Enhanced detection of myeloperoxidase activity in deep tissues through luminescent excitation of near-infrared nanoparticles

The noninvasive detection of myeloperoxidase (MPO)-mediated oxidative stress in deep tissue inflammatory foci has been hampered by poor penetration of luminol-emitted short wavelength light due to tissue absorption and scattering. To circumvent this, Daniel Ansaldi and his colleagues have adopted a chemiluminescence resonance energy transfer approach whereby near-infrared (NIR) nanoparticles are used to red-shift luminol-emitted blue light to the NIR. Improved in vivo detectability of MPO is demonstrated in a lipopolysaccharide-induced pulmonary inflammation model, as well as in deep tissue tumor metastases. A previous study reported the use of luminol for the detection of myeloperoxidase (MPO) activity using optical imaging in infiltrating neutrophils under inflammatory disease conditions. The detection is based on a photon-emitting reaction between luminol and an MPO metabolite. Because of tissue absorption and scattering, however, luminol-emitted blue light can be efficiently detected from superficial inflammatory foci only. In this study we report a chemiluminescence resonance energy transfer (CRET) methodology in which luminol-generated blue light excites nanoparticles to emit light in the near-infrared spectral range, resulting in remarkable improvement of MPO detectability in vivo . CRET caused a 37-fold increase in luminescence emission over luminol alone in detecting MPO activity in lung tissues after lipopolysaccharide challenge. We demonstrated a dependence of the chemiluminescent signal on MPO activity using MPO-deficient mice. In addition, co-administration of 4-aminobenzoic acid hydrazide (4-ABAH), an irreversible inhibitor of MPO, significantly attenuated luminescent emission from inflamed lungs. Inhibition of nitric oxide synthase with a nonspecific inhibitor, L -NAME, had no effect on luminol-mediated chemiluminescence production. Pretreatment of mice with MLN120B, a selective inhibitor of IKK-2, resulted in suppression of neutrophil infiltration to the lung tissues and reduction of MPO activity. We also demonstrated that CRET can effectively detect MPO activity at deep tissue tumor foci due to tumor development–associated neutrophil infiltration. We developed a sensitive MPO detection methodology that provides a means for visualizing and quantifying oxidative stress in deep tissue. This method is amenable to rapid evaluation of anti-inflammatory agents in animal models.