The Influence of the Auxiliary Ligand in Monofunctional Pt(II) Anticancer Complexes on the DNA Backbone

Monofunctional platinum complexes offer a promising alternative to cisplatin in cancer chemotherapy, showing a unique mechanism of action. Their ability to induce minor helix distortions effectively inhibits DNA transcription. In our study, we synthesized and characterized three monofunctional Pt(II) complexes with the general formula [Pt(en)(L)Cl]NO₃, where en = ethylenediamine, and L = pyridine (py), 2-methylpyridine (2-mepy), and 2-phenylpyridine (2-phpy). The hydrolysis rates of [Pt(en)(py)Cl]NO₃ (1) and [Pt(en)(2-mepy)Cl]NO₃ (2) decrease with the bulkiness of the auxiliary ligand with k() = 2.28 ± 0.15 × 10^-4 s^-1 and k() = 8.69 ± 0.98 × 10^-5 s^-1 at 298 K. The complex [Pt(en)(2-phpy)Cl]Cl (3) demonstrated distinct behavior. Upon hydrolysis, an equilibrium (K_eq = 0.385 mM) between the complexes [Pt(en)(2-phpy)Cl]⁺ and [Pt(en)(2-phpy-H⁺)]⁺ was observed with no evidence (NMR or HR-ESI-MS) for the presence of the aquated complex [Pt(en)(2-phpy)(H₂O)]²⁺. Despite the kinetic similarities between phenanthriplatin and (2), complexes (1) and (2) exhibit minimal activity against A549 lung cancer cell line (IC₅₀ > 100 μΜ), whereas complex (3) exhibits notable cytotoxicity (IC₅₀ = 41.11 ± 2.1 μΜ). In examining the DNA binding of (1) and (2) to the DNA model guanosine (guo), we validated their binding through guoN7, which led to an increased population of the C3'-endo sugar conformation, as expected. However, we observed that the rapid transition ²E (C2'-endo) ↔ ³E (C3'-endo), in the case of [Pt(en)(py)(guo)](NO₃)₂ ([1-guo]), slows down in the case of [Pt(en)(2-mepy)(guo)](NO₃)₂ ([2-guo]), resulting in separate signals for the two conformers in the ¹H NMR spectra. This phenomenon arises from the steric hindrance between the methyl group of pyridine and the sugar moiety of guanosine. Notably, this hindrance is absent in [2-(9-MeG)] (9-MeG = 9-methylguanine), probably due to the absence of a bulky sugar unit in 9-MeG. In the case of (3), where the bulkiness of the substitution on the pyridine is further increased by a phenyl group, we observed a notable proximity between 9-MeGH8 and the phenyl ring of 2-phpy. Considering that only (3) exhibited good cytotoxicity against the A549 cancer cell line, it is suggested that auxiliary ligands, L, with an extended aromatic system and proper orientation in complexes of the type cis-[Pt(en)(L)Cl]NO₃, may enhance the cytotoxic activity of such complexes.