Gold and silver complexes of BIS (phosphino) hydrazine ligands as potential anti-tumour agents
Date
2008-07-10T12:19:47Z
Authors
Kriel, Frederik Hermanus
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Abstract
Cancer is presently responsible for about 25 % of all deaths in developed countries. It
has a substantial impact on a patient's quality of life and many cancer treatments (such
as chemotherapy) may have severe side-effects. New therapies that display a
minimum of the drastic side-effects of current drugs are constantly sought after.
The first comprehensive studies of the anti-tumour potential of gold compounds,
including gold drugs, were published in the mid-to-late 1980’s. Gold(I) bisphosphine
compounds of bis(diphenylphosphino)ethane (dppe) and related ligands with a
tetrahedral coordination geometry turned out to be a particularly potent class of
compounds. Clinical trials were, however, not pursued owing to the acute toxicity
associated with them. The high toxicity of [Au(dppe)2]+ is attributable to the high
lipophilicity of the cations which results in non-selective uptake into mitochondria in
all cells. The tumour selectivity of tetrahedral gold(I) complexes was found to hinge
on a fine balance between its lipophilic and hydrophilic character.
One way of introducing the more hydrophilic character to a ligand or complex,
necessary for selectivity, is the addition of heteroatoms. To this end, one of the aims
of this project was to synthesise a range of ligands where the lipophilic ethane bridge
of dppe was replaced by a hydrazine bridge. It was hoped that the addition of the
nitrogen heteroatoms would increase the hydrophilic character of the ligands, thus
rendering more selective drug candidates. Further modifications identified included
the initial phenyl substituents on the phosphorous centres could be enriched by
replacing the phenyl with either O-methylanisol or N,N-dimethylaniline. In both cases
four extra heteroatoms per ligand were added, oxygen in the case of anisol and
nitrogen in the case of aniline. With dimethylhydrazine or diethylhydrazine as starting
material six ligands were synthesised and characterised. Crystallisation efforts
afforded a x-ray crystal structure of bis(diphenylphosphino)diethylhydrazine. In
contrast to the stability of the diethylhydrazine ligands, the dimethylhydrazine ligands
were found to be prone to decomposition, at times even under an inert atmosphere.
The aim of AuTEK Biomed is the inclusion of metals, in this case gold(I) and
silver(I), to the above mentioned ligands (L) to enhance activity. To this effect, six
phosphine-bridged gold complexes (ClAu-L-AuCl), six bischelated gold complexes
(AuL2Cl), six phosphine-bridged silver complexes (NO3Ag-L-AgNO3) and six
bischelated silver complexes (AgL2NO3) were attempted.
It was found that the ClAu-L-AuCl complexes are very stable and readily crystallise
from THF. Five of the six complexes could be characterised by x-ray crystallography.
The sixth complex, that of bis(di(N,N-dimethylanaline)phosphino)dimethylhydrazine,
could not be isolated due to the breakdown of the ligand upon addition of the metal
starting material. Succesful preparation of five of the six AuL2Cl complexes was
accomplished. These complexes where found to be slightly hygroscopic and storage
under an inert atmosphere was required. Efforts to crystallise these complexes led to
the crystallisation of conglomerates of micro crystals, which could not be analysed
crystallographicly due to the disordered nature of the micro crystals.
The synthesis of the bischelated gold complex of bis(di(N,N-dimethylanaline)-
phosphino)diethylhydrazine led to an interesting phenomenon which was studied
further. Upon complexation, the expected yellow complex spontaneously turned
bright red even when the solvent was removed. Efforts to crystallise this complex was
not successful until the counter ion (Cl-) was replaced with that of NO3
-. The crystal
structure revealed that gold(I) had spontaneously oxidised to gold(III) and this crystal
structure represents the first known complex to our knowledge of gold(III) chelated to
four phosphorous centres.
NO3Ag-L-AgNO3 complexes were mostly unstable and regularly led to the formation
of a silver mirror on the reaction vessel. Even with this inherent instability of these
complexes, it was possible to characterise three of the complexes to some extent and
to analyse the complex of bis(diphenylphosphino)diethylhydrazine by
x-ray crystallography. The molecular structure showed intricate connectivity between
the complexes leading to polymeric structures. The tetrahedral silver complexes were
markedly more stable and five of the complexes could be isolated and characterised.
The molecular structure of bis(bis(diphenylphosphino)diethylhydrazine)silver nitrate
could be determined.Thirteen of the complexes were found to be sufficiently stable to be investigated as
potencial drug candidates and were subjected to in vitro anti-tumour screening. These
thirteen complexes were firstly screened against three cell lines, namely the cancerous
HeLa and Jurkat cell lines and Lymphocytes obtained from healthy human donors.
These results were analysed and selectivity for toxicity towards cancerous cell-lines
vs the healthy cell line was calculated.
Five complexes were identified as suitable for investigation on a further range of cell
lines based on the specificity they displayed. Two of the complexes were of the type
ClAu-L-AuCl, while the other three were of the type AgL2NO3. The five compounds
were screened against A2780, A2780cis, Colo, MCF-7 and MCF-12 cell lines from
which new selectivity profiles were calculated.
Due to the mechanism of activity of [Au(dppe)2]+ being described as apoptosis
induced through the disruption of mitochondrial membrane potential, so leading to
cell arrest, it was decided to test whether these analogues would work through the
same mode of action. To this end, the best two complexes, namely ClAu(MeO-4-
Ph)2PN(Et)N(Et)P(Ph-4-OMe)2AuCl and [{(MeO-4-Ph)2PN(Me)N(Me)P(Ph-4-
OMe)2}2 Ag]+.NO3
-, selected through the analysing of selectivity calculations were
subjected to the mitochondrial membrane potential and apoptosis assays. From the
mitochondrial membrane potential assay it could be proven that these complexes do
indeed affect the mitochondrial membrane potential by lowering the potential
necessary to the cell for producing ATP. Results from the apoptosis assays were
inconclusive and remain as future work.
The hypothesis of this theses is that it is possible to fine tune the
lipophilic/hydrophilic balance of tetrahedral Group 11 complexes to improve
selectivity towards cancerous cells above healthy cells. This has to a certain extent
been proven through the increased selectivities of these complexes when compared to
[Au(dppe)2]+. It has to be mentioned that even though increased selectivities are
observed for these complexes, the selectivities are not profound enough to merit further investigation into the anti-tumour activity of these complexes.