As we move towards an era of personalized medicine, molecular imaging contrast agents are likely to see an increasing presence in routine clinical practice. Magnetic resonance (MR) imaging has garnered particular interest as a platform for molecular imaging applications due its ability to monitor anatomical changes concomitant with physiologic and molecular changes. One promising new direction in the development of MR contrast agents involves the labeling and/or loading of nanoparticles with gadolinium (Gd). These nanoplatforms are capable of carrying large payloads of Gd, thus providing the requisite sensitivity to detect molecular signatures within disease pathologies. In this review, we discuss some of the progress that has recently been made in the development of Gd-based macromolecules and nanoparticles and outline some of the physical and chemical properties that will be important to incorporate into the next generation of contrast agents, including high Gd chelate stability, high “relaxivity per particle” and “relaxivity density”, and biodegradability.

Eu(S-THP) 3+ is the first PARACEST agent that functions through exchange of hydroxyl groups with water protons in aqueous solution. The CEST spectrum of this complex is highly pH dependent and is modulated by the presence of phosphate esters, as shown for diethyl phosphate which forms an outersphere complex and by methyl phosphate which forms an innersphere complex with Eu(S-THP) 3+ . The sensitivity of the alcohol proton environment to interactions with these anions shows that this complex is promising as a responsive PARACEST MRI contrast agent.The development of magnetic resonance imaging (MRI) contrast agents that report on their environment through specific molecular recognition events is an active area of research. 1 To address this, we reported on Ln(III) macrocyclic complexes with alcohol exchangeable protons, but these complexes functioned as PARACEST agents only in water/acetonitrile mixtures. 17 The alcohol proton exchange rate constant was predicted to be too large to observe a CEST effect. Here we show for the first time that a Eu(III) macrocyclic complex with alcohol groups, Eu(S-THP) 3+ (Chart 1) acts as a PARACEST agent in pure water under controlled pH. In addition, the CEST spectrum of this complex is selectively responsive to two biologically important classes of phosphate esters. The modulation of the CEST effect is unexpectedly mediated by an outersphere phosphate diester or an innersphere phosphate monoester complex as shown by direct excitation Eu(III) luminescence spectroscopy. These differences may provide a basis for designing selectively responsive PARACEST agents.The CEST spectrum of Eu(S-THP) 3+ as shown in Figure 1 was recorded by applying a presaturation pulse in 1 ppm increments. There is a CEST feature at about 6 ppm downfield of bulk water that arises from the alcohol group as shown by the corresponding alcohol proton resonance ( Figure S1). A pronounced pH-dependence is observed for the CEST spectrum of Eu(S-THP) 3+ over the pH range of 4.5 to 7.3 with an optimum pH of 5.9 ( Figure S2). This pH dependence is characteristic of base catalyzed exchange with a low pH optimum due to the acidic alcohol protons. 18 In addition, anionic ligands such as phosphate esters modulate the pH dependence of the CEST effect.Titration of Eu(S-THP) 3+ with diethylphosphate (DEP) in buffered solution, pH 6.6 and 100 mM NaCl increases the intensity of the existing CEST alcohol peak (Figure 1a). A plot of the Eu(S-THP) 3+ CEST response as a function of DEP concentration ( Figure S3a) shows that even one equivalent of DEP changes the CEST effect. By contrast, addition of methylphosphate (MP) to Eu(S-THP) 3+ changes the CEST spectrum in two ways (Figure 1b). The alcohol CEST peak of Eu(S-THP) 3+ decreases and a new CEST peak at about 8 ppm grows in, corresponding to a new alcohol proton resonance ( Figure S4). A plot of the intensity of the new CEST peak as a function of MP is fit to a 1:1 binding curve ( Figure S3b) with a dissociation constant of 10 mM. These phosphate ester complexes of E...

Gd-conjugated dendrimer nanoclusters (DNCs) are a promising platform for the early detection of disease; however, their clinical utility is potentially limited due to safety concerns related to nephrogenic systemic fibrosis (NSF). In this paper, biodegradable DNCs were prepared with polydisulfide linkages between the individual dendrimers to facilitate excretion. Further, DNCs were labeled with pre-metalated Gd chelates to eliminate the risk of free Gd becoming entrapped in dendrimer cavities. The biodegradable polydisulfide DNCs possessed a circulation half-life of > 1.6 h in mice and produced significant contrast enhancement in the abdominal aorta and kidneys for as long as 4 h. The DNCs were reduced in circulation as a result of thiol-disulfide exchange and the degradation products were rapidly excreted via renal filtration. These agents demonstrated effective and prolonged in vivo contrast enhancement and yet minimized Gd tissue retention. Biodegradable polydisulfide DNCs represent a promising biodegradable macromolecular MRI contrast agent for magnetic resonance angiography and can potentially be further developed into target specific MRI contrast agents.

The Eu(III) complex of (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (S-THP) is studied as a sensor for biologically relevant anions. Anion interactions produce changes in the luminescence emission spectrum of the Eu(III) complex, in the 1H NMR spectrum, and correspondingly, in the PARACEST spectrum of the complex (PARACEST = paramagnetic chemical exchange saturation transfer). Direct excitation spectroscopy and luminescence lifetime studies of Eu(S-THP) give information about the speciation and nature of anion interactions including carbonate, acetate, lactate, citrate, phosphate and methylphosphate at pH 7.2. Data is consistent with the formation of both innersphere and outersphere complexes of Eu(S-THP) with acetate, lactate and carbonate. These anions have weak dissociation constants that range from 19–38 mM. Citrate binding to Eu(S-THP) is predominantly innersphere with a dissociation constant of 17 μM. Luminescence emission peak changes upon addition of anion to Eu(S-THP) show that there are two distinct binding events for phosphate and methylphosphate with dissociation constants of 0.3 mM and 3.0 mM for phosphate and 0.6 mM and 9.8 mM for methyl phosphate. Eu(THPC) contains an appended carbostyril derivative as an antenna to sensitize Eu(III) luminescence. Eu(THPC) binds phosphate and citrate with dissociation constants that are 10-fold less than that of the Eu(S-THP) parent, suggesting that functionalization through a pendent group disrupts the anion binding site. Eu(S-THP) functions as an anion responsive PARACEST agent through exchange of the alcohol protons with bulk water. The alcohol proton resonances of Eu(S-THP) shift downfield in the presence of acetate, lactate, citrate and methylphosphate, giving rise to distinct PARACEST peaks. In contrast, phosphate binds to Eu(S-THP) to suppress the PARACEST alcohol OH peak and carbonate does not markedly change the alcohol peak at 5 mM Eu(S-THP), 15 mM carbonate at pH 6.5 or 7.2. This work shows that the Eu(S-THP) complex has unique selectivity toward binding of biologically relevant anions and that anion binding results in changes in both the luminescence and PARACEST spectra of the complex.

Lanthanide(III) complexes of macrocycles 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (THED) and (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (S-THP) were studied as chemical exchange saturation transfer (CEST) agents for magnetic resonance imaging (MRI) applications. The four hyperfine-shifted alcohol protons of these Ln(III) complexes gave rise to a single 1H resonance in wet d3-acetonitrile that was separated from the bulk water resonance (Δω) by 8 ppm (Ce), 2 ppm (Nd), 7 ppm (Eu) or 17 ppm (Yb). A CEST peak corresponding to the alcohol protons was observed for all Ln(THED)3+ or Ln(S-THP)3+ complexes except Nd(III) at low water concentrations (< 1%). In 100% aqueous buffered solutions, the CEST hydroxyl peak is observed for the Eu(III), Ce(III) and Yb(III) complexes over a range of pH values. The optimal pH range for the CEST effect of each complex is related to the pKa of the hydroxyl/water ligands of the complex. Optimum pH values for the CEST effect from alcohol proton exchange are pH = 6.0 for Ce(S-THP)3+, pH = 4.5 for Eu(THED)3+, and pH= 3.0 for Yb(S-THP)3+.