Rational Design of Fluorophores for In Vivo Applications - Progress in Molecular Biology and Translational Science - page 69

properties of cyanines for imaging applications, but this strategy has not yet
been tested for other cyanines or in vivo . Similarly, substitution of the mer-
ocyanine dye (i.e., cyanine analog having an oxygen atom at one of the ter-
minal heterocycles instead of nitrogen) with an electron-withdrawing cyano
group increases its chemo- and photostability. 85
Contrary to the results described above, where chemo- and photo-
stability have been improved by substitution of cyanines with electron-
withdrawing groups, it has also been reported that substitution of
cycloheptamethine with electron-donating groups at the nitrogen slightly
inhibits the photobleaching of the resulting fluorophores. 86
2.3.2 Encapsulation
Encapsulation of cyanines inside larger organic molecules, such as dendri-
mers or cyclodextrins, or inside nanoparticles would protect the fluorophore
from interacting with external agents that may quench fluorescence or
decompose the molecule, such as solvents, oxygen, degrading enzymes,
proteins, etc. Moreover, encapsulation would prevent cyanines from
aggregation and improve their water solubility and, in many cases, pharma-
codynamic properties. As the simplest way of encapsulation is PEGylation,
attachment of poly(ethyleneglycol) chains can be considered. Brechbiel
demonstrated that conjugation of the amine-reactive ICG analog with
PEG of average molecular weight of 3400 g/mol improves the water
solubility and prevents aggregation of cyanine, both in the free form as well
as when the resulting PEGylated fluorophore is attached to the antibody
panitumumab. 55 PEGylation of cyanine does not affect the targeting ability
of the cyanine-antibody conjugate; hence PEGylation seems to be a viable
strategy to improve the performance of cyanine fluorophores for in vivo
applications.
Properties of cyanines can be also improved by the attachment of large
water-solubilizing dendrons. Fr´chet and Achilefu have examined the cypate
encapsulated within a covalently attached polyester dendrimer with poly(eth-
ylene oxide) branches on its periphery. 44 Such nanoencapsulated fluorophore
exhibits higher water solubility, diminished aggregation, and higher quantum
yield in aqueous solution than cypate itself. Moreover, it shows improved
pharmacokinetics (longer plasma circulation, low accumulation in normal tis-
sue) and better stability toward cytochrome P450-catalyzed oxidation com-
pared to the clinically approved ICG. Weck studied the properties of
cycloheptamethine cyanines substituted at the central carbon atoms with both
short-chain substituents and large dendrons. 74 The results indicate that the