sensitizers presented in Fig. 8 typically use different experimen-
tal conditions, namely, a variety of drug doses and formulations,
broad-band and laser light sources, drug-to-light intervals and
light doses, and cell lines and animal-models. The rich diversity
of studies is dictated by the specific properties of each sensitizer
and by the requirements of the clinical cases. This diversity is
welcome because each type of cancer (or clinical case) will cer-
tainly require a specific therapy and the multiparametric
approach offered by PDT may have the flexibility to offer rele-
vant clinical options to various types of cancers. However, the
scatter of the experimental details makes the comparison
between the performances of the sensitizers very problematic.
The aim of this part of our work is only to present some selected
examples of PDT with new bacteriochlorin sensitizers and is not
intended to be representative of all the therapeutic opportunities
they may offer. Thus, the examples presented below are
organized in terms of comparable experimental conditions to
illustrate how the design of the photochemical properties of
sensitizers is reflected in their performance.
Table V presents the limits for cytotoxicity in the dark of por-
phyrin, chlorin, and bacteriochlorin sensitizers and their photo-
toxicity under irradiation, in the same lab, by a filtered 500-W
halogen lamp with a nearly uniform spectral density from 610
to 800 nm (54,96,100). The only exception in that table is Tookad,
which was irradiated with a 250-W halogen lamp, with similar
filters but in another lab ( 106 ). Direct comparison between struc-
turally related porphyrins and bacteriochlorins shows that they
have similar cytotoxicities in the dark but that the bacteri-
ochlorins are much more phototoxic. In fact, their phototoxicity
exceeds what could be expected from the amount of light
absorbed, given bye[PS], and the singlet oxygen quantum yield.
Note that H 2 TClPPOH has e[PS]FD¼7.5 103 cm^1 , whereas
H 2 TClPBOH hase[PS]FD¼ 5 104 cm^1. Thus, considering only
the amount of singlet oxygen generated by these sensitizers, for
the same incident light dose, the bacteriochlorin should be 6.7
times more potent than the porphyrin. In reality, taking into con-
sideration the drug concentrations and light doses required to
kill 90% of melanoma cells, H 2 TClPBOH is ca. 65 times more
potent than H 2 TClPPOH. This difference cannot be assigned to
different intracellular localizations because the two sensitizers
have similar structures andn-octanol:water partition coefficients
(POW). In view of the reactions of bacteriochlorins with molecular
oxygen, it is likely that their increased phototoxicity with respect
to porphyrins is related to the generation of the hydroxyl
radical in water. A word of caution must be said about the limited
222 LUIS G. ARNAUT