title: A gadolinium chelate for detection of beta-glucuronidase: a self-immolative approach.
abstract: 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.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60201, USA.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Duimstra
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Femia
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meade
mesh_terms: D000818:Animals; D002417:Cattle; D002614:Chelating Agents / Q000138:chemical synthesis / Q000737:chemistry*; D005682:Gadolinium / Q000737:chemistry*; D005966:Glucuronidase / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013997:Time Factors
chemical_list: D002614:Chelating Agents; D009942:Organometallic Compounds; D005682:Gadolinium; D005966:Glucuronidase