Antimicrobial peptides as infection imaging agents: better than radiolabeled antibiotics

Abstract

Nuclear medicine imaging techniques offer whole body imaging for localization of number and site of infective foci inspite of limitation of spatial resolution. The innate human immune system contains a large member of important elements including antimicrobial peptides to combat any form of infection. However, development of antibiotics against bacteria progressed rapidly and gained popularity over antimicrobial peptides but even powerful antimicrobials failed to reduce morbidity and mortality due to emergence of mutant strains of bacteria resulting in antimicrobial resistance. Differentiation between infection and inflammation using radiolabeled compounds with nuclear medicine techniques has always been a dilemma which is still to be resolved. Starting from nonspecific tracers to specific radiolabeled tracers, the question is still unanswered. Specific radiolabeled tracers included antibiotics and antimicrobial peptides which bind directly to the bacteria for efficient localization with advanced nuclear medicine equipments. However, there are merits and demerits attributed to each. In the current paper, radiolabeled antibiotics and radiolabeled peptides for infection localization have been discussed starting with the background of primitive nonspecific tracers. Radiolabeled antimicrobial peptides have certain merits compared with labeled antibiotics which make them superior agents for localization of infective focus.

Figure 1

The membrane target of antimicrobial peptides and the basis of their specific binding.

Figure 2

Structure of UBI 29–41.

Figure 3

Biodistribution of  ^99mTc UBI 29–41 in a normal rabbit at 30 minutes after injection.

Figure 4

^99mTc-UBI (29–41) scintigram of rabbit with Staphylococcus aureus thigh muscle infection (arrow). Maximum tracer uptake visualized at 60 minutes after tracer injection.

Figure 5

Anterior whole body image at 30 minutes after ^99mTc-UBI 29–41 injection in normal human subject showing kidneys (dotted arrows), liver (solid arrows), and urinary bladder (ball arrow).

Figure 6

Positive ^99mTc-UBI 29–41 scan in a patient with infection in medial aspect of right hand (arrow). Maximum focal increased tracer uptake was seen at 30 minutes after tracer injection.