Photoresponsive molecular tools for emerging applications of light in medicine

Abstract

Light-based therapeutic and imaging modalities, which emerge in clinical applications, rely on molecular tools, such as photocleavable protecting groups and photoswitches that respond to photonic stimulus and translate it into a biological effect. However, optimisation of their key parameters (activation wavelength, band separation, fatigue resistance and half-life) is necessary to enable application in the medical field. In this perspective, we describe the applications scenarios that can be envisioned in clinical practice and then we use those scenarios to explain the necessary properties that the photoresponsive tools used to control biological function should possess, highlighted by examples from medical imaging, drug delivery and photopharmacology. We then present how the (photo)chemical parameters are currently being optimized and an outlook is given on pharmacological aspects (toxicity, solubility, and stability) of light-responsive molecules. With these interdisciplinary insights, we aim to inspire the future directions for the development of photocontrolled tools that will empower clinical applications of light.

Fig. 1. Overview of the photoresponsive molecules for emerging biomedical applications. In blue is shown the metastable state (photoswitches) or the uncaged product (photocleavable protecting groups). In orange is shown the stable state (photoswitches) or the photocleavable protecting groups (PPG).

Fig. 2. Key properties of light-responsive tools for biomedical applications: molecular photoswitches (a and b) and photocleavable protecting groups (PPGs, c). (a) Idealized UV-Vis spectra of the stable (S) and metastable (MS) isomers of a molecular photoswitch, showing the band separation that enables selective addressing of isomer with light of appropriate wavelengths (λ₁, λ₂). (b) Stages of light-induced isomerisation of a molecular photoswitches. (1) Switching from the stable to metastable state using light of λ₁ wavelength. (2) Switching from the metastable to stable state using light of λ₂ wavelength. (3) In the dark, the system relaxes to the thermal equilibrium. (c) Uncaging of PPG-modified compound is a first-order process, which proceeds with a kinetic constant that depends on the light intensity I, molar attenuation coefficient of the PPG at the irradiation wavelength ε and quantum yield for the uncaging process ϕ.

Fig. 3. Therapeutic scenarios, for the use of reversibly (1–4) and irreversibly (5) photocontrolled systems for medical application, concerning the situations when the compound is to be activated outside (1) or inside (2–5) of the body, and taking into consideration if the stable (3) or metastable (1, 2 and 4) form of the switch is more active in the biological system. The intensity of colour in the background of the graphs represents the increasing biological activity that can be explored with changing the concentration of the metastable isomer (blue line). See Section 2.1 in the text for the explanation of the abbreviations used.

Fig. 4. Azobenzene-derived photoswitchable antibiotic 1. (a) Compound trans-1 is a weakly potent antibiotic, which can be isomerized to its more active cis-1 form using UV irradiation. (b) The cis form has a half-life of ∼2 h, and isomerizes back to the trans form, in a first order process characterised by the recovery of the absorbance band at 368 nm. (c) Freshly activated antibiotic 1 prevents the bacterial growth, as indicated by the lack of increase of optical density in bacterial culture in time. Increasing the time between activation and addition to bacterial culture (0–3 h) results in partial thermal re-isomerisation to the less active trans-1 isomer and is accompanied by the drop in potency to inhibit bacterial growth. After 3 h, the sample loses its potency and behaves the same as the negative control. Reproduced from ref. 21 with permission from Springer Nature, Copyright 2013.

Fig. 5. Key photoswitch parameters for scenario 2. (a) Light- and temperature-induced changes in the content of the metastable, biologically active isomer (ideal situation). In panels (b–d), consequences of insufficient photoswitching cross section (panel b), PSD(MS) (panel c) and thermal relaxation rate (panel d) are shown.

Fig. 6. (a) Azosulfonylurea 2 used for the control of pancreatic beta cell function. (b) The UV-spectrum of compound 2 in the dark (black line) and under irradiation with 520 nm light (green line) (c) cycles of photoswitching between cis- and trans-2, with in green irradiation with 520 nm and in black under dark conditions. (d) Rodent islets treated with compound 2, under dark or under light (560 nm) conditions, the response is increasing of insulin secretion. No difference was obtained between dark conditions and the control (Con, 5 mM glucose-alone). Reproduced from ref. 25 with permission from The Royal Society of Chemistry, Copyright 2015.

Fig. 7. (a) Photoswitchable blocking of voltage-gated potassium channels by compound 3, which shows higher potency in the trans state, as an example of compounds useful in scenario 3. Reproduced from ref. 31 with permission from Wiley-VCH, Copyright 2009. (b) “Sign inversion” approach used when the more potent photoisomer (in red) is showing higher thermal stability. (c) The potassium channel blocker/opener diazocine 4 designed through “sign inversion” approach by Trads et al.

Fig. 8. Key photoswitch parameters for scenario 4. (a) Light- and temperature-induced changes in the content of the metastable, biologically active isomer (ideal situation). In panels (b–e), consequences of insufficient forward photoswitching cross section (panel b), PSD(MS) (panel c), PSD(S) (panel d) and insufficient backward photoswitching cross section (panel e) are shown.

Fig. 9. The photoresponsive MRI contrast agent 5. In the trans isomer shows no MRI contrast, while upon isomerization with λ = 505 nm to the cis isomer the contrast is switched on. This process can be reversed by irradiation with λ = 435 nm light. Reproduced from ref. 37 with permission from PCCP Owner Societies, Copyright 2019.

Fig. 10. Examples of application of scenario 5 in drug activation, delivery and medical imaging. (a) PPG-protected proto-oncogene tyrosine-protein kinase (RET) inhibitor 6, which prevents motoneuron extension and axonal pathfinding during development in zebrafish. (b) Quantification of axonal phenotypes in the different treatments with compound c6 (caged-6) and 6 in different concentrations and with different post fertilization (hpf) incubation times (n = number of axonal processes quantified). Reproduced from ref. 39 with permission from Springer Nature, Copyright 2015. (c) BODIPY-protected dopamine derivative activation for controlling heart rhythm. (d) Light-responsive supramolecular valves for photocontrolled drug release from mesoporous nanoparticles. Reproduced from ref. 42 with permission from the American Chemical Society, Copyright 2016. (e) Supramolecular block copolymers as novel UV and NIR responsive nanocarriers based on a photolabile coumarin unit. Reproduced with permission from ref. 44 from Elsevier, Copyright 2020. (f) A light-responsive liposomal agent for MRI contrast enhancement and monitoring of cargo delivery, showing the increase of fluorescence intensity upon light-induced release of model cargo. Reproduced from ref. 45 with permission from The Royal Chemical Society, Copyright 2019. (g) Left: furyl-substituted DAE undergoes retro-Diels–Alder upon ring opening under irradiation with λ = 400 nm light, resulting in a release gradual of compound 13. Right: the gradual release rate of maleimide (13) can be reversibly activated by switching compound 11 to its open form with λ = 400 nm light and back to the closed form with λ = 313 nm light. Reproduced from ref. 43 with permission from Wiley-VCH, Copyright 2015.

Fig. 11. Extension of the π-conjugation as a method to red-shift the activation wavelength of molecular photoswitches (a and c) and photocages (b).

Fig. 12. Introduction of electron withdrawing and donating substituents in conjugated positions in photoswitches to create a “push–pull” system and extend the absorption spectra towards the red/NIR region in (a) hemithioindigos and azo-BF₂ switches; (b) azobenzenes and diarylethenes.

Fig. 13. Overview of photoresponsive molecules discussed in the engineering of photostationary state distributions (PSD).

Fig. 14. Overview of the photoresponsive molecules to discussed for engineering the half-life.

Fig. 15. Overview of discussed photoswitches for the stability in water and in the cellular environment.

Fig. 16. Overview of discussed PPGs for the stability in water and in the cellular environment. For compound 95, the accompanying figure is reproduced from ref. 87 with permission from The Royal Society of Chemistry, Copyright 2016.

Fig. 17. Overview of the photoresponsive molecules discussed in the context of toxicity.

Fig. 18. Overview of photoresponsive molecules discussed in the context of controlling aqueous solubility. (a) Introduction of PEG chains to improve solubility; reprinted from ref. 36 with permission from American Chemical Society, Copyright 2015. (b) Introduction of a solubilizing group to improve solubility. (c–d) Controlling biological distribution; (e) Modulating biological activity through solubility. (f) Drug release through light-induced increase in polarity; reprinted from ref. 102 with permission from American Chemical Society, Copyright 2014.

Ilse M. Welleman

Mark W. H. Hoorens

Ben L. Feringa

Hendrikus H. Boersma

Wiktor Szymański