Peptidoglycan-Targeted [18F]3,3,3-Trifluoro-d-alanine Tracer for Imaging Bacterial Infection

Alexandre M Sorlin; Marina López-Álvarez; Jacob Biboy; Joe Gray; Sarah J Rabbitt; Junaid Ur Rahim; Sang Hee Lee; Kondapa Naidu Bobba; Joseph Blecha; Mathew F L Parker; Robert R Flavell; Joanne Engel; Michael Ohliger; Waldemar Vollmer; David M Wilson

doi:10.1021/jacsau.3c00776

Peptidoglycan-Targeted [¹⁸F]3,3,3-Trifluoro-d-alanine Tracer for Imaging Bacterial Infection

JACS Au. 2024 Feb 26;4(3):1039-1047. doi: 10.1021/jacsau.3c00776. eCollection 2024 Mar 25.

Authors

Alexandre M Sorlin¹, Marina López-Álvarez¹, Jacob Biboy², Joe Gray², Sarah J Rabbitt¹, Junaid Ur Rahim¹, Sang Hee Lee¹, Kondapa Naidu Bobba¹, Joseph Blecha¹, Mathew F L Parker^{1

3}, Robert R Flavell^{1

4

5}, Joanne Engel^{6

7}, Michael Ohliger^{1

8}, Waldemar Vollmer^{2

9}, David M Wilson¹

Affiliations

¹ Department of Radiology, Biomedical Imaging University of California, San Francisco, San Francisco, California 94158, United States.
² The Centre for Bacterial Cell Biology, Newcastle University Newcastle, Newcastle upon Tyne NE2 4AX, United Kingdom.
³ Department of Psychiatry, Renaissance School of Medicine at Stony Brook University, Stony Brook, New York 11794, United States.
⁴ UCSF Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, San Francisco, California 94158, United States.
⁵ Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California 94158, United States.
⁶ Department of Medicine, University of California, San Francisco, San Francisco, California 94158, United States.
⁷ Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, California 94158, United States.
⁸ Department of Radiology, Zuckerberg San Francisco General Hospital, San Francisco, California 94110, United States.
⁹ Institute for Molecular Bioscience, The University of Queensland, Brisbane 4072, Australia.

Abstract

Imaging is increasingly used to detect and monitor bacterial infection. Both anatomic (X-rays, computed tomography, ultrasound, and MRI) and nuclear medicine ([¹¹¹In]-WBC SPECT, [¹⁸F]FDG PET) techniques are used in clinical practice but lack specificity for the causative microorganisms themselves. To meet this challenge, many groups have developed imaging methods that target pathogen-specific metabolism, including PET tracers integrated into the bacterial cell wall. We have previously reported the d-amino acid derived PET radiotracers d-methyl-[¹¹C]-methionine, d-[3-¹¹C]-alanine, and d-[3-¹¹C]-alanine-d-alanine, which showed robust bacterial accumulation in vitro and in vivo. Given the clinical importance of radionuclide half-life, in the current study, we developed [¹⁸F]3,3,3-trifluoro-d-alanine (d-[¹⁸F]-CF₃-ala), a fluorine-18 labeled tracer. We tested the hypothesis that d-[¹⁸F]-CF₃-ala would be incorporated into bacterial peptidoglycan given its structural similarity to d-alanine itself. NMR analysis showed that the fluorine-19 parent amino acid d-[¹⁹F]-CF₃-ala was stable in human and mouse serum. d-[¹⁹F]-CF₃-ala was also a poor substrate for d-amino acid oxidase, the enzyme largely responsible for mammalian d-amino acid metabolism and a likely contributor to background signals using d-amino acid derived PET tracers. In addition, d-[¹⁹F]-CF₃-ala showed robust incorporation into Escherichia coli peptidoglycan, as detected by HPLC/mass spectrometry. Based on these promising results, we developed a radiosynthesis of d-[¹⁸F]-CF₃-ala via displacement of a bromo-precursor with [¹⁸F]fluoride followed by chiral stationary phase HPLC. Unexpectedly, the accumulation of d-[¹⁸F]-CF₃-ala by bacteria in vitro was highest for Gram-negative pathogens in particular E. coli. In a murine model of acute bacterial infection, d-[¹⁸F]-CF₃-ala could distinguish live from heat-killed E. coli, with low background signals. These results indicate the viability of [¹⁸F]-modified d-amino acids for infection imaging and indicate that improved specificity for bacterial metabolism can improve tracer performance.