# Photodynamic Therapy and the Development of  Metal-Based Photosensitisers

Authors: Leanne B. Josefsen; Ross W. Boyle
Journal: Metal-Based Drugs
 (2008)

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## Abstract

Photodynamic therapy (PDT) is a treatment modality that has been used in the successful treatment of a number of diseases and disorders, including age-related macular degeneration (AMD), psoriasis, and certain cancers. PDT uses a combination of a selectively localised light-sensitive drug (known as a photosensitiser) and light of an appropriate wavelength. The light-activated form of the drug reacts with molecular oxygen to produce reactive oxygen species (ROS) and radicals; in a biological environment these toxic species can interact with cellular constituents causing biochemical disruption to the cell. If the homeostasis of the cell is altered significantly then the cell enters the process of cell death. The first photosensitiser to gain regulatory approval for clinical PDT was Photofrin. Unfortunately, Photofrin has a number of associated disadvantages, particularly pro-longed patient photosensitivity. To try and overcome these disadvantages second and third generation photosensitisers have been developed and investigated. This Review highlights the key photosensitisers investigated, with particular attention paid to the metallated and non-metallated cyclic tetrapyrrolic derivatives that have been studiedin vitro and in vivo; those which have entered clinical trials; and those that are currently in use in the clinic for PDT.


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## Body

## 1. Photodynamic Therapy: Background

The use of light in the   treatment of disease has been known for  many centuries and can be traced
back over 4000 years to the ancient Egyptians [1]. The Egyptian people used a
combination of the orally ingested Amni Majus plant and sunlight to
successfully manage vitiligo: a skin disorder of unknown cause. The
active ingredient of this plant (psoralen, Figure 1) is now successfully employed
in the worldwide treatment of psoriasis [1–4].Figure 1
Structure of psoralen.Photodynamic therapy (PDT, see Glossary) is a treatment involving light and
a chemical substance (a photosensitiser), used in conjunction with molecular
oxygen to elicit cell death. More explicitly, photodynamic
therapy is a selective treatment modality for the local destruction of diseased
cells and tissue. The selectivity is based on the ability of the
photosensitiser to preferentially accumulate in the diseased tissue and
efficiently generate singlet oxygen or other highly reactive species such as
radicals, which induce target cell death.The principle of photodynamic therapy is based on a multi-stage process (Figure2). The first of
these stages (Figure  2(a))  sees the administration of a photosensitiser with negligible
dark toxicity, either systemically or topically, in the absence of light. When the optimum ratio of photosensitiser in diseased versus healthy tissue is achieved, the photosensitiser is
(Figure  2(c))  activated by exposure to a carefully regulated dose of light which is shone directly onto the diseased tissue for a specified
length of time. The light dose is regulated in order to allow a sufficient amount
of energy to be delivered to activate the photosensitiser, but at the same time
the dose should be small enough to minimise damage inflicted on neighbouring
healthy tissue. It is the activated form
of the photosensitiser which evokes a toxic response in the tissue, resulting
in cell death. The success of photodynamic therapy lies in the prolonged
accumulation of photosensitiser in diseased tissue, relative to the more rapid
clearance from normal tissue cells.Photosensitiser administration.
(a)
(b)
(c)
(d)Photodynamic therapy is commonly practiced in the treatment of a number of cancers, including those present in the head and neck, the lungs, bladder, and
particular skin cancers [5–16].
It has also been successfully used in the
treatment of non-cancerous conditions such as age-related macular degeneration
(AMD),  psoriasis, atherosclerosis, and has shown some efficacy in anti-viral treatments
including herpes [2, 3, 5–7, 9, 17, 18, 19, 20].Photodynamic therapy carries advantages for both the patient and the physician:
the need for delicate surgery and lengthy recuperation periods is minimised,
along with minimal formation of scar tissue and disfigurement. However, photodynamic
therapy is not without its drawbacks: a major limitation is the associated general
photosensitisation of skin tissue.


## 2. History of Photodynamic Therapy

Reports of contemporary photodynamic therapy came first in the investigations led by
Finsen in the late nineteenth century [8].
Finsen successfully demonstrated phototherapy by employing heat-filtered light from a carbon-arc lamp (the “Finsen lamp”) in the treatment of a tubercular condition of the skin known as
lupus vulgaris, for which he won the Nobel Prize in Physiology or
Medicine in 1903 [8]. But it was not until the early twentieth
century that reports of photodynamic therapy for the treatment of cancer patients (with
solid tumours) were made by von Tappeiner’s group in Munich
[2, 4, 6, 9].
In 1913  another German scientist, Meyer-Betz, described the major stumbling block of
photodynamic therapy. After injecting himself with haematoporphyrin (Hp, a photosensitiser),
he swiftly experienced a general skin sensitivity upon exposure to sunlight—a
problem still persistent with many of todays’ photosensitisers [2, 3, 7, 18].Further studies, investigating the accumulation of haematoporphyrin and the purified
haematoporphyrin derivative (HpD) in tumours, culminated in the late 1980s with
the photosensitiser  Photofrin   (Figure3). A photosensitiser which,
after further purification, was first given approval in 1993 by the Canadian
health agency for use against bladder cancer and later in Japan, USA  and parts
of Europe for use against certain cancers of the oesophagus and non-small cell
lung cancer [3–9, 17, 18, 21, 22].Figure 3
Structure of     Photofrin,n
=
1–9.Photofrin  was
far from ideal and carried with it the disadvantages of prolonged patient
photosensitivity and a weak long-wavelength absorption (630 nm) [6, 7, 21]. This
led to the development of improved (second generation) photosensitisers,
including  Verteporfin (a benzoporphyrin derivative, also known as  Visudyne) and
more recently, third-generation photosensitisers based around targeting
strategies, such as antibody-directed photosensitisers [4, 5, 7, 18, 19, 23–25].


## 3. Cyclic Tetrapyrrolic Chromophores and Photosensitisers

Cyclic tetrapyrrolic molecules are good examples of fluorophores (see Glossary) and photosensitisers.
Photosensitisers are molecules, which, when excited by light energy, can utilise the energy
to induce photochemical reactions to produce lethal toxic agents. In a cellular environment,
these agents (reactive oxygen species (ROS) and radicals) ultimately result in cell
death and tissue destruction (Figure4) [5–9].
Photosensitisers are absorbed into cells all over the body and alone are harmless, that is, in the absence of light,  and usually oxygen they have no  effect on healthy  or abnormal tissue. Ideally, they should be retained by diseased tissue,
particularly tumours, for longer periods of time in comparison to healthy tissue; thus it is
important to carefully time light exposure and ensure that activation only occurs when
the ratio of photosensitiser is greater in diseased tissue than in healthy tissue, thereby
minimising unwanted damage to surrounding non-cancerous cells
[3, 19].Figure 4
Photosensitiser initiated cell death.Photosensitisers also have alternative applications. They have been employed in the
sterilisation of blood plasma and water in order to remove blood-borne viruses and microbes
and have been considered for agricultural uses, including herbicides and insecticides
[5, 9, 26–28].


## 4. Photochemistry: Photochemical Processes

Only when a photosensitiser is in its excited state (3Psen*)
can it interact with molecular oxygen (3O2) and produce radicals
and activated oxygen species (ROS), crucial to the Type II mechanism which is
thought to predominate in PDT (see below). These species include singlet oxygen
(1O2), hydroxyl radicals
(OH
•), and superoxide (O
2
−) ions and can interact with
cellular components including unsaturated lipids;  amino acid residues;  and
nucleic acids. If sufficient oxidative damage ensues, this will result in
target-cell death (only within the immediate area of light  illumination).


## 5. Photochemical Mechanisms

When a chromophore, such as a cyclic tetrapyrrolic
molecule, absorbs a photon of electromagnetic radiation in the form of light
energy, an electron is promoted into a higher-energy molecular orbital, elevating
the chromophore from the ground state (S0) into a short-lived,
electronically excited state (Sn) composed of a number of
vibrational sub-levels (S
n
′). The excited chromophore can lose
energy by rapidly decaying through these sub-levels via  internal conversion (IC) to populate the first excited
singlet state (S1),  before quickly relaxing back to the ground state (Figure 5).Figure 5
Modified Jablonski energy diagram.The decay from the excited singlet state (S1)
to the ground state (S0) is via
fluorescence   (S
1
→
S
0).
Singlet state lifetimes of excited fluorophores are very short (τ
fl. = 10−9–10−6 seconds) since transitions
between the same spin states (S
→
S or T
→
T)
conserve the spin multiplicity of the electron and, according to the Spin Selection
Rules, are therefore considered “allowed” transitions [3, 6, 8]. Alternatively, an excited singlet state
electron (S1) can undergo spin inversion and populate the
lower-energy first excited triplet state (T1) via  intersystem crossing (ISC); a
spin-forbidden process, since the spin of the electron is no longer conserved
[29–34]. The excited electron can then undergo a second spin-forbidden
inversion and depopulate the excited triplet state (T1) by decaying
back to the ground state (S0) via  phosphorescence (T
1
→
S
0) [29–34]. Owing
to the spin-forbidden triplet to singlet transition, the lifetime of
phosphorescence (τ
P
=
10
−
3
−
1 second) is considerably longer than that of
fluorescence.


## 6. Photosensitisers and Photochemistry

Tetrapyrrolic  photosensitisers in the excited singlet state (1Psen*, S
>
0)
are relatively efficient at undergoing intersystem crossing and can
consequently have a high triplet-state quantum yield, Box 2 (Φ
T 0.62 (tetraphenylporphyrin (TPP), methanol)), 0.83 (etiopurpurin, benzene), 0.71
(tetrasulphonated TPP, D2O), and 0.47 (tetrasulphonated zinc
phthalocyanine, methanol))  [8, 35, 36]. The longer lifetime of this species is
sufficient to allow the excited triplet state photosensitiser to interact with
surrounding bio-molecules, including cell membrane constituents [5, 17].Box 1Box 2


## 7. Photochemical Reactions

Excited triplet-state
photosensitisers can react in two ways defined as Type-I and Type-II processes.
Type-I processes can involve the excited singlet or triplet photosensitiser (1Psen*,
S1; 3Psen*, T1), however due to the
short lifetime of the excited singlet state, the photosensitiser can only react
if it is intimately associated with a substrate, in both cases the interaction
is with readily oxidisable or reducable substrates. Type-II processes involve
the direct interaction of the excited triplet photosensitiser (3Psen*,
T1) with molecular oxygen (3O2, Σ
g
3)
[5–8, 17, 18, 37].Type-I  processes can be divided into two further mechanisms;
Type I(i) and Type I(ii). The first of these mechanisms (i) involves
the transfer of an electron (oxidation) from a substrate molecule to the
excited state photosensitiser (Psen*), generating a photosensitiser
radical anion (Psen
•
−) and a substrate radical cation
(Subs
•
+). The majority of the radicals produced
from Type-I(i) reactions react instantaneously with oxygen, generating a
complex mixture of oxygen intermediates. For example, the photosensitiser
radical anion can react instantaneously with molecular oxygen (3O2)
to generate a superoxide radical anion (O
2
•
−), which can go on to produce the
highly reactive hydroxyl radical (OH
•, Figure 6), initiating a cascade of cytotoxic free radicals; this process is common in
the oxidative damage of fatty acids and other lipids [17, 18]. Some of the more
common Type-I(i) reactions are shown in Figure 6.Figure 6
Type-I process (i).The second Type-I process (ii) involves the transfer
of a hydrogen atom (reduction) to the excited state photosensitiser (Psen*).
This generates free radicals capable of rapidly reacting with molecular oxygen
and creating a complex mixture of reactive oxygen intermediates, including
reactive peroxides (Figure 7). Once again, this can trigger a torrent of cytotoxic
events, culminating in cell damage and death.Figure 7
Type-I process (ii).On the other hand, Type-II processes involve the
direct interaction of the excited triplet state photosensitiser (3Psen*)
with ground state molecular oxygen (3O2, Σ
3
g,
Figure 8); a spin allowed transition—the excited state
photosensitiser and ground state molecular oxygen are of the same spin state
(T, Figure 5).Figure 8
Type-II process.When the excited photosensitiser collides with a
molecule of molecular oxygen, a process of triplet-triplet annihilation takes
place (3Psen*
→
1Psen and 3O2
→
1O2).
This inverts the spin of one of molecular oxygens (3O2) outermost
antibonding electrons, generating two forms of singlet oxygen (Δ
g
1 and Σ
g
1, Figure 9), while
simultaneously depopulating the photosensitiser’s excited triplet state (T
1
→
S
0, Figure 5). The higher-energy
singlet oxygen state (Σ
g
1, 157
kJ mol
−
1
>
Σ
g
3) is very short-lived (Σ
g
1
≤
0.33 milliseconds  (methanol),
undetectable in H2O/D2O) and rapidly relaxes to the lower-energy
excited state (Δ
g
1, 94
kJ mol
−
1
>
Σ
g
3) [36]. It is, therefore, this lower-energy form of
singlet oxygen (Δ
g
1) which is implicated in
cell injury and cell death [38].Figure 9
Molecular orbital diagram of oxygen (Σ
3
g, Δ
g
1,
and Σ
g
1).The highly-reactive oxygen species (1O2)
produced via  the Type-II
process act near to their site of generation and within a radius of action of
approximately 20 nm, with a typical lifetime of approximately 40 nanoseconds in
biological systems [2, 7, 17]. However, it has recently been suggested that (over
a 6 microsecond period) singlet oxygen can diffuse up to approximately 300 nm  in vivo [39–41]. Singlet
oxygen can theoretically only interact with proximal molecules and structures
within this radius [17]. ROS are known to initiate a large number of
reactions with biomolecules, including amino acid residues in proteins, such as
tryptophan; unsaturated lipids like cholesterol and nucleic acid bases,
particularly guanosine and guanine derivatives, Box 3, with the latter base more
susceptible to ROS [2, 5, 8, 17, 36, 42–45]. These
interactions cause damage and potential destruction to cellular membranes and
enzyme deactivation, culminating in cell death [8].Box 3Box 4It is highly probable that in the presence of
molecular oxygen, and as a direct result of the photoirradiation of the
photosensitiser molecule, both Type-I and II pathways play a pivotal role in
disrupting cellular mechanisms and cellular structure. Nevertheless, there is
considerable evidence to suggest that
the Type-II photo-oxygenation process predominates in the induction of cell
damage, a consequence of the interaction between the irradiated photosensitiser
and molecular oxygen [2, 5, 8, 18, 43, 46]. It has been suggested,
however, that cells  in vivo are
partially protected against the effects of photodynamic therapy by the presence
of singlet oxygen  scavengers (such as histidine) and that certain skin cells
are somewhat resistant to photodynamic therapy in the absence of molecular
oxygen; further supporting the proposal that the Type-II process is at the
heart of photoinitiated cell death [17, 43, 47, 48].The efficiency of Type-II processes is dependent upon the triplet state lifetimeτ
T (see Glossary)
and the triplet quantum yield (Φ
T) of the photosensitiser.
Both of these parameters have been implicated in the effectiveness of a
photosensitiser in phototherapeutic medicine; further supporting the distinction
between Type-I and Type-II mechanisms. However, it is worthy to note that the
success of a photosensitiser is not exclusively dependent upon a Type-II
process taking place. There are a number of photosensitisers whose excited
triplet lifetimes are too short to permit a Type-II process to occur. For
example, the copper metallated octaethylbenzochlorin photosensitiser (Figure 10)
has a triplet state lifetime of less than 20 nanoseconds and is still deemed to
be an efficient photodynamic agent [13, 43].Figure 10
A copper metallated photosensitiser.


## 8. Photosensitisers—Ideal Photosensitisers

Although a  number of different photosensitising compounds,
such as methylene blue (see Glossary), rose bengal,
and acridine (Figure11), are known to be efficient singlet oxygen generators (and
therefore potential photodynamic therapy agents), a large number of
photosensitisers are cyclic tetrapyrroles or structural derivatives of this
chromophore; in particular porphyrin, chlorin, bacteriochlorin, expanded porphyrin, and
phthalocyanine (PCs)
derivatives (Figure 12). This is possibly because cyclic tetrapyrrolic
derivatives have an inherent similarity to the naturally occurring porphyrins
present in living matter—consequently they have little or no toxicity
in  the absence of light [2, 5, 17, 18, 36, 44, 49].Examples of non-porphyrinic photosensitisers.Porphine, chlorine, bacteriochlorin, expanded porphyrin, and phthalocyanine
structures.Porphyrins are a group of naturally occurring and
intensely coloured compounds, whose name is drawn from the Greek wordporphura, the Greek word
for purple [50–52]. These molecules are known to be involved in a number
of biologically important roles, including oxygen transport and photosynthesis,
and have applications in a number of fields, ranging from fluorescence imaging
to medicine [2, 5, 17, 42]. Porphyrins are classified as tetrapyrrolic
molecules, with the heart of the skeleton a heterocyclic macrocycle, known as a
porphine. The fundamental porphine frame consists of four pyrrolic sub-units
linked on opposing sides (α-positions, numbered 1, 4, 6, 9, 11, 14,
16, and 19, Figure 13) through four methine (CH) bridges (5, 10, 15, and 20),
known as the meso-carbon
atoms/positions (Figure 13). The resulting conjugated planar macrocycle may be
substituted at the meso- and/or β-positions
(2, 3, 7, 8, 12, 13, 17, and 18): if the meso- and β-hydrogens are substituted with non-hydrogen
atoms or groups, the resulting compounds are known as porphyrins.Figure 13
Porphine macrocycle.The inner two
protons of a free-base porphyrin can be removed by strong bases such as
alkoxides, forming a dianionic molecule; conversely, the inner two pyrrolenine nitrogens can
be protonated with acids such as trifluoroacetic acid affording a dicationic intermediate
(Figure14). The tetradentate anionic species can readily form
complexes with most metals.Porphyrin dianionic and dicationic species.


## 9. Porphyrin Absorption Spectroscopy

On account of their highly conjugated skeleton,
porphyrins have a characteristic ultra-violet visible (UV-VIS) spectrum (Figure15). The spectrum typically consists of an intense, narrow absorption band (ε
>
200
000 l mol−1cm−1) at around 400 nm, known as the
Soret or B band, followed by four longer wavelength (450–700 nm), weaker absorptions (ε
>
20
000 l mol−1cm−1 (free-base porphyrins)) referred to as the Q bands [6, 17, 50, 53, 54].Figure 15
Typical porphyrin absorption spectrum [55, (modified)].The Soret band arises from astrong  electronic transition from the (porphyrin) ground state to the second excited singlet state (S
0
→
S
2, Figure 16); whereas the Q band is a result of a weak  transition to the first excited singlet state (S
0
→
S
1).
The dissipation of energy via
internal conversion (IC) is so rapid that fluorescence is only observed from depopulation of the first excited singlet state to the lower-energy ground state (S
1
→
S
0).Figure 16
Modified Jablonski energy diagram.


## 10. Second-Generation Photosensitisers

## 10.1. Ideal Photosensitiser Properties

The key characteristic of any photodynamic sensitiser is its ability to preferentially accumulate in diseased tissue and,via the generation of cytotoxic species, induce a desired biological effect. In
particular, a good photodynamic sensitiser should adhere to the following criteria:(i)
have strong absorption with a high extinction
coefficient in the red/near infrared region of the electromagnetic spectrum (600–850 nm)—allows deeper tissue penetration [5–7, 17, 36, 43],
(ii)
be effective generators of singlet
oxygen and other ROS,
(iii)
have suitable photophysical characteristics: a high-quantum yield of triplet formation (Φ
T
≥
0.5); a high singlet oxygen quantum yield (Φ
Δ
≥
0.5); a relatively long triplet state lifetime (τ
T, microsecond range); and a high triplet-state energy (≥ 94 KJ mol−1) [3, 8, 18, 36, 56]. To date the parameters Φ
T
=
0.83 and Φ
Δ
=
0.65 (haematoporphyrin); Φ
T
=
0.83 and Φ
Δ
=
0.72 (etiopurpurin); and Φ
T
=
0.96 and Φ
Δ
=
0.82 (tin etiopurpurin) have been achieved
[2, 36],
(iv)
have minimum dark toxicity and negligible cytotoxicity in the absence of light,
(v)
exhibit greater retention in diseased/target tissue over healthy tissue,
(vi)
present rapid clearance from the body,
(vii)
be single, well-characterised
compounds, with a known and constant composition,
(viii)
have a short and high yielding synthetic route (with easy translation into multi-gram scales/reactions),
(ix)
have a simple and stable drug formulation,
(x)
be soluble in biological media,
allowing direct intravenous administration and transport to the intended target. Failing this, a hydrophilic delivery system should be sought enabling efficient and effective transportation of the photosensitiser to the target sitevia the bloodstream.While the major disadvantages associated with the first generation photosensitisers HpD and  Photofrin (skin
sensitivity and weak absorption at 630 nm) have not prevented the treatment of some cancers and other diseases, they have markedly reduced the successful application of these photosensitisers to a wider field of disease. The development of second generation photosensitisers, designed to minimise the drawbacks of the first generation photosensitisers, was key to the development
of photodynamic therapy. A number of new photosensitisers were therefore developed to overcome these short comings.5-Aminolaevulinic Acid
The 5-Aminolaevulinic acid (ALA) is a prodrug used in
the clinic to treat and image a number of superficial cancers and tumours (see Tables2 and 3) [5–9, 11, 17, 18]. ALA on its own is not a photosensitiser, but a key
precursor in the biosynthesis of the naturally occurring porphyrin, haem (Scheme
1).
Haem is synthesised in every energy-producing cell in the body
and is a key structural component of haemoglobin, myoglobin, and other haemproteins. The
immediate precursor to haem is protoporphyrin IX (PPIX), an effective
photosensitiser. Haem itself is not a photosensitiser, due to the coordination
of a paramagnetic ion (iron; see Glossary; see also diamagnetic species) in the centre of the macrocycle, causing
significant reduction in excited state lifetimes [5–9, 11].
The haem molecule is synthesised from glycine and
succinyl coenzyme A (succinyl CoA). The rate-limiting step in the biosynthesis
pathway is controlled by a tight (negative) feedback mechanism in which the
concentration of haem regulates the production of ALA. However, this controlled
feedback can be by-passed by artificially adding excess exogenous ALA to cells.
The cells respond by producing PPIX (photosensitiser) at a faster rate than the
ferrochelatase enzyme can convert it to haem [5–9, 11, 17, 18].
ALA, marketed  as Levulan (DUSA Pharmaceuticals
Incorporated, Toronto, Canada), has shown promise in photodynamic
therapy (tumours)via both
intravenous and oral administration, as well as through topical administration
in the treatment of malignant and non-malignant dermatological conditions,
including psoriasis, Bowen’s disease, and Hirsutism (Phase   II/III
clinical trials, see Glossary) [5–9, 11, 18].
ALA  shows a more rapid accumulation in comparison to other intravenously
administered sensitisers [5–9, 11]. Typical peak tumour accumulation levels   post-administration for
PPIX are usually achieved within several hours; compare this with other
(intravenously administered) photosensitisers which may take up to 96 hours to
reach peak levels and one of the main advantages of ALA can be clearly seen. ALA
is also excreted
more rapidly from the body (∼
24 hours) than other photosensitisers, minimising
patient photosensitivity [5–8, 11].
In an attempt to overcome the poor bioavailability
when ALA is applied topically, esterified ALA
derivatives with
improved pharmacological properties have been examined [5–8, 11]. A methyl ALA ester (Metvix) is now being marketed by Photocure ASA (Oslo, Norway)
as a potential photosensitiser for basal cell carcinoma and other skin lesions
[5, 6, 9, 17]. Benzyl (Benvix) and hexyl ester (Hexvix) derivatives are also
registered by Photocure ASA for the treatment of gastrointestinal cancers and
for the diagnosis of bladder cancer [9].Scheme 1
Simplified haem biosynthesis.Verteporfin
The second generation photosensitiser, benzoporphyrin
derivative monoacid ring A (BPD-MA, Figure17) has been developed by QLT Phototherapeutics
(Vancouver, Canada) under the trade name Visudyne (Verteporfin, for injection)
and, in collaboration with Ciba Vision Corporation (Duluth, GA, USA), has
undergone Phase   III clinical trials  (USA) for the photodynamic
treatment of wet age-related macular degeneration (AMD, see Glossary) and cutaneous non-melanoma
skin cancer [3, 5–7, 9, 57, 58, 59]. Verteporfin is currently marketed by Novartis
Pharmaceuticals Corporation (NJ,
USA).
The chromophore of BPD-MA has a red-shifted and
intensified long-wavelength absorption maxima at approximately 690 nm. Tissue
penetration by light at this wavelength is 50% greater than that achieved for
Photofrin (λ
max.
=
630 nm) [5, 60].
Verteporfin has further advantages over the first
generation sensitiser Photofrin. It is rapidly absorbed by the tumour (optimal
tumour-normal tissue ratio 30–150 minutes post-intravenous
injection) and is rapidly cleared from the body, minimising patient
photosensitivity (1-2 days) [5, 61].Figure 17
Verteporfin.Purlytin
Tin etiopurpurin, a chlorin photosensitiser (Figure18), is marketed
under the trade name Purlytin by Miravant Medical Technologies (Santa Barbara,
Calif, USA) [5–9, 62]. Purlytin has also undergone Phase   II
clinical trials  (USA) for cutaneous metastatic breast cancer and Kaposi’s
sarcoma in patients with AIDS (acquired immunodeficiency syndrome) [3, 7].
Purlytin has been used successfully to treat the non-malignant conditions
psoriasis and restenosis [5].
Chlorins (Figure12) are distinguished from the
parent porphyrins by a reduced exocyclic double bond. The result of the reduced
bond is a decrease in the symmetry of the conjugated macrocycle, leading to an
increased absorption in the long-wavelength portion of the visible region of
the electromagnetic spectrum (650–680 nm). More
correctly, Purlytin is a purpurin; a degradation product of chlorophyll [8, 9, 12].
Purlytin has a tin atom chelated in its central
cavity which causes a red-shift of approximately 20–30 nm (with respect to
Photofrin and non-metallated etiopurpurin,λ
max.
SnEt
2
=
650 nm) [6, 9, 12]. Purlytin has been reported to localise in skin and produce a photoreaction
7–14 days post-administration
[6, 9].Figure 18
Purlytin.Foscan
Tetra(m-hydroxyphenyl)chlorin
(mTHPC, Figure 19) has been developed
and entered into clinical trials (USA and Europe) under the trade name Foscan
by Scotia Pharmaceutics (Guildford, Surrey, UK) and BioLitec Pharma Limited (Dublin, Ireland) [3, 5–9, 11, 18]. Foscan, also known as Temoporfin,
has been evaluated as a phototherapeutic agent against head and neck cancers in
these trials [5]. It has also been investigated in clinical trials for
malignant and non-malignant diseases, including gastric and pancreatic cancers,
hyperplasia, field sterilisation after cancer surgery and for the control of
antibiotic-resistant bacteria, in the USA, Europe, and the Far East [5, 9, 11].
Foscan has a singlet oxygen quantum yield comparable
to other chlorin photosensitisers but the low drug and light doses (approximately
0.1 mg kg−1 and as low as 5 J cm−2, resp.) required to
achieve photodynamic responses (equivalent to Photofrin, 2–5 mg kg−1,
100–200 J cm−2; therefore Foscan is approximately 100 times more photoactive than
Photofrin), potentially make Foscan one of the most potent second generation
photosensitisers currently under investigation [5, 7, 9].
Unfortunately, Foscan can render patients photosensitive
for up to 20 days after initial illumination [6, 63, 64]. One solution to this
problem would be to use lower drug doses.Figure 19
Foscan.Lutex
Lutetium texaphyrin, marketed under the trade name
Lutex and Lutrin (Pharmacyclics, Calif, USA), is a “texas-sized” porphyrin [5–9, 18, 65, 66].
Texaphyrins (first synthesised in 1987 by Sessler and his group) are expanded
porphyrins that have a penta-aza core (Figure 20). The result of this macrocyclic
modification is a strong absorption in the 730–770 nm region of
the electromagnetic spectrum [9, 12]. This region is particularly important
since tissue transparency is optimal in this range. As a result, Lutex-based
PDT can (potentially) be carried out more effectively at greater depths and on
larger tumours [5, 6].
Lutex has entered Phase   II
clinical trials (USA) for evaluation against breast cancer and malignant
melanomas [6, 67].
A Lutex derivative, Antrin, has also undergone Phase I
clinical trials (USA) for the prevention of restenosis (see Glossary) of vessels after cardiac
angioplasty by photoinactivating foam cells that accumulate within arteriolar
plaques [6, 68]. A second Lutex derivative, Optrin, is in Phase I trials for AMD
[5].
Texaphyrins are being developed further by Pharmacyclics as
radiosensitisers (Xcytrin, see Glossary) and chemosensitisers (see Glossary) [5]. Xcytrin, a gadolinium texaphyrin (motexafin gadolinium), has been evaluated in Phase III clinical trials against brain metastases and Phase I clinical trials (USA) for primary brain tumours [5].Figure 20
Lutex.ATMPn
9-Acetoxy-2,7,12,17-tetrakis-(β-methoxyethyl)-porphy\-cene
(Figure 21) has been evaluated by Glaxo Dermatology (GlaxoWellcome, NC, USA)
and Cytopharm (Calif, USA) as a photodynamic therapy agent for dermatological
applications against psoriasis vulgaris and superficial non-melanoma skin cancer
[5, 69–72].Figure 21
ATMPn.Zinc Phthalocyanine CGP55847
A liposomal formulation of zinc phthalocyanine
(CGP55847, Figure22), developed by QLT Phototherapeutics (Vancouver, Canada)
and sponsored by Ciba Geigy (Novartis, Basel, Switzerland), has undergone clinical
trials (Phase I/II, Switzerland) against squamous cell carcinomas of the upper
aerodigestive tract [5, 18, 73, 74]. Phthalocyanines (PCs) (Figure 12) are related
to tetra-aza porphyrins. Instead of four bridging carbon atoms at the meso-positions, as for the porphyrins,
PCs have four nitrogen atoms linking the pyrrolic sub-units together. PCs
further differ from porphyrins through the presence of an extended conjugate
pathway: a benzene ring is fused to the β-positions
of each of the four-pyrrolic sub-units. These benzene rings act to strengthen
the absorption of the chromophore at longer wavelengths (with respect to
porphyrins). The absorption band of PCs is almost two orders of magnitude
stronger than the highest Q band of haematoporphyrin [12]. These favourable
characteristics, along with the ability to selectively functionalise their
peripheral structure, make PCs favourable photosensitiser candidates [10, 75–78].
A sulphonated aluminium PC derivative (Photosense, Figure23) has also entered clinical trials (Russian Academy of Medical Sciences, and
the surgical clinic of the Moscow Medical Academy, Moscow, Russia) against
skin, breast, and lung malignancies and cancer of the gastrointestinal tract [5, 18, 79–81].
Sulphonation significantly increases PC solubility in polar solvents including
water, circumventing the need for alternative delivery vehicles [9, 12, 18, 82].
A third PC under investigation is a silicon complex, PC4.
This photosensitiser is being examined for the sterilisation of blood
components at the New York Blood Centre (VI Technologies Incorporated (Vitex),
Melville, NY, USA), against human colon, breast, and ovarian cancers and
against glioma [5, 83–89].
A shortcoming of many of the metallo-PCs is their tendency
to aggregate in aqueous buffer (pH 7.4), resulting in a decrease, or total loss,
of their photochemical activity. This behaviour can be minimised in the presence
of detergents [12].
Metallated cationic porphyrazines (PZ), including
PdPZ+, CuPZ+, CdPZ+, MgPZ+, AlPZ+,
and GaPZ+, have been developed and also tested in vitro  on V-79 (Chinese hamster
lung fibroblast) cells. Results have suggested these photosensitisers are
capable of inducing substantial dark toxicity [12].Figure 22
Zinc phthalocyanine CGP55847.Figure 23
Photosense.Naphthalocyanines
Naphthalocyanines (NCs, Figure24) are an extended PC
derivative. They have an additional benzene ring attached to each isoindole sub-unit
on the periphery of the PC structure. Subsequently, NCs absorb strongly at even
longer wavelengths (approximately 740–780 nm) than PCs
(670–780 nm), further
increasing the depth NC photosensitisers can be effectively used at. This absorption
in the near infrared region makes NCs good candidates for photodynamic
treatment of highly pigmented tumours, including melanomas, which present
significant problems with respect to transmission of visible light.
However, a number of problems are associated with NC
photosensitisers. NCs are generally less stable than their PC relatives: they
readily decompose in the presence of light and oxygen; and metallo-NCs, which
lack axial ligands, have a tendency to form H-aggregates in solution [12, 90].
These aggregates are photoinactive, thus compromising the photodynamic efficacy
of NCs [12]. The main investigations into NCs as photodynamic therapy agents
have been carried out by Kenney and co-workers, van Lier’s group and the Bulgarian
Academy of Sciences (Sofia, Bulgaria) (see below).Figure 24
A naphthalocyanine.Functional Groups
Altering the peripheral functionality of
porphyrin-type chromophores can also have an effect on photodynamic activity.
Diamino platinum porphyrins show high anti-tumour activity,
demonstrating the combined effect of the cytotoxicity of the platinum complex
and the photodynamic activity of the porphyrin species [12, 91].
Positively charged PC derivatives have also been
investigated [12, 64, 76, 77]. Cationic species are believed to selectively
localise in the vital sub-cellular organelle, the mitochondrion. Mitochondria
are key to the survival of a cell; being the site of oxidative phosphorylation,
and hence are potentially important PDT targets.
Zinc and copper cationic derivatives have been
investigated. Although, the positively charged zinc complexed PC was found to
be less photodynamically active than its neutral counterpartin vitro  against V-79 cells [12].
Water-soluble cationic porphyrins bearing
nitrophenyl, aminophenyl, hydroxyphenyl, and/or pyridiniumyl functional groups
exhibit varying cytotoxicity to cancer cellsin vitro, depending on the nature of the metal ion (Mn, Fe, Zn,
Ni), and on the number and type of functional groups [12, 77, 92]. The manganese
pyridiniumyl derivative has shown the highest photodynamic activity, while the
nickel analogue is photoinactive (Figure 25) [12, 92].
Another metallo-porphyrin complex, the iron chelate,
was found to be more photoactive (towards HIV and simian immunodeficiency virus
in MT-4 cells) than the manganese complexes; the zinc derivative was found to
be photoinactive [12, 93].
The hydrophilic sulphonated porphyrins and PCs
(AlPorphyrin and AlPC) compounds were tested for photodynamic activity [94].
The disulphonated analogues (with adjacent substituted sulphonated groups, Figure 26) exhibited greater photodynamic activity than their di-(symmetrical), mono-,
tri- and tetra-sulphonated counterparts; tumour activity increased with
increasing degree of sulphonation [8, 78].Figure 25
Water-soluble cationic metallated porphyrins.Figure 26
5,10-Di-(4-sulphonatophenyl)-15,20-diphenylporphyrinato aluminium chloride.


## 11. Third-Generation Photosensitisers

The poor
solubility of many photosensitisers in aqueous media, particularly at
physiological pH, prevents their intravenous delivery directly into the
bloodstream. It would be advantageous therefore, if a delivery model could be
conceived which would allow the transportation of these (otherwise potentially
useful) photosensitisers to the site of diseased tissue.Work has
recently focused on designing systems to effect greater selectivity and
specificity on the photosensitiser in order to enhance cellular uptake [7, 38]. A number of possible delivery strategies have been suggested, ranging
from the use of oil-in-water (o/w) emulsions to liposomes and nanoparticles as
potential carrier vehicles [3, 7, 18, 36, 95, 96]. There is concern however, that although the use of these systems may increase the therapeutic
effect observed as a result of photodynamic therapy, the carrier system may
inadvertently decrease the “observed” singlet oxygen quantum yield (Φ
Δ) of the encapsulated
photosensitiser: the singlet oxygen generated by the photosensitiser would have
to diffuse out of the carrier system; and since it (singlet oxygen) is believed
to have a narrow radius of action, singlet oxygen may not reach the target and
elicit its desired effect [18]. It may also be possible that,  if the size of the
carrier is not sufficiently small or that the carrier system does not fully
dissolve in physiological media, the incidence/exciting light may not be
appropriately absorbed and light scattering may be significant, thus
inadvertently reducing the singlet oxygen yield. An alternative delivery method
which would remove this problem is the use of targeting moieties. Typical
targeting strategies have included the investigation of photosensitisers
directly attached to biologically active molecules such as antibodies [23–25].
These third generation photosensitisers are currently showing promise (in vitro) against colorectal tumour cells [24].Metallation
A wide range of metals have been used to form
complexes with photosensitiser macrocycles, with variable photodynamic results.
A number of the second generation photosensitisers described earlier contain a
chelated central metal ion. The main metals which have been used are transition
metals, although a number of photosensitisers co-ordinated to group 13 (Al,
AlPcS4) and group 14 (Si, SiNC, and Sn, SnEt2) metals
have also been synthesised.
There seems to be no consistent observation as to the
potential success of metallated photosensitisers. Indeed, a wide range of
photosensitisers are metallated, but the metal ion does not confer definite
photoactivity on the photosensitiser. Copper (II), cobalt (II), iron (II), and
zinc (II) complexes of Hp are all photoinactive in contrast to metal-free
porphyrins [12]. Yet the reverse has been observed for texaphyrin and PC
photosensitisers; only the metallo-complexes have demonstrated efficient
photosensitisation [12].
The presence and nature of the central metal ion,
bound by a number of photosensitisers, strongly influences the photophysical
properties of the photosensitiser [12, 64, 77]. Chelation of paramagnetic metals
to a PC chromophore appears to shorten triplet lifetimes (down to nanosecond
range), generating variations in the triplet quantum yield and triplet lifetime
of the photoexcited triplet state of the metallated PC (mPC) [12, 64, 77, 97].
Intersystem crossing (ISC) is an important parameter
of photosensitisers. The triplet quantum yield and lifetime of a
photosensitiser are directly related to the efficiency of singlet oxygen
generation; a key component in the success of a photosensitiser [97].
Certain heavy metals are known to enhance ISC.
Generally, diamagnetic metals promote ISC and have a long triplet lifetime [64, 77, 97].
In contrast, paramagnetic species deactivate excited states, reducing the
excited-state lifetime and preventing photochemical reactions from taking place
[97]. However, there are well-known exceptions to this generalisation,
including copper octaethylbenzochlorin [13].
For many of the metallated paramagnetic texaphyrin
species, triplet-state lifetimes are down in the nanosecond range [97]. These
results are also mirrored by metallated PCs. PCs metallated with diamagnetic
ions, such as Zn2+, Al3+, and Ga3+, generally
yield photosensitisers with desirable quantum yields and lifetimes (Φ
T 0.56, 0.50 and 0.34 and τ
T 187, 126 and 35 μs,
resp.) [12, 97]. The ZnPC photosensitiser (ZnPcS4) has a singlet
oxygen quantum yield of 0.70; nearly twice that of most other mPCs (Φ
Δ at least 0.40) [12, 18]. Hence,
the latter diamagnetic complexes should be strong candidates for PDT.
Since the heavy metal effect (see Glossary) is known to promote ISC,
theoretically, it should be possible to enhance the photophysical properties (Φ
T,
Φ
Δ, and τ
T)
of any photosensitiser via
metallation. In practice, this is not the case. Only one metallo-porphyrin
photosensitiser (copper octaethylbenzochlorin) has shown photodynamic promise,
the remaining efficient porphyrin photosensitisers are metal-free [13]. The
reverse of this behaviour is observed for PCs and texaphyrins; only the
(diamagnetic) metallated complexes have exhibited potential as photosensitisers
[10, 12]. The metal-free analogues have shown no promise as photosensitisers [12].Expanded Metallo-Porphyrins
Expanded porphyrins have a larger central binding
cavity, increasing the number of potential metals it can accommodate.
Diamagnetic metallo-texaphyrins have shown encouraging
photophysical properties; high triplet quantum yields and efficient generation
of singlet oxygen [12, 64, 77]. In particular, the zinc and cadmium derivatives
have shown triplet quantum yields close to unity [12]. In contrast, the
paramagnetic metallo-texaphyrins, Mn-Tex, Sm-Tex, and Eu-Tex, have undetectable
triplet quantum yields. This behaviour is parallel with that observed for the
corresponding metallo-porphyrins [12].
The cadmium-texaphyrin derivative has shownin vitro photodynamic activity against
human leukemia cells and Gram positive (Staphylococcus)
and Gram negative (Escherichia coli)
bacteria [98–101]. Although follow-up studies have been limited with this
photosensitiser due to the toxicity of the complexed cadmium ion.
A zinc-metallatedseco-porphyrazine (Figure 27) has been developed with a high
quantum singlet oxygen yield (Φ
Δ 0.74) [102]. This expanded porphyrin-like photosensitiser has shown the best
singlet oxygen photosensitising ability of any of the reported seco-porphyrazines. Platinum and
palladium derivatives have also been synthesised with singlet oxygen quantum
yields of 0.59 and 0.54, respectively, (Figure 27) [102].Zinc metallatedseco-porphyrazine, platinum, and palladium derivatives.Metallochlorins/Bacteriochlorins
The tin (IV) purpurins were found to be more active
when compared with analogous zinc (II) purpurins, when evaluated against human
cancers [5–7, 9, 18, 103, 104].
Sulphonated benzochlorin derivatives have
demonstrated a reduced phototherapeutic response against murine leukemia L1210
cellsin vitro and transplanted
urothelial cell carcinoma in rats, whereas the tin (IV) metallated
benzochlorins exhibited an increased photodynamic effect in the same tumour
model (Figure 28) [105].
The previously mentioned copper octaethylbenzochlorin
(Figure10) demonstrated an unexpected result. Despite an undetectable triplet
state, it appears to be more photoactive towards leukemia cells in vitro
and a rat bladder tumour model [106–108]. Suggestions for this unusual effect
have pointed to interactions between the cationic iminium group and
biomolecules [109]. Such interactions may allow electron-transfer reactions to
take place  via the short-lived
excited singlet state and lead to the formation of radicals and radical ions.
The copper-free derivative exhibited a tumour response with short intervals
between drug administration and photodynamic therapy. Increased in vivo activity was observed with
the zinc benzochlorin analogue [109].Sulphonated and tin (IV) benzochlorin derivatives.Metallo-Phthalocyanines
The photophysical properties of PCs are strongly
influenced by the presence and nature of the central metal ion [12, 18, 64, 77]. Co-ordination
of transition metal ions gives metallo-complexes with short triplet lifetimes (nanosecond
range), resulting in different triplet quantum yields and lifetimes (with
respect to the non-metallated analogues) [12]. The diamagnetic metals, such as
zinc, aluminium, and gallium, generate metallo-phthalocyanines (mPc) with high
triplet quantum yields (Φ
T
≥
0.4)
and short lifetimes (ZnPCS
4
τ
T
=
490 Fs and AlPcS
4
τ
T
=
400 Fs) and high singlet oxygen quantum yields (Φ
Δ ≥ 0.7)
[12, 18, 64, 77, 110]. As a result, ZnPc and AlPc have been evaluated as second
generation photosensitisers active against certain tumours [12].Metallo-Naphthocyaninesulfobenzo-Porphyrazines (M-NSBP)
Aluminium has been successfully coordinated to M-NSBP
(Figure29). The resulting complex has shown photodynamic activity against
EMT-6 tumour-bearing Balb/c mice (disulphonated analogue demonstrated greater
photoactivity than the mono-derivative) [111].Figure 29
An aluminium naphthalocyaninesulfobenzoporphyrazine.Metallo-Naphthalocyanines
Wöhrle and co-workers (Bulgaria) have concentrated their investigations on a zinc NC with various amido substituents.
They observed the best phototherapeutic response (Lewis lung carcinoma in mice)
with a tetrabenzamido  analogue [112–114]. Kenney’s group in the USA have studied
complexes of silicon (IV) NCs (Figure 30) with two axial ligands in
anticipation the ligands would minimise aggregation [115]. In particular, they
investigated the disubstituted analogues as potential photodynamic agents [116, 117].
Kenney’s results suggested that a siloxane NC substituted with two
methoxyethyleneglycol ligands is an efficient photosensitiser against Lewis
lung carcinoma in mice and that SiNC[OSi(i-Bu)2-n-C18H37]2 is effective against Balb/c mice MS-2 fibrosarcoma cells [118, 119]. van Lier
and his group in Canada
have also extensively investigated siloxane NCs as agents for photodynamic
therapy [5, 76]. van Lier’s research on these compounds suggests that they are
efficacious photosensitisers against EMT-6 tumours in Balb/c mice also [120, 121].
The ability of certain metallo-NC derivatives (AlNc) to generate singlet oxygen
is weaker than the analogous (sulphonated) metallo-PCs (AlPC); reportedly 1.6–3 orders of
magnitude less [12].
It can be seen from the above examples that
generalisation(s) between the nature of the parent chromophore; the
presence/absence of a central metal ion; and the desirable photophysical
properties required for a successful photosensitiser are difficult to make. In
the porphyrin systems, the zinc ion appears to hinder the photodynamic activity
of the compound; whereas, in the higher/expandedπ-systems, dyes chelated with the same
metal ion are observed to form complexes with good to high
photophysical/photodynamic properties.
In order to try and address these observations,
Sessler and his group undertook an extensive study into the metallated
texaphyrins, investigating the “influence of large metal cations on the
photophysical properties of texaphyrins.” They particularly studied “the effect
of metal cations on the photophysical properties of coordinating ligands.” The
group concentrated on the lanthanide (III) metal ions, Y, In, Lu, Cd, Nd, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb [97].
Sessler and co-workers observed that when diamagnetic
Lu (III) was complexed to texaphyrin, an effective photosensitiser (Lutex) was
generated. When they substituted the paramagnetic Gd (III) ion for the Lu
metal, photodynamic activity was lost. As a result, the group investigated a
range of diamagnetic and paramagnetic ions [97].
Sessler further reported a correlation between the
excited-singlet and triplet state lifetimes and the rate of ISC of the
diamagnetic texaphyrin complexes, Y(III), In (III), and Lu (III), and the
atomic number of the cation [97].
Paramagnetic metallo-texaphyrins were observed to display
rapid ISC. Greater effects on the rates of triplet decay were also observed,
and the triplet lifetimes were strongly affected by the choice of metal centre
[97]. The diamagnetic ions (Y, In, and Lu) were recorded as having triplet
lifetimes ranging from 187, 126, and 35 μs, respectively. Comparable lifetimes
for the paramagnetic species (Eu-Tex 6.98 μs, Gd-Tex 1.11, Tb-Tex
<
0.2, Dy-Tex
0.44
×
10
−
3, Ho-Tex 0.85
×
10
−
3, Er-Tex 0.76
×
10
−
3,
Tm-Tex 0.12
×
10
−
3, and Yb-Tex 0.46) were obtained [97].
Sessler and his group were only able to measure the
triplet quantum yields for three of the paramagnetic complexes (see Table1).
The results were significantly lower than the diamagnetic metallo-texaphyrins [97].
The data collected from Sessler and co-workers experiments
suggests that, in general, singlet oxygen quantum yields closely follow the
triplet quantum yields.
Their experimental data leads to the conclusion that various
diamagnetic and paramagnetic texaphyrins investigated have independent
photophysical behaviour with respect to a complex’s magnetism. The diamagnetic
complexes were characterised by relatively high fluorescence quantum yields,
excited-singlet and triplet lifetimes, and singlet oxygen quantum yields; in
distinct contrast to the paramagnetic species investigated [97].
Results suggested that the +2 charged diamagnetic species exhibit
a direct relationship between their fluorescence quantum yields, excited state
lifetimes, rate of ISC, and the atomic number of the metal ion. The greatest
diamagnetic ISC rate was observed for Lu-Tex; a result ascribed to the heavy
atom effect. The heavy atom effect also held for the Y-Tex, In-Tex, and Lu-Tex
triplet quantum yields and lifetimes. The triplet quantum yields and lifetimes
both decreased with increasing atomic number. The singlet oxygen quantum yield
correlated with this observation [97].
The photophysical properties displayed by the paramagnetic
species were more complex. A simple correlation between the observed
data/behaviour and the number of unpaired electrons located on the metal ion
could not be made. For example, the ISC rates and the fluorescence lifetimes
gradually decreased with increasing atomic number, the Gd-Tex, and Tb-Tex
chromophores showed (despite having a larger number of unpaired electrons)
slower rates of ISC and longer lifetimes than Ho-Tex or Dy-Tex. Sessler
suggested that charge transfer or intermolecular energy transfer is taking
place from higher excited states (such as S2) [97].Table 1
Quantum triplet yields
of texaphyrin metallated with paramagnetic and diamagnetic lanthanides (data reproduced from [97]).
Paramagnetic MTex
Φ
T
Diamagnetic MTex
Eu-Tex
0.090
0.563
Y-Tex
Gd-Tex
0.156
0.500
In-Tex
Yb-Tex
0.126
0.340
Lu-TexTable 2
Summary of a collection of different photosensitiser
types and their absorption data.
CLASS OF PHOTOSENSITISER
LONGEST
EXTINCTION
DRUGS IN
DRUGS
WAVELENGTH
COEFFICIENT/
CLINICAL TRIAL
APPROVED FOR
ABSORPTION/nm
M−1cm−1
(Phase I–III)
PDT (PRECLINICAL
AND CLINICAL)
Porphyrins
620–640
3,500
—
Photofrin
Levulan
Metvix
(Expanded Porphyrins)
ATMPn
Porphycenes Texaphyrins
610–650   730–770
50,000\ \ \ \ \ \       40,000
Lu-Tex
Optrin
Antrin
Xcytrin
Benzvix
Hexvix
Chlorins
650–690
40,000
Foscan
Visudyne
Puryltin
Bacteriochlorins
730–800
150,000
—
—
Phthalocyanines
680–780
200,000
CGP55847
—
PC4
Photosense
Naphthalocyanines
740–780
250,000
—
—Table 3
Summary of a range of photosensitisers and their clinical applications.
TRADE NAME
MARKETING
COMPANY
PRE-/CLINICAL APPLICATION
COUNTRIES APPROVED IN
Photofrin
QLT Phototherapeutics
Oesophageal, lung, bladder and cervical dysplasia
Canada (1993),
The Netherlands (1994),
Japan (1994),
USA (1995),
France  (1996),
Germany  (1997),
Finland (1999),
UK (1999),
Sweden (2000),
Italy (2000),
Ireland (2000),
Poland   (2000)
Levulan
DUSA Pharmaceuticals
Actinic  keratosis
USA (1999),
Actinic
keratosis and
Sweden (2001),
basal-cell carcinoma
Europe (2001)
Metvix
Photocure ASA
Actinic  keratosis and
Sweden (2001),
basal-cellcarcinoma
Europe (2001)
Visudyne
QLT Phototherapeutics
Wet-AMD
Europe (2001),
USA (2000),
Canada
(2000),
Subfoveal choroidal neovascularisation
Europe (2001),
USA(2001),
Canada  (2001)
ATMPn
GlaxoWellcome and Cytopharm
Psorasis and non-melanoma
skin cancer
Germany  (1997)
Purlytin
Miravant Medical Technologies
Psorasis and restenosis
USA   (1998)
Foscan
BioLitec Pharmaceuticals
Head and neck cancers
Europe (2001)Figure 30
A siloxane naphthalocyanine.


## Summary

A variety of second generation photosensitisers have
been developed and evaluated against a range of clinical applications (see Tables2 and 3). The metallation of a number of these chromophores has generated a variety
of photosensitisers with improved photophysical properties. The effectiveness
of these metallo-photosensitisers depends largely (but not definitively) on the
nature of the co-ordinated central metal ion. Chromophores chelated to
diamagnetic transition metals and lanthanide ions have shown the greatest
potential as photodynamic agents, a consequence of the heavy metal effect
enhancing the rate of ISC. As a result, a number of these metallated
tetrapyrrole-based macrocycles are currently photosensitisers of choice,
particularly the zinc (II), aluminium (III), and tin (IV) complexes.



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*Source: 276109-2008-09-07.xml*