Design, Preparation, and Evaluation of a Novel 99mTcN Complex of Ciprofloxacin Xanthate as a Potential Bacterial Infection Imaging Agent

Recently developed radiopharmaceuticals for bacterial infection imaging

BACKGROUND

Infection remains a major cause of morbidity and mortality, regardless of advances in antimicrobial therapy and improved knowledge of microorganisms. With the major global threat posed by antimicrobial resistance, fast and accurate diagnosis of infections, and the reliable identification of intractable infection, are becoming more crucial for effective treatment and the application of antibiotic stewardship. Molecular imaging with the use of nuclear medicine allows early detection and localisation of infection and inflammatory processes, as well as accurate monitoring of treatment response. There has been a continuous search for more specific radiopharmaceuticals to be utilised for infection imaging. This review summarises the most prominent discoveries in specifically bacterial infection imaging agents over the last five years, since 2019.

MAIN BODY

Some promising new radiopharmaceuticals evaluated in patient studies are reported here, including radiolabelled bacterial siderophores like [68Ga]Ga-DFO-B, radiolabelled antimicrobial peptide/peptide fragments like [68Ga]Ga-NOTA-UBI29-41, and agents that target bacterial synthesis pathways (folic acid and peptidoglycan) like [11C]para-aminobenzoic acid and D-methyl-[11C]-methionine, with clinical trials underway for [18F]fluorodeoxy-sorbitol, as well as for 11C- and 18F-labelled trimethoprim.

CONCLUSION

It is evident that a great deal of effort has gone into the development of new radiopharmaceuticals for infection imaging over the last few years, with remarkable progress in preclinical investigations. However, translation to clinical trials, and eventually clinical Nuclear Medicine practice, is apparently slow. It is the authors' opinion that a more structured and harmonised preclinical setting and well-designed clinical investigations are the key to reliably evaluate the true potential of the newly proposed infection imaging agents.

Recent Advancements in Radiopharmaceuticals for Infection Imaging

COVID-19 pandemic has heightened the interest toward diagnosis and treatment of infectious diseases. Nuclear medicine, with its powerful scintigraphic, single photon emission computer tomography (SPECT), and positron emission tomography (PET) imaging modalities, has always played an important role in diagnosis of infections and distinguishing them from the sterile inflammation. In addition to the clinically available radiopharmaceuticals, there has been a decades-long effort to develop more specific imaging agents with some examples being radiolabeled antibiotics and antimicrobial peptides for bacterial imaging, radiolabeled antifungals for fungal infections imaging, radiolabeled pathogen-specific antibodies, and molecular engineered constructs. In this chapter, we discuss some examples of the work published in the last decade on developing nuclear imaging agents for bacterial, fungal, and viral infections to generate more interest among nuclear medicine community toward conducting clinical trials of these novel probes, as well as toward developing novel radiotracers for imaging infections.

Radiosynthesis, quality control, biodistribution, and infection-imaging study of a new 99mTc-labeled ertapenem radiopharmaceutical

Ertapenem is a member of carbapenem antibiotics used for the treatment of moderate-to-severe intra-abdominal, urinary tract, acute pelvic, and post-surgical gynecologic infections. The antibacterial activity of ertapenem is mediated through binding to penicillin-binding proteins which results in inhibiting the cross-linking of the peptidoglycan layer of the bacterial cell wall. Therefore, ertapenem can be labeled with technetium-99m (^99mTc), a gamma emitter radionuclide, for the diagnosis of deep-seated bacterial infections, such as urinary tract, intra-abdominal, osteomyelitis, and post-surgical gynecologic infections. The labeling procedure was carried out by varying the reaction conditions, such as the amount of the ligand and reducing agent, pH, reaction time and temperature, and radioactivity. At optimized reaction conditions more than 93% ^99mTc–ertapenem radioconjugate was obtained. ^99mTc–ertapenem was found 90% intact in saline medium up to 6 h, while 88% intact in human blood serum up to 3 h. Biodistribution study showed target-to-non-target ratios of 2.91 ± 0.19, 2.39 ± 0.31, and 1.23 ± 0.22 in S. aureus, E. coli, and turpentine oil-infected rat models, respectively. The SPECT scintigraphy showed high uptake of ^99mTc–ertapenem in bacterial-infected abscesses, and low counts were recorded in normal and turpentine oil-inflamed tissues. In conclusion, ^99mTc–ertapenem can be a potent infection-imaging agent, which can diagnosis deep-seated bacterial infections at early stage but need further pre-clinical evaluation in variety of infection models.

Highlights of the Latest Developments in Radiopharmaceuticals for Infection Imaging and Future Perspectives

COVID-19 pandemic has heightened the interest toward diagnosis and treatment of infectious diseases. Nuclear medicine with its powerful scintigraphic, single photon emission computer tomography (SPECT) and positron emission tomography (PET) imaging modalities has always played an important role in diagnosis of infections and distinguishing them from the sterile inflammation. In addition to the clinically available radiopharmaceuticals there has been a decades-long effort to develop more specific imaging agents with some examples being radiolabeled antibiotics and antimicrobial peptides for bacterial imaging, radiolabeled anti-fungals for fungal infections imaging, radiolabeled pathogen-specific antibodies and molecular engineered constructs. In this opinion piece, we would like to discuss some examples of the work published in the last decade on developing nuclear imaging agents for bacterial, fungal, and viral infections in order to generate more interest among nuclear medicine community toward conducting clinical trials of these novel probes, as well as toward developing novel radiotracers for imaging infections.

99m Tc-labeled antibiotics for infection diagnosis: Mechanism, action, and progress

Discovery of penicillin marked a turning point in the history of infection therapy which also led to the emergence of bacterial resistance. It is now 100 years to fight with ever-muted variants of pathogens by developing more and more antibiotics. Since 1987 to todate, no successful class of antibiotic was introduced; this three decade period is known as "the discovery void" period. While, the clinically approved antibiotics are gradually dying in front of bacterial resistance due to which bacterial infections are appearing leading cause of death and disability. Nuclear medicine imaging technique is the strongest modality to diagnose and follow-up of deep-seated and complicated infections. However, the selection of radiolabeled antimicrobial agents plays critical role in gaining sensitivity and specificity of the imaging results. This review comprises of two main sections; first section explains antibiotic targets, and second section explains the imaging efficacy of ^99m Tc-labeled antimicrobial agents against bacterial infection along with the emphasis on progress and update of ^99m Tc-labeled antibiotics as infection imaging probes. The review, in conclusion, could be an acceleration for radiopharmaceutical chemists for designing and developing ^99m Tc-labeled antimicrobial agents to improve infection imaging quality.

Synthesis and Evaluation of Novel Norfloxacin Isonitrile 99mTc Complexes as Potential Bacterial Infection Imaging Agents

To develop potential technetium-99m single-photon emission computed tomography (SPECT) imaging agents for bacterial infection imaging, the novel norfloxacin isonitrile derivatives CN4NF and CN5NF were synthesized and radiolabeled with a [^99mTc][Tc(I)]⁺ core to obtain [^99mTc]Tc-CN4NF and [^99mTc]Tc-CN5NF. These compounds were produced in high radiolabeling yields and showed hydrophilicity and good stability in vitro. The bacterial binding assay indicated that [^99mTc]Tc-CN4NF and [^99mTc]Tc-CN5NF were specific to bacteria. Compared with [^99mTc]Tc-CN4NF, biodistribution studies of [^99mTc]Tc-CN5NF showed a higher uptake in bacteria-infected tissues than in turpentine-induced abscesses, indicating that [^99mTc]Tc-CN5NF could distinguish bacterial infection from sterile inflammation. In addition, [^99mTc]Tc-CN5NF had higher abscess/blood and abscess/muscle ratios. SPECT image of [^99mTc]Tc-CN5NF showed that there was a clear accumulation in the infection site, suggesting that it could be a potential bacterial infection imaging radiotracer.