Macrophages as a potential tumor-microenvironment target for noninvasive imaging of early response to anticancer therapy.

As a result of therapy-induced apoptosis, peripheral blood monocytes are recruited to tumors, where they become tumor-associated macrophages (TAMs). To date, few studies have investigated noninvasive molecular imaging for assessment of macrophage infiltration in response to therapy-induced apoptosis. Here, noninvasive assessment of changes in tumor accumulation of TAMs was proposed as a new way to measure early tumor response to anticancer therapy. Three different nanoparticles, QD710-Dendron quantum dots (QD710-D), Ferumoxytol, and PG-Gd-NIR813, were used for near-infrared fluorescence imaging, T2-weighted magnetic resonance imaging, and dual optical/T1-weighted MR imaging, respectively, in the MDA-MB-435 tumor model. Treatment with Abraxane induced tumor apoptosis and infiltrating macrophages. In spite of markedly different physicochemical properties among the nanoparticles, in vivo imaging revealed increased uptake of all three nanoparticles in Abraxane-treated tumors compared with untreated tumors. Moreover, imaging visualized increased uptake of QD710-D in MDA-MB-435 tumors but not in drug-resistant MDA-MB-435R tumors grown in the mice treated with Abraxane. Our results suggest that infiltration of macrophages due to chemotherapy-induced apoptosis was partially responsible for increased nanoparticle uptake in treated tumors. Noninvasive imaging techniques in conjunction with systemic administration of imageable nanoparticles that are taken up by macrophages are a potentially useful tool for assessing early treatment response.