Exogenous Molecular Probes for Targeted Imaging in Cancer: Focus on Multi-modal Imaging

Abstract

Cancer is one of the major causes of mortality and morbidity in our health care system. Molecular imaging is an emerging methodology for the early detection of cancer, and the development of exogenous molecular probes that can be labeled for multi-modality imaging is critical to this process. Today, molecular imaging is at crossroad, and new targeted imaging agents are expected to broadly expand our ability to detect pre-malignant lesions. This integrated imaging strategy will permit clinicians to not only localize lesions within the body, but also to visualize the expression and activity of specific molecules. This information is expected to have a major impact on diagnosis, therapy, drug development and understanding of basic cancer biology. At this time, a number of molecular probes have been developed by conjugating various labels to affinity ligands for targeting in different imaging modalities. This review will describe the current status of exogenous molecular probes for optical, nuclear and MRI imaging platforms. Furthermore, we will also shed light on how these techniques can be used synergistically in multi-modal platforms and how these techniques are being employed in current research.

Figure 1

Hemoglobin and water in tissue absorb light over a broad spectral regime, creating an optical window in the NIR spectral band between 650 and 900 nm that represent an optimal tradeoff between image resolution and tissue penetration for in vivo imaging. (Reproduced from [17] with permission)

Figure 2

Chemical structures of common non-specific optical contrast agents used in clinical research.

Figure 3

(Left panel) Chemical structure of ITCC conjugated somatostatin (R1), octreotate (R2) and M2M7 (R3). (Right panel) In vivo fluorescence images of RIN38/SSTR2 tumor-bearing nude mice before intravenous injection of (A) R2 and (B) R3. Fluorescence images acquired 6 h after intravenous injection of (C) R2 and (D) R3 at a dose of 0.02 μmol/kg body weight are shown. (Reprinted with the permission from [54].)

Figure 4

(A) FITC conjugated peptide probe. (B) Whole body fluorescence image of a mouse whose abdomen has been surgically exposed, revealing a subcutaneous H69 tumor at 24 h after the administration of the FITC-labeled peptide. (C) Fluorescence image of an excised tumor and adjacent normal organs: (1) H69 tumor nodule; (2) skin; (3) liver; (4) lung; (5) pancreas; (6) kidney; and (7) spleen at 24 h post-administration of the FITC-conjugate peptide. (Reprinted with permission from [55].)

Figure 5

(Top panel) Chemical structure of phage derived peptide with sequence VRPMPLQ conjugated with fluorescein via an aminohexanoic linker. (Middle Panel) Histology of colonic (dysplasia) adenoma (A) and normal mucosa (B) stained with H&E. Scale bars, 50 μm. (Bottom panel) In vivo validation of peptide binding. (A) Conventional white light endoscopic image of colonic adenoma. (B) In vivo confocal image following topical administration of peptide probe, demonstrating preferential binding to dysplastic crypt (left half) and no binding to normal crypt (right half). (C) Confocal image with control peptide (scrambled sequence QLMRPPV) shows no binding to dysplastic crypt, scale bar 20 μm. (Reprinted from [64] with permission.)

Figure 6

(Top panel) Chemical structure of the candidate peptide ASYNYDA selected using techniques of phage display to target high-grade dysplasia (HGD) in Barret’s esophagus. (Bottom panel) In vivo localization of peptide binding to HGD on wide-area fluorescence endoscopy. (A) Conventional white light image of the distal esophagus shows endoscopic evidence of intestinal metaplasia but no sign of pre-malignant lesions (dysplasia). (B) Narrow band image (NBI) of the same region shows improved spectral contrast, highlighting intestinal metaplasia but not dysplasia. (C) In vivo fluorescence image following topical administration of affinity peptide reveals preferential binding to HGD. (Reprinted from [20] with permission.)

Figure 7

Targeted imaging with fluorescent-labeled anti-EGFR antibody. (A) White light image of colonic neoplasia on colonoscopy. (B) Confocal microendoscopy image reveals increased fluorescence intensity within the lesion (arrows), scale bar 100 μm. (C) Mean fluorescence intensity on human specimens was 0.25 ± 0.16 for normal mucosa vs. 2.12 ± 0.30 for neoplasia (p < 0.002). (D) Immuohistochemistry demonstrates EGFR expression in multiple tumor cells (arrows). (Reprinted with the permission from [74].)

Figure 8

Chemical structures of fluorophore conjugates with small molecule ligands; (a) Cy5.5 dye conjugated with folic acid via a PEG linker; (b) bone-targeted IRDye78 pamidronate Pam78; (c) IRDye78 conjugate with PSMA (Prostate Specific Membrane Antigen) ligand GPI; (d) progesterone receptor antagonist mifepristone labeled with FITC; (e) Cy 5.5 dye conjugated photoprobe for ETAR receptor; (f) Barbiturate based Cy5.5 conjugated probe for monitoring MMPs; (g) Hydroxamic based Cy5.5 conjugated probe for MMPs and (h) computationally screened Cy5.5 labeled probe for α_vβ₃ integrins.

Figure 9

Bioluminescence images of mice with double xenograft tumors, HT 1080 (left) and KB (right), collected 6.5 h after injection of (A) Pyro-Peptide-Folate (PPF), revealing targeting of the folate receptor positive KB tumor and of (B) Pyro-Peptide (PP) without folic acid, demonstrating no signal for either HT1080 or KB tumor. (Reprinted from [78] with permission.)

Figure 10

Schematic diagram of enzyme activated methoxypolyethylene modified poly-L-lysine probe. In the dormant state, the proximity of the fluorochrome molecules to each other results in fluorescence quenching. The presence of over expressed enzymes (e.g., MMP or protease) cleaves the probes, releasing intense fluorescence. (Modified and reprinted from [83] with permission).

Figure 11

Various types of nanomaterials used as contrast agents for targeted imaging: (A) gold nanoparticle; (B) carbon nanotube; (C) liposome; (D) micelle; (E) quantum dot; and (F) dendrimer.

Figure 12

(Top panel) Chemical structures of the anti-HER2 affibody molecules. (Bottom panel) Decay corrected coronal microPET images of nu/nu mice bearing SKOV3 (white arrows) and MDA-MB-435 tumor (red arrows) at 1, 4, and 20 h after tail vein injection of (A) ⁶⁴Cu-DOTA-(ZHER2:477)2 and (B) ⁶⁴Cu-DOTAZHER2:477. Decay corrected coronal microPET images of SKOV3 bearing mice that were pretreated with (C) PBS or (D) 300 μg of Herceptin 48 h before probe administration. Images at 1, 3.5, and 24 h after injection of ⁶⁴Cu-DOTA-ZHER2:477 are shown. Yellow arrows indicate location of kidneys. (Reprinted from [105] with permission.)

Figure 13

(A) Schematic representation of triple functional probe HSA-IONPs for PET/optical and MR imaging; (B) Representative in vivo NIR images of a mouse injected with the probe. Images were acquired 1, 4 and 18 h post-injection; (C) In vivo PET images of the mouse are shown after injection at 1, 4 and 18 h; (D) MR images acquired before and 18 h post-injection are shown. (Reprinted from [139] with permission.)