Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Zhuo Zhang, Alvaro A. Ordonez, Peter Smith-Jones, Hui Wang, Kayla R. Gogarty, Fereidoon Daryaee, Lauren E. Bambarger, Yong S. Chang, Sanjay K. Jain, Peter J. Tonge, Juri G. Gelovani.
5-[18F]F-pyrazinamide (5-[18F]F-PZA), a radiotracer analog of the first-line tuberculosis drug pyrazinamide (PZA), was employed to determine the biodistribution of PZA using PET imaging and ex vivo analysis. 5-[18F]F-PZA was synthesized in 60 min using a halide exchange reaction. The overall decay-corrected yield of the reaction was 25% and average specific activity was 2.6 × 106 kBq (70 mCi)/μmol. The biodistribution of 5-[18F]F-PZA was examined in a pulmonary Mycobacterium tuberculosis mouse model, where rapid distribution of the tracer to the lung, heart, liver, kidney, muscle, and brain was observed. The concentration of 5-[18F]F-PZA was not significantly different between infected and uninfected lung tissue. Biochemical and microbiological studies revealed substantial differences between 5-F-PZA and PZA. 5-F-PZA was not a substrate for pyrazinamidase, the bacterial enzyme that activates PZA, and the minimum inhibitory concentration for 5-F-PZA against M. tuberculosis was more than 100-fold higher than that for PZA.
PZA is a well-established anti-tuberculosis drug and a critical component of first- and second-line drug regimens against tuberculosis (TB), due to its unique ability to synergize with other TB drugs to shorten treatment duration [1, 2]. PZA is active against Mycobacterium tuberculosis, but not other mycobacterial or non-mycobacterial species [3, 4]. It is considered a pro-drug that is converted to the active form pyrazinoic acid (POA) by a mycobacterial amidase, pyrazinamidase (PZase), encoded by pncA . While a majority of PZA-resistant isolates harbor pncA mutations , recent data suggest that host-mediated bioactivation of PZA also occurs . Although the mechanism of PZA still remains to be fully elucidated [5, 8], the lack of definitive in vitro targets [4, 5, 9–11], and surprisingly high in vitro minimum inhibition concentration (MIC) against M. tuberculosis [12–14], suggests that PZA may also act through other mechanisms. Owing to the potent anti-inflammatory activity of the PZA analog nicotinamide , it has been hypothesized that the synergistic effects of PZA (or its active metabolite) may at least, in part, be due to anti-inflammatory effects.
All protocols were approved by the Johns Hopkins and / or Stony Brook University Biosafety, Radiation Safety and Animal Care and Use Committees. All in vitro and animal experiments with M. tuberculosis were performed according to biosafety procedures in the animal biological safety level-3 (ABSL-3) facility at Johns Hopkins University School of Medicine.
5-F-PZA and 5-[18F]F-PZA were synthesized by halogen exchange from 5-Cl-PZA. 5-[18F]F-PZA was synthesized in approximately 60 min from [18F]fluoride in an overall decay-corrected yield of 25% and with an average specific activity of 2.6 × 106 kBq (70 mCi)/μmol. Analytical HPLC demonstrated that after purification there was a single peak on the radioactive HPLC chromatography, suggesting that the final tracer is pure (Fig 3A). In addition, co-injection of 5-F-PZA with the purified tracer resulted in the appearance of an UV absorption peak with the same retention time as that detected using the inline radioactivity detector (Fig 3B). Analytical HPLC thus verified that the radiosynthesis was successful and the radioactive peak that was collected contained 5-[18F]F-PZA.
Molecular imaging provides noninvasive and rapid evaluation of disease processes. Dynamic PET can provide multi-compartment analyses at multiple time points, allowing the characterization of radiolabeled drug concentrations in multiple tissues simultaneously. Previous studies with whole-body PET bioimaging in murine models of TB have provided valuable information about the pharmacokinetics of the first-line TB drugs isoniazid (2-[18F]fluoroisonicotinic acid hydrazide, ) and rifampin (11C-rifampin, ).
In conclusion, we present a successful synthesis of radiolabeled 5-[18F]F-PZA. However, 5-F-PZA was not converted into 5-F-POA by M. tuberculosis PZase, but the possibility of other host-mediated activation pathways has not been ruled out. 5-[18F]F-PZA distributes to various peripheral organs rapidly after intravenous administration but with no significant accumulation in any of these organs. The fast elimination of 5-[18F]F-PZA in combination with the lack of sustained and specific interaction with mycobacterial targets, and the defluorination observed in mice might limit the imaging potential of 5-[18F]F-PZA for TB.