Towards medical imaging of drug photoactivation: Development of light responsive magnetic resonance imaging and chemical exchange saturation transfer contrast agents

Abstract

In recent years, the use of light to selectively and precisely activate drugs has been developed along the fundamental concepts of photopharmacology. One of the key methods in this field relies on transiently silencing the drug activity with photocleavable protecting groups (PPGs). To effectively utilize light-activated drugs in future medical applications, physicians will require a reliable method to assess whether light penetrates deep enough into the tissues to activate the photoresponsive theragnostic agents. Here, we describe the development and evaluation of magnetic resonance (MR) imaging agents that allow for the detection of light penetration and drug activation in the tissues using non-invasive whole-body magnetic resonance imaging (MRI) and chemical exchange saturation transfer (CEST)-MRI modalities. The approach relies on the use of PPG-protected MR contrast agents, which upon irradiation with light change their imaging signal. A Gadolinium(III)-based MRI contrast agent is presented that undergoes a significant change in relaxivity (25%) upon uncaging, providing a reliable indicator of light-induced cargo release. Additionally, we introduce the first light-responsive CEST-MRI imaging agent, enabling positive signal enhancement (off-to-on) upon light activation, offering a novel approach to visualize the activation of photoactive agents in living tissues. This research provides a proof-of-principle for the non-invasive, whole-body imaging of light penetration and drug activation with high temporal resolution characteristic of MR methods.

Keywords: CEST; MRI; photochemistry; photocleavable protecting groups; photopharmacology.

FIGURE 1

Schematic representation of the functioning of light responsive CAs for imaging the light penetration and activation of a model photocaged drug. (a) Upon irradiation with light, the light responsive CAs creates a change in the MR imaging signal. (b) Schematic explanation of activatable MRI CA design and the synthesized molecules 1‐Gd and 3‐Gd: upon irradiation with light, the photocleavable protecting group is removed and the newly formed contrast agents 2‐Gd and 4‐Gd show a difference in relaxivity. (c) Schematic explanation of the CEST‐MRI working principle and the designed molecules 1‐Yb and 3‐Yb, upon irradiation the photocleavable protecting group is removed and the newly formed contrast agents 2‐Yb and 4‐Yb have protons to participate in CEST. (d) The outline of the CA structures presented in this work with different R‐groups and PPGs attached.

SCHEME 1

Synthetic route toward compounds 1‐Gd and 1‐Yb.

SCHEME 2

Synthetic route towards compound 3‐Yb.

FIGURE 2

Photochemical evaluation of the uncaging process of compounds 1‐Yb, 1‐Gd and 3‐Yb, with the overall spectrum shown in the left and an expanded spectrum, highlighting the isosbestic points and most pronounced signal change, shown on the right side of each panel. (a) UV‐Vis absorption spectra of 1‐Yb (water, 100 μM, 25°C, pH 7), freshly prepared solutions, and solutions after irradiation (λ = 365 nm) for the times indicated in figure. (b) UV‐Vis absorption spectra of 1‐Gd (water, 100 μM, 25°C, pH 7), freshly prepared solutions, and solutions after irradiation (λ = 365 nm) for the times indicated in figure. (c) UV‐Vis absorption spectra of 3‐Yb (water, 20 μM, 25°C, pH 7), freshly prepared solutions, and solutions after irradiation (λ = 390 nm) for the times indicated in figure. For an expanded spectrum, see Figures S16–S18.

FIGURE 3

NMRD relaxometric analysis of 1‐Gd (1.0 mM) in water at 37°C, pH 7.4. Error bars represent the uncertainty of fitting the T ₁ curve to the experimental data. (a) The activation of 1‐Gd with irradiation of 365 nm light towards 2‐Gd. (b) NMRD profile of 1‐Gd at t = 0 and after 3 days in the dark (c) NMRD profiles of 1‐Gd upon irradiation of the sample with light (λ = 365 nm). (d) The decrease in the relaxivity of 1‐Gd at 10 MHz in response to irradiation with light (λ = 365 nm) for the indicated times. (e) The molar relaxivity of 1‐Gd with and without light irradiation (λ = 365 nm) at time points t = 0 min and t = 192 min, at 4.7 T.

FIGURE 4

The NMR‐Z profiles for 1‐Yb and 3‐Yb. (a) Z‐spectra of the solutions of 1‐Yb at t = 0 and t = 3 h incubation at 37°C in the dark (50 mM in water with 10% D₂O, pH 7.40, B0 = 11.7 T, satpwr = 28 dB, satdly = 2 s). (b) Z‐spectra of the solution of 1‐Yb at t = 0 min, and after irradiation (λ = 365 nm) for t = 30, 60, 120 and 180 min at 37°C (50 mM in water with 10% D₂O, pH 7.40, B0 = 11.7 T, satpwr = 28 dB, satdly = 2 s). (c) Z‐spectra of the solutions of 3‐Yb at t = 0 and t = 2 h at 37°C in the dark (30 mM in water with 10% D₂O, pH 7.40, B0 = 11.7 T, satpwr = 28 dB, satdly = 2 s). (d) Z‐spectra of the solution of 3‐Yb at t = 0 s, and after irradiation (λ = 400 nm) t = 60, 120, 180, 360 and 720 s at 37°C (30 mM in water with 10% D₂O, pH 7.40, B0 = 11.7 T, satpwr = 28 dB, satdly = 2 s). The complete spectra are presented in the Supporting Information file (Figures S6–S7 for 1‐Yb and Figures S10 and S11 for 3‐Yb).