Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism.

Multi-responsive functional molecules are key for obtaining user-defined control of the properties and functions of chemical and biological systems. In this respect, pH-responsive photochromes, whose switching can be directed with light and acid-base equilibria, have emerged as highly attractive molecular units. The challenge in their design comes from the need to accommodate application-defined boundary conditions for both light- and protonation-responsivity. Here we combine time-resolved spectroscopic studies, on time scales ranging from femtoseconds to seconds, with density functional theory (DFT) calculations to elucidate and apply the acidochromism of a recently designed iminothioindoxyl (ITI) photoswitch. We show that protonation of the thermally stable Z isomer leads to a strong batochromically-shifted absorption band, allowing for fast isomerization to the metastable E isomer with light in the 500-600 nm region. Theoretical studies of the reaction mechanism reveal the crucial role of the acid-base equilibrium which controls the populations of the protonated and neutral forms of the E isomer. Since the former is thermally stable, while the latter re-isomerizes on a millisecond time scale, we are able to modulate the half-life of ITIs over three orders of magnitude by shifting this equilibrium. Finally, stable bidirectional switching of protonated ITI with green and red light is demonstrated with a half-life in the range of tens of seconds. Altogether, we designed a new type of multi-responsive molecular switch in which protonation red-shifts the activation wavelength by over 100 nm and enables efficient tuning of the half-life in the millisecond-second range.

Cancer-Associated Fibroblasts as Players in Cancer Development and Progression and Their Role in Targeted Radionuclide Imaging and Therapy.

Cancer Associated Fibroblasts (CAFs) form a major component of the tumour microenvironment, they have a complex origin and execute diverse functions in tumour development and progression. As such, CAFs constitute an attractive target for novel therapeutic interventions that will aid both diagnosis and treatment of various cancers. There are, however, a few limitations in reaching successful translation of CAF targeted interventions from bench to bedside. Several approaches targeting CAFs have been investigated so far and a few CAF-targeting tracers have successfully been developed and applied. This includes tracers targeting Fibroblast Activation Protein (FAP) on CAFs. A number of FAP-targeting tracers have shown great promise in the clinic. In this review, we summarize our current knowledge of the functional heterogeneity and biology of CAFs in cancer. Moreover, we highlight the latest developments towards theranostic applications that will help tumour characterization, radioligand therapy and staging in cancers with a distinct CAF population.

Structural Aspects of Photopharmacology: Insight into the Binding of Photoswitchable and Photocaged Inhibitors to the Glutamate Transporter Homologue.

Photopharmacology addresses the challenge of drug selectivity and side effects through creation of photoresponsive molecules activated with light with high spatiotemporal precision. This is achieved through incorporation of molecular photoswitches and photocages into the pharmacophore. However, the structural basis for the light-induced modulation of inhibitory potency in general is still missing, which poses a major design challenge for this emerging field of research. Here we solved crystal structures of the glutamate transporter homologue GltTk in complex with photoresponsive transport inhibitors-azobenzene derivative of TBOA (both in trans and cis configuration) and with the photocaged compound ONB-hydroxyaspartate. The essential role of glutamate transporters in the functioning of the central nervous system renders them potential therapeutic targets in the treatment of neurodegenerative diseases. The obtained structures provide a clear structural insight into the origins of photocontrol in photopharmacology and lay the foundation for application of photocontrolled ligands to study the transporter dynamics by using time-resolved X-ray crystallography.

Photoresponsive molecular tools for emerging applications of light in medicine.

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.

Light-controlled inhibition of BRAFV600E kinase.

Metastatic melanoma is amongst the most difficult types of cancer to treat, with current therapies mainly relying on the inhibition of the BRAFV600E mutant kinase. However, systemic inhibition of BRAF by small molecule drugs in cancer patients results - paradoxically - in increased wild-type BRAF activity in healthy tissue, causing side-effects and even the formation of new tumors. Here we show the development of BRAFV600E kinase inhibitors of which the activity can be switched on and off reversibly with light, offering the possibility to overcome problems of systemic drug activity by selectively activating the drug at the desired site of action. Based on a known inhibitor, eight photoswitchable effectors containing an azobenzene photoswitch were designed, synthesized and evaluated. The most promising inhibitor showed an approximately 10-fold increase in activity upon light-activation. This research offers inspiration for the development of therapies for metastatic melanoma in which tumor tissue is treated with an active BRAFV600E inhibitor with high spatial and temporal resolution, thus limiting the damage to other tissues.

Iminothioindoxyl as a molecular photoswitch with 100 nm band separation in the visible range.

Light is an exceptional external stimulus for establishing precise control over the properties and functions of chemical and biological systems, which is enabled through the use of molecular photoswitches. Ideal photoswitches are operated with visible light only, show large separation of absorption bands and are functional in various solvents including water, posing an unmet challenge. Here we show a class of fully-visible-light-operated molecular photoswitches, Iminothioindoxyls (ITIs) that meet these requirements. ITIs show a band separation of over 100 nm, isomerize on picosecond time scale and thermally relax on millisecond time scale. Using a combination of advanced spectroscopic and computational techniques, we provide the rationale for the switching behavior of ITIs and the influence of structural modifications and environment, including aqueous solution, on their photochemical properties. This research paves the way for the development of improved photo-controlled systems for a wide variety of applications that require fast responsive functions.

Reversible, Spatial and Temporal Control over Protein Activity Using Light.

In biomedical sciences, the function of a protein of interest is investigated by altering its net activity and assessing the consequences for the cell or organism. To change the activity of a protein, a wide variety of chemical and genetic tools have been developed. The drawback of most of these tools is that they do not allow for reversible, spatial and temporal control. Here, we describe selected developments in photopharmacology that aim at establishing such control over protein activity through bioactive molecules with photo-controlled potency. We also discuss why such control is desired and what challenges still need to be overcome for photopharmacology to reach its maturity as a chemical biology research tool.

Structure-kinetic relationship studies of cannabinoid CB2 receptor agonists reveal substituent-specific lipophilic effects on residence time.

A decade ago, the drug-target residence time model has been (re-)introduced, which describes the importance of binding kinetics of ligands on their protein targets. Since then, it has been applied successfully for multiple protein targets, including GPCRs, for the development of lead compounds with slow dissociation kinetics (i.e. long target residence time) to increase in vivo efficacy or with short residence time to prevent on-target associated side effects. To date, this model has not been applied in the design and pharmacological evaluation of novel selective ligands for the cannabinoid CB2 receptor (CB2R), a GPCR with therapeutic potential in the treatment of tissue injury and inflammatory diseases. Here, we have investigated the relationships between physicochemical properties, binding kinetics and functional activity in two different signal transduction pathways, G protein activation and ß-arrestin recruitment. We synthesized 24 analogues of 3-cyclopropyl-1-(4-(6-((1,1-dioxidothiomorpholino)methyl)-5-fluoropyridin-2-yl)benzyl)imidazoleidine-2,4-dione (LEI101), our previously reported in vivo active and CB2R-selective agonist, with varying basicity and lipophilicity. We identified a positive correlation between target residence time and functional potency due to an increase in lipophilicity on the alkyl substituents, which was not the case for the amine substituents. Basicity of the agonists did not show a relationship with affinity, residence time or functional activity. Our findings provide important insights about the effects of physicochemical properties of the specific substituents of this scaffold on the binding kinetics of agonists and their CB2R pharmacology. This work therefore shows how CB2R agonists can be designed to have optimal kinetic profiles, which could aid the lead optimization process in drug discovery for the study or treatment of inflammatory diseases.