Near-infrared phosphorescent polymeric nanomicelles: efficient optical probes for tumor imaging and detection

Abstract

We report a formulation of near-infrared (near-IR) phosphorescent polymeric nanomicelles and their use for in vivo high-contrast optical imaging, targeting, and detection of tumors in small animals. Near-IR phosphorescent molecules of Pt(II)-tetraphenyltetranaphthoporphyrin (Pt(TPNP)) were found to maintain their near-IR phosphorescence properties when encapsulated into phospholipid nanomicelles. The prepared phosphorescent micelles are of approximately 100 nm size and are highly stable in aqueous suspensions. A large spectral separation between the Pt(TPNP) absorption, with a peak at approximately 700 nm, and its phosphorescence emission, with a peak at approximately 900 nm, allows a dramatic decrease in the level of background autofluorescence and scattered excitation light in the near-IR spectral range, where the signal from the phosphorescent probe is observed. In vivo animal imaging with subcutaneously xenografted tumor-bearing mice has resulted in high contrast optical images, indicating highly specific accumulation of the phosphorescent micelles into tumors. Using optical imaging with near-IR phosphorescent nanomicelles, detection of smaller, visually undetectable tumors has also been demonstrated.

Figure 1

Structure of the Pt(TPNP) and the scheme showing the procedure used to prepare polymeric nanomicelles encapsulating the Pt(TPNP) within PEG-modified phopholipid micelle.

Figure 2

Representative TEM images of the Pt(TPNP)/DSPE-PEG/PC nanomicelles.

Figure 3

Absorption (A), emission (B) spectra and emission decays (C) from Pt(TPNP) in CHCl₃ (1) and DSPE-PEG/PC nanomicelles (2-4) at room temperature. Concentrations of Pt(TPNP): 2.5 μM (1, 2), 12.5 μM (3), 25 μM (4). D. Pt(TPNP)/DSPE-PEG/PC phosphorescence spectra in presence of oxygen (solid line) and under nitrogen purging (dashed line). Inset shows change in the phosphorescence intensity at 900 nm with turning on/off N₂ bubbling.

Figure 4

A. PL images of the tumored nude mouse at various time points (2, 24, 96 hrs) post-injection with the nanomicelles. B. Spectral profiles used to unmix images shown in the panel A. C – Bright field and PL images of the major organs resected from mouse 96 hrs post-injection.

Figure 5

Bright field (A), phosphorescence (B) and combined (C) images of the nude mouse xenografted with a subcutaneous Panc 1 tumor in its early phase of growth. (1 week after tumor cells injection). Images were taken 24 hrs after iv injection of the Pt(TPNP)/DSPE-PEG/PC nanomicelles.