Multilocus Binding Increases the Relaxivity of Protein-Bound MRI Contrast Agents

Facile Synthesis of Weakly Ferromagnetic Organogadolinium Macrochelates-Based T1 -Weighted Magnetic Resonance Imaging Contrast Agents

To surmount the major concerns of commercial small molecule Gd chelates and reported Gd-based contrast agents (GBCAs) for magnetic resonance imaging (MRI), a new concept of organogadolinium macrochelates (OGMCs) constructed from the coordination between Gd3+ and macromolecules is proposed. A library of macromolecules were screened for Gd3+ coordination, and two candidates [i.e., poly(acrylic acid) (PAA), and poly(aspartic acid) (PASP)] succeeded in OGMC formation. Under optimized synthesis conditions, both Gd-PAA12 and Gd-PASP11 OGMCs are outstanding T1 -weighted CAs owing to their super high r1 values (> 50 mm-1  s-1 , 3.0 T) and ultralow r2 /r1 ratios (< 1.6, 3.0 T). The ferromagnetism of OGMCs is completely different from the paramagnetism of commercial and reported GBCAs. The ferromagnetism is very weak (Ms  < 1.0 emu g-1 ) leading to a low r2 , which is preferred for T1 MRI. Gd3+ is not released from the OGMC Gd-PAA12 and Gd-PASP11, ensuring biosafety for in vivo applications. The safety and T1 -weighted MRI efficiencies of the OGMC Gd-PAA12 and Gd-PASP11 are tested in cells and mice. The synthesis method of the OGMCs is facile and easy to be scaled up. Consequently, the OGMC Gd-PAA12 and Gd-PASP11 are superior T1 -weighted CAs with promising translatability to replace the commercial Gd chelates.

In Situ Gastric pH Imaging with Hydrogel Capsule Isolated Paramagnetic Metallo-albumin Complexes

Abnormal gastric pH (pH > 3) has instructive significance for early diagnosis of various diseases, including cancer. However, for low patient compliance, limited penetration depth, high dependence on physiological function or unsafety issue, in situ noninvasive monitoring gastric pH is challenged. Herein, we developed a hydrogel capsule isolated human serum albumin-manganese complex (HSA-Mn) for in situ magnetic resonance imaging (MRI) gastric pH monitoring for the first time. In this strategy, the rotation motion restriction of Mn²⁺ after binding to HSA significantly increased the R₁ (longitudinal relaxation rate) signal, and its high correlation with protonation imparted the HSA-Mn system sensitive responsiveness to varying pH (R₁(pH 7)/R₁(pH 1) = 8.2). Moreover, a screw jointed hydrogel capsule with signal confinement and internal standard abilities was designed. Such a nanoporous hydrogel capsule with size selectivity to surrounding molecules enabled a stable and sensitive response to different pH simulated gastric fluid within 0.5 h. In addition, with the unique structural outline and stable MRI characteristics, the capsule could also work as an internal standard, which facilitates the collection of signals and trace of the capsule in vivo. Through validating in a rabbit model, the precise abnormal gastric pH recognition capacity of the HSA-Mn hydrogel capsule was amply confirmed. Hence, the hydrogel capsule isolated HSA-Mn system strategy with great biocompatibility could be expected to be a potent tool for in situ anti-disturbance MRI of gastric pH in future clinical application.

Structure - relaxivity relationships among targeted MR contrast agents

Paramagnetic gadolinium(III) complexes are widely used to increase contrast in magnetic resonance (MR) images. Contrast enhancement depends on the concentration of the gadolinium complex and on its relaxivity, an inherent property of the complex. Increased relaxivity results in greater image contrast or the ability to detect the contrast agent at a lower concentration. Increasing relaxivity enables imaging of abundant molecular targets.Relaxivity depends on the structure of the complex, kinetics of inner-sphere and second sphere water exchange, and on the rotational dynamics of the molecule. The latter, and in some cases the former, properties of the complex change when it is bound to its target. All of these properties can be rationally tuned to enhance relaxivitry. In this Microreview we summarize our efforts in understanding and optimizing the relaxivity of contrast agents targeted to serum albumin and to fibrin.

Serum albumin targeted, pH-dependent magnetic resonance relaxation agents

The objective of this work was the synthesis of serum albumin targeted, Gd(III)-based magnetic resonance imaging (MRI) contrast agents exhibiting a strong pH-dependent relaxivity. Two new complexes (Gd-glu and Gd-bbu) were synthesized based on the DO3A macrocycle modified with three carboxyalkyl substituents α to the three ring nitrogen atoms, and a biphenylsulfonamide arm. The sulfonamide nitrogen coordinates the Gd in a pH-dependent fashion, resulting in a decrease in the hydration state, q, as pH is increased and a resultant decrease in relaxivity (r(1)). In the absence of human serum albumin (HSA), r(1) increases from 2.0 to 6.0 mM(-1)  s(-1) for Gd-glu and from 2.4 to 9.0 mM(-1)  s(-1) for Gd-bbu from pH 5 to 8.5 at 37 °C, 0.47 T, respectively. These complexes (0.2 mM) are bound (>98.9 %) to HSA (0.69 mM) over the pH range 5-8.5. Binding to albumin increases the rotational correlation time and results in higher relaxivity. The r(1) increased 120 % (pH 5) and 550 % (pH 8.5) for Gd-glu and 42 % (pH 5) and 260 % (pH 8.5) for Gd-bbu. The increases in r(1) at pH 5 were unexpectedly low for a putative slow tumbling q=2 complex. The Gd-bbu system was investigated further. At pH 5, it binds in a stepwise fashion to HSA with dissociation constants K(d1)=0.65, K(d2)=18, K(d3)=1360 μM. The relaxivity at each binding site was constant. Luminescence lifetime titration experiments with the Eu(III) analogue revealed that the inner-sphere water ligands are displaced when the complex binds to HSA resulting in lower than expected r(1) at pH 5. Variable pH and temperature nuclear magnetic relaxation dispersion (NMRD) studies showed that the increased r(1) of the albumin-bound q=0 complexes is due to the presence of a nearby water molecule with a long residency time (1-2 ns). The distance between this water molecule and the Gd ion changes with pH resulting in albumin-bound pH-dependent relaxivity.

Visible drug delivery by supramolecular nanocarriers directing to single-platformed diagnosis and therapy of pancreatic tumor model

Nanoparticle therapeutics are promising platforms for cancer therapy. However, it remains a formidable challenge to assess their distribution and clinical efficacy for therapeutic applications. Here, by using multifunctional polymeric micellar nanocarriers incorporating clinically approved gadolinium (Gd)-based magnetic resonance imaging contrast agents and platinum (Pt) anticancer drugs through reversible metal chelation of Pt, simultaneous imaging and therapy of an orthotopic animal model of intractable human pancreatic tumor was successfully performed without any serious toxicity. The strong tumor contrast enhancement achieved by the micelles correlated with the 24 times increase of r(1) of the Gd chelates, the highest for the formulations using clinically approved Gd chelates reported to date. From the micro-synchrotron radiation X-ray fluorescence spectrometry scanning of the lesions, we confirmed that both the Gd chelates and Pt drugs delivered by the micelles selectively colocalized in the tumor interior. Our study provides new insights for the design of theranostic micelles with high contrast enhancement and site-specific clinical potential.