Fluorescent Macromolecular Sensors of Enzymatic Activity for In Vivo Imaging - Progress in Molecular Biology and Translational Science - page 352

shown its utility as an i.v.-administered, nonspecific fluorescent contrast
agent for applications in intraoperative angiography and tumor/sentinel
lymph node mapping and staging (reviewed in Refs. 36,37 ). In addition,
5-aminolevulinic acid (which is not itself fluorescent but leads to the
accumulation of fluorescent porphyrins in malignant tissue) has also been
used to demarcate tumor margins in neurosurgical procedures and has been
judged to contribute to positive clinical outcomes. 38,39 Improvements in
clinical fluorescence detection systems 40,41 and the use of more advanced
imaging methods such as fluorescence lifetime imaging (FLI) 42-45

should

further strengthen the standing of fluorescence imaging in the clinical

diagnosis and treatment of disease.
Fluorescence is a property of organic and inorganic materials in which
electromagnetic energy (usually in the form of visible or ultraviolet light)
is absorbed by the material and results in the emission of light (photons).
In general, the excitation light is of shorter wavelength (higher energy) than
the emission light. Certain tissues and molecules of the body naturally ex-
hibit fluorescence, and a growing number of clinical applications harness this
property to aid in interventions such as endoscopy and surgery. The most
routine use of fluorescence for clinical detection and diagnosis is in the ap-
plication of autofluorescence imaging (AFI) as a component of endoscopic
exams. AFI detects the natural tissue fluorescence that is emitted by biomol-
ecules such as collagen, flavins, and porphyrins. The emission wavelength is
altered by changes in the metabolic state of the tissue, which can be used as
an indicator of malignancy. Also under current development are methods
that rely on the administration of fluorescent dyes and fluorescently tagged
macromolecules which then are distributed throughout the body. The suc-
cess of the latter approach depends on the ability of the fluorescent probe to
relay accurate and precise information about physiological processes and dis-
ease. This, in turn, is largely dependent on the ability of the probe to target
the tissue or disease process of interest and to then render a signal that has
sufficient intensity to be detected. Although these characteristics vary for
the individual probes developed to date, generalizations can be made in
terms of what characteristics are necessary for fluorescent probes to be useful
in a clinical setting. As described in Ref. 46 , an ideal fluorescence sensor for
in vivo imaging of disease would be characterized by (1) a peak fluorescence
close to 700-900 nm; (2) high quantum yield; (3) narrow excitation/
emission spectrum; (4) high chemical and photostability; (5) nontoxicity;
(6) excellent biocompatibility, biodegradability, or excretability; (7) avail-
ability of monofunctional derivatives for conjugations; and (8) commercial