Development of a novel nonaromatic small-molecule MR contrast agent for the blood pool

Magnetic Resonance Macromolecular Agents for Monitoring Tumor Microvessels and Angiogenesis Inhibition

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using macromolecular contrast media enables assessments of the tumor vasculature based on the differential distribution of the contrast agent within normal and pathologic tissues. Quantitative assays of both morphologic and functional properties can provide useful diagnostic insight into tissue angiogenesis. The use of MRI enhanced with macromolecular agents for the characterization of tumor microvessels has been experimentally demonstrated in a range of malignant tumor types. Kinetic analysis of DCE-MRI data can be used to estimate microvascular permeability and tumor blood volume. By measuring these functional tumor properties, an accurate, noninvasive, and quantitative description of the microcirculation of individual tumors can be acquired, improving the specificity of imaging examinations for cancer diagnosis and for treatment and follow up. The noninvasive MRI assessment of tumor angiogenesis can be applied in the diagnostic differentiation between benign and malignant tumors and can also provide means for in vivo monitoring of antitumor therapy. In this review, the potential clinical applications and limitations of various macromolecular contrast agents applied for evaluations of tumor angiogenesis, with and without drug interventions, are discussed.

Albumin-binding MR blood pool agents as MRI contrast agents in an intracranial mouse glioma model

Intravenous MRI contrast agents are commonly used to improve the detection of intracranial tumors and other central nervous system (CNS) lesions for diagnosis and treatment planning. Two small-molecule, albumin-binding blood pool contrast agents (MP-2269 and MS-325) of potential clinical significance were evaluated at 1.5 Tesla in a mouse glioma model and compared with an extracellular contrast agent (OptiMARK). Tumor image contrast was significantly enhanced and long-lived following administration of 30 micromole/kg of the blood pool agents: specifically, contrast enhancement peaked slowly at 25-30 min following administration, remained constant for >3 hr, and returned to baseline within 20 hr. Comparable but "transient" enhancement was achieved using 100 micromole/kg OptiMARK: specifically, contrast enhancement peaked rapidly at 2-5 min following administration and then declined over 40 min. The blood pool contrast agents demonstrated an approximately threefold increased dose-effectiveness and a lengthened window of tumor contrast enhancement in comparison to commonly available extracellular contrast agents. This demonstrates the potential of alternative contrast-enhanced (CE) MRI examination protocols for tumor detection.

The effect of a phosphodiester linking group on albumin binding, blood half-life, and relaxivity of intravascular diethylenetriaminepentaacetato aquo gadolinium(III) MRI contrast agents

Amphiphilic gadolinium complexes were investigated as potential magnetic resonance imaging (MRI) contrast agents. A series of complexes was synthesized in order to study the effect of hydrophilic phosphodiester groups on albumin binding, relaxivity, and blood half-life in rats. Thus, compound 11a, a diethylenetriaminepentaacetato aquo gadolinium(III) (Gd-DTPA) derivative with an octyl substituent, was synthesized and compared to 5b, the analogous octyl derivative containing a phosphodiester linkage between the gadolinium chelate and the alkyl chain. Likewise, 11b, a naphthyl Gd-DTPA derivative, was compared to the naphthyl phosphodiester derivative 5c. A direct comparison is not available for 5a, a 4,4-diphenylcyclohexyl phosphodiester Gd-DTPA derivative; however, its pharmacokinetic properties mirror those of the other phosphodiester derivatives. Although the introduction of the phosphodiester moiety decreased log P by approximately 1.7 units, albumin binding data obtained in 4.5% human serum albumin (HSA) indicated that derivatives containing the phosphodiester group exhibited somewhat higher albumin affinity than their alkyl analogues (54 +/- 5 and 44 +/- 4% for 5b and 11a, respectively; 40 +/- 4 and 30 +/- 3% for 5c and 11b, respectively). Both classes of agents were characterized by enhanced relaxivity in the presence of 4.5% HSA (r1 = 16-42 mM(-1) s(-1) at 20 MHz and 37 degrees C) as compared with the relaxivity values measured in phosphate-buffered saline (PBS) alone (r1 = 4.6-6.6 mM(-1) s(-1) at 20 MHz and 37 degrees C). Pharmacokinetic data indicated that compound 5b had a half-life of 14.3 +/- 1.8 min in the rat as compared with a half-life of 6.20 +/- 0.04 min for the non-phosphodiester analogue 11a. Similarly, the half-life obtained for the phosphodiester 5c was 14.3 +/- 1.7 min as compared with a half-life of 6.80 +/- 0.03 min for 11b. The percent biliary excretion was significantly lower for the phosphodiester compounds than for non-phosphodiester analogues (17.7 +/- 4.0 and 66.9 +/- 3.4% for 5b and 11a, respectively; 17.0 +/- 1.6 and 64.3 +/- 9.0% for 5c and 11b, respectively). The percent biliary excretion (15.8 +/- 4.4%) and plasma half-life in the rat (23.1 +/- 2.9 min) for 5a are consistent with the extended plasma half-life of the other phosphodiester derivatives. Taken together, the enhanced relaxivity and extended blood half-life of the phosphodiester derivatives support the concept of using endogenous albumin binding to achieve blood pool-like properties for small-molecule magnetic resonance imaging (MRI) contrast agents.