Magnetic resonance imaging (MRI) is one of the most impressive non-invasive imaging modality for diagnosis [1, 2]. MRI is based on the nuclear magnetic resonance (NMR) signal generated by hydrogen nuclei of water molecules and its changes that are dependent on the water distribution in tissues. MRI contrast agents improve diagnostic accuracy by providing physiological information along with the exquisitely high anatomic detail [3, 4]. In general, contrast agents consist of a paramagnetic metal center, typically gadolinium (III), which must be chelated with an appropriate ligand molecule, since the free metal ions are toxic at quantity required for diagnosis [5, 6]. Gd(III) diethylenetriaminepentaacetic acid (Gd(III)DTPA) and Gd(III) 1,4,7,10-tetraazacyclododecan-1,4,7,10-tetraacetic acid (Gd(III)DOTA) are clinically used as contrast agents due to their thermodynamic stability . Among Gd-chelate complexes, it has been known that the Gd-DOTA complex has much greater stability than Gd-DTPA in physiological conditions. The half-life of Gd-DOTA is estimated to be over 1,000 years at pH 7.4 [8, 9]. However, these low molecular weight contrast agents cannot distinguish effectively disease tissues from normal tissues . In recent years, contrast agents with improved characteristics, such as increased efficacy and organ specificity, have been developed.
Several approaches to slow the rotational motion of gadolinium-based contrast agents and thus to improve their relaxation efficiency have been reported in the literature. Macromolecular Gd complexes have been developed by conjugating these Gd chelates to biocompatible polymer such as dendrimers , linear polymers [12–14], or proteins [15, 16] for exhibition of more effective relaxation as well as prolongation of their intravascular retention time . For these purposes, various hydrophobic groups have been introduced into the DTPA molecule through amidation of monoacid tetraester DTPA derivatives. The paramagnetic complexes of these compounds have been studied for the detection of tumors and myocardinal infections. In addition, incorporation of certain amphiphilic gadolinium(III) complexes into liposomes showed enhanced proton relaxibility [18, 19].
Another interesting strategy for the development of new types of macromolecular contrast reagents is the synthesis of amphiphilic gadolinium(III) complexes that can form spontaneously micelles [20, 21]. We recently synthesized biocompatible amphiphilic derivatives of DOTA with hydrophobic alkyl chains, whose gadolinium(III) were incorporated into DOTA of micelles. The resulting micelles showed an increase in relaxibility relative to DOTA-Gd (Omniscan®) because of the lower mobility of the paramagnetic complex inside micelles.
Polymeric micelles very promising for MRI contrast agents. Because the polymeric micelles is an associate of many block copolymer chains, block copolymers with well-controlled molecular weight can be excreted through kidney filtration after dissociation of the polymeric micelles into block copolymer chains. Therefore, a low risk of chronic toxicity is expected to present itself and is expected to stem from polymeric micelles complete excretion over a long time period. In addition, micelles are known to mimic the phospholipid structure of the membranes and hence can be good candidates as hepatocyte-specific agents .
Polyhydroxyethylaspartamide (PHEA) is a synthetic polymer having protein-like structure, obtained by the reaction of ethanolamine with polysuccinimide (PSI), itself prepared by thermal polycondensation of d,l-aspartic acid. PHEA has good biopharmaceutical properties as drug carrier such as high water solubility, multifunctionality, and low cost of production [23–29].
In this paper, we synthesized amphiphilic graft derivatives of PHEA by the introduction of hydrophobic hexadecylamine (C16) and using ethylenediamine (ED) as a linker of DOTA-Gd. DOTA was conjugated to PHEA-mPEG-C16-ED, and then Gd was chelated to the PHEA-mPEG-C16-ED-DOTA. MR contrast enhancing ability of PHEA-mPEG-C16-ED-DOTA-Gd was investigated in vitro and in vivo. The synthesized graft amphiphilic copolymers have demonstrated to form polymeric micelles in aqueous solution with 180 nm.