Introduction
Scintigraphic detection of infection has the advantage of whole body imaging, which might be of great value in cases of occult infection. However, the critical issue is differentiation between infectious and non-infectious inflammatory processes. 67Ga-citrate, being the most primitive radiotracer has high sensitivity for inflammation but is not specific for infection. Labelled leucocytes with 111In-oxine or 99mTc-hexamethylene propyleneamine oxime are considered gold standard in nuclear medicine for imaging infection and inflammation. Their use is limited in neutropenic patients. Radiolabelled monoclonal antibodies against surface antigens of granulocytes carry a risk of induction of human anti-mouse antibodies. Similarly antibody fragments, interleukins and platelet factor 4 have certain limitations. Cytokines and chemotactic peptides are highly immunogenic and cytotoxic.
Ubiquicidin Imaging Introduction:
Antimicrobial peptides are found in abundance in mammals, birds, amphibians, insects and plants as a part of innate immnunity against infection. These preferentially bind to a broad spectrum of microorganisms and can be produced by genetically engineered bacteria or by pepetide synthesis. The basic interaction between the peptide and the bacteria is based on the cationic domains of peptide and negatively charged surface of the bacteria.
Ubiduicidin - Radiotracer
Ubiquicidin 29-41, having amino acid sequence of TGRAKRRMQYNRR and weight of 1693 Da, is a synthetic anti-microbial peptide fragment that originally was isolated from mouse macrophages. This peptide labeled with Technitium discriminated well between bacterial inflammation and inflammation induced by lipopolysaccharides of bacterial origin. It has affinity for candida albicans and low affinity for E coli as compared to Staphylococcus aureus infection. A specific mechanism exists for the bacterial intracellular accumulation of Tc99m UBI29-41, as it does not concentrate in tumour cells. 99mTc UBI 29-41 is free from hazards of handling blood products- the major disadvanatge of labelled leucocytes. Other advantage is applicability to leukopenic patients and low probability of resistance to antimicrobial peptides. It is highly sensitive and specific agent for localizing infective foci in bone and soft tissue of humans. The optimum imaging time for delineation between infectious and inflammatory process is 30 minutes after intravenous administration of tracer. No adverse effects are observed in the studies performed.
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