title: Cerenkov-specific contrast agents for detection of pH in vivo.
abstract: We report the design, testing, and in vivo application of pH-sensitive contrast agents designed specifically for Cerenkov imaging. Radioisotopes used for PET emit photons via Cerenkov radiation. The multispectral emission of Cerenkov radiation allows for selective bandwidth quenching, in which a band of photons is quenched by absorption by a functional dye. Under acidic conditions, (18)F-labeled derivatives emit the full spectrum of Cerenkov light. Under basic conditions, the dyes change color and a wavelength-dependent quenching of Cerenkov emission is observed.
  
  
  METHODS
  Mono- and di-(18)F-labeled derivatives of phenolsulfonphthalein (phenol red) and meta-cresolsulfonphthalein (cresol purple) were synthesized by electrophilic fluorination. Cerenkov emission was measured at different wavelengths as a function of pH in vitro. Intramolecular response was measured in fluorinated probes and intermolecular quenching by mixing phenolphthalein with (18)F-FDG. Monofluorocresol purple (MFCP) was tested in mice treated with acetazolamide to cause urinary alkalinization, and Cerenkov images were compared with PET images.
  
  
  RESULTS
  Fluorinated pH indicators were produced with radiochemical yields of 4%-11% at greater than 90% purity. Selective Cerenkov quenching was observed intramolecularly with difluorophenol red or monofluorocresol purple and intermolecularly in phenolphthalein (18)F-FDG mixtures. The probes were selectively quenched in the bandwidth closest to the indicator's absorption maximum (λmax) at pHs above the indicator pKa (the negative logarithm of the acid dissociation constant). Addition of acid or base to the probes resulted in reversible switching from unquenched to quenched emission. In vivo, the bladders of acetazolamide-treated mice exhibited a wavelength-dependent quenching in Cerenkov emission, with the greatest reduction occurring near the λmax. Ratiometric imaging at 2 wavelengths showed significant decreases in Cerenkov emission at basic pH and allowed the estimation of absolute pH in vivo.
  
  
  CONCLUSIONS
  We have created contrast agents that selectively quench photons emitted during Cerenkov radiation within a given bandwidth. In the presence of a functional dye, such as a pH indicator, this selective quenching allows for a functional determination of pH in vitro and in vivo. This method can be used to obtain functional information from radiolabeled probes using multimodal imaging. This approach allows for the imaging of nonfluorescent chromophores and is generalizable to any functional dye that absorbs at suitable wavelengths.
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Julie
authors: J
authors: Czupryna
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Alexander V
authors: AV
authors: Kachur
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Eric
authors: E
authors: Blankemeyer
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Anatoliy V
authors: AV
authors: Popov
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Alejandro D
authors: AD
authors: Arroyo
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Joel S
authors: JS
authors: Karp
authors: Small Animal Imaging Facility, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania delikatn@mail.med.upenn.edu.
authors: Edward J
authors: EJ
authors: Delikatny
mesh_terms: D000086:Acetazolamide / Q000737:chemistry; D000818:Animals; D003287:Contrast Media / Q000737:chemistry*; D004195:Disease Models, Animal; D005462:Fluorine Radioisotopes / Q000737:chemistry*; D019788:Fluorodeoxyglucose F18; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008956:Models, Chemical; D064847:Multimodal Imaging; D055095:Optics and Photonics; D020113:Phenolphthalein / Q000737:chemistry; D010637:Phenolsulfonphthalein / Q000031:analogs & derivatives* / Q000737:chemistry; D017785:Photons; D049268:Positron-Emission Tomography; D011868:Radioisotopes / Q000737:chemistry*; D019275:Radiopharmaceuticals
chemical_list: D003287:Contrast Media; D005462:Fluorine Radioisotopes; D011868:Radioisotopes; D019275:Radiopharmaceuticals; C000597687:monofluorocresol purple; D019788:Fluorodeoxyglucose F18; D020113:Phenolphthalein; D010637:Phenolsulfonphthalein; D000086:Acetazolamide
keywords: Cerenkov imaging; electrophilic fluorination; metacresol purple; pH measurement; phenolphthalein; ratiometric imaging
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