also significant amounts of superoxide ion and, in aqueous sol-
utions, hydroxyl radicals. These ROS are very useful in the PDT
of cancer and bacteriochlorins rank among the most phototoxic
sensitizers presently available. The structure of these macrocycles
also allows for the tuning of their intracellular localization and
improvement of their biocompatibility and pharmacokinetics.
The rich photochemistry of porphyrin derivatives can serve
many other useful applications. A much less explored avenue in
photomedicine is the use of porphyrin derivatives for tumor imag-
ing( 115 ). Their intrinsic affinity for tumors and their remarkable
photochemical properties suggest various roles in the imaging of
neoplastic diseases. Recent work described fluorescence imaging
with porphyrin derivatives (116,117), but their low fluorescence
quantum yields and the diffusional nature of light propagation
in tissues strongly limit deep tissue imaging with these
sensitizers. Alternatively, it was proposed to use porphyrin
derivatives as contrast agents in photoacoustic tomography
(PAT) ( 118 ). The design of better contrast agents for PAT should
incorporate paramagnetic metal ions that rapidly quench the
excited states and rapidly convert all the absorbed energy into
heat and, consequently, generate intense acoustic waves.
In view of the ability to predict the effect of substituents in the
macrocycle and of the flexibility of the synthetic methods avail-
able, it is likely that much improved sensitizers and probes will
be made available for photomedicine in the coming years.
ACKNOWLEDGMENTSThis work was supported by ERA Chemistry (FCT—Fundação
para a Ciência e a Tecnologia—project no. 0002/2008) and
FEDER (COMPETE program).
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DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 229