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.

Preparation, biological evaluation and radiolabeling of [99mTc]-technetium tricarbonyl procainamide as a tracer for heart imaging in mice

This study focuses on the synthesis and preliminary bio-evaluation of [99mTc]-technetium tricarbonyl procainamide ([99mTc]-technetium tricarbony PA) as a viable cardiac imaging agent. The compound, [99mTc]-technetium tricarbony PA, was synthesized by labelling procainamide with a [99mTc]-technetium tricarbonyl core, yielding a high radiochemical yield and radiochemical purity of 98%. Under optimal circumstances, high radiochemical yield and purity were obtained utilizing [99mTc]-technetium tricarbonyl core within 30 min of incubation at pH 9, 200 µg substrate concentration, and 100 °C reaction temperature. The heart showed a high absorption of 32.39 ± 0.88% of the injected dose/g organ (ID/g), confirming the suitability of [99mTc]-technetium tricarbonyl PA as a viable complex for heart imaging.

Enhancement of the thermal and physicochemical properties of styrene butadiene rubber composite foam using nanoparticle fillers and electron beam radiation

This study investigates the physicochemical and thermal properties of styrene–butadiene rubber (SBR) nanocomposite foam. Nano-calcium carbonate (CaCO3) was prepared from eggshells (ESs) waste. Sponge rubber nanocomposites were prepared and were irradiated by electron beam (EB) radiation at 25, 75, and 150 kGy. Their physicochemical properties, including foam density, compression set (CS), hardness, abrasion loss, and expansion ratio, and their thermal stability were investigated. The physicochemical properties were enhanced by adding 2.5 phr of a foaming agent. Among the composites examined, the foam composites containing nano-CaCO3 had the lowest CS, abrasion loss, and expansion ratio and the highest hardness and foam density. The results confirmed that the thermal stability was improved by incorporating nano-CaCO3 into the SBR foam and as the radiation dose increased. The sponge containing nanoclay demonstrated an intermediate behavior, whereas that with CaCO3 nanoparticles showed low average cell diameter and size and high cell wall thickness. The radiation process enhanced the foam density, CS, abrasion loss, hardness, and thermal property of the developed nanocomposites by inducing the formation of intermolecular crosslinks within the composite matrix. The results showed that physicochemical properties improved by increasing the radiation dose at 25 kGy.

Radioiodination of balsalazide, bioevaluation, and characterization as a highly selective radiotracer for imaging of ulcerative colitis in mice

This work focuses on tracking ulcerative colitis in mice. High labeling yield and radiochemical purity were achieved for the formation of a [^125/131 I]balsalazide radiotracer at optimum conditions of oxidizing agent content (chloramines-T [Ch-T], 75 μg), substrate amount (100 μg), pH of reaction mixture (6), reaction time (30 min), and temperature (37°C), using radioactive iodine-125 (200-450 MBq). The radiolabeled compound, [^125/131 I]balsalazide, was stable in serum and saline solution during 24 h. Balsalazide is acting as a peroxisome proliferator-activated receptor (PPARγ). Biodistribution studies were carried in normal and ulcerated colon mice. High uptake of 75 ± 1.90% injected dose/g organ (ID/g) observed in ulcerated mice confirmed the suitability of [¹³¹ I]balsalazide as a novel radiotracer for ulcerative colitis imaging in mice.

Preparation, characterization, and bioevaluation of 99mTc-famotidine as a selective radiotracer for peptic ulcer disorder detection in mice

This work focuses on tracking peptic ulcer localized in mice. The formation of a [99mTc]dithiocarbamate of famotidine complex at optimum conditions of reaction temperature (37 °C), reaction time (30 min), pH of the reaction mixture (5), amount of substrate (100 µg), amount of reducing agent (tin (II) content, 50 µg), was achieved using radioactive Tc-99m (250–400 MBq), with labelling yield of 98% and high radiochemical purity. The in-vitro stability of [99mTc]dithiocarbamate of famotidine complex was shown to be high in rat serum for up to 8 h. Normal and ulcerated mice were used in biodistribution studies. Famotidine works by blocking histamine-2-receptor antagonists (H2RAs). The high absorption of [99mTc]dithiocarbamate of famotidine complex in stomach in amount of 27.15% injected dose/g organ (ID/g) observed in ulcerated mice for up to 24 h demonstrated its usefulness as a novel radiotracer for stomach imaging.

Visualizing and quantifying antimicrobial drug distribution in tissue

The biodistribution and pharmacokinetics of drugs are vital to the mechanistic understanding of their efficacy. Measuring antimicrobial drug efficacy has been challenging as plasma drug concentration is used as a surrogate for tissue drug concentration, yet typically does not reflect that at the intended site(s) of action. Utilizing an image-guided approach, it is feasible to accurately quantify the biodistribution and pharmacokinetics within the desired site(s) of action. We outline imaging modalities used in visualizing drug distribution with examples ranging from in vitro cellular drug uptake to clinical treatment of microbial infections. The imaging modalities of interest are: radio-labeling, magnetic resonance, mass spectrometry imaging, computed tomography, fluorescence, and Raman spectroscopy. We outline the progress, limitations, and future outlook for each methodology. Further advances in these optical approaches would benefit patients and researchers alike, as non-invasive imaging could yield more profound insights with a lower clinical burden than invasive measurement approaches used today.

Radiosynthesis and in silico bioevaluation of 131 I-Sulfasalazine as a highly selective radiotracer for imaging of ulcerative colitis

This study demonstrated the tracking of ulcerative colitis, which is considered a stressful immune disease. Although there are many ways to test for this disease including dependence on gases, dyes, and painful anal endoscopy, these treatment modalities have many disadvantages. Hence, it is the utmost need of time to discover new methods to detect this chronic immune disease and to avoid the defects of traditional methodologies. Sulfasalazine (SSD) was labeled with iodine-131 (half-life: 8 days, Energy: 971 keV) under optimum reaction conditions including the amount of reducing agent, pH factor, chloramine-T (Ch-T) amount, and incubation period. Characterization was performed using ¹ H/ ¹³ C-NMR, ESI-MS, and HPLC (UV/ Radio) techniques. The biodistribution study was performed in normal and ulcerative mice models, and in silico molecular docking study was performed to evaluate the possible mechanism of action to target peroxisome proliferator-activated receptor gamma (PPARγ). The high radiolabeling yield of [¹³¹ I]-sulfasalazine ([¹³¹ I]-SSD) was achieved ≥90% through the direct labeling method with radioactive iodine-131 in the presence of chloramine-T (100 μg). The radiotracer [¹³¹ I]-SSD was observed to be stable in normal saline and freshly eluted serum up to 12 hr at ambient temperature (37℃ ± 2℃). The radiotracer [¹³¹ I]-SSD showed the highest uptake in the targeted organ (i.e., ulcerative colon) which was observed to be ≥75% injected dose per gram (% ID/g) organ for 24 hr postinjection (p.i). Furthermore, in silico data collected from molecular modeling analysis of SSD and [¹³¹ I]-SSD with antimicrobial protein (PDB code: 3KEG) and peroxisome proliferator-activated receptor gamma (PPARγ) (PDB code: 4XTA) showed azoreductase activity and high binding potential for PPAR-γ site, respectively. The results of biological studies obtained in this study enlighten the usefulness of radiotracer [¹³¹ I]-SSD as a potential imaging agent for ulcerative colitis.

Radioiodinated esomeprazole as a model for peptic ulcer localization

This work focuses on tracking stomach ulcer localized in mice. High labeling yield and radiochemical purity were achieved for the formation of a [125I]esomeprazole radiotracer at optimum conditions of oxidizing agent content (chloramines-T (Ch-T), 100 μg), substrate amount (Esom, 100 μg), pH of reaction mixture (6), reaction time (30 min) and temperature (37 °C), using radioactive iodine-125 (200–450 MBq). The radiolabeled compound, [125I]esom, was stable in serum and saline solution during 24 h. Esom is acting as a histamine-2-receptor antagonist (H2RA). Biodistribution studies were carried in normal and ulcerated mice. High uptake of 78.12 ± 0.80% injected dose/g organ (ID/g) observed in ulcerated mice confirmed the suitability of [125I]esomeprazole as a novel radiotracer for stomach imaging.

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 silico and in vivo study of radio-iodinated nefiracetam as a radiotracer for brain imaging in mice

This study presents development and characterization of a radiotracer, [125I]iodonefiracetam ([125I]iodoNEF). Labeling with high yield and radiochemical purity was achieved through the formation of a [125I]iodoNEF radiotracer after investigating many factors like oxidizing agent content (chloramines-T (Ch-T)), substrate amount (Nefiracetam (NEF)), pH of reaction mixture, reaction time and temperature. Nefiracetam (NEF) is known as nootropic agent, acting as N-methyl-d-aspartic acid receptor ligand (NMDA). The radiolabeled compound was stable, and exhibited the logarithm of the partition coefficient (log p) value of [125I]iodonefiracetam as 1.85 (lipophilic). Biodistribution studies in normal mice confirmed the suitability of the [125I]iodoNEF radiotracer as a novel tracer for brain imaging. High uptake of 8.61 ± 0.14 percent injected dose/g organ was observed in mice

Synthesis, characterization, and bioevaluation of 99mTc nitrido-oxiracetam as a brain imaging model

In this work, the radiotracer [99mTc]nitrido-oxiracetam complex was labeled in the presence of 99mTc-nitrido as a core. In order to get the highest radiochemical purity, many effective factors have been studied such as temperature of the reaction, time of the reaction, the pH of the reaction mixture, substrate amount, and stability to give high percent more than 99%. Finally, biodistribution studies have been indicated the convenience of [99mTc]nitrido-oxiracetam as a new radiotracer that could be used in brain imaging. Giving a maximum uptake of 10.6% at 30 min post injection.

Radiochemical Approaches to Imaging Bacterial Infections: Intracellular versus Extracellular Targets

The discovery of penicillin began the age of antibiotics, which was a turning point in human healthcare. However, to this day, microbial infections are still a concern throughout the world, and the rise of multidrug-resistant organisms is an increasing challenge. To combat this threat, diagnostic imaging tools could be used to verify the causative organism and curb inappropriate use of antimicrobial drugs. Nuclear imaging offers the sensitivity needed to detect small numbers of bacteria in situ. Among nuclear imaging tools, radiolabeled antibiotics traditionally have lacked the sensitivity or specificity necessary to diagnose bacterial infections accurately. One reason for the lack of success is that the antibiotics were often chelated to a radiometal. This was done without addressing the ramifications of how the radiolabeling would impact probe entry to the bacterial cell, or the mechanism of binding to an intracellular target. In this review, we approach bacterial infection imaging through the lens of bacterial specific molecular targets, their intracellular or extracellular location, and discuss radiochemistry strategies to guide future probe development.

Currently Available Radiopharmaceuticals for Imaging Infection and the Holy Grail

Infection is ubiquitous. However, its management is challenging for both the patients and the health-care providers. Scintigraphic imaging of infection dates back nearly half a century. The advances in our understanding of the pathophysiology of disease at cellular and molecular levels have paved the way to the development of a large number of radiopharmaceuticals for scintigraphic imaging of infection. These include radiolabeling of blood elements such as serum proteins, white blood cells (WBCs), and cytokines, to name a few. Infectious foci have also been imaged using a radiolabeled sugar molecule by taking advantage of increased metabolic activity in the infectious lesions. Literature over the years has well documented that none of the radiopharmaceuticals and associated procedures that facilitate imaging infection are flawless and acceptable without a compromise. As a result, only a few compounds such as 99mTc-hexamethylpropyleneamineoxime, 18F-FDG, the oldest but still considered as a gold standard 111In-oxine, and, yes, even 67Ga-citrate in some countries, have remained in routine clinical practice. Nonetheless, the interest of scientists and physicians to improve the approaches to imaging and to the management of infection is noteworthy. These approaches have paved the way for the development of numerous, innovative radiopharmaceuticals to label autologous WBCs ex vivo or even those that could be injected directly to image infection or inflammation without direct involvement of WBCs. In this review, we briefly describe these agents with their pros and cons and place them together for future reference.

Radioiodinated famotidine as a new highly selective radiotracer for peptic ulcer disorder detection, diagnostic nuclear imaging and biodistribution

Famotidine was labeled with iodine-125 to obtain 125I-famotidine (125I-fam) as an agent for ulcer imaging. The radiochemical yield of 125I-famotidine reached approximately 98.5 ± 0.23% at optimum conditions of pH, oxidizing agent, reaction time and the amount of substrate. 125I-fam was stable for 48 h. Different chromatographic techniques were used to determine the radiochemical yield and purity. Intravenous biodistribution studies of 125I-fam revealed high concentration in the stomach ulcer, reaching about 65.9 ± 0.28% of the total injected dose at 30 min post injection. This concentration of 125I-fam in stomach ulcer makes this agent promising for stomach ulcer imaging.

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.