Improving the delivery of 5-aminolaevulinic acid in photodynamic therapy (PDT): synthesis and biological studies of novel peptide prodrugs

The main aim of this project is to design new drugs for photodynamic therapy (PDT) which is a treatment that can be used for destroying cancers and killing infectious bacteria. The key feature feature of PDT is that it uses a light-activated drug known as a 'photosensitiser'. When light activates the drug this results in a toxic effect to tumour cells or bacteria leading to destruction of the tumour or healing of the bacterial infection. In effect, the light 'switches on' the drug but only in areas exposed to light. We propose to look at a new type of drug based on 5-aminolaevulinic acid (ALA). This is a natural compound present in many cells but normally at very low levels. However if cells or bacteria are exposed to high concentrations of ALA then a photosensitiser is produced (a porphyrin) which absorbs light over a wide range from red to blue and can therefore be used for PDT treatment. For example, a skin tumour can be treated by first covering the tumour with a lotion containing the ALA which is then left for a few hours. After ALA has seeped into the tumour, light is shone onto it for a few minutes to kill the tumour cells. Alternatively, a solution of ALA can be injected or given orally for the treatment of tumours inside the body where an endoscope can be used to guide the light onto the tumour. One main problem with using ALA however is that it is not quickly taken up by cells. We plan to change its chemical structure so that cells or bacteria can absorb it more easily, specifically by attaching amino acids to the ALA to help its uptake by cells. After entering the cell the drug would break down to release the ALA. If the modified ALA drug can enter cells more easily then in principle it should be more effective for tumour treatment, and since amino acids are natural compounds which make up proteins these new drugs should be safe to use. As well as treating cancers, there are several types of bacterial or even fungal infection which can be treated using PDT. One advantage of PDT for treating bacteria is that bacteria resistant to antibiotics can be treated. For example, burn wounds often become infected and in this case the PDT drug can be applied to the wounds and then illuminated after the drug has penetrated the infected area. If the new types of ALA drug can be taken up more efficiently by the bacteria then again the PDT treatment should be more effective. We also hope to design modified ALA drugs that are broken down specifically by bacteria and not by healthy cells by attaching amino acids whose structures are the mirror image of those which are normally found naturally.

Photodynamic therapy (PDT)is an emerging therapy for the treatment for cancer and various other human disorders. In PDT, destruction of tumours or pathogenic organisms is achieved with light following the administration of a light-activated photosensitising drug which is ideally selectively retained in, or targeted to diseased tissue relative to normal adjacent tissue. The exogenous administration of 5-aminolaevulinic acid (ALA) has attracted considerable interest for PDT since it is a naturally occurring compound present in prokaryotic and eukaryotic cells which can be metabolised to a porphyrin photosensitiser, protoporphyrin IX (PpIX)via the haem biosynthetic pathway. ALA-PDT is a powerful approach for both the detection and treatment of cancers, in particular the treatment of basal cell carcinomas using topical ALA administration, and the visualisation of early tumours in hollow organs. Moreover, it has considerable potential for use in antimicrobial applications since since various Gram-positive and Gram-negative bacteria, yeasts and fungi are able to assimilate exogenous ALA for porphyrin synthesis, thus rendering them susceptible to photosensitisation. Currently, a key limitation of ALA-PDT is the zwitterionic nature of ALA at physiological pH which limits its passage through cellular membranes and other biological barriers. Furthermore, the selective delivery and release of ALA in specific tissues is difficult to achieve. To address these issues, this proposal aims to develop novel ALA peptide prodrugs with improved cellular uptake and targeting properties and to demonstrate the effectiveness of these compounds in a variety of cell lines and microorganisms. The project will build on preliminary synthetic and biological studies that indicate that a variety of ALA peptide prodrugs are more effectively internalised than ALA itself in cell and tissue explant models and are effectively converted to ALA by intracellular esterases and peptidases, leading to PpIX production and photocytotoxicity upon irradiation.