Synthesis and testing of Palladium and Platinum phosphine complexes with potential mitochondrial targeting anti-cancer properties

Abstract

The main theme of this thesis focuses on the preparation of palladium and platinum phosphine complexes that possess the potential to act as anti-cancer agents. The design of the complexes was based on the known compound, [Au(dppe)2]Cl which was shown to have an anti-mitochondrial mode of action on cancer cells. Major problems were experienced in the synthesis of these novel palladium and platinum compounds as the five phosphine ligands required diverse reaction conditions. Instability was the major hindrance as decomposition occurred during purification. This led to the substitution of the counter-ion (Cl-) with PF6-. The complexes prepared in this study were varied in lipophilicity as the gold complex was found to be non-selective due to high lipophilicity. In total, six compounds were prepared, purified and tested for potency against a panel of cancer cell lines as well as normal cells.

The most lipophilic compound, [Au(dppe)2]Cl, was non-selective as it exhibited the highest toxicity to both cancerous and normal cells. In general, in vitro studies showed that palladium complexes were more toxic than the platinum analogues. These novel compounds were also non-toxic to both resting and stimulated lymphocytes signifying high selectivity for cancer cells. Three compounds, Pg 3, Pg 4a and Pg 8 exhibited high toxicity and were hence tested as such on murine cancer cell lines. Pg 8, with intermediate lipophilicity, showed toxicity against a larger number of cancer cell lines and this led to further investigations in an attempt to determine its mode of action.

Analysis of the effects of Pg 8 on the mitochodria showed that it did not depolarise the mitochondrial membrane potential. A seven day analysis showed that while it did not have any effect on the mitochondrial membrane potential, it depolarised the plasma membrane potential from day 4. In contrast, [Au(dppe)2]Cl depolarised the mitochondrial membrane potential as expected. Pg 8 was shown to induce apoptosis and necrosis on Jurkat cells after exposure for 48 h. It was also shown to induce cell cycle arrest (after 48 h) as it caused blockade in the S-phase. In contrast, [Au(dppe)2]Cl caused a blockade in the G0/G1 phase.

Uptake studies with radiolabelled Pg 8, [103Pd(d2pyrpe)2][PF6]2, showed that it accumulated significantly in Jurkat cells. Biodistribution studies in Wistar rats demonstrated that it was mostly taken up in the spleen followed by the liver. However, it was excreted faster than [198Au(dppe)2]Cl as this latter compound accumulated significantly in the lungs followed by the spleen, small intestine and liver. Acute toxicity studies in Balb/c mice showed that Pg 8 was less toxic than [Au(dppe)2]Cl. The latter compound (at 3 and 6 ìM) caused a significant reduction of total body weight over a 5-day period. Toxicity was evident as it was also shown to cause elevation of liver enzymes (AST and GGT), contrary to the results obtained from the mice treated with Pg 8 (at 3, 6, 12 and 15 ìM).

Preparation of a patent for the synthesis as well as anti-cancer properties of the novel compound, [Pd(d2pyrpe)2][PF6]2 (Pg 8) is currently in progress.