| Popis: |
Cancer is one of the most common causes of death in the Western world. Hence, the\ud development of novel effective treatment modalities to combat this disease is important.\ud Combretastatins are powerful anticancer drugs that are being evaluated in phase I/II clinical\ud trials. Combretastatins act by binding to p-tubulin, preventing microtubule assembly and\ud inhibiting angiogenesis in developing tumours. Most widely studied is the natural product\ud c/5(Z)-combretastatin A4 that was originally isolated from the African bush willow\ud Combretum caffrum by Pettit and co-workers. Chemically Combretastatins are substituted\ud stilbenes, exhibiting major activity in the cis-form, whereas the corresponding fr-am'-isomers\ud are ~ 2 - 3 orders of magnitude less cytotoxic.\ud Combretastatins undergo reversible photoisomerization upon irradiation with UV/visible\ud light. Here a novel form of two-photon phototherapy is proposed, in which the "inactive"\ud fnms(£)-combretastatins are used as anticancer pro-drugs that may be activated (converted to\ud the cw-isomer) by localized irradiation with light at the tumour site. The use of near-infrared\ud (NIR) light to trigger a nonlinear two-photon excited photoactivation process provides deeper\ud light penetration into tissues and minimizes cellular autofluorescence compared with\ud UV/visible light.\ud The work described in this thesis is highly multidisciplinary. A major task was the synthesis\ud of a range of combretastatin derivatives as target compounds for the novel form of\ud two-photon excited anticancer phototherapy proposed herein. The cytotoxicity of the drug\ud candidates was assessed on live mammalian cell lines. Photochemical properties such as\ud fluorescence quantum yields, fluorescence lifetimes, photoisomerization quantum yields and\ud two-photon absorption characteristics of the combretastatin derivatives were investigated\ud spectroscopically. The fact that the trans-isomers fluoresce in most cases, whereas ds-isomers\ud are virtually non-fluorescent has allowed multiphoton fluorescence lifetime imaging (FLIM)\ud of the real-time uptake of the inactive pro-drugs into live mammalian cells, as well as\ud monitoring their intracellular distribution and concentration. Two-photon excitation of\ud ^-Combretastatins on monolayers of live cells led to effective conversion of the inactive\ud pro-drugs to the highly cytotoxic Z-derivatives leading to apoptosis in the irradiated areas\ud within 24 h. In conclusion, the two-photon excited activation of £-combretastatins on live\ud cells was demonstrated, making £-combretastatins promising pro-drug candidates for\ud two-photon excited anticancer phototherapy. |
