Potocny, Andrea Michelle2021-04-072021-04-072019https://udspace.udel.edu/handle/19716/28875More than 14 million people worldwide received a new cancer diagnosis in 2012. The traditional forms of treatment for this disease – radiation treatments, chemotherapy and surgical excision – are plagued by debilitating side effects. Furthermore, the more than 8 million cancer-related deaths that occurred in 2012 reveal that treatment is often ineffective. Photodynamic therapy (PDT), which combines light with a photoactive drug to produce toxic singlet oxygen (1O2), holds promise as an alternative treatment for certain types of cancers because it is less invasive and causes fewer side effects. Macrocyclic tetrapyrroles such as porphyrins, chlorins, bacteriochlorins, and phthalocyanines have received extensive study as potential photoactive drugs for PDT; however, no compound known to date possesses all of the desired properties. The lack of ideal 1O2 photosensitizers has prevented the widespread adoption of PDT as a mainstream cancer treatment, and inspired ongoing research aimed at developing more promising candidates. Our group has contributed to this effort by synthesizing several new bipyridylBODIPY and linear tetrapyrrole metal complexes and studying their photophysical properties and 1O2 generating abilities. The most promising complex was then carefully modified in order to tailor its properties for use in PDT, and investigated as a photochemotherapeutic agent against triple negative breast cancer cells. ☐ A series of three Re(bpy)(CO)3Cl complexes with BODIPY groups appended to either the 4,4’-, 5,5’- or 6,6’- positions of the bipyridine ligand had been synthesized previously in our group. The three isomers position the BODIPY fluorophores at varying distances from the rhenium atom and we hypothesized that the complex with the shortest BODIPY ─ rhenium distances might populate the triplet excited state more effectively and act as the most efficient 1O2 photosensitizer. To test this, the steady state absorption and emission properties of the three complexes were studied and their 1O2 quantum yields were measured. All three compounds showed the strong visible absorption features characteristic of the BODIPY chromophores; however, the fluorescence quantum yields decreased substantially compared to those of the three bipyridylBODIPY ligands. The complex with the shortest average BODIPY ─ rhenium distance, which was the 6,6’-isomer, had the smallest fluorescence quantum yield and was the only one to produce measureable phosphorescence. Additionally, while all three complexes generated 1O2, the highest 1O2 quantum yield was obtained from the 6,6’-isomer. These results indicate that the heavy rhenium metal is able to enhance intersystem crossing in the BODIPY groups, and this effect is most pronounced when the fluorophores are positioned closest to the metal atom. These results also imply that the most effective visible light-absorbing Re(bpy)(CO)3Cl-based 1O2 photosensitizers should be obtained by appending chromophores to the 6,6’-positions of the bipyridyl ligand. ☐ A stable a,c-biladiene (DMBil1), which can be obtained from a facile three step synthetic process, and its zinc complex (Zn[DMBil1]) had been synthesized previously in our group. Low 1O2 quantum yields were measured for both compounds, so we hoped to enhance intersystem crossing to the triplet state and improve the 1O2 quantum yield by developing homologous complexes containing heavier metal atoms. A new platinum derivative (Pt[DMBil1]) was synthesized and its electrochemical characteristics, photophysical properties and 1O2 quantum yield were studied along with those of a palladium derivative (Pd[DMBil1]) which had previously been synthesized, but had not been thoroughly characterized. Density functional theory calculations were also carried out for the two complexes to learn about the electronic transitions responsible for their absorption features and visualize the molecular orbitals involved. Pd[DMBil1] and Pt[DMBil1] both showed two reversible 1e– reductions and two 1e– oxidations, all of which were ligand-centered. Both complexes were also efficient light absorbers between ~350-550 nm, and their longer wavelength absorption features were found to arise from a complex mixture of ligand centered, metal centered, metal to ligand and ligand to metal charge transfers involving a number of highly delocalized molecular orbitals. Emission studies revealed evidence of a clear heavy atom effect; Pd[DMBil1] and Pt[DMBil1] emitted both fluorescence and phosphorescence in contrast to DMBil1 and Zn[DMBil1], which only emitted fluorescence. The two heavy metal analogs also produced 1O2 with impressive quantum yields comparable to those of commercial photosensitizers used currently in PDT. ☐ Although Pd[DMBil1] and Pt[DMBil1] are highly efficient 1O2 photosensitizers that are easily synthesized, their insolubility in aqueous solutions would make them challenging to work with in biological environments. We hoped to obtain a biocompatible derivative of Pd[DMBil1] by introducing poly(ethylene)glycol (PEG) substituents to the photosensitizer. By exploiting the susceptibility of the pentafluorophenyl groups to nucleophilic aromatic substitution at the para position, several PEGylated derivatives were synthesized. Pd-PEGDMBil-1 has a three monomer long PEG chain attached at the para-position of one pentafluorophenyl substituent while Pd-PEGDMBil-2 is di-substituted with a three monomer long PEG chain attached to each of the two pentafluorophenyl substituents. A third PEGylated derivative, Pd[DMBil1]-PEG750, was mono-substituted with a longer PEG chain that had an average molecular weight of 750 Da. The absorption and emission properties of the three PEGylated derivatives in methanol did not deviate significantly from those of Pd[DMBil1] suggesting that introduction of the PEG functionalities had little effect on the electronic properties of the tetrapyrrole chromophore. While the three new derivatives continued to produce 1O2 with high quantum yields in methanol, Pd[DMBil1]-PEG750 was the only complex capable of efficient 1O2 production in aqueous solutions. This was attributed to the fact that Pd-PEGDMBil-1 and Pd- PEGDMBil-2 still showed very poor solubility in water. The effects of treating triple negative breast cancer cells with Pd[DMBil1]-PEG750 were investigated, and these experiments revealed that Pd[DMBil1]-PEG750 caused very low levels of cellular toxicity in the dark, but it became ~5300 times more toxic upon activation with λ > 500 nm light and produced predominantly apoptotic cell death. ☐ Even though Pd[DMBil1]-PEG750 possesses many of the attributes of an excellent photochemotherapeutic agent for use in PDT, its failure to absorb between ~650-850 nm, which demarcates the therapeutic window of wavelengths capable of propagating deeper into biological tissues, makes it poorly suited for treatment of most solid tumors. To address this problem without the need for extensive modifications to the photosensitizer, we used the synthetic strategy developed for Pd[DMBil1]-PEG750 to produce a new derivative, Pd[DMBil1]-PEG5000-SH, with a longer thiol-terminated PEG chain to enable facile conjugation to silica core gold shell nanoparticles (NS) that absorb well within the therapeutic range of wavelengths and emit upconverted photoluminescence that can be absorbed by the photosensitizer. The NS also emit heat under near IR light, which we expected would endow the nanoshell-photosensitizer conjugates (NS-PS) with a complimentary photothermal effect and permit dual photodynamic and photothermal therapy upon activation with a single wavelength of light. As observed previously with the other PEGylated derivatives, Pd[DMBil1]- PEG5000-SH retained the photophysical properties of the parent Pd[DMBil1]-based photosensitizer. After confirming the ability of the new photosensitizer derivative to produce 1O2, it was conjugated to the surface of the gold nanoshells (NS). The NS-PS conjugates showed a clear ability to sensitize 1O2 under 800 nm pulsed laser irradiation and emitted heat as effectively as NS that were functionalized with methoxy-terminated PEG suggesting that the upconverted photoluminescence was successfully activating the photosensitizer and that the conjugates should mediate both PDT and PTT as hoped.BODIPYPhotodynamic TherapyRhenium bipyridyltricarbonylchlorideSinglet OxygenTetrapyrroleTriplet photosensitizerThesis1245422169https://doi.org/10.58088/bf28-x8692021-02-20en