title: Tumor targeting by an aptamer.
abstract: Aptamers are small oligonucleotides that are selected to bind tightly and specifically to a target molecule. We sought to determine whether aptamers have potential for in vivo delivery of radioisotopes or cytotoxic agents.
  
  
  METHODS
  TTA1, an aptamer to the extracellular matrix protein tenascin-C, was prepared in fluorescent and radiolabeled forms. After in vivo administration, uptake and tumor distribution of Rhodamine Red-X-labeled aptamer was studied by fluorescence microscopy. In glioblastoma (U251) and breast cancer (MDA-MB-435) tumor xenografts, biodistribution and imaging studies were performed using TTA1 radiolabeled with (99m)Tc. Tenascin-C levels and tumor uptake were studied in a variety of additional human tumor xenografts. To assess the effect of radiometal chelate on biodistribution, mercapto-acetyl diglycine (MAG(2)) was compared with diethylenetriaminepentaacetic acid and with MAG(2)-3,400-molecular-weight PEG (PEG(3,400)).
  
  
  RESULTS
  Intravenous injection of fluorescent aptamer TTA1 produced bright perivascular fluorescence in a xenografted human tumor within 10 min. In the ensuing 3 h, fluorescence diffused throughout the tumor. Labeled with (99m)Tc, TTA1 displayed rapid blood clearance, a half-life of less than 2 min, and rapid tumor penetration: 6% injected dose (%ID)/g at 10 min. Tumor retention was durable, with 2.7 %ID/g at 60 min and a long-lived phase that stabilized at 1 %ID/g. Rapid tumor uptake and blood clearance yielded a tumor-to-blood ratio of 50 within 3 h. Both renal and hepatic clearance pathways were observed. Using the (99m)Tc-labeled aptamer, images of glioblastoma and breast tumors were obtained by planar scintigraphy. Aptamer uptake, seen in several different human tumors, required the presence of the target protein, human tenascin-C. Modification of the MAG(2) radiometal chelator dramatically altered the uptake and clearance patterns.
  
  
  CONCLUSIONS
  TTA1 is taken up by a variety of solid tumors including breast, glioblastoma, lung, and colon. Rapid uptake by tumors and rapid clearance from the blood and other nontarget tissues enables clear tumor imaging. As synthetic molecules, aptamers are readily modified in a site-specific manner. A variety of aptamer conjugates accumulate in tumors, suggesting imaging and potentially therapeutic applications.
authors.affiliation: NeXstar Pharmaceuticals, Boulder, Colorado, USA. bhicke@gmail.com
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Hicke
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Stephens
authors.forename: Ty
authors.initials: T
authors.lastname: Gould
authors.forename: Ying-Fon
authors.initials: YF
authors.lastname: Chang
authors.forename: Cynthia K
authors.initials: CK
authors.lastname: Lynott
authors.forename: James
authors.initials: J
authors.lastname: Heil
authors.forename: Sandra
authors.initials: S
authors.lastname: Borkowski
authors.forename: Christoph-Stephan
authors.initials: CS
authors.lastname: Hilger
authors.forename: Gary
authors.initials: G
authors.lastname: Cook
authors.forename: Stephen
authors.initials: S
authors.lastname: Warren
authors.forename: Paul G
authors.initials: PG
authors.lastname: Schmidt
mesh_terms: D000818:Animals; D052157:Aptamers, Nucleotide / Q000737:chemistry* / Q000493:pharmacokinetics; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry; D005260:Female; D005456:Fluorescent Dyes / Q000737:chemistry; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D008856:Microscopy, Fluorescence; D009368:Neoplasm Transplantation; D009374:Neoplasms, Experimental / Q000000981:diagnostic imaging / Q000378:metabolism*; D009942:Organometallic Compounds / Q000737:chemistry; D015609:Organotechnetium Compounds / Q000737:chemistry* / Q000493:pharmacokinetics; D004369:Pentetic Acid / Q000737:chemistry; D011485:Protein Binding; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000737:chemistry* / Q000493:pharmacokinetics; D012235:Rhodamines / Q000737:chemistry; D019063:Tenascin / Q000378:metabolism; D014018:Tissue Distribution; D014183:Transplantation, Heterologous
chemical_list: D052157:Aptamers, Nucleotide; D002614:Chelating Agents; D004151:Dipeptides; D005456:Fluorescent Dyes; D009942:Organometallic Compounds; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; D012235:Rhodamines; D019063:Tenascin; C510949:technetium 99m mercapto-acetyl diglycine; D004369:Pentetic Acid