Activatable Probes For The Imaging Of Biological Parameters Using Cerenkov Radiation
Cerenkov radiation is a multispectral photon emission caused by beta particles from decaying radionuclides breaking the speed of light in a dielectric medium. These photons can be absorbed by chromophores or fluorophores, which can be designed to activate and report on the status of a particular tissue in vivo. Breast cancer is one of the major causes of death and the top diagnosed cancer in women. Development of probes to report on tumor health and, possibly, drug response would be of great benefit to the diagnosis and prognosis of the patient. The molecules developed and presented in this thesis focus on the identification of two major characteristics of cancer: the acidic pH of the tumor microenviroment and the deregulated energy production of tumor cells. The acidity of the tumor microenvironment has been associated with a more aggressive phenotype, increased metastatic potential and, to an extent, drug resistance, especially to weak-base drugs. In this part of the thesis, two pH indicator molecules (naphthofluorescein and bromothymol blue) were electrophilically fluorinated to develop pH sensitive analogs that modulate the Cerenkov radiation arising from the 18F-label. The deregulation of energy production has been determined to be one of the emerging Hallmarks of Cancer. In this section, a redox and viability dye (resazurin, also known as Alamar Blue) was fluorinated to develop a probe capable of identifying the metabolic activity of the tumor cells by monitoring the reduction of resazurin to a highly fluorescent molecule (resorufin). A primary focus of this thesis is the development of the Cerenkov imaging field. Two techniques are employed: Selective Bandwidth Quenching (SBQ) and Cerenkov Radiation Energy Transfer (CRET). SBQ uses optical filters to detect the attenuation of Cerenkov radiation within a specific wavelength range as a result of the interaction of these photons with a chromophore, like bromothymol blue. This attenuation is correlated to the concentration of the chromophore; which in the case of pH indicators, is dependent on the pH of the solution. CRET uses optical filters to detect fluorescence arising from the interaction of Cerenkov radiation with a fluorophore, like naphthofluorescein and reduced resazurin. CRET can also be correlated to the concentration of the probe. With the development of these probes, this thesis expands the knowledge on Cerenkov radiation and the Cerenkov imaging field and its viability as a routine imaging technique both on the bench and in the clinic. It also validates Cerenkov imaging as a functional bridge between optical and nuclear imaging.
Ian A. Blair