Studies on batch formulation of a kit for the preparation of the 99m Tc-Ubiquicidin (29-41): An infection imaging agent

Integrating a covalent probe with ubiquicidin fragment enables effective bacterial infection imaging

Developing potent and novel bacterial imaging agents remains formidable due to the rapid development of bacterial resistance. Ubiquicidin and its derivatives are the most studied antimicrobial peptides that bind to anionic membranes of a broad range of bacterial pathogens. Studies reveal that UBI (29-41) labeled with ^99mTc and ⁶⁸Ga could distinguish sterile inflammation from infection. A significant challenge that remains for cationic peptides is their poor salt tolerance. The present study deliberates the increment of UBI (29-41) peptide interaction with the bacterial membrane by incorporating 2-acetylphenylboronic acid (2-APBA) as a covalent probe and developing infection imaging probes with improved retention at the target. Given that both ^99mTc-UBI (29-41) and ^99mTc-UBI (29-41)-2-APBA peptide complexes are stable in serum over 16 h, ^99mTc-UBI (29-41)-2-APBA shows enhanced uptake in S. aureus cells as compared to ^99mTc-UBI (29-41). SPECT imaging in a mouse model of infection exhibited a higher target to non-target ratio after 2 h in the case of ^99mTc-UBI (29-41)-2-APBA. The present study reveals a synergistic mechanism of target binding through covalent conjugation and non-covalent interaction, which could be a potential strategy for improving bacterial infection imaging. As a proof of concept, ^99mTc-UBI (29-41)-2-APBA elicits our hypothesis by in vivo imaging of bacterial infection.

In Vivo Microbial Targeting of 99mTc-Labeled Human β-Defensin-3 in a Rat Model of Infection

OBJECTIVE

Differentiation of infection from aseptic inflammation represents a major clinical issue. None of the commercially available compounds (labeled granulocytes, antigranulocyte antibodies, Ga-citrate, labeled immunoglobulin G, F-FDG) is capable of this differentiation, producing a nonnegligible false-positive rate. Recently, our group reported on a reliable labeling procedure of the antimicrobial peptide human β-defensin 3 (HBD-3) with Tc. The aim of this study was to evaluate in vivo Tc-HBD-3 uptake in a rat model of infection.

METHODS

Recombinant HBD-3 was radiolabeled with Tc. Radiolabeling yield and specific activity of the compound were calculated. Chromatographic behavior and biological activity of Tc-HBD-3 were also assessed. An experimental model involving Staphylococcus aureus-induced infection and carrageenan-induced aseptic inflammation was performed in 5 Wistar rats. Serial planar scintigraphic acquisitions were performed from 15 to 180 minutes after Tc-HBD-3 intravenous administration. Radiotracer uptake was evaluated qualitatively and semiquantitatively as a target-to-nontarget ratio.

RESULTS

Radiolabeling yield of Tc-HBD-3 was 70% with a specific activity of 6 to 8 MBq/μg. A significant and progressive Tc-HBD-3 uptake was observed in the site of S. aureus-induced infection, with a maximum average target-to-nontarget ratio of 5.7-fold higher in the infection site compared with an inflammation site observed at 140 minutes.

CONCLUSIONS

In vivo imaging with Tc-HBD-3 in a rat model of S. aureus-induced infection demonstrated favorable uptake in the infection site compared with sterile inflammation and background. These promising results, together with previous ex vivo uptake and toxicity assessment, suggest the potential of Tc-HBD-3 as a novel agent for specific infection imaging.

68Ga-NOTA-ubiquicidin fragment for PET imaging of infection: From bench to bedside

This study explores the possibility of formulation of a cold kit for fast and easy preparation of a PET radiopharmaceutical, ⁶⁸Ga-NOTA-UBI (29-41) for clinical translation. In this study, Circular dichroism (CD) spectroscopy to study conformation of NOTA-UBI (29-41) and its comparison with conformation of UBI (29-41) was done. Pharmaceutical grade cold kits of NOTA-UBI (29-41) were formulated for radiolabeling with ⁶⁸Ga and necessary quality control tests were carried out. ⁶⁸Ga-NOTA-UBI (29-41) could be prepared in >90% radiochemical yield and radiochemical purity using cold kits of NOTA-UBI (29-41). In vitro and in vivo evaluation of ⁶⁸Ga-NOTA-UBI (29-41) was done to demonstrate specificity of the agent for imaging infection. Kits were utilized for preparation of patient dose of ⁶⁸Ga-NOTA-UBI (29-41). Simple instant thin layer chromatography (ITLC) method for estimating radiolabeling yield of ⁶⁸Ga-NOTA-UBI (29-41) at hospital radiopharmacy was demonstrated. Clinical evaluation was done in patients with suspected infection. 148-185 MBq of ⁶⁸Ga-NOTA-UBI (29-41) was injected intravenously in three patients. ⁶⁸Ga-NOTA-UBI (29-41) uptake could clearly delineate infection foci from non target normal tissues. This is the first report on formulation of a cold kit of NOTA-UBI (29-41) for preparation of ⁶⁸Ga labeled NOTA-UBI(29-41) at hospital radiopharmacy for infection imaging. Initial clinical evaluation reveal it to be a prospective agent for imaging infection.

Gallium-68 labeled Ubiquicidin derived octapeptide as a potential infection imaging agent

INTRODUCTION

Gallium-68 based infection imaging agents are in demand to detect infection foci with high spatial resolution and sensitivity. In this study, Ubiquicidin derived octapeptide, UBI (31-38) conjugated with macrocyclic chelator NOTA was radiolabeled with ⁶⁸Ga to develop infection imaging agent.

METHODS

Circular dichroism (CD) spectroscopy was performed to study conformational changes in UBI (31-38) and its NOTA conjugate in a "membrane like environment". Radiolabeling of NOTA-UBI (31-38) with ⁶⁸Ga was optimized and quality control analysis was done by chromatography techniques. In vitro evaluation of ⁶⁸Ga-NOTA-UBI (31-38) in S. aureus and preliminary biological evaluation in animal model of infection was studied. Initial clinical evaluation in three patients with suspected infection was carried out.

RESULTS

⁶⁸Ga-NOTA-UBI (31-38) was prepared in high radiochemical yields and high radiochemical purity. In vitro evaluation of ⁶⁸Ga-NOTA-UBI (31-38) complex in S. aureus confirmed specificity of the agent for bacteria. Biodistribution studies with ⁶⁸Ga-NOTA-UBI (31-38) revealed specific uptake of the complex in infected muscle compared to inflamed muscle with T/NT ratio of 3.24 ± 0.7 at 1 h post-injection. Initial clinical evaluation in two patients with histopathologically confirmed infective foci conducted after intravenous injection of 130-185 MBq of ⁶⁸Ga-NOTA-UBI (31-38) and imaging at 45-60 min post-injection revealed specific uptake at the sites of infection and clearance from vital organs. No uptake of tracer was observed in suspected infection foci in one patient, which was proven to be aseptic and served as negative control.

CONCLUSION

This is the first report on ⁶⁸Ga labeled NOTA-UBI (31-38) fragment for prospective infection imaging.

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.