High Enantiomeric Excess In-Loop Synthesis of d-[methyl-¹¹C]Methionine for Use as a Diagnostic Positron Emission Tomography Radiotracer in Bacterial Infection

Abstract

Currently, there exists no accurate, noninvasive clinical imaging method to detect living bacteria in vivo. Our goal is to provide a positron emission tomography (PET) method to image infection by targeting bacteria-specific metabolism. Standard of care methodologies detect morphologic changes, image immunologic response to infection, or employ invasive tissue sampling with associated patient morbidity. These strategies, however, are not specific for living bacteria and are often inadequate to detect bacterial infection during fever workup. As such, there is an unmet clinical need to identify and validate new imaging tools suitable for noninvasive, in vivo (PET) imaging of living bacteria. We have shown that D-[methyl-11 C]methionine (D-[ 11 C]Met) can distinguish active bacterial infection from sterile inflammation in a murine infection model and is sensitive to both Gram-positive and Gram-negative bacteria. Here, we report an automated and >99% enantiomeric excess (ee) synthesis of D-[ 11 C]Met from a linear D-homocysteine precursor, a significant improvement over the previously reported synthesis utilizing a D-homocysteine thiolactone hydrochloride precursor with approximately 75−85% ee. Furthermore, we took additional steps toward applying D-[ 11 C]Met to infected patients. D-[ 11 C]Met was subject to a panel of clinically relevant bacterial strains and demonstrated promising sensitivity to these pathogens. Finally, we performed radiation dosimetry in a normal murine cohort to set the stage for translation to humans in the near future.

Arabinofuranose‐derived positron‐emission tomography radiotracers for detection of pathogenic microorganisms

“…For d ‐2‐ 18 F‐AF the highest radioactivity was retained for Proteus mirabilis , Salmonella typhimurium , Enterococcus faecalis and S. epidermidis (approximately 7‐8 Bq/10 million cells). The uptake levels of d ‐2‐ 18 F‐AF in these four bacteria were similar (<3‐fold difference based on incorporated radioactivity/10 million cells) to data recently obtained for d ‐[methyl‐ 11 C]methionine 33 . Evaluation of l ‐2‐ 18 F‐AF showed the highest uptake in K. pneumoniae , Acinetobacter baumannii , and M. marinum that retained between 3.5 and 4 Bq/10 million cells.…”

Pathophysiology and Molecular Imaging of Diabetic Foot Infections

“…One analog of D-methionine, D-[methyl-11 C]methionine, has been used to differentiate sterile inflammation from E. coli and S. aureus infections in a murine model, reporting an SUV max 8-10 times higher (SUV max = 0.8% ID/cc) in E.coli and S. aureus compared to [ 11 C]L-methionine (SUV max = 0.1% ID/cc) [146]. An in vitro study demonstrated the broad sensitivity of D-[methyl 11 C] methionine across a panel of clinically relevant DFI pathogens, including E. coli, P. aeruginosa, P. mirabilis, S. aureus, S. epidermidis, and E. faecalis, with the greatest uptake occurring in S. aureus and P. aeruginosa [147]. A recent study compared the uptake of D-5-[ 11 C]glutamine and L-5-[ 11 C]glutamine isomers in a murine dual pathogen myositis model with localized MRSA and E.coli infections.…”

Radiotracers for Bone Marrow Infection Imaging

“…A 2017 study has, however, demonstrated that both E. coli and S. aureus have high uptake of D-methionine, while the D-form has low uptake in animal and human tissue, for which reason D-[ 11 C]methionine in a mouse model was demonstrated to have much more infection-specific uptake than L-[ 11 C]methionine [ 53 ]. The group has since then developed a new synthesis for D-[ 11 C]methionine, performed in vitro tests showing uptake of the tracer in a broad range of clinically relevant bacteria, and are preparing for in-human evaluation of D-[ 11 C]methionine in patients with, e.g., vertebral discitis-osteomyelitis [ 54 ].…”