Heavy metal-based complexes: activity, neurotoxicity and photon activation therapy's effect in human tumour cells

Cisplatin (CDDP) is an anticancer drug widely used in clinic for the treatment of several solid tumours. Despite its high effectiveness CDDP treatment is still limited by severe side effects and by inherited or acquired resistance phenomena, only partially overcome by the use of new platinum drugs (i.e. oxaliplatin and carboplatin). These drawbacks have stimulated the search of alternative strategies based on different metals offering a better toxicity profile while maintaining the same level of efficacy as platinum-based treatments. In this study we evaluated the cytotoxicity and neurotoxicity of new promising water soluble anticancer complexes based on copper [Cu(PTA)4PF6] [Cu(thp)4][PF6] and gold [Au(PTA)4PF6] [Au(thp)4][PF6] on A549 non-small cell lung cancer (NSCLC) and IGROV-1 ovarian human cancer cells. IC50 at 48 hours of treatment was calculated by SRB assay while the neurotoxicity was tested evaluating the interference of the compounds with neurite elongation. CDDP was used as drug reference with respect to our newly developed drugs. In our model CDDP resulted neurotoxic at concentrations achievable in plasma of patients treated with the same drugs. Similarly the gold-based compound [Au(PTA)4PF6] was neurotoxic at lower concentration than IC50 calculated for the tested cancer cell lines. On the contrary, both copper-based compounds and [Au(thp)4][PF6] were neurotoxic at higher concentrations with respect to the IC50 obtained in tumour cell lines tested. We then tested the efficacy of synchrotron radiation (SR) to trigger the Auger effect in A549 and IGROV-1 cells containing a high Z-number element, like platinum, gold, and copper. CDDP and the new metal-based drugs were used as carrier of heavy Z-elements. The irradiation was performed with monochromatic beams above the K-shell edge of the different elements. Using keV X-rays, the high photoelectric cross sections of the heavy-Z elements result in substantial interactions, producing fluorescence light and free electrons. The experiments were carried out at the ID17 beamline of the European Synchrotron Radiation Facility, where a high-fluence monochromatic beam adjustable from 20 to 180 keV is available. Irradiation of cells pre-treated with CDDP or [Cu(PTA)4PF6] concentrations allowing roughly 90% of cell survival induced an enhancement in cellular death with respect to drug and irradiation alone. With the other compounds no cell death enhancement was observed. Finally we studied the homologous recombination repair pathways. SR in combination with CDDP resulted in a significative nuclear relocalization of RAD51 and BRCA1 proteins with respect to untreated control A549 and IGROV-1 cells as well as in cells treated with SR or CDDP alone. Similar results were observed in IGROV-1 cells pre-treated with [Cu(PTA)4PF6]. Our results suggest that SR-enhanced CDDP activity might allow the use of a reduced dose of CDDP thus achieving side effects minimization due to the exposure of normal cells/tissues to less toxic doses. Furthermore, considering the anticancer activity and the neurotoxic profile of [Cu(PTA)4PF6], our data suggest that copper-based drugs represent new and promising compounds in anticancer treatment also in combination with SR.