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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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texaphyrin complexes 379 are stable and are formed with 1:1 metal-to-
ligand stoichiometry. Such complexes are detectable in vivo by MRI. Thus,
monitoring the selective accumulation of gadolinium-texaphyrin in tumor
tissues by MRI enables the possibility of treatment planning and
subsequent monitoring of cancer response to radiation therapy. Selective
localization of such compounds in tumors was confirmed through the MRI-
43/Porphyrins as Photosensitizers in Photodynamic Therapy
205
Scheme 67. Some chlorin dimers prepared for PDT.
Me Me Me Me
322
324
contrast enhancement afforded by the paramagnetic nature of the
agent.301302
Besides their use as MRI agents, gadolinium(III) texaphyrin complexes
can also be used as radiation sensitizers because such complexes can
easily be reduced. The rationale for radio-sensitization is that like
molecular oxygen, the easy-to-reduce gadolinium-texaphyrin would be able
to capture hydrated electrons and thus increase the concentration of
hydroxyl radicals available after exposure to a given dose of ionizing
radiation. As judged from experiments involving tumor cells in vitro and
SMT-F and EMT6 tumors in mice, the gadolinium(III) texaphyrin complex 379
was also found to be an efficient radiation sensitizer.303
B. VINYLOGOUS PORPHYRINS
Berger and LeGoff have shown that insertion of an odd number of carbon
units alternatively between the pyrrolic rings of porphyrin could give
rise to a series of porphyrin vinyl analogues, the so-called platyrins
380.304 In this class of compounds, the additional carbon units were
introduced between the pyrroles as extended bridging units. Franck and
coworkers reported the biomimetic syntheses of a family of vinylogous
porphyrins 381-383 (Scheme 75)si,305 as possible PDT agents. These
investigators also reported a first synthesis of [22 7i]coproporphyrin II
system 381.306 This compound was synthesized with the belief that the
caboxylic acid functionalities in the lower half of the hematoporphyrin
are important due to their role in solubility and stability among
Photofrin components. Therefore, incorporation of several carboxylic acid
groups into an expanded porphyrin, coupled with the more intense long-
wavelength absorption of the resultant expanded porphyrin, could generate
an improved photosensitizer. Compared to Photofrin and
isohematoporphyrin, this compound achieved slightly improved
production without any significant increase in vitro photosensitizing
activity.
Franck and Nonn further explored their approach to synthesize the
first [26 ] tetravinylogous porphyrin 382, as well as the related 34
system 383.81 With photophysical properties and high thermal and
photochemical stability, these porphyrins have attracted some attention
as potential candidates for PDT. The [26 n\ porphyrin 382 has a strong
absorption at 783 nm (s = 28,000) and does generate singlet oxygen ( =
0.14). Further enlargement of conjugation of
26 n to 34 n porphyrin 383 leads to a further bathochrome shift (A,max =
997 nm, ?==24,000). However, the singlet-oxygen yields of these compounds
were low because the triplet-state energy of the 34 n system was
significantly smaller than the excitation energy.
X. Miscellaneous Porphyrins and Chlorins
A. GLYCOSYLATED PORPHYRINS AND CHLORINS
For an ideal photosensitizer, solubility in water is a major requirement,
in addition to long-wavelength absorption. Therefore, in recent years, a
number of carbohydrate derivatives of various photosensitizers have been
synthesized. It was shown recently that some carbohydrate-substituted
porphyrins exhibit enhanced selectivity to cancer cells. Although the
mechanism of sensitizer uptake is not clear, there is evidence that
hydrophobic and amphiphilic porphyrins associate strongly to low-density
lipoprotein (LDL) and are introduced into the tumor cells by receptor
mediated endocytosis. Amphiphilic porphyrins may be incorporated into the
plasma membranes of the tumor cells. Such incorporation of a
photosensitizer into a plasma membrane leads to a high quantum yield of
cell deactivation.307
Hombrecher et al. reported the preparation of a galactopyranosyl-
cholesteryloxy substituted porphyrin 385308 (Scheme 76). The vesicle-
forming properties of this
206
Pandey and Zheng
Scheme 68. Preparation of BPD dimers via trifflic acid method.
Me .^>^~~ Me Me
Me
V-C02Me H
N-A/le 'C02Me
CO2M6 CO2M6
325
C02Me C02Me C02Me C02Me
327
photosensitizer were investigated by light-scatterring experiments and by
electron microscopy. Several tetra-and octa-glycoconjugated
tetraphenylporphyrins 386-390 were also reported by Yano and
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