Photodynamic therapy (PDT) utilises light, oxygen and organic macrocycles, called
photosensitisers, to produce reactive oxygen species that can kill malignant cells.
Conventional PDT is associated with side effects that have stifled its advance and
widespread use. These include low tumour selectivity, slow blood clearance and poor
formulation. We proposed that an antibody fragment could be used to carry the
photosensitiser to the target cells, significantly overcoming these limitations.
Pyropheophorbide-a (PPa) was synthetically modified to enhance its water solubility
obtaining two compounds, PS1 and PS4 each more water soluble than PPa. The use of
Sonogashira coupling and short polyethylene glycol chains gave PS1, whereas the use of
Suzuki coupling and a single positive charge gave PS4. The singlet oxygen quantum yields
of these were improved compared to PPa with that of PS4 being 1.5 times higher than PPa.
The in vitro characterisation of PPa, PS1 and PS4 using cytotoxicity assays did not correlate
with their photophysical characterisation. PS4 was significantly less potent than PPa and
PS1 on SKOV3 and KB human cancer cell lines. Confocal microscopy aided further
characterisation using stains for intracellular organelles. PS1 was found to localise primarily
in the ER and Golgi apparatus, similarly to PPa, while PS4 was found to localise mainly in
the lysosomes.
PS1 was conjugated to C6.5(-k), an anti-HER2 single chain variable fragment (scFv) using
lysine coupling, to obtain a photoimmunoconjugate that was characterised in vitro and
subsequently in vivo. In vitro characterisation showed increased potency and specificity but
non-specific cell death attributed to the non-covalently bound photosensitiser was observed.
However, in vivo therapy studies showed that the C6.5(-k)-PS1 photoimmunoconjugate
could be used to cure SKOV3 subcutaneous tumours in nude mice, validating the use of
targeted PDT as a successful targeted therapy with the potential to lower the effective drug
dose and minimise side effects.