title: In vivo imaging of protease activity in arthritis: a novel approach for monitoring treatment response.
abstract: OBJECTIVE
  Sensitive noninvasive strategies for monitoring treatment response in rheumatoid arthritis (RA) would be valuable for facilitating appropriate therapy and dosing, evaluating clinical outcome, and developing more effective drugs. Because different proteases are highly up-regulated in RA and contribute significantly to joint destruction, in the present study we investigated whether such enzymes are suitable in vivo imaging biomarkers for early evaluation of treatment response in a murine model of RA.
  
  
  METHODS
  Using a protease-activated near-infrared fluorescence (NIRF) imaging "smart" probe, we examined the presence and distribution of fluorescence in arthritic joints of mice with collagen-induced arthritis by both noninvasive fluorescence imaging and histology. Proteases that target the Lys-Lys cleavage site, including cathepsin B, activate probe fluorescence. Treatment monitoring data were obtained following methotrexate (MTX) therapy.
  
  
  RESULTS
  Twenty-four hours after intravenous injection of the protease sensor, affected toes and paws of arthritic mice showed significantly higher fluorescence intensity than did toes and paws of healthy mice. Fluorescence from the protease probe and cathepsin B antibody histologic staining were localized in the vast majority of cells in the inflamed synovium. In arthritic animals treated with MTX (35 mg of MTX/kg 48 hours prior to probe injection), a significantly lower fluorescent signal (inflamed paws 50%, inflamed toes 70%) was observed as compared with untreated arthritic animals.
  
  
  CONCLUSIONS
  Protease-activated NIRF probes are sensitive means of imaging the presence of target enzymes in arthritic joints and can be used for early monitoring of treatment response to antirheumatic drugs such as MTX.
authors.affiliation: Massachusets General Hospital, Harvard Medical School, Charlestown 02129, USA.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wunder
authors.forename: Ching-Hsuan
authors.initials: CH
authors.lastname: Tung
authors.forename: Ulf
authors.initials: U
authors.lastname: Müller-Ladner
authors.forename: Ralph
authors.initials: R
authors.lastname: Weissleder
authors.forename: Umar
authors.initials: U
authors.lastname: Mahmood
mesh_terms: D000818:Animals; D001169:Arthritis, Experimental / Q000139:chemically induced / Q000188:drug therapy / Q000201:enzymology; D001172:Arthritis, Rheumatoid / Q000188:drug therapy* / Q000201:enzymology; D015415:Biomarkers / Q000032:analysis; D003094:Collagen; D004195:Disease Models, Animal; D010450:Endopeptidases / Q000378:metabolism*; D005453:Fluorescence; D006651:Histocytochemistry; D008297:Male; D008727:Methotrexate / Q000627:therapeutic use; D051379:Mice; D008811:Mice, Inbred DBA; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
chemical_list: D015415:Biomarkers; D003094:Collagen; D010450:Endopeptidases; D008727:Methotrexate