Optical imaging of mammary and prostate tumors in living animals using a synthetic near infrared zinc(II)-dipicolylamine probe for anionic cell surfaces

Abstract

In vivo optical imaging shows that a fluorescent imaging probe, comprised of a near-infrared fluorophore attached to an affinity group containing two zinc(II)-dipicolylamine (Zn-DPA) units, targets prostate and mammary tumors in two different xenograft animal models. The tumor selectivity is absent with control fluorophores whose structures do not have appended Zn-DPA targeting ligands. Ex vivo biodistribution and histological analyses indicate that the probe is targeting the necrotic regions of the tumors, which is consistent with in vitro microscopy showing selective targeting of the anionic membrane surfaces of dead and dying cells.

Figure 1

Micrographs (60X magnification) of Jurkat cells treated with cytotoxic camptothecin (10 μM) for 3.5 h and stained simultaneously with Annexin V-Alexa Fluor 488, and probe 1 (10 μM). Brightfield image of the entire field of cells (A); cells stained with Annexin V-Alexa Fluor 488 (B); cells stained with probe 1 (C); overlay of images A, B, and C (D). No staining of healthy cells was observed in the absence of camptothecin.

Figure 2

X-ray and fluorescence overlay image of a rat prostate tumor model at 24 h post-injection of probe 1 (A). The image was acquired at a 190 mm field of view. Bar graph showing ex vivo tissue distribution of probes 1, 2, and 3 (B). The values represent the mean (n=3), ± standard error of the mean. *P<0.0005. This imaging data is representative of four replicate studies using independent cohorts.

Figure 3

Ex vivo analysis of probe 1 localization in rat prostate tumor. Excised PAIII prostate tumors were sliced along the longest axis, and a 30 mm field of view generated the representative near-IR fluorescence intensity image (A). Representative co-registered micrographs of a 5 μm histological slice of tumor core; the brightfield image (B) shows necrotic cells as darker regions that colocalize with near-IR fluorescence intensity image from probe 1 (C). Scale bar = 100 μm.

Figure 4

Representative overlay image of a nude mouse with an EMT-6 mammary tumor. Brightfield and fluorescence intensity images were acquired 24 hrs following injection of probe 1 (A). The fluorescence intensity is plotted along the z-axis of this 3D surface diagram. Images were taken at an 80 mm field of view. Bar graph showing ex vivo tissue distribution of probe 1 and control 2 (B). The values represent the mean (n=3), ± standard error of the mean. *P<0.005. This imaging data is representative of three replicate studies using independent cohorts.