Design and validation of bioorthogonal probes for cellular disease models

Well-validated chemical probes enable testing of biological hypotheses, investigation of target tractability and translatability to the clinical phase. Consequently, these important tool compounds play a key role in the drug discovery process. Moreover, their use for target validation may ultimately help to decrease the attrition rate encountered by new molecular entities in clinical trials. Despite the number of publications describing well-validated chemical probes, for several reasons, it remains a difficult challenge to identify and select the appropriate high quality molecule enabling the desired biological and pharmacological studies in the relevant disease model. Various reports describe sets of principles to be fulfilled by high quality chemical probes, which mainly rely on verifying that chemical probes are potent, engage their intended targets in the relevant cellular model, have sufficient exposure at the desired site of action and express functional pharmacological activities by a selective modulation of their targets. Classical chemical probes include LMW ligands, which inform on the functional consequences of interacting with a particular bio-logical target in a model system. To get information on other parameters (such as sub-cellular distribution of target or compound) different tools may need to be used, often requiring specific chemical functionalization in order to observe the molecule using the currently available technologies. Compared to classical chemical probes, bioorthogonal probes can also be small molecules that elicit a functional response but with the additional advantage of being able to undergo reaction with a variety of chemical reporters (e.g. fluorophores) in situ, in bio-logical model systems such as cells and animals. Therefore, this increases their versatility; for example their visualization using imaging technologies (bioluminescence and fluorescence imaging) can provide important insights on compound permeability, in-tracellular distribution, and potentially co-localization with its protein target in relevant cellular disease models. Hence, the use of a bespoke bioorthogonal probe may answer key biological questions, which are usually only addressed by using multiple classical probes. Herein, we describe the design and the synthesis of bioorthogonal probes to study the inhibition of the p53-Mdm2 protein-protein interaction. Furthermore, we report the development of a set of assays based on fluorescence and bioluminescence techniques enabling the validation of these molecules in a cellular osteosarcoma model. Our approach for chemical probe design and validation in cellular disease models may be applied in principle to a wide range of novel drug discovery projects to provide early mechanistic understanding.