Synthesis of N₄'-[¹⁸F]fluoroalkylated ciprofloxacin as a potential bacterial infection imaging agent for PET study

Supramolecular pyrrole radical cations for bacterial theranostics

Bacterial infections with emerging resistance to antibiotics require urgent development of antibacterial agents with new core skeletons. Recently, a series of antibacterial agents have been reported based on positively charged organic groups, such as ammonium, guanidine, and phosphonium groups, which can selectively bind and destroy negatively charged bacterial membranes. To achieve imaging-guided precise antibacterial therapy, these positively charged organic groups usually require further decoration with imaging modalities, such as fluorescence. However, most fluorophores with electron-closed shell structures usually suffer from tedious synthetic procedures for preparation. We herein prepare a series of positively charged and deep-red fluorescent supramolecular pyrrole radical cations (P˙+-CB[7]) based on the simple mixing of pyrroles and CB[7] in water under air. The readily available deep-red fluorescent P˙+-CB[7] can not only be used for selective imaging and killing of live Gram-positive bacteria with excellent biocompatibility, but also for imaging of dead Gram-negative bacteria killed by drugs and in vivo monitoring of phagocytosis of bacteria by innate immune cells in zebrafish. It is believed that the deep-red fluorescent pyrrole radical cations as a new core skeleton are promising in bacterial theranostics.

Direct 18F-Fluorosulfurylation of Phenols and Amines Using an [18F]FSO2+ Transfer Agent Generated In Situ

We report the direct radiofluorosulfurylation method for the synthesis of ¹⁸F-labeled fluorosulfuryl derivatives from phenols and amines using an [¹⁸F]FSO₂⁺ transfer agent generated in situ. Nucleophilic radiofluorination is achieved even in a hydrous organic medium, obviating the need for azeotropic drying and the use of cryptands. This unprecedented, operationally simple isotopic functionalization facilitates the reliable production of potential radiotracers for positron emission tomography, rendering facile access to SuFEx radiochemistry.

Chimeric Tobramycin-Based Adjuvant TOB-TOB-CIP Potentiates Fluoroquinolone and β-Lactam Antibiotics against Multidrug-Resistant Pseudomonas aeruginosa

According to the World Health Organization, antibiotic resistance is a global health threat. Of particular importance are infections caused by multidrug-resistant Gram-negative bacteria including Escherichia coli, Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa for which limited treatment options exist. Multiple and simultaneously occurring resistance mechanisms including outer membrane impermeability, overexpression of efflux pumps, antibiotic-modifying enzymes, and modification of genes and antibiotic targets have made antibiotic drug development more difficult against these pathogens. One strategy to cope with these challenges is the use of outer membrane permeabilizers that increase the intracellular concentration of antibiotics when used in combination. In some circumstances, this approach can rescue antibiotics from resistance or repurpose currently marketed antibiotics. Tobramycin-based hybrid antibiotic adjuvants that combine two outer membrane-active components have been previously shown to potentiate antibiotics by facilitating transit through the outer membrane, resulting in increased antibiotic accumulation within the cell. Herein, we extended the concept of tobramycin-based hybrid antibiotic adjuvants to tobramycin-based chimeras by engineering up to three different membrane-active antibiotic warheads such as tobramycin, 1-(1-naphthylmethyl)-piperazine, ciprofloxacin, and cyclam into a central 1,3,5-triazine scaffold. Chimera 4 (TOB-TOB-CIP) consistently synergized with ciprofloxacin, levofloxacin, and moxifloxacin against wild-type and fluoroquinolone-resistant P. aeruginosa. Moreover, the susceptibility breakpoints of ceftazidime, aztreonam, and imipenem were reached using the triple combination of chimera 4 with ceftazidime/avibactam, aztreonam/avibactam, and imipenem/relebactam, respectively, against β-lactamase-harboring P. aeruginosa. Our findings demonstrate that tobramycin-based chimeras form a novel class of antibiotic potentiators capable of restoring the activity of antibiotics against P. aeruginosa.

Antibiotic-Derived Radiotracers for Positron Emission Tomography: Nuclear or "Unclear" Infection Imaging?

The excellent features of non-invasive molecular imaging, its progressive technology (real-time, whole-body imaging and quantification), and global impact by a growing infrastructure for positron emission tomography (PET) scanners are encouraging prospects to investigate new concepts, which could transform clinical care of complex infectious diseases. Researchers are aiming towards the extension beyond the routinely available radiopharmaceuticals and are looking for more effective tools that interact directly with causative pathogens. We reviewed and critically evaluated (challenges or pitfalls) antibiotic-derived PET radiopharmaceutical development efforts aimed at infection imaging. We considered both radiotracer development for infection imaging and radio-antibiotic PET imaging supplementing other tools for pharmacologic drug characterization; overall, a total of 20 original PET radiotracers derived from eleven approved antibiotics.

Physicochemical and Biological Study of 99mTc and 68Ga Radiolabelled Ciprofloxacin and Evaluation of [99mTc]Tc-CIP as Potential Diagnostic Radiopharmaceutical for Diabetic Foot Syndrome Imaging

This paper presents the application of ciprofloxacin as a biologically active molecule (vector) for delivering diagnostic radiopharmaceuticals to the sites of bacterial infection. Ciprofloxacin-based radioconjugates containing technetium-99m or gallium-68 radionuclides were synthesised, and their physicochemical (stability, lipophilicity) and biological (binding study to Staphylococcus aureus and Pseudomonas aeruginosa) properties were investigated. Both the tested radiopreparations met the requirements for radiopharmaceuticals, and technetium-99m-labelled ciprofloxacin turned out to be a good radiotracer for the tomography of diabetic foot syndrome using SPECT.

Phosphate-Based Self-Immolative Linkers for the Delivery of Amine-Containing Drugs

Amine-containing drugs often show poor pharmacological properties, but these disadvantages can be overcome by using a prodrug approach involving self-immolative linkers. Accordingly, we designed l-lactate linkers as ideal candidates for amine delivery. Furthermore, we designed linkers bearing two different cargos (aniline and phenol) for preferential amine cargo release within 15 min. Since the linkers carrying secondary amine cargo showed high stability at physiological pH, we used our strategy to prepare phosphate-based prodrugs of the antibiotic Ciprofloxacin. Therefore, our study will facilitate the rational design of new and more effective drug delivery systems for amine-containing drugs.

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

In order to seek novel technetium-99m bacterial infection imaging agents, a ciprofloxacin xanthate (CPF2XT) was synthesized and radiolabeled with [^99mTcN]²⁺ core to obtain the ^99mTcN-CPF2XT complex, which exhibited high radiochemical purity, hydrophilicity, and good stability in vitro. The bacteria binding assay indicated that ^99mTcN-CPF2XT had specificity to bacteria. A study of biodistribution in mice showed that ^99mTcN-CPF2XT had a higher uptake in bacterial infection tissues than in turpentine-induced abscesses, indicating that it could distinguish bacterial infection from sterile inflammation. Compared to ^99mTcN-CPFXDTC, the abscess/blood and abscess/muscle ratios of ^99mTcN-CPF2XT were higher and the uptakes of ^99mTcN-CPF2XT in the liver and lung were obviously decreased. The results suggested that ^99mTcN-CPF2XT would be a potential bacterial infection imaging agent.

Spontaneous conversion of O-tosylates of 2-(piperazin-1-yl)ethanols into chlorides during classical tosylation procedure

A direct conversion of piperazinyl ethanols into chlorides via a classical O-tosylation protocol is observed. The acceleration of the transformation by the piperazine unit is demonstrated. It is found that the reaction goes via the corresponding O-tosylate, which converts spontaneously into chloride with different rate depending on the substrate structure. In the case of pirlindole derivative, partially aromatized chloride formation was observed upon prolongation and/or increased excess of tosyl chloride.

Hydrogen-bond promoted nucleophilic fluorination: concept, mechanism and applications in positron emission tomography

Due to the tremendous interest in carbon-fluorine bond-forming reactions, research efforts in this area have been dedicated to the development of facile processes to synthesize small fluorine-containing organic molecules. Among others, PET (Positron Emission Tomography) is one of the most important applications of fluorine chemistry. Recognizing the specific requirements of PET processes, some groups have focused on fluorination reactions using alkali metal fluorides, particularly through SN2-type reactions. However, a common "misconception" about the role of protic solvents and hydrogen bonding interactions in this class of reactions has hampered the employment of these excellent promoters. Herein, we would like to review recent discoveries in this context, showing straightforward nucleophilic fluorination reactions using alkali metal fluorides promoted by protic solvents. Simultaneous dual activation of reacting partners by intermolecular hydrogen bonding and the enhancement of the "effective fluoride nucleophilicity", which is Nature's biocatalytic approach with the fluorinase enzyme, are the key to this unprecedentedly successful nucleophilic fluorination.

(68) Ga-labeled Ciprofloxacin Conjugates as Radiotracers for Targeting Bacterial Infection

With an aim of developing a bacteria-specific molecular imaging agent, ciprofloxacin has been modified with a propylamine spacer and linked to two common bifunctional chelators, p-SCN-Bz-DOTA and p-SCN-Bz-NOTA. The two ciprofloxacin conjugates, CP-PA-SCN-Bz-DOTA (1) and CP-PA-SCN-Bz-NOTA (2), were radiolabeled with (68)Ga in >90% radiochemical yield and were moderately stable in vitro for 4 h. The efficacy of (68)Ga-1 and (68)Ga-2 has been investigated in vitro in Staphylococcus aureus cells where bacterial binding of the radiotracers (0.9-1.0% for (68)Ga-1 and 1.6-2.3% for (68)Ga-2) could not be blocked in the presence of excess amount of unlabeled ciprofloxacin. However, uptake of radiotracers in live bacterial cells was significantly higher (p < 0.01) than that in non-viable bacterial cells. Bacterial infection targeting efficacy of (68)Ga-1 and (68)Ga-2 was tested in vivo in rats where the infected muscle-to-inflamed muscle ((68)Ga-1: 2 ± 0.2, (68)Ga-2: 3 ± 0.5) and infected muscle-to-normal muscle ratios ((68)Ga-1: 3 ± 0.4, (68)Ga-2: 6.6 ± 0.8) were found to improve at 120 min p.i. Fast blood clearance and renal excretion was observed for both the radiotracers. The two (68)Ga-labeled infection targeting radiotracers could discriminate between bacterial infection and inflammation in vivo and are worthy of further detailed investigation as infection imaging agents at the clinical level.

Targeted imaging of bacterial infections: advances, hurdles and hopes

Bacterial infections represent an increasing problem in modern health care, in particular due to ageing populations and accumulating bacterial resistance to antibiotics. Diagnosis is rarely straightforward and consequently treatment is often delayed or indefinite. Therefore, novel tools that can be clinically implemented are urgently needed to accurately and swiftly diagnose infections. Especially, the direct imaging of infections is an attractive option. The challenge of specifically imaging bacterial infections in vivo can be met by targeting bacteria with an imaging agent. Here we review the current status of targeted imaging of bacterial infections, and we discuss advantages and disadvantages of the different approaches. Indeed, significant progress has been made in this field and the clinical implementation of targeted imaging of bacterial infections seems highly feasible. This was recently highlighted by the use of so-called smart activatable probes and a fluorescently labelled derivative of the antibiotic vancomycin. A major challenge remains the selection of the best imaging probes, and we therefore present a set of target selection criteria for clinical implementation of targeted bacterial imaging. Altogether, we conclude that the spectrum of potential applications for targeted bacterial imaging is enormous, ranging from fundamental research on infectious diseases to diagnostic and therapeutic applications.

Investigation of 6-[¹⁸F]-fluoromaltose as a novel PET tracer for imaging bacterial infection

Despite advances in the field of nuclear medicine, the imaging of bacterial infections has remained a challenge. The existing reagents suffer from poor sensitivity and specificity. In this study we investigate the potential of a novel PET (positron emission tomography) tracer that overcomes these limitations.

Methods

6-[¹⁸F]-fluoromaltose was synthesized. Its behavior in vitro was evaluated in bacterial and mammalian cultures. Detailed pharmacokinetic and biodistribution profiles for the tracer were obtained from a murine model.

Results

6-[¹⁸F]-fluoromaltose is taken up by multiple strains of pathogenic bacteria. It is not taken up by mammalian cancer cell lines. 6-[¹⁸F]-fluoromaltose is retained in infected muscles in a murine model of bacterial myositis. It does not accumulate in inflamed tissue.

Conclusion

We have shown that 6-[¹⁸F]-fluoromaltose can be used to image bacterial infection in vivo with high specificity. We believe that this class of agents will have a significant impact on the clinical management of patients.

Infective severe acute pancreatitis: A comparison of 99mTc-ciprofloxacin scintigraphy and computed tomography

AIM: To evaluate ^99mTc-ciprofloxacin scintigraphy compared with computed tomography (CT) for detecting secondary infections associated with severe acute pancreatitis (SAP) in swine.

METHODS: Six healthy swine were assigned to a normal control group (group A, n = 6). SAP was induced in group B (n = 9) and C (n = 18), followed by inoculation of the resulting pancreatic necroses with inactive Escherichia coli (E. coli) (group B) and active E. coli (group C), respectively. At 7 d after inoculation, a CT scan and a series of analyses using infecton imaging (at 0.5, 1, 2, 3, 4 and 6 h after the administration of 370 MBq of intravenous infecton) were performed. The scintigrams were visually evaluated and semi-quantitatively analyzed using region of interest assignments. The differences in infecton uptake and changes in the lesion-background radioactive count ratios (L/B) in the 3 groups were recorded and compared. After imaging detection, histopathology and bacterial examinations were performed, and infected SAP was regarded as positive. The imaging findings were compared with histopathological and bacteriological results.

RESULTS: In group A, 6 animals survived without infection in the pancreas. In group B, 7/9 swine survived and one suffered from infection. In group C, 15/18 animals survived with infection. Hence, the number of normal, non-infected and infected SAP swine was 6, 6 and 16, respectively. The sensitivity, specificity, accuracy, positive predictive value and negative predictive value of the infecton method were 93.8% (15/16), 91.7% (11/12), 92.9% (26/28), 93.8% (15/16) and 91.7% (11/12), whereas these values for CT were 12.5% (2/16), 100.0% (12/12), 50.0% (14/28), 100.0% (2/2) and 46.2% (12/26), respectively. The changes in L/B for the infected SAP were significantly different from those of the non-infected and normal swine (P < 0.001). The mean L/B of the infectious foci at 0.5, 1, 2, 3, 4 and 6 h was 1.17 ± 0.10, 1.71 ± 0.30, 2.46 ± 0.45, 3.36 ± 0.33, 2.04 ± 0.37 and 1.1988 ± 0.09, respectively. At 3 h, the radioactive counts (2350.25 ± 602.35 k) and the mean L/B of the infectious foci were significantly higher than that at 0.5 h (P = 0.000), 1 h (P = 0.000), 2 h (P = 0.04), 4 h (P = 0.000) and 6 h (P = 0.000).

CONCLUSION: ^99mTc-ciprofloxacin scintigraphy may be an effective procedure for detecting SAP secondary infections with higher sensitivity and accuracy than CT.

Molecular imaging: an innovative force in musculoskeletal radiology

OBJECTIVE

A review of the innovative role molecular imaging plays in musculoskeletal radiology is provided. Musculoskeletal molecular imaging is under development in four key areas: imaging the activity of osteoblasts and osteoclasts, imaging of molecular and cellular biomarkers of arthritic joint destruction, cellular imaging of osteomyelitis, and imaging generators of musculoskeletal pain.

CONCLUSION

Together, these applications suggest that next-generation musculoskeletal radiology will facilitate quantitative visualization of molecular and cellular biomarkers, an advancement that appeared futuristic just a decade ago.