Platinum(IV) Complexes of trans-1,2-diamino-4-cyclohexene: Prodrugs Affording an Oxaliplatin Analogue that Overcomes Cancer Resistance

Abstract

Six platinum(IV) compounds derived from an oxaliplatin analogue containing the unsaturated cyclic diamine trans-1,2-diamino-4-cyclohexene (DACHEX), in place of the 1,2-diaminocyclohexane, and a range of axial ligands, were synthesized and characterized. The derivatives with at least one axial chlorido ligand demonstrated solvent-assisted photoreduction. The electrochemical redox behavior was investigated by cyclic voltammetry; all compounds showed reduction potentials suitable for activation in vivo. X-ray photoelectron spectroscopy (XPS) data indicated an X-ray-induced surface reduction of the Pt(IV) substrates, which correlates with the reduction potentials measured by cyclic voltammetry. The cytotoxic activity was assessed in vitro on a panel of human cancer cell lines, also including oxaliplatin-resistant cancer cells, and compared with that of the reference compounds cisplatin and oxaliplatin; all IC₅₀ values were remarkably lower than those elicited by cisplatin and somewhat lower than those of oxaliplatin. Compared to the other Pt(IV) compounds of the series, the bis-benzoate derivative was by far (5-8 times) the most cytotoxic showing that low reduction potential and high lipophilicity are essential for good cytotoxicity. Interestingly, all the complexes proved to be more active than cisplatin and oxaliplatin even in three-dimensional spheroids of A431 human cervical cancer cells.

Keywords: anticancer drugs; cisplatin; oxaliplatin; platinum(IV) prodrugs; trans-1,2-diamino-4-cyclohexene.

Scheme 1

Synthesis of cis,trans,cis-[Pt(OXA)(OH)₂(DACHEX)] (1), cis,trans,cis-[Pt(OXA)(AcO)₂(DACHEX)] (AcO = acetate) (2), and cis,trans,cis-[Pt(OXA)(BzO)₂(DACHEX)] (BzO = benzoate) (3). a) Excess H₂O₂, 70 °C, 2 h in the dark; b) Ac₂O, 80 °C, 16 h in the dark; c) Acetone, Pyridine, PhC(O)Cl, 4 h reflux in the dark.

Figure 1

¹H-NMR (700 MHz, ¹H) of 1 in DMSO-d_6. * indicates solvent residual peaks.

Figure 2

¹H-NMR spectrum (700 MHz, ¹H) of 2 in DMSO-d_6. * indicates solvent residual peaks, x indicates an impurity (acetone) present in the solvent.

Figure 3

¹H-NMR spectrum (700 MHz, ¹H) of 3 in acetone-d_6. * indicates residual solvent peaks.

Scheme 2

Synthesis of cis,trans,cis-[Pt(OXA)Cl₂(DACHEX)] (4). a) CH₃CN, PhICl₂, 60 °C, 1 h.

Scheme 3

Synthesis of cis,trans,cis-[Pt(OXA)(AcO)Cl(DACHEX)] (5) and cis,trans,cis-[Pt(OXA)(OH)Cl(DACHEX)] (6). a) Acetic acid, N-chloro-succinimide, room temperature, dark, 4 h; b) water, N-chloro-succinimide, room temperature, dark, 3 h.

Figure 4

¹H-NMR (700 MHz, ¹H) spectrum of 5 in CD₃OD. * indicates residual solvent peaks.

Figure 5

¹H-NMR spectrum (700 MHz, ¹H) of 6 in DMSO-d₆. * indicates residual solvent peaks. The second signals relative to the methynic protons H_c/c’ and to the methylenic protons H_d/d’ are overlapping with solvent signals.

Figure 6

Cyclic voltammograms of Pt(IV) complexes 1–6 (red) recorded at a glassy carbon electrode in buffered aqueous solution (PBS, pH = 7.4, I = 0.1) containing 5 mM NaCl as supporting electrolyte and 5·10⁻⁴ M Pt(IV) complex. Scan rate 20 mVs⁻¹. The CV plot of the blank solution is overlaid in grey.

Figure 7

Pt4f XPS spectrum of complex 2.

Figure 8

Pt4f XP spectra for complexes 1–6 (a) acquired at the onset of the experiments and (b) after 30 min irradiation. On each spectrum is shown the number that identifies the complex.

Figure 9

(A). Cellular uptake (a) and correlation between cytotoxicity and cellular platinum levels (a’) in treated A431 cancer cells. A431 cells were incubated for 24 h with 0.5 µM of 1–6. The error bars indicate standard deviation (SD). (B). Nuclear DNA platination levels (b) and correlation between cellular uptake and DNA-platination levels (b’) in drug-treated A431 cells. A431 cells were treated for 24 h with 0.5 µM of 1–6. DNA was extracted and quantified, and the amount of Pt bound to DNA was estimated using graphite furnace atomic absorption spectrometry (GF–AAS). Error bars indicate SD.