Cyanine-Flavonol Hybrids for Near-Infrared Light-Activated Delivery of Carbon Monoxide

Abstract

Carbon monoxide (CO) is an endogenous signaling molecule that controls a number of physiological processes. To circumvent the inherent toxicity of CO, light-activated CO-releasing molecules (photoCORMs) have emerged as an alternative for its administration. However, their wider application requires photoactivation using biologically benign visible and near-infrared (NIR) light. In this work, a strategy to access such photoCORMs by fusing two CO-releasing flavonol moieties with a NIR-absorbing cyanine dye is presented. These hybrids liberate two molecules of CO in high chemical yields upon activation with NIR light up to 820 nm and exhibit excellent uncaging cross-sections, which surpass the state-of-the-art by two orders of magnitude. Furthermore, the biocompatibility and applicability of the system in vitro and in vivo are demonstrated, and a mechanism of CO release is proposed. It is hoped that this strategy will stimulate the discovery of new classes of photoCORMs and accelerate the translation of CO-based phototherapy into practice.

Keywords: CO release; cyanine; near-infrared light; photoCORM; photorelease.

Figure 1

(a) Flavonol‐based 1 (left) and BODIPY‐based 2 (right) photoCORMs. (b) The concept of fusing a flavonol photoCORM moiety and a heptamethine cyanine dye into the conjugated system proposed in this work. Structural elements suitable for further modifications are depicted. (c) Photochemical release of CO from cyanine‐flavonol hybrids 5 a–b prepared and studied herein. Trimethylammonium groups installed in 5 b facilitate solubility in aqueous media. Counter anions are omitted for clarity.

Scheme 1

Synthesis of the cyanine‐flavonol hybrids 5 a–b. Reaction conditions: (i) 3‐methyl‐2‐butanone, AcOH, 110 °C, 97 %; (ii) BBr₃, CH₂Cl₂, 0 °C to rt, 95 %; (iii) AcCl, Et₃N, CH₂Cl₂, rt, 59 %; (iv) AlCl₃, neat, 190 °C, 80 %; (v) NaOH, 30 % H₂O₂, MeOH, 0 °C. 11 a: 22 %. 11 b: 38 %; (vi) 12 a: MeOTf, CH₂Cl₂, 86 % or 12 b: 1) MeI, MeCN, 100 °C; 2) AgOTf, MeOH 81 %; (vii) enamine 13, AcONa, EtOH, reflux. 5 a: 55 %. 5 b: 51 %.

Figure 2

(a) UV/Vis absorption (solid) and emission (dotted) spectra of the hybrids 5 a (red) and 5 b (blue). (b) Irradiation of 5 b (c≈3.8×10⁻⁶  m) at 820 nm in aerated methanol followed by UV/Vis spectroscopy at 10 min intervals (from blue to red lines). The inset depicts a kinetic trace of the absorption at 793 nm superimposed with the liberation of CO in time (black). (c) Total chemical yields of CO produced in the dark or upon exhaustive irradiation of 5 a (red) or 5 b (blue) in methanol or PBS (pH 7.4, 10 mm, I=100 mm) at 770 or 820 nm, respectively. The error bars represent the standard deviation of the mean from four independent samples. (d) Time‐dependent CO release from 5 b (c≈4×10⁻⁶  m); blue line: irradiated at 770 nm in methanol; red line: irradiated at 820 nm in methanol; black line: irradiated at 820 nm in PBS. The CO released to the headspace was determined by GC and is expressed as the total chemical yield.

Figure 3

CO release from 5 b upon irradiation in vivo. A control group of eight animals (only vehicle application) is depicted in blue. Another group of six animals (red) received an intraperitoneal injection of 5 b (50 μmol kg⁻¹ of body weight) in vehicle (saline 10 μL g⁻¹, 5 % DMSO) and was irradiated with 780 nm light focused on the abdominal area for 2 h. The amounts of CO are expressed as the relative amounts of COHb in the total amount of Hb in the blood (w _COHb in %; left ordinate) or in pmol mg⁻¹ of fresh organ tissue (right ordinate). The horizontal lines represent the median, the boxes show the interquartile range. A statistically significant increase of CO was observed in the blood, liver, and heart tissues of the irradiated mice (* P value ≤0.05 vs. “control group”; n=6).

Scheme 2

The proposed mechanism of photochemical CO release from 5 a–b. (a) The major orthogonal photodecarbonylation pathways of model flavonols 1 and 14 under aerobic conditions. (b) Sequential photodecarbonylation from 5 a–b at both terminal flavonol units.