Photoinduced reduction of Pt(IV) within an anti-proliferative Pt(IV)-texaphyrin conjugate

Abstract

In an effort to increase the stability and control the platinum reactivity of platinum-texaphyrin conjugates, two Pt(IV) conjugates were designed, synthesized, and studied for their ability to form DNA adducts. They were also tested for their anti-proliferative effects using wild-type and platinum-resistant human ovarian cancer cell lines (A2780 and 2780CP, respectively). In comparison to an analogous first-generation Pt(II) chimera, one of the new conjugates provided increased stability in aqueous environments. Using a combination of (1) H NMR spectroscopy and FAAS (flameless atomic-absorption spectrometry), it was found that the Pt(IV) center within this conjugate undergoes photoinduced reduction to Pt(II) upon exposure to glass-filtered daylight, resulting in an entity that binds DNA in a controlled manner. Under conditions in which the Pt(IV) complex is reduced to the corresponding Pt(II) species, these new conjugates demonstrated potent anti-proliferative activity in both test ovarian cancer cell lines.

Keywords: conjugates; drug design; photo-activation; platinum; texaphyrin.

Figure 1

Partial RP-HPLC chromatograms (top) and ¹H-NMR spectra (bottom, D₂O, pH = 7, 300 K, 400 MHz) of complex 3 recorded in the presence of 5′-GMP (2 equiv.) before (A) and after being exposed to laboratory light for 2 days (B); (C) cisplatin incubated in presence of 5′-GMP (2 equiv.) for 10 h at 37°C; circle = free 5′-GMP; triangle = Pt^II-coordinated 5′-GMP; black square = Pt^IV-coordinated succinate anion; square = free succinate dianion.

Figure 2

(A) Schematic representation of the photo-activation of 4 followed by formation of a DNA-Pt adduct. Note: The star represents the texaphyrin moiety; (B) stability (RP-HPLC analysis) of 4 in PBS solution in the dark (black circles) or exposed to laboratory light (black squares), The designation 1 indicates the hydrolysis phase (absence of light), while the 2 indicates the phase corresponding to light exposure; (C) quantification by FAAS (Pt) and by nanodrop (DNA) of the number of Pt-DNA adducts formed after reaction of 4 with DNA in the dark (black circles) or exposed to natural light (black squares).

Scheme 1

Structures of compounds considered in this study.

Scheme 2

Synthesis of the platinum(IV)-texaphyrin conjugates 4 and 5. R1 = (OCH2CH2)3OCH3. Conditions: i = 4,4′-dimethoxytriphenylmethylchloride (DMT-Cl), diisopropylethylamine (DIPEA) in dichloromethane, 30%; ii = triphenylphosphine, phthalimide, diisopropylazocaboxylate in dichloromethane, methylamine in methanol/acetonitrile, 25%; iii = N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC•HCl), N-hydroxysuccinimide (NHS) and 3 in water. Yields were 40% (4) and 20% (5).