A study of Pyropheophorbide-a based photosensitisers for photodynamic therapy

Photodynamic therapy (PDT) is an approved but under-development modality for cancer therapy. Conventional, non-targeted PDT consists of administration of a photosensitiser to the patient, passive accumulation in the tumour, localised photosensitiser activation using visible light and subsequent cellular destruction via reactive oxygen species (ROS) and/or free radicals. A major limitation of PDT is its poor selectivity for tumours and a suboptimal pharmacokinetic / biodistribution profile. This leads to low potency and patient photosensitivity. This is being addressed with the development of antibody targeted PDT; the conjugation of multiple photosensitisers to a tumour-specific antibody. For effective antibody conjugation photosensitisers are preferable if highly water soluble. PPa (Pyropheophorbide-a) is an effective photosensitiser and more soluble versions have been prepared to aid in its conjugation. The work presented in this thesis explores the differences these novel, soluble photosensitisers exhibit in terms of toxicity, cell death mechanisms and uptake in an in vitro model of traditional PDT and also in terms of improvements in their bio-conjugation. Data presented shows chemical manipulation of PPa significantly affects the biochemical properties important for effective PDT. PEG-modified PPa (PS-1, with enhanced solubility over PPa) as an un-conjugated drug exhibits similar overall toxicity to PPa, along with similar hallmarks for caspase-independent apoptosis, however this equivalent toxicity is achieved despite a significantly reduced cellular uptake. Its toxicity therefore is higher per internalised molecule than its parent compound. This is hypothesised to be due to enhanced retention of photo-physical properties in a cellular environment. Cationic PPa (PS4, with even greater enhanced solubility over PPa) exhibits a loss in cytotoxicity in comparable levels of free photosensitiser studied; this is hypothesised to be due to decreased uptake in addition to an altered cellular localisation. Derivatives of PS1 activated for conjugation showed greatly enhanced reaction with antibody compared to PPa showing its promise for improved targeted PDT, PS4 proved too difficult to chemically activate in the time frame of this project.Open Acces