Stereoselective11C Labeling of a “Native” Tetrapeptide by Using Asymmetric Phase-Transfer Catalyzed Alkylation Reactions

Insights into Peptidoglycan-Targeting Radiotracers for Imaging Bacterial Infections: Updates, Challenges, and Future Perspectives

The unique structural architecture of the peptidoglycan allows for the stratification of bacteria as either Gram-negative or Gram-positive, which makes bacterial cells distinguishable from mammalian cells. This classification has received attention as a potential target for diagnostic and therapeutic purposes. Bacteria's ability to metabolically integrate peptidoglycan precursors during cell wall biosynthesis and recycling offers an opportunity to target and image pathogens in their biological state. This Review explores the peptidoglycan biosynthesis for bacteria-specific targeting for infection imaging. Current and potential radiolabeled peptidoglycan precursors for bacterial infection imaging, their development status, and their performance in vitro and/or in vivo are highlighted. We conclude by providing our thoughts on how to shape this area of research for future clinical translation.

Enantioselective Synthesis of Carbon-11-Labeled Amino Acids and Peptides

Radiolabeled amino acids (AAs), their derivatives, and peptides are essential radiotracers in nuclear imaging. Despite its potential, the preparation of enantiopure radiopharmaceuticals poses several challenges, demanding a great need for rapid and stereocontrolled reactions. This chapter describes a highly stereoselective carbon-11 alkylation of Schiff bases, to obtain radiolabeled AAs and small peptides. The method uses chiral quaternary ammonium salt phase-transfer catalyst with two alkylating agents, namely, [11C]methyl iodide and [11C]benzyl iodide. This methodology allows the radiolabeling of AAs and peptides with excellent regioselectivity and enantiomeric or diastereomeric excess.

Recent Developments in Carbon-11 Chemistry and Applications for First-In-Human PET Studies

Positron emission tomography (PET) is a molecular imaging technique that makes use of radiolabelled molecules for in vivo evaluation. Carbon-11 is a frequently used radionuclide for the labelling of small molecule PET tracers and can be incorporated into organic molecules without changing their physicochemical properties. While the short half-life of carbon-11 (¹¹C; t_½ = 20.4 min) offers other advantages for imaging including multiple PET scans in the same subject on the same day, its use is limited to facilities that have an on-site cyclotron, and the radiochemical transformations are consequently more restrictive. Many researchers have embraced this challenge by discovering novel carbon-11 radiolabelling methodologies to broaden the synthetic versatility of this radionuclide. This review presents new carbon-11 building blocks and radiochemical transformations as well as PET tracers that have advanced to first-in-human studies over the past five years.

Sensing Living Bacteria in Vivo Using d-Alanine-Derived 11C Radiotracers

Incorporation of d-amino acids into peptidoglycan is a unique metabolic feature of bacteria. Since d-amino acids are not metabolic substrates in most mammalian tissues, this difference can be exploited to detect living bacteria in vivo. Given the prevalence of d-alanine in peptidoglycan muropeptides, as well as its role in several antibiotic mechanisms, we targeted this amino acid for positron emission tomography (PET) radiotracer development. d-[3-11C]Alanine and the dipeptide d-[3-11C]alanyl-d-alanine were synthesized via asymmetric alkylation of glycine-derived Schiff-base precursors with [11C]methyl iodide in the presence of a cinchonidinium phase-transfer catalyst. In cell experiments, both tracers showed accumulation by a wide variety of both Gram-positive and Gram-negative pathogens including Staphylococcus aureus and Pseudomonas aeruginosa. In a mouse model of acute bacterial myositis, d-[3-11C]alanine was accumulated by living microorganisms but was not taken up in areas of sterile inflammation. When compared to existing clinical nuclear imaging tools, specifically 2-deoxy-2-[18F]fluoro-d-glucose and a gallium citrate radiotracer, d-alanine showed more bacteria-specific uptake. Decreased d-[3-11C]alanine uptake was also observed in antibiotic-sensitive microbes after antimicrobial therapy, when compared to that in resistant organisms. Finally, prominent uptake of d-[3-11C]alanine uptake was seen in rodent models of discitis-osteomyelitis and P. aeruginosa pneumonia. These data provide strong justification for clinical translation of d-[3-11C]alanine to address a number of important human infections.

Synthesis and Preclinical Evaluation of the First Carbon-11 Labeled PET Tracers Targeting Substance P1-7

Two potent SP_1–7 peptidomimetics have been successfully radiolabeled via [¹¹C]CO₂-fixation with excellent yields, purity, and molar activity. l-[¹¹C]SP_1–7-peptidomimetic exhibited promising ex vivo biodistribution profile. Metabolite analysis showed that l-[¹¹C]SP_1–7-peptidomimetic is stable in brain and spinal cord, whereas rapid metabolic degradation occurs in rat plasma. Metabolic stability can be significantly improved by substituting l-Phe for d-Phe, preserving 70% more of intact tracer and resulting in better brain and spinal cord tracer retention. Positron emission tomography (PET) scanning confirmed moderate brain (1.5 SUV; peak at 3 min) and spinal cord (1.0 SUV; peak at 10 min) uptake for l- and d-[¹¹C]SP_1–7-peptidomimetic. A slight decrease in SUV value was observed after pretreatment with natural peptide SP_1–7 in spinal cord for l-[¹¹C]SP_1–7-peptidomimetic. On the contrary, blocking using cold analogues of l- and d-[¹¹C]tracers did not reduce the tracers’ brain and spinal cord exposure. In summary, PET scanning of l- and d-[¹¹C]SP_1–7-peptidomimetics confirms rapid blood–brain barrier and blood–spinal-cord barrier penetration. Therefore, further validation of these two tracers targeting SP_1–7 is needed in order to define a new PET imaging target and select its most appropriate radiopharmaceutical.

Small Molecule PET Tracers in Drug Discovery

The process of discovering and developing a new pharmaceutical is a long, difficult, and risky process that requires numerous resources. Molecular imaging techniques such as PET have recently become a useful tool for making decisions along a drug candidate's development timeline. PET is a translational, noninvasive imaging technique that provides quantitative information about a potential drug candidate and its target at the molecular level. Using this technique provides decisional information to ensure that the right drug candidate is being chosen, for the right target, at the right dose within the right patient population. This review will focus on small molecule PET tracers and how they are used within the drug discovery process. PET provides key information about a drug candidate's pharmacokinetic and pharmacodynamic properties in both preclinical and clinical studies. PET is being used in all phases of the drug discovery and development process, and the goal of these studies are to accelerate the process in which drugs are developed.