Synthesis and biological studies of multifunctional dendritic conjugates of synergistically acting therapeutic agents

Many interesting drug candidates are limited in their therapeutic applications by properties such as low water solubility, rapid elimination from the body or inability to cross cell membranes. The investigation of ways to improve the targeting of these potential pharmaceuticals to a cell is therefore a major current area of medicinal chemistry research and development. Over the past few years a class polymers with multiple side-branches known as dendrimers have emerged as a promising means for enhancing and optimising drug delivery. The key advantage of using a dendrimer is that many drug molecules can be linked to the side-branches on the periphery of a dendrmer which can then deliver a high drug payload to a tumour cell for example. A further advantage is that the structure of a dendrimer is well-defined with a known size and drug loading, which makes them attractive to the pharmaceutical industry. In this project we propose to synthesise dendrimers containing a natural compound known as 5-aminolaevulinic acid (ALA), which is used for photodynamic therapy (PDT) of cancer. This treatment involves shining light onto a tumour (eg a skin tumour) to activate a photosensitising drug in the tumour resulting in the production of free radicals that are toxic to cancer cells. ALA is used for this therapy because once inside cells it is converted to a light-activated compound known as a porphyrin. To produce one porphyrin molecule, eight ALA molecules are combined through reactions involving a series of cellular enzymes. However ALA is not readily taken up by cells which limits its therapeutic efficacy in tumour treatment. For treating thicker skin tumours such as nodular basal cell carcinomas better penetration and higher cellular porphyrin levels throughout the tumour are required. In this project we propose a novel means of enhancing ALA uptake and porphyrin levels inside cells. New ALA derivatives would be synthesized in which ALA is linked with iron-binding compounds which, in combination with ALA, can induce a greater than additive or synergistic enhancement in porphyrin levels. These iron-binding or chelating compounds (HPOs) were developed at KCL originally to treat patients suffering from metabolic deficiencies which caused a build-up of iron in the body. Since these compounds are capable of reducing levels of free iron within cells, they should also inhibit the natural conversion of the photoactive porphyrin into a photoinactive form, called protohaem. Therefore iron chelation can result in a build-up of the photoactive porphyrin intermediate in the cells and potentially improve the therapeutic outcome. Since the porphyrin is fluorescent, we are able to use fluorescence detection to demonstrate the synergistic enhancement in porphyrin levels. We have recently carried out proof-of-principle studies on new compounds incorporating these two agents to demonstrate the feasibility of this approach. Our aim is to prepare conjugates of ALA and HPO molecules, starting from small single conjugates of both drugs increasing to nanoparticale size dendrimers. Incorporation of these relatively small molecules bound via ester linkages within dendrimers will enable a high payload of the bioactive agents to be codelivered to cells, and avoid the need to administer the drugs separately which would be limited by their differing pharmacological properties. The smaller compounds will be tested for surface or topical application on tissue, whereas the larger ones would be designed for oral and intravenous administration. The project would involve a concerted effort bringing together groups at KCL, Essex and UCL with complementary skills and experience in chemistry, biochemistry and photobiology, and would draw on expertise available at MedPharm Ltd in drug formulation and tissue explant studies. Although the work is focused on photodynamic therapy, the same principles may serve as a template for other agents in multiple drug therapy.