Dual radioisotope - Fluorescence nanoprobe for mapping pH in vivo

Objectives We recently reported a dextran-based nanoprobe NP1, which remains ‘‘silent’’ in the extracellular environment in vivo at physiological pH, but ‘‘turns on’’ after it enters the target tumor with an acid microenvironment. In this study we incorporated In-111 into the established pH-responsive NP1 to test its pharmacokinetics and molecular property in terms of acidosis responsibility simultaneously with a single tracer injection.

Methods 111In-DOTA was conjugated to the nanoprobe via physiologically stable bonds, the radioactivity therefore represented the location of the nanoprobe. The self-quenching between IR783 fluorophores labeled in 111In-NP1 is expected to result in a low background in normal tissues and be activated in the acidic tumor microenvironment. We examined the biodistribution of this dual labeling tracer in 8 mice bearing U89MG tumor xenografts. In order to determine the intratumoral distribution of the NIRF signal, autoradiography with a Beta imager and NIRF microscopy of the dual labeled probe were performed.

Results 111InNP1 predominantly located in the liver, kidney and spleen. The hepatic uptake was about 9.6 times higher than uptake in tumor at 2 h post injection, even at 24 h PI the radioactivity concentration in the liver was about 8.7 times higher than in the tumor. However, the strongest NIR fluorescence was found in tumor, the normalized NIR fluorescence in tumor was 3.1 times higher than that in liver at 24 h PI. The biodistribution data also indicated that considerable amount of the nanoprobe was retained in the main organs from 2 to 24 h post injection, but little NIRF signal emanated from these tissues in contrast to tumor. Ex vivo optical imaging showed that the NIR fluorescence predominately located in the central area of the fresh tumor sections, while the radioactivity of the 111InNP1 was predominantly located in the tumor margin area after the injection.

Conclusions The concentration independent NIR fluorescence of 111In-NP1 in tumor verified that pH activated NIR fluorescence resulted in high specificity tumor targeting.

Research Support NIH S10RR02252