), cleaved Tau (ab269557), and S100B (OKEH00537)] and cellular metabolite tryptophan (KA1916) were quantified using their respective ELISA kits (Abcam, Novus Biologicals and Aviva systems biology).
Image analysis
Human images were analyzed using Mirada XD™ 3.6.8 (Mirada Medical) and PMOD version 3.402 (PMOD Technologies LLC) while the animal images were analyzed using VivoQuant 2020 (Invicro). Three-dimensional volumes of interest (VOIs) were drawn using the CT as a reference, and the PET data was extracted as time-activity curves (TACs), which were used to calculate tissue AUCs and represented as AUCtissue/plasma ratios7,8. PET activity was converted from tissue volume to tissue mass using tissue density from the Hounsfield units (CT). Heatmap overlays were created using AMIRA 5.2.1 (Visage Imaging, Inc.) and AMIDE 1.0.6 (Andreas Loening).
Pharmacokinetic modeling
The pharmacokinetic modeling of bedaquiline, pretomanid, linezolid, and sutezolid was based on a published mPBPK framework, which included plasma and lung compartments55. This model was extended to include the CNS by integrating human physiological parameters56. Antibiotic-specific parameters (Supplementary Tables S15–18) were sourced from existing literature models55,57. For sutezolid, these parameters were fine-tuned utilizing digitized pharmacokinetic data. Drug uptake in the lung and brain was explained using the effect compartment model, where exposure within these compartments is defined by the drug transport rates and penetration ratios. The transport rates are governed by blood flow rates, which in turn depend on the volume of the respective organ. Antibiotic penetration coefficients were calculated from the PET data as tissue-to-plasma AUC ratios (AUCtissue/plasma). Since the penetration coefficients are derived from drug concentration in the target tissue compartments, this inherently accounts only for the free drug that is not bound to plasma proteins and is free to enter the tissue compartments. Any possible drug accumulation over the duration of treatment was investigated for all antibiotics. The models were validated through their ability to accurately predict the observed human 18F-pretomanid PET data or the published (digitized) human pharmacokinetic data (Supplementary Table S19).
Monte Carlo simulations were performed based on the mPBPK to predict tissue exposure at various oral doses for the antibiotics under study. One thousand virtual subjects were simulated for each antibiotic, at several dose levels to cover a comprehensive range. The simulations integrated a 40% interindividual variability, anticipating significant differences in drug exposures among patients in clinical settings. Classically, therapeutic targets for antibiotics are chosen based on free drug levels achieved in the tissue of interest. However, data on free drug levels in brain tissue is lacking for most antibiotics, and it is also difficult to estimate free drug tissue levels for highly protein-bound antibiotics (e.g., pretomanid and bedaquiline). Fortunately, there is excellent data correlating antibiotic exposures required for optimal bacterial killing in lung tissues28,55,58,59,60,61, and we therefore chose lung tissue antibiotic exposures achieved with standard oral dosing for patients with pulmonary TB, as the therapeutic target for optimal brain exposures. All analyses were performed using Pumas® version 2.0 (Pumas-AI).
Statistical analysis
Data were analyzed using Prism 10.2.2 (GraphPad). The linear trapezoidal rule was used to calculate PET-derived AUCs. Bacterial burden (CFU) is represented on a logarithmic scale (base 10) as mean ± SD, and comparisons were made using a two-tailed student t test and ANOVA with Sidak’s multiple comparison correction. All other data are represented as median ± IQR and comparisons were made using a two-tailed Mann-Whitney U test. P values ≤ 0.05 were considered statistically significant. No sex or gender-based analyses were performed in humans or animal studies.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
Data availability
All data are available in the main text or the supplementary materials. Source data are provided with this paper.
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Acknowledgements
We thank all the subjects who participated in the study. We would also like to thank Jeff Leal (Johns Hopkins Hospitals) for curating the human imaging data and Kelly Flavahan (Johns Hopkins Hospitals) for assistance with PET/CT imaging. This work was funded by the U.S. National Institutes of Health R01-AI145435-A1 (S.K.J.), R01-AI153349 (S.K.J.), R01-HL131829 (S.K.J.), R21-AI149760 (S.K.J.), and K08-AI139371 (E.W.T.).
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X.C. and S.K.J. conceptualized and designed the studies. M.O.S. and L.S.C. developed and performed the manual radiotracer syntheses with assistance from B.J. J.S.F. and K.M. developed the precursors for 18F-sutezolid and 18F-linezolid. C.A.P. performed mass spectrometry analysis. X.C., O.J.N.-M., M.O.S., and M. Singh performed the mouse studies. M. Singh performed the cytokine, tryptophan, and brain injury marker studies. E.W.T. performed the studies in the rabbit model of TB meningitis. S.K.J. wrote the protocol for the human studies. O.J.N.-M., M. Shah, E.W.T., and S.K.J. recruited and consented the human subjects. B.A. and V.D.I. performed the pharmacokinetic modeling. X.C. analyzed the data in the manuscript. M.O.S. analyzed the mass spectrometry. O.J.N.-M. analyzed the imaging data with help from E.W.T. and S.K.J. S.K.J. provided funding and supervised the project. S.K.J. wrote the manuscript with substantial input from all co-authors.
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Vijay D. Ivaturi is a co-founder and Vijay D. Ivaturi and Bhavatharini Arun are employees of Pumas-AI, which commercializes Pumas and Lyv software. All other authors declare that they have no competing interests.
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Chen, X., Arun, B., Nino-Meza, O.J. et al. Dynamic PET reveals compartmentalized brain and lung tissue antibiotic exposures of tuberculosis drugs. Nat Commun 15, 6657 (2024). https://doi.org/10.1038/s41467-024-50989-4
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DOI: https://doi.org/10.1038/s41467-024-50989-4
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