Evaluation of 99mTc-Cefuroxime axetil for imaging of inflammation

Current Status of SPECT Radiopharmaceuticals for Specific Bacteria Imaging

Imaging infection still represents a challenge for researchers. Despite nuclear medicine (NM) offers valuable tools able to discriminate between infections and inflammation, there is an unmet clinical need to develop new strategies able to specifically target the causative pathogen, to select the best antimicrobial treatment for each patient and to accurately assess therapeutic efficacy. These aspects are commonly addressed by microbiology or histology but the diagnosis often relies on invasive procedures that are prone to contamination or sample bias and do not reflect the spatial heterogeneity of the infective process. Therefore, in the era of personalized medicine and treatment, a lot of efforts are in play to improve a personalized diagnosis. Molecular imaging is an ideal candidate for this purpose and, indeed, research is going fast to this direction aiming to find more selective and proper antimicrobial treatments and to overcome broad-spectrum antibiotic use, which still represents the major cause of bacterial drug-resistance. Several approaches for specifically image bacteria have been proposed and provided encouraging perspectives in preclinical studies. Nevertheless, the majority of these promising approaches are still confined in "bench stages" and crucial issues still need to be addressed before their translation in clinical practice. This review will focus on radiolabeled antibiotics for SPECT imaging of bacteria, their mechanisms of action, their potentiality and limitations for "bed-side" applications.

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.

In vitro evaluation of 99m Tc-sultamicillin for infection imaging

Early detection of the site of infection non-invasively with radiolabeled molecules is important for the success of treatment. Technetium-99m labeled antibiotics have the potential to discriminate between bacterial infection and sterile inflammation. Sultamicillin is the tosylate salt of the double ester of sulbactam plus ampicillin. In this study, sultamicillin was labeled with ^99m Tc according to the stannous chloride method. Quality control studies of radiolabeled sultamicillin were performed by radiochromatographic methods. In vitro binding assays were performed in live and heat-killed gram-positive Staphylococcus aureus and gram-negative Escherichia coli strains. The radiolabeling yield of ^99m Tc-sultamicillin was determined as 97.8% ± 3.1% (n = 5). The maximum bacterial uptake of ^99m Tc-sultamicillin was 80.7% ± 11.00% at 4 h for living S. aureus and 93.2% ± 4.40% at 2 h for E. coli. Bacterial uptake study results show that sultamicillin has the potential to be a nuclear imaging agent, especially in infections caused by gram-negative E. coli and gram-positive S. aureus.

Recent Progress in the Molecular Imaging of Tumor-Treating Bacteria

Bacterial cancer therapy (BCT) approaches have been extensively investigated because bacteria can show unique features of strong tropism for cancer, proliferation inside tumors, and antitumor immunity, while bacteria are also possible agents for drug delivery. Despite the rapidly increasing number of preclinical studies using BCT to overcome the limitations of conventional cancer treatments, very few BCT studies have advanced to clinical trials. In patients undergoing BCT, the precise localization and quantification of bacterial density in different body locations is important; however, most clinical trials have used subjective clinical signs and invasive sampling to confirm bacterial colonization. There is therefore a need to improve the visualization of bacterial densities using noninvasive and repetitive in vivo imaging techniques that can facilitate the clinical translation of BCT. In vivo optical imaging techniques using bioluminescence and fluorescence, which are extensively employed to image the therapeutic process of BCT in small animal research, are hard to apply to the human body because of their low penetrative power. Thus, new imaging techniques need to be developed for clinical trials. In this review, we provide an overview of the various in vivo bacteria-specific imaging techniques available for visualizing tumor-treating bacteria in BCT studies.

Evaluation of 99mTechnetium-Vancomycin Imaging Potential in Experimental Rat Model for the Diagnosis of Infective Endocarditis

BACKGROUND

Infective endocarditis (IE) is an infection of the heart's endocardial surface. In recent years, nuclear imaging methods have gained importance in the diagnosis of IE. The present study aims to investigate the imaging potential of ^99mTc-labeled vancomycin (^99mTc-Vancomycin) as a new agent that would enable the diagnosis of IE in its early stages when it is difficult to diagnose or has small vegetation in the experimental rat model.

METHODS

^99mTc-Vancomycin scintigraphy was evaluated for its accumulation in IE with Staphylococcus aureus performed in an experimental rat model. Serial planar scintigraphic and biodistribution analysis of infected vegetations are compared to rats with sterile vegetations. The heart was identified as an infected organ, the liver was identified as a non-infected organ and the heart/liver uptake ratio (T / NT ratio) was compared between infective endocarditis and sterile endocarditis groups.

RESULTS

Planar scintigrams (in vivo measurements) showed more uptake in the heart of rats in the infective endocarditis group compared to the uptake in the heart of rats in the sterile endocarditis group, but this difference was not statistically significant (p>0.05). From the ex vivo measurements, the ^99mTc-Vancomycin heart uptake increased significantly (p = 0.016), liver uptake was significantly decreased (p = 0.045) and the T/NT ratio was significantly higher (p = 0.014) in the infective endocarditis group compared to the sterile endocarditis group.

CONCLUSION

In this experimental study, ^99mTc-Vancomycin scintigraphy ensured the detection of ex vivo infected tissue in a rat model of IE. In addition, the absence of significant ^99mTc-Vancomycin uptake in the sterile endocarditis group indicates that this agent targeted the infected tissue instead of the sterile inflammatory tissue. Finally, this agent should also be evaluated with animal- specific imaging devices.

99mTc-radiolabeled Levofloxacin and micelles as infection and inflammation imaging agents

Easy and early detection of infection and inflammation is essential for early and effective treatment. In this study, PEGylated micelles were designed and both micelles and Levofloxacin were radiolabeled with 99mTcO4 - to develop potential radiotracers for detection of infection/inflammation. Radiolabeling efficiency, in vitro stability and bacterial binding of 99mTc-Levofloxacin and 99mTc-micelles were compared. The aim of this study is to formulate and compare 99mTc-Levofloxacin and 99mTc-micelles as infection and inflammation agents having different mechanisms for the accumulation at infection and inflammation site. PEGylated micelles were designed with a particle size of 80 ± 0.7 nm and proper characterization properties. High radiolabeling efficiency was achieved for 99mTc-Levofloxacin (96%) and 99mTc-micelles (87%). The radiolabeling efficiency was remained stable with some insignificant alterations for both radiotracers at 25 °C for 24 h. Although in vitro bacterial binding of 99mTc-levofloxacine was higher than 99mTc-micelles, 99mTc-micelles may also be evaluated potential agent due to long circulation and passive accumulation mechanisms at infection/inflammation site. Both radiopharmaceutical agents exhibit potential results in design, characterization, radiolabeling efficiency and in vitro bacterial binding point of view.

Ciprofloxacin: from infection therapy to molecular imaging

Diagnosis of deep-seated bacterial infection remains a serious medical challenge. The situation is becoming more severe with the increasing prevalence of bacteria that are resistant to multiple antibiotic classes. Early efforts to develop imaging agents for infection, such as technetium-99m (^99mTc) labeled leukocytes, were encouraging, but they failed to differentiate between bacterial infection and sterile inflammation. Other diagnostic techniques, such as ultrasonography, magnetic resonance imaging, and computed tomography, also fail to distinguish between bacterial infection and sterile inflammation. In an attempt to bypass these problems, the potent, broad-spectrum antibiotic ciprofloxacin was labeled with ^99mTc to image bacterial infection. Initial results were encouraging, but excitement declined when controversial results were reported. Subsequent radiolabeling of ciprofloxacin with ^99mTc using tricarbonyl and nitrido core, fluorine and rhenium couldn't produce robust infection imaging agent and remained in discussion. The issue of developing a robust probe can be approached by reviewing the broad-spectrum activity of ciprofloxacin, labeling strategies, potential for imaging infection, and structure-activity (specificity) relationships. In this review we discuss ways to accelerate efforts to improve the specificity of ciprofloxacin-based imaging.

99mTc-prulifloxacin in artificially infected animals

Aim: The radiosynthesis of 99mTc-Prulifloxacin (99mTc-PRN) was assessed in terms of stability, binding with Staphylococcus aureus (S. aureus), biodistribution in rats (RT) and scintigraphic profile in rabbits (RB). Animals, material, methods: 99mTc-PRN was synthesized by mixing 25 μg of stannous fluoride (SnF2) with 18.5 MB of sodium pertechnetate. Thereafter, 0.5 mg of the prufloxacin (PRN) was added to the reaction mixture and the pH was set at 5.1 with 0.01 mol/l HCl. The reaction mixture was incubated at room temperature. The same process was repeated by increasing the concentration of the stannous fluoride from 25 to 250 μg, sodium pertechnetate from 18,5 to 185 MBq and the PRN from 0.5 to 5 mg. The radiochemical stability of the 99mTc-PRN was investigated in higher concentration of the cystein. In-vitro binding investigation was performed using living and heat killed S. aureus to verify specificity of the 99mTc-PRN. Biodistribution was evaluated in artificially infected rats and scintigraphic precision in rabbits at different interval. Results: The 99mTc-RPN prepared by mixing 2 mg of PRN, 74 MBq sodium pertechnetate, 100 μg stannous fluoride at pH 5.4, appeared to be more than 90% stable with a maximum radiochemical yield of 98.15 ± 0.25% at 30 min. The 99mTc-PRN showed higher stability in serum and satisfactory in-vitro binding to living as compared to heat killed S. aureus. 14.25 ± 0.15% of the injected dose was accumulated in the infected muscle of the model RT. Infected to normal muscle ratio was 5.12 and inflamed to normal muscle was 1.2. The biodistribution was validated by the scintigraphic localization of infection in rabbits. Conclusion: This investigation of 99mTc-PRN confirmed its momentous radiochemical immovability in saline, serum, preferential in-vitro binding to living bacteria, higher uptake in the infected muscle of model RT and precise localization in the infected muscle of model RB.

Technetium-99m based small molecule radiopharmaceuticals and radiotracers targeting inflammation and infection

^99mTc-labeled antibiotics, antifungal drugs, antimicrobial peptides and COX-2 inhibitors are comprehensively reviewed.

Imaging bacteria with radiolabelled quinolones, cephalosporins and siderophores for imaging infection: a systematic review

Bacterial infections are still one of the main causes of patient morbidity and mortality worldwide. Nowadays, many imaging techniques, like computed tomography or magnetic resonance imaging, are used to identify inflammatory processes, but, although they recognize anatomical modifications, they cannot easily distinguish bacterial infective foci from non bacterial infections. In nuclear medicine, many efforts have been made to develop specific radiopharmaceuticals to discriminate infection from sterile inflammation. Several compounds (antimicrobial peptides, leukocytes, cytokines, antibiotics…) have been radiolabelled and tested in vitro and in vivo, but none proved to be highly specific for bacteria. Indeed factors, including the number and strain of bacteria, the infection site, and the host condition may affect the specificity of tested radiopharmaceuticals. Ciprofloxacin has been proposed and intensively studied because of its easy radiolabelling method, broad spectrum, and low cost, but at the same time it presents some problems such as low stability or the risk of antibiotic resistance. Therefore, in the present review studies with ciprofloxacin and other radiolabelled antibiotics as possible substitutes of ciprofloxacin are reported. Among them we can distinguish different classes, such as cephalosporins, fluoroquinolones, inhibitors of nucleic acid synthesis, inhibitors of bacterial cell wall synthesis and inhibitors of protein synthesis; then also others, like siderophores or maltodextrin-based probes, have been discussed as bacterial infection imaging agents. A systematic analysis was performed to report the main characteristics and differences of each antibiotic to provide an overview about the state of the art of imaging infection with radiolabelled antibiotics.

Radiotracers used for the scintigraphic detection of infection and inflammation

Over the last forty years, a small group of commercial radiopharmaceuticals have found their way into routine medical use, for the diagnostic imaging of patients with infection or inflammation. These molecular radiotracers usually participate in the immune response to an antigen, by tagging leukocytes or other molecules/cells that are endogenous to the process. Currently there is an advancing effort by researchers in the preclinical domain to design and develop new agents for this application. This review discusses radiopharmaceuticals used in the nuclear medicine clinic today, as well as those potential radiotracers that exploit an organism's defence mechanisms to an infectious or inflammatory event.

99mTc-Novobiocin: a~novel radiotracer for infection imaging

The radiosynthesis of 99mTc-Novobiocin (99mTc-NBN) complex and its suitability as a radiotracer for infection imaging was assessed. The radiochemical purity (RCP) of the 99mTc-NBN complex was determined using radio-TLC and radioactive HPLC and biodistribution was studied in artificially infected (A.I.) rats and rabbit, using single well gamma counter (SWGRC) interface with scalar count rate meter (SCRM) and Gamma Camera (γ-CM). The maximum RCP observed for the preparation having 2 mg of NBN, 111 MBq of sodium pertechnetate (Na99mTcO4) and 125 μL of SnF2 (1 μg/μL in 0.01 N HCl) at a pH 5.6 was 98.97±0.40% and remained stable >90% up to 120 min. The activity of the 99mTc-NBN in the infected muscle (TI) was significantly increased from 6.50±0.15 to 19.00±0.17% and decreased in the inflamed muscle (TII), normal muscle (NT), blood, liver, spleen, stomach and intestine within 120 min. The TI/NT and TII/NT ratios were 7.60±1.08 and 1.60±1.14. The Whole Body Static (WBS) images of A.I. rabbit were obtained at 30, 40, 50 and 60 min after the I.V. administration of 111 MBq of 99mTc-NBN to the A.I. rabbit. The stability in saline and serum, higher TI/NT, lower TII/NT ratios and WBS images confirmed the feasibility of the 99mTc-NBN complex as an infection imaging agent.