Photodynamic Therapy (PDT) is a relatively novel oncological treatment modality, in which a patient is administered
a photosensitive drug, called a photosensitizer. After allowing sufficient time for biodistribution, the
cancerous area is irradiated with light of the appropriate wavelength, activating the photosensitizer to produce
highly reactive singlet oxygen, which produces a highly localized cell kill. The efficacy of PDT is determined by a)
the intensity of the light b) the local concentration of the photosensitizer, and c) the availability of oxygen. However,
with the clinical application of PDT, the patient is simply administered a body mass dependent quantity
of photosensitizer, and then the target area is administered a prescribed amount of radiant energy (joules per
cubic centimetre). For treatment of superficial malignancies, PDT has many successes; however, interstitial PDT
(PDT of solid, internal malignancies) has inconsistent outcomes mostly due to the inability to predict, calculate
or measure the variables that affect PDT: the radiation dose, oxygen concentration, and the photosensitizer
concentration. We have developed sophisticated methods to determine the behaviour of light in homogeneous
biological tissues. Tissue oxygen levels can be replenished by fractionating the light dose - allowing areas of your
target tissue to go through a "dark" cycle during PDT. However, to date, there has not been an accurate method
of determining tissue photosensitizer concentrations in-vivo.
We are researching the efficacy of a novel hypocrellin derivative, SL-052. Like other photosensitizers available,
SL-052 shows strong therapeutic photodynamic activity when irradiated by 635 nm light. Like most photosensitizers,
SL-052 exhibits fluorescent activity, but SL-052 also shows strong fluorescent emission at 725nm when
excited by 635 nm. The intensity of the fluorescent emission can been correlated with the local concentration of
the photosenstizer. However, many clinically available photosensitizers require that fluorescence is excited using
a wavelength of light much shorter than the therapeutic wavelength. This characteristic allows us to monitor the
availability of the photosensitizer during PDT and to correlate the outcome of PDT to the observed fluorescence.
In this paper, we monitor the temporal distribution of SL-052 in the Dunning R3327-AT cell line grown on
the flank of a Fisher Copenhangen rats.