Four new
palladium(II) complexes with the formula Pd(L)(2), where L are
quinoxaline-2-carbonitrile N(1),N(4)-dioxide derivatives, were synthesized as a contribution to the chemistry and pharmacology of
metal compounds with this class of pharmacologically interesting bioreductive
prodrugs. Compounds were characterized by elemental, conductometric and thermogravimetric analyses, fast atom bombardment mass spectrometry (FAB-MS) and electronic, Fourier transform infrared (FTIR) and (1)H-nuclear magnetic resonance spectroscopies. The complexes were subjected to cytotoxic evaluation on V79 cells in hypoxic and aerobic conditions. In addition, a preliminary study on interaction with plasmid
DNA in normoxia was performed. Complexes showed different in vitro
biological behavior depending on the nature of the substituent on the
quinoxaline ring. Pd(L1)(2) and Pd(L2)(2), where L1 is 3-aminoquinoxaline-2-carbonitrile N(1),N(4)-dioxide and L2 is 3-amino-6(7)-methylquinoxaline-2-carbonitrile N(1),N(4)-dioxide, showed non selective cytotoxicity, being cytotoxic either in hypoxic or in aerobic conditions. On the other hand, Pd(L3)(2), where L3 is 3-amino-6(7)-chloroquinoxaline-2-carbonitrile N(1),N(4)-dioxide, resulted in vitro more potent
cytotoxin in
hypoxia (P=5.0 microM) than the corresponding free
ligand (P=9.0 microM) and
tirapazamine (P=30.0 microM), the first bioreductive cytotoxic
drug introduced into clinical trials. In addition, it showed a very good selective cytotoxicity in hypoxic conditions, being non-cytotoxic in normoxia. Its hypoxic cytotoxicity relationship value, HCR, was of the same order than those of other
hypoxia selective
cytotoxins (i.e., Mitomycine C,
Misonidazole and the N-
oxide RB90740). Interaction of the complexes with plasmid
DNA in normoxia showed dose dependent ability to relax the negative supercoiled forms via different mechanisms. Pd(L2)(2) introduced a scission event in
supercoiled DNA yielding the circular relaxed form. Meanwhile, both Pd(L1)(2) and Pd(L3)(2) produced the loss of negative supercoils rendering a family of topoisomers with reduced electrophoretic mobility. Pd(L3)(2) showed a more marked effect than Pd(L1)(2). Indeed, for the highest doses assayed, Pd(L3)(2) was even able to introduce positive supercoils on the plasmid
DNA.