Stereoselective synthesis of oxime containing Pd(ii) compounds: highly effective, selective and stereo-regulated cytotoxicity against carcinogenic PC-3 cells

Abstract

New palladium compounds [Pd{(1S,4R)-NOH^NH(R)}Cl₂] (R = Ph 1a or Bn 1b), [Pd{(1S,4R)-NOH^NH(R)}{(1S,4R)-NO^NH(R)}][Cl] (R = Ph 2a or Bn 2b) and corresponding [Pd{(1R,4S)-NOH^NH(R)}Cl₂] (R = Ph 1a′ or Bn 1b′) and [Pd{(1R,4S)-NOH^NH(R)}{(1R,4S)-NO^NH(R)}][Cl] (R = Ph 2a′ or Bn 2b′) have been synthesized. Novel compounds 1a, 1b, and 2b (and 1a′, 1b′, and 2b′) were obtained in solution as a mixture of diastereomers whose relative ratios depend on the solvent and the nature of the amino substituent. In contrast, the synthetic reactions of derivatives 2a and 2a′ were stereospecific, and afforded single enantiomers of absolute configuration (S_N,1S_C,4R_C)-(R_N,1S_C,4R_C) and (R_N,1R_C,4S_C)-(S_N,1R_C,4S_C), respectively. All compounds have been fully characterized by NMR and IR spectroscopy, time-dependent UV-spectroscopy, ESI-HR-MS in water, and CHN elemental analysis. Absolute configurations of the major epimers of 1a and 1a′, both epimers of 1b and enantiomer 2a′, were determined by single crystal X-ray crystallography, and confirmed by 2D NOESY NMR experiments in solution. Additionally, the pH-dependent stability of 2b in water was assessed by ¹H-NMR spectroscopy. Metal derivatives have been tested in vitro against three human cancer (prostate PC-3, cervical HeLa, and breast MCF-7) cell lines. The highest anticancer activities were shown by palladium compound 2a′ in all cancer cells, with IC₅₀ values up to 80 times lower than those found for cisplatin. The cytotoxicity of 2a and 2a′’ is stereo-dependent, with IC₅₀ values that differ significantly for each enantiomer in all the cell lines tested. The cytotoxic activity of 2a and 2a′ was further evaluated against the non-tumorigenic human prostate RWPE-1 cell line, revealing a selectivity index (SI) of ca. 30 for derivative 2a′. DNA interactions have been investigated by equilibrium dialysis, fluorescence resonance energy transfer (FRET) DNA melting assays, and viscometric titrations, pointing to groove and/or external binding. Cell cycle assay on PC-3 cells after treatment with 2a or 2a′ shows cell cycle arrest in the S and G2/M phases, especially when the cells are treated with compound 2a′.