Muhammad Sohaib, Samina Roohi and Zain Khurshid

Department of Medical Sciences, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore Islamabad, Pakistan

Abstract:

Background: The delineation of the site of infection and its extent is crucial for its clinical management and the response to therapy. Imaging with labelled antibiotics may help to establish the presence localization of infection. In this quest we present formulation and bio-distribution of a radiolabeled 3rd generation cephalosporin, ^99mTc-ceftriaxone as a new infection specific radiopharmaceutical.

Aims and objectives: This study was conducted to label ceftriaxone with ^99mTc; to optimize its labelling conditions; to ascertain its stability in-vitro; to ascertain its bio-distribution in-vivo in normal and infection inoculated animal models.

Materials and methods: Freshly eluted ^99mTc was used for labeling ceftriaxone. Labeling yield was determined with paper and instant thin layer chromatography. Optimization of labeling conditions like pH, stannous chloride and ceftriaxone concentration was performed by altering the pH level of formulation to 3, 5, 7, 9 and 11, adding different amounts 10, 20, 40, 50 and 60µg of SnCl₂.2H₂O and various concentrations 10, 20, 30, 40 and 50 mg of ceftriaxone. The labeled compound was boiled for 10 minutes and the nature of compound was verified through HPLC in both pre and post boiling states. Stability of the compound was determined at room temperature with and without human serum. In-vitro binding of the compound was also assessed. Bio-distribution studies were carried out in normal Sprague Dawley male rats and compared with animals inoculated with S.aureus and E.coli infections and inflammation with turpentine oil. Scintigraphy was performed in E.coli infected rats and oil inflamed rabbits.

Results: ^99mTc-ceftriaxone prepared at pH 7 by adding 30mg of ligand in the presence of 50µg SnCl₂.2H₂O and boiling for ten minutes gave maximum labeling yield (96 ± 1.76%). The stability at room temperature both with and without human serum was more than 90% at 24 hours after labelling. In-vitro binding revealed maximum binding of 68% and 47% with E.coli and S.aureus respectivelyafter 4-hour incubation. Bio-distribution studies in normal rats showed the uptake of labeled ceftriaxone in various organs with maximum uptake in kidneys followed by hepatobiliary system and gut. Mean target to non-target ratios in infection and inflammation models, four hours after injection, were 12.66±2.59, 2.36±0.30 and 1.44±0.53 for E.coli, S.aureus and oil inflammation respectively. Scintigraphic findings also correlated with bio-distribution studies.

Conclusions: In this study we could efficiently label ceftriaxone with ^99mTc.  Biodistribution studies and scintigraphy demonstrated that it is an infection specific agent, with more specificity for E.coli.