Anti-cancer properties of gold(III) mesoporphyrin IX and gold(III) tetraphenylporphyrins with pegylation and glycosylation

Abstract

In recent years, enormous effort has been devoted to evaluate the anti-cancer properties of new gold(III)-based complexes in virtue of the prominent paradigm of cisplatin. Gold(III) porphyrin [Au(TPP)]Cl was shown to be stable under physiological conditions; its multifaceted anti-cancer properties including potent in vitro cytotoxicity against various human cancer cell lines and effective in vivo anti-tumor activities on multiple cancer types render the gold(III) porphyrin a promising anti-cancer candidate for further development. Through structural modification and drug formulation, further enhancements of the anti-cancer activities of gold(III) porphyrin complexes in the aspects of selectivity towards cancerous cells, bioavailability and therapeutic window are envisioned. In this thesis, different strategies were adopted to modify the gold(III) porphyrinato scaffold, including (1) incorporation of structurally distinct porphyrinato ligands, (2) PEGylation, and (3) glycosylation; the cytotoxicity and in vivo anti-cancer activities of these types of modified gold(III) porphyrin complexes were examined. A gold(III) complex of mesoporphyrin IX dimethyl ester (AuMesoIX), which contains no meso-substituents, was synthesized, and the chemical and biological activities of this complex were described. With the use of AuMesoIX, conjugation of a meso-unsubstituted gold(III) porphyrin complex to biological thiols (glutathione and cysteine containing peptides) was identified for the first time. This reactivity is presumably attributed to the strong electron-withdrawing effect of the gold(III) center for activating the meso-carbon atoms towards nucleophilic aromatic substitution by thiols. AuMesoIX exhibited potent cytotoxicity to cancer cells with significant reactive oxygen species (ROS)-production and accumulation of polyubiquitinated proteins. These cellular events were associated with inhibitions of cellular thiol-enzymes including thioredoxin reductases and deubiquitinases in cancer cells. AuMesoIX also formed nanoparticles in aqueous buffer and displayed enhanced cellular uptake. Moreover, AuMesoIX was shown to significantly suppress tumor growth in vivo in two independent mouse models with no apparent toxicity. Amphiphilic gold(III) porphyrins [Au(TPPCOOPEG5000OCH3)]Cl and [Au(TPPCONHPEG5000OCH3)]Cl bearing hydrophilic poly(ethylene glycol) (PEG) polymer were prepared. Both PEGylated conjugates self-assemble into spherical nanostructure in physiological buffer. The nano-sized polymeric conjugate, [Au(TPPCOOPEG5000OCH3)]Cl, dissociates and releases the cytotoxic [Au(TPPCOOH)]+ in slightly acidic buffer solutions, in the cellular environment and under in vivo conditions through the cleavage of ester bond. A favorable uptake of this conjugate and an enhanced selectivity in killing cancer cells over non-tumorigenic cells were also observed. More importantly, this PEGylated conjugate significantly suppressed the tumor growth of two independent xenografts (especially the cisplatin-resistant model) with minimal systemic toxicity and improved biodistribution compared to that with the complex without PEG-conjugation. In addition, nanocomposites formed via co-assembly of the PEGylated conjugate with doxorubicin demonstrated a synergistic anti-cancer effect on doxorubicin-resistant cancer cells. A panel of gold(III) tetraphenylporphyrin prodrugs based on different cleavable linkages with glucose units having potential cancer-targeting effect are also described. With the disulfide linker, the glucose-appended conjugate Au-S-Glu was shown to accumulate in lung cancer cells, which overexpress glucose transporter, and undergo reductive cleavage by cellular thiols to release the active gold(III) porphyrin moiety. This cytotoxic gold(III) porphyrin prodrug significantly suppresses tumor growth in vivo in association with the enhanced tumor accumulation in comparison with the unmodified gold(III) porphyrin complex.