Phthalocyanines are tetrapyrrolic macrocycles that, unlike
the porphyrins, have nitrogen atoms linking the individual
pyrrole units instead of methine bridges. The
periphery of the macrocycle is extended by benzene rings,
which strengthens the absorption at longer wavelengths
compared to porphyrins such as Photofrin.
have long been used as dyes and colouring agents
and have recently also been used as photoconducting
agents in photocopying machines. They have been extensively
studied as PDT agents because of their favourable
photophysical properties and because their properties
(such as solubility) can be altered through the addition of
substituents to the periphery of the macrocycle.
Ciba–Geigy Ltd (Basel, Switzerland), in partnership with
QLT PhotoTherapeutics, has developed a liposomal preparation
of zinc phthalocyanine (CGP55847) that has been in
Type I/II clinical trials in Switzerland for patients with
squamous cell carcinomas of the upper aerodigestive tract.
Attempts have also been made to develop a topical formulation
of this photosensitizer for use in treating psoriasis.
The oncological centre of the Russian Academy of Medical
Sciences (Moscow, Russia) and the surgical clinic of
the Moscow Medical Academy (Moscow, Russia) are currently
carrying out trials using a mixture of sulphonated
aluminium phthalocyanine derivatives (Photosense)
against malignancies such as skin, breast, lung and gastrointestinal
cancer. The addition of the sulphonate
groups to the periphery of the phthalocyanine greatly
increases the solubility of these compounds, removing the
need for liposomal delivery vehicles, and success with
Photosense has been relatively promising.
A silicon-based phthalocyanine (Pc4) is also being
studied for the sterilization of blood components by V.I.
Technologies (Vitex, Melville, NY, USA), who are based at
the New York Blood Center. Preclinical results with this
drug have been very promising and it is hoped that it will
enter clinical trials by the end of 1999.
Addition of a second benzene ring to the periphery of
the phthalocyanine produces naphthalocyanines.
These compounds absorb at a higher wavelength than
phthalocyanines (770 nm versus 680 nm), thus increasing
the therapeutic depth that can be achieved and rendering
them potential photosensitizers for highly pigmented
tumours such as melanomas. Significant work has been
carried out evaluating these compounds as photosensitizers
for PDT  and they are being pushed
towards clinical trials in Bulgaria by the Bulgarian Academy
of Sciences (Sofia, Bulgaria).