The effect of metal based complexes on the survival of aerobic and hypoxic chinese hamster ovary cells, in vitro.

Abstract

It is well established that many solid tumours are heterogeneous with respect to oxygenation, and contain regions of hypoxic cells, which due to their inherent resistance to ionizing radiation, limit the success of radiotherapy. Numerous chemicals and drugs have been investigated over the past few decades as potential radiosensitizers. The most notable of these being the organometallic compound, cis-diammine dichloroplatinum(II). The clinical success of this drug led to the synthesis of other types of organic cytotoxic metal-containing drugs. Prof. J.C. Swart from the University of the Orange Free State supplied seventeen novel iridium, ferrecenium and rhodium complexes, which I screened for cytotoxic activity against the CHO cell line. The two most cytotoxic complexes namely, [Rh(fctca)(cod)] and [Rh(fctfa)(cod)], were tested for radiosensitizing potential against aerobic and hypoxic CHO cells in the presence of an 8 MV photon beam by the MTT assay adapted to our laboratory conditions. The ferrocene betadiketones co-ordinated to them, Hfctca and Hfctfa and the Ir compliment of [Rh(fctfa)(cod)] namely, [Ir(fctfa)(cod)] were also assessed by the MTT assay. Interestingly, neither the ferrocene nor the iridium complexes showed noteworthy sensitization, which suggests that the rhodium is responsible for the efficacy observed. The radiosensitizing potential of the most active complex, [Rh(fctfa)(cod)] and cisplatin were also confirmed by the use of the more traditional clonogenic assay. Not only did the MTT assay deliver results comparable to the clonogenic technique, but one of the complexes [Rh(fctfa)(cod)] showed radiosensitizing potential against hypoxic CHO cells, equal to that of cisplatin. The rhodium complex, [Rh(fctfa)(cod)] was also tested for radiosensitization properties against the CHO cell line in the presence of a p(66)/Be neutron beam. Results indicated that [Rh(fctfa)(cod)] sensitizes cells to radiation possibly by inhibition of cell inactivation mechanisms that are normally associated with repairable damage. Consequent work done on the flow cytometer where direct DNA damage after irradiation (8 MV photon beam) and drug treatment, was assessed on aerobic CHO cells by a technique adapted to our laboratory showed no significant increase in the forward angle scattered light (FSC) parameter which is an indication of radiation induced strand breaks. Furthermore, [Rh(fctfa)(cod)] showed a significantly greater increase in the side angle scattered light (SSC) parameter, which is an indication of the binding ability of the complex, compared to cisplatin, after treatment with different concentrations of the drugs. Results obtained from enumerating micronuclei frequencies after drug treatment and radiation confirmed that both cisplatin and [Rh(fctfa)(cod)] are more active under hypoxic conditions, with [Rh(fctfa)(cod)] responsible for more micronuclei per binucleated cell. In conclusion, I have established that [Rh(fctfa)(cod)] has a cytotoxic activity comparable to that of cisplatin and that it sensitizes preferentially hypoxic CHO cells to radiation in the clinically relevant dose range. I have also identified the probable action by which [Rh(fctfa)(cod)] sensitizes CHO cells to radiation as being inhibition of repair capacity. Furthermore, results suggest that this complex binds covalently to DNA base pairs. The complex [Rh (fctfa) (cod)] , has so far proven to possess interesting radiosensitizing potential which must be exploited for eventual therapeutic benefit.