In vivo magnetomotive optical molecular imaging using targeted magnetic nanoprobes

Abstract

Dynamic magnetomotion of magnetic nanoparticles (MNPs) detected with magnetomotive optical coherence tomography (MM-OCT) represents a new methodology for contrast enhancement and therapeutic interventions in molecular imaging. In this study, we demonstrate in vivo imaging of dynamic functionalized iron oxide MNPs using MM-OCT in a preclinical mammary tumor model. Using targeted MNPs, in vivo MM-OCT images exhibit strong magnetomotive signals in mammary tumor, and no significant signals were measured from tumors of rats injected with nontargeted MNPs or saline. The results of in vivo MM-OCT are validated by MRI, ex vivo MM-OCT, Prussian blue staining of histological sections, and immunohistochemical analysis of excised tumors and internal organs. The MNPs are antibody functionalized to target the human epidermal growth factor receptor 2 (HER2 neu) protein. Fc-directed conjugation of the antibody to the MNPs aids in reducing uptake by macrophages in the reticulo-endothelial system, thereby increasing the circulation time in the blood. These engineered magnetic nanoprobes have multifunctional capabilities enabling them to be used as dynamic contrast agents in MM-OCT and MRI.

Fig. 1.

MM-OCT principle. (A) Schematic of the experimental setup. (B) TEM of the MNPs used in the study. (C) Close-up of the sample arm with focusing lens, electromagnet, and specimen showing axial magnetomotive displacements of MNPs and corresponding change in the OCT interferogram. (D) (I) B-mode image through a MNP-silicone phantom. (II–IV) Representative zoomed-in images of M-mode scans along the dashed red line marked in (I) showing magnetomotion in the sample on application of a (II) sinusoidal, (III) pulse, and (IV) sweep of frequencies of the magnetic field.

Fig. 2.

In vivo (A) MM-OCT and (B) OCT images. MM-OCT signal (green channel) is superposed on the OCT signal (red channel). PB-stained sections of (C, D) tumors and (E, F) livers from (I) targeted MNP-injected, (II) nontargeted MNP-injected, and (III) saline-injected rats. PB-stained sections in D and F, at 40×) represent the boxed regions in C and E, at 10×, respectively. (G) Immunohistochemical-stained sections from (I) tumor from a targeted MNP-injected rat, (II) tail injection site from a targeted MNP-injected rat, and (III) tumor from a saline-injected rat.

Fig. 3.

MM-OCT signal intensities observed in vivo and ex vivo for the three cases in study. Comparison of (A) in vivo tumor data and (B) ex vivo tumor data for the three cases is shown. Statistical analysis based on analysis of variance indicate statistically significant MM-OCT signals from tumor of rat injected with targeted MNPs compared to tumor from rat injected with nontargeted MNPs or from rat injected with saline (p = 0.0001 and p < 0.0001, respectively). (C) Comparison of ex vivo MM-OCT signal data from tumors and internal organs for each case. For rats injected with targeted MNPs (C Left), statistical analysis shows a significant MM-OCT signal from tumor only, compared to the internal organs (p < 0.0001). For rats injected with nontargeted MNPs (C Center), MM-OCT signal strength is significantly higher in liver (p < 0.0001), and weak signals were obtained from lung and spleen. Note that elevated signals from spleen are recognized as falsely positive due to the presence of the iron protein ferritin. For rats injected with saline (C Right), there is only statistically significant MM-OCT signal from spleen (p < 0.0001) compared to the internal organs, due to the presence of ferritin.

Fig. 4.

Coronal slices from a 3D MRI dataset with corresponding T2* maps and plots of T2* values before injection and 6 h postinjection from (A) rat injected with targeted MNPs, (B) rat injected with nontargeted MNPs, and (C) rat injected with saline. Plots (right column) represent T2* values present within the segmented 3D volume of the corresponding tumors noted by the arrows.