Targeted probes for cardiovascular MRI

Abstract

Molecular MRI plays an important role in studying molecular and cellular processes associated with heart disease. Targeted probes that recognize important biomarkers of atherosclerosis, apoptosis, necrosis, angiogenesis, thrombosis and inflammation have been developed. This review discusses the properties of chemically different contrast agents including iron oxide nanoparticles, gadolinium-based nanoparticles or micelles, discrete peptide conjugates and activatable probes. Numerous examples of contrast agents based on these approaches have been used in preclinical MRI of cardiovascular diseases. Clinical applications are still under investigation for some selected agents with highly promising initial results. Molecular MRI shows great potential for the detection and characterization of a wide range of cardiovascular diseases, as well as for monitoring response to therapy.

Keywords: Cardiovascular; MRI; atherosclerosis; gadolinium; iron oxide nanoparticles; micelles; molecular imaging; smart probes; targeted peptides.

Figure 1

Chemical structures of some Gd(III) containing contrast agents discussed in this review. These include acyclic chelates such as Gd-DTPA and GdDTPA-bis(methylamide) (Gd-DTPA-BMA); macrocyclic chelates such as Gd-DOTA and Gd-HPDO3A; the albumin binding agent MS-325; the micelle forming amphiphile Gadofluorine M; and activatable agents such as GdDTPA-bis-5HT.

Figure 2

Schematic representation of the synthesis of targeted and fluorescently labeled superparamagnetic iron oxide nanoparticles. Coprecipitation of ferrous and ferric salts in an alkaline medium in the presence of a biocompatible surface complexing agent such as dextran gives a polydisperse mixture of nanoparticles that is fractionated using sizing columns and/or by magnetic separation. The dextran coating is then crosslinked and functionalized. These functionalized nanoparticles can be conjugated to targeting ligands and fluorescent reporters.

Figure 3

Synthesis of VCAM-1 targeting nanoparticles. Crosslinked iron oxide nanoparticles (CLIO) are first labeled with Cy5.5. The other amino groups on the surface of the nanoparticles are reacted with activated iodoacetic acid to create pendant iodides, which can then react with the thiol groups on the targeting peptide.

Figure 4

Synthesis of CLIO-Cy5.5-Annexin V. CLIO-Cy5.5 was reacted with the SPDP linker which contains a disulfide linkage. The pyridine thiol in this disulfide bond can be exchanged with the cysteine thiol in Annexin V.

Figure 5

(a – c) Schematic representation of Gd based agents. (a) Targeting peptide/vector conjugated to Gd chelates; (b) Gd loaded micelles conjugated to targeting ligand and fluorescent label; (c) Gd loaded liposomes conjugated to targeting ligand and fluorescent label; (d, e) commonly used micelle/liposome constituents: (d) palmitoyloleoyl phosphatidylcholine (POPC); (e) 1,2-Dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE).

Figure 6

Structure of EP-3533. Amino acids are represented by their one-letter abbreviations. The targeting peptide is highlighted in gray. The amino acids whose side chains are critical for binding are shown in bold. The amino acid positions that can be replaced with lysine for conjugation of Gd chelates with no effect on binding are shown in italics.

Figure 7

Mixed solid phase, solution phase synthesis of the fibrin targeted peptide-gadolinium chelate conjugate EP-2104R.

Figure 8

MRI of thrombus in the thoracic aorta post injection of EP-2104R. The thrombus appears bright on an inversion recovery black blood gradient echo sequence after EP-2104R injection. Arrow shows the localization of the clot. A corresponding multiplanar reconstruction from a contrast-enhanced CT indicates a filling defect consistent with the thrombus in the MR image. At the cranial end of the plaque a small calcification is also visible (arrowhead). (From Spuentrup et al. [56] with kind permission of Springer Science+Business Media).

Figure 9

Examples of types of Gd micelle forming amphiphiles. (a) Amphiphiles where the hydrophilic part solely consists of the Gd chelate conjugated to fatty acyl chains. Structure shown is GdDTPA-bistrearylamide (GdDTPA-BSA). (b) Amphiphiles where the Gd chelate is conjugated to the hydrophilic part of an existing amphiphile such as phosphoethanolamine (PE). Structure shown is GdDOTA-distearoyl-phosphoethanolamine (GdDOTA-DSPE).

Figure 10

Activatable probes. (a) A general schematic of the process of activation for “smart” probes. (b) A specific example of a “smart” probe activated by the thrombin-activatable fibrinolysis inhibitor (TAFI). A Gd³⁺ prodrug complex with poor affinity for human serum albumin (HSA) and concominant low relaxivity is transformed by the TAFI enzyme to a species with stronger affinity to HSA and higher relaxivity. The relaxivity in HSA solution increased from 9.8 to 26.5 mM^-1s^-1 upon activation by TAFI.