Rational Design of Fluorophores for In Vivo Applications - Progress in Molecular Biology and Translational Science - page 88

suggestions that, for hydroporphyrins, the fluorescencequantumyield increases
at the expense of triplet state formation. 185
Vinogradov and coworkers studied the phototoxicity of phosphores-
cent dendritic benzoporphyrin used as in vivo oxygen probes. 208 In this
case, there is a particular concern about phototoxicity of the probe, as
the side product of phosphorescent oxygen sensing is a singlet oxygen.
They found negligible phototoxicity of their probes, which they attributed
to the inability of dendritic benzoporphyrins to penetrate cellular mem-
branes and produce singlet oxygen inside the cellular organelles. Their
studies pointed out the importance of probe localization on their toxicity
and demonstrated that even a potentially highly photocytotoxic probe can
be safely used in vivo .
The elegant and general solution to overcome the problem of singlet ox-
ygen generation by tetrapyrrolic fluorophores has been proposed by Moore
and coworkers. 209 They prepared the covalently linked carotene-porphyrin
dyads and found that carotenoids effectively quenched the triplet state of
porphyrins (and other tetrapyrrolic compounds) by energy transfer, thereby
making them incapable of producing singlet oxygen (the same principle has
been also used for the design of a quenched, protease-activatable,
chlorophyll-based photosensitizer for anticancer therapy 210 ). This general
approach seems to be a viable strategy when phototoxicity is a problem.
5.8. Phthalocyanines
Phthalocyanines are benzoporphyrin congeners having nitrogen rather than
carbon bridging atoms. Phthalocyanines exhibit strong and sharp absorption
in red spectral window and, in contrast to porphyrins, show high fluorescence
quantum yields. Expansion of the aromatic system by fusing additional benzo
ringgives naphthocyanineswith absorption andemissionabove 700 nm. 211-219
Phthalocyanines exhibit also excellent chemo- and photostability compared to
the other near-IR fluorophores. Phthalocyanines have been used as
fluorophores in polymerase chain reaction (PCR) 211 and molecular
beacons, 212 and broadly examined as photosensitizers in photodynamic
therapy, 167 whereas their use as a fluorophore for in vivo imaging has been
rather neglected.
One of the reasons for this neglect is the fact that phthalocyanines, because of
their planar, hydrophobic structure, are difficult to solubilize in water and tend
to aggregate in aqueous solution. Their solubility in organic solvents is usually
low as well, which makes their purification and handling rather difficult. The
few available methods for synthesis and derivatization of phthalocyanines do
not offer much flexibility in preparing functional derivatives, especially