New classes of physiologically responsive magnetic resonance (MR) contrast agents are being developed that are activated by enzymes, secondary messengers, pH, and temperature. To this end, we have prepared a new class of enzyme-activated MR contrast agents using a self-immolative mechanism and investigated the properties of these agents using novel in vitro assays. We have synthesized in nine steps a Gd(III) agent 1 that is activated by the oncologically significant beta-glucuronidase. 1 consists of Gd(III)DO3A (DO3A = 1,4,7-tricarboxymethylene-1,4,7,10-tetraazacyclododecane) bearing a pendant beta-glucuronic acid moiety connected by a self-immolative linker to the macrocycle. LC-MS analysis reveals that 1 is enzymatically processed as predicted by bovine liver beta-glucuronidase, generating 2-aminoethylGdDO3A, 2. Compound 2 was prepared independently in a four-step synthetic procedure. Complex 1 displays a decrease in relaxivity upon titration with bicarbonate anion. The relaxivity increases when 1 is converted to 2 in a buffer mimicking in vivo anion concentrations (Parker, D. In Crown Compounds: Towards Future Applications; Cooper, S. R., Ed.; VCH: New York, 1992; pp 51-67) by 17%, while the relaxivity decreases by 27% for the same experiment in human blood serum. Hydrolytic kinetics catalyzed by bovine liver beta-glucuronidase at interstitial pH = 7.4 fit the Michaelis-Menten model with k cat/Km = 74.9 +/- 10.9 M(-1) s(-1). Monitoring of bulk water proton T1 during incubation with enzyme shows an increase in T1 that mirrors results obtained through the relaxivity measurements of compounds 1 and 2.

We report a mechanistic investigation of an isomeric series of β-galactosidase-activated magnetic resonance contrast agents. Our strategy focuses on the synthesis of macrocyclic caged-complexes that coordinatively saturate a chelated lanthanide. Enzyme cleavage of the complex results in an open coordination site available for water that creates a detectable MR contrast agent. The complexes consist of a DO3A Gd(III) chelator modified with a galactopyranose at the N-10 position of the macrocycle. We observed significant differences in relaxometric properties and coordination geometry that can be correlated to subtle variations of the linker between the macrocycle and the galactopyranose. After synthesis and purification of the R, S, and racemic mixtures of complexes 1 and 3 and measurement of the hydration number, water residence lifetime, and longitudinal relaxation rates, we propose mechanisms for water exclusion from the lanthanide in the precleavage state. While the stereochemistry of the linker does not influence the agents' properties, the mechanism of water exclusion for each isomer is significantly influenced by the position of modification. Data for one series with a methyl group substituted on the sugar-macrocycle linker at the R-position suggests a steric mechanism where the galactopyranose sugar blocks water from the Gd(III) center. In contrast, our observations for a second series with methyl substitution at the β position of the sugar-macrocycle linker are consistent with a mechanism in which a bidentate anion occupies two available coordination sites of Gd(III) in the precleavage state.

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