Guillaume Lessene

Transitioning a candidate therapeutic target from bench to bedside requires significant time and financial investment, yet clinical success remains low often due to poor on-target toxicity assessment during preclinical validation. Tag-degraders provide a tool to improve target validation by enabling degradation of any protein of interest via a degron-tag. This drug-based, reversible, and dose-dependent method of protein removal can mimic degrader-based drug treatments and assess the implications of target protein depletion in vivo. However, each degrader has a distinct pharmacokinetic profile that will influence its effectiveness across tissues. To create a resource to enable the most appropriate choice of tag-degrader, we benchmark the dTAG, HaloPROTAC, and NanoTAC systems in vivo by employing a transgenic mouse expressing a reporter protein targetable by these tag-degraders. Through various treatment regimes, we characterise each degrader profile across a panel of 20 tissues and organs, highlighting the superior degradation by dTAG molecules, and identify differences between degradation in whole tissues versus single cell populations. Using an FKBP^F36V knock-in mouse expressing 65K-FKBP^F36V, we reveal target specific degradation kinetics, and a critical requirement for 65K in mice. Together, this resource will assist researchers in choosing the right degrader and tag for their own applications.

BH3 mimetic drugs that selectively target the pro-survival BCL2 proteins are highly promising for cancer treatment, most notably for treating blood cancers. Venetoclax, which inhibits BCL2, is now approved for treating chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML). Preferably, robust and validated assays would identify patients most likely to benefit from therapy with venetoclax itself or with inhibitors of other pro-survival proteins. A sophisticated method that has been developed is the BH3 profiling assay. In this assay, permeabilized, instead of intact, cells are treated for a few hours with inhibitors of the pro-survival BCL2 proteins, and the resultant mitochondrial depolarization measured. Sensitivity to a specific inhibitor (e.g., venetoclax or other BH3 mimetics) is then used to infer the reliance of a tumor (e.g., CLL) on one or more pro-survival BCL2 proteins. However, we found that this methodology cannot reliably identify such dependencies. In part, this is because almost all cells express multiple pro-survival BCL2 proteins that restrain BAX and BAK which must be inhibited before mitochondrial depolarization and apoptosis can proceed. Using genetic and pharmacological tools across multiple cell line models of blood cancer, we demonstrated that selective BCL2 inhibitors have important flow-on effects that includes the redistribution of BH3-only proteins to ancillary pro-survival proteins not directly engaged by the inhibitor. These secondary effects, critical to the biological action of selective inhibitors, were not accurately recapitulated in permeabilized cells, probably due to the limited time frame possible in such assays or the altered biophysical conditions when cells are permeabilized. While we could consistently define the sensitivity of a tumor cell to a particular BH3 mimetic drugs using intact cells, this was not reliable with permeabilized cells. These studies emphasize the need to carefully evaluate assays on permeabilized cells undertaken with inhibitors of the pro-survival BCL2 proteins.

The COVID-19 pandemic caused by the coronavirus SARS-CoV-2 has highlighted the vulnerability of a globally connected population to zoonotic viruses. The FDA-approved coronavirus antiviral Paxlovid targets the essential SARS-CoV-2 main protease, Mpro. Whilst effective in the acute phase of a COVID infection, Paxlovid cannot be used by all patients, can lead to viral recurrence, and does not protect against post-acute sequelae of COVID-19 (PASC), commonly known as long COVID, an emerging significant health burden that remains poorly understood and untreated. Alternative antivirals that are addressing broader patient needs are urgently required. We here report our drug discovery efforts to target PLpro, a further essential coronaviral protease, for which we report a novel chemical scaffold that targets SARS-CoV-2 PLpro with low nanomolar activity, and which exhibits activity against PLpro of other pathogenic coronaviruses. Our lead compound shows excellent in vivo efficacy in a mouse model of severe acute disease. Importantly, our mouse model recapitulates long-term pathologies matching closely those seen in PASC patients. Our lead compound offers protection against a range of PASC symptoms in this model, prevents lung pathology and reduces brain dysfunction. This provides proof-of-principle that PLpro inhibition may have clinical relevance for PASC prevention and treatment going forward.

Defective apoptosis mediated by B cell lymphoma 2 antagonist/killer (BAK) or B cell lymphoma 2–associated X protein (BAX) underlies various pathologies including autoimmune and degenerative conditions. On mitochondria, voltage-dependent anion channel 2 (VDAC2) interacts with BAK and BAX through a common interface to inhibit BAK or to facilitate BAX apoptotic activity. We identified a small molecule (WEHI-3773) that inhibits interaction between VDAC2 and BAK or BAX revealing contrasting effects on their apoptotic activity. WEHI-3773 inhibits apoptosis mediated by BAX by blocking VDAC2-mediated BAX recruitment to mitochondria. Conversely, WEHI-3773 promotes BAK-mediated apoptosis by limiting inhibitory sequestration by VDAC2. In cells expressing both pro-apoptotic proteins, apoptosis promotion by WEHI-3773 dominates, because activated BAK activates BAX through a feed-forward mechanism. Loss of BAX drives resistance to the BCL-2 inhibitor venetoclax in some leukemias. WEHI-3773 overcomes this resistance by promoting BAK-mediated killing. This work highlights the coordination of BAX and BAK apoptotic activity through interaction with VDAC2 that may be targeted therapeutically.

The COVID-19 pandemic caused by the coronavirus SARS-CoV-2 has highlighted the vulnerability of a globally connected population to zoonotic viruses. The only FDA-approved coronavirus antiviral, Paxlovid, targets the essential SARS-CoV-2 main protease, Mpro. Whilst effective in the acute phase of an infection, Paxlovid cannot be used by all patients, can lead to viral recurrence, and does not protect against post-acute sequelae of COVID-19 (PASC), commonly known as Long COVID, an emerging significant health burden that remains poorly understood and untreated. Alternative antivirals that are addressing broader patient needs are urgently required. We here report our drug discovery efforts to target PLpro, a further essential coronaviral protease, for which we report a novel chemical scaffold that targets SARS-CoV-2 PLpro with low nanomolar activity, and which exhibits activity against PLpro of other pathogenic coronaviruses. Our lead compound shows excellent in vivo efficacy in a mouse model of severe acute disease. Importantly, our mouse model recapitulates long-term pathologies matching closely those seen in PASC patients, including lung, heart, gut and brain dysfunction. Our lead compound offers protection against PASC in this model, prevents lung pathology and reduces brain dysfunction, providing a potential treatment option for PASC sufferers going forward.

Defective apoptosis mediated by BAK or BAX underlies various human pathologies including autoimmune and degenerative conditions. The mitochondrial channel protein VDAC2 interacts with BAK and BAX through a common interface to either inhibit BAK or to facilitate BAX apoptotic activity. Using a newly developed small molecule (WEHI-3773) that inhibits the interaction between VDAC2 and BAK or BAX, we reveal contrasting effects on BAX and BAK apoptotic activity. WEHI-3773 inhibits apoptosis mediated by BAX by blocking VDAC2-mediated BAX recruitment to mitochondria. Conversely, WEHI-3773 primes BAK for apoptosis by impairing its inhibitory sequestration by VDAC2 on the mitochondrial membrane. In cells expressing both BAX and BAK, repressing their association with VDAC2 promotes apoptosis, because once BAK is activated, it further activates BAX through a feed-forward mechanism. In some leukemias, mutation or loss of BAX is a key driver of resistance to the BH3-mimetic anti-cancer drug venetoclax. Strikingly, promoting BAK-mediated killing by small molecule dissociation of the VDAC2 interaction can overcome this resistance in different leukemia models. These data reveal a hitherto unappreciated level of coordination of BAX and BAK apoptotic activity through their interaction with VDAC2 that may be targeted therapeutically.

Papain-like protease (PLpro) is an attractive drug target for SARS-CoV-2 because it is essential for viral replication, cleaving viral poly-proteins pp1a and pp1ab, and has de-ubiquitylation and de-ISGylation activities, affecting innate immune responses. We employ Deep Mutational Scanning to evaluate the mutational effects on PLpro enzymatic activity and protein stability in mammalian cells. We confirm features of the active site and identify mutations in neighboring residues that alter activity. We characterize residues responsible for substrate binding and demonstrate that although residues in the blocking loop are remarkably tolerant to mutation, blocking loop flexibility is important for function. We additionally find a connected network of mutations affecting activity that extends far from the active site. We leverage our library to identify drug-escape variants to a common PLpro inhibitor scaffold and predict that plasticity in both the S4 pocket and blocking loop sequence should be considered during the drug design process.

SARS-CoV-2, the causative agent of COVID-19, is responsible for the recent global pandemic and remains a major source of mortality. Papain-like protease (PLpro) is a target for SARS-CoV-2 inhibitor development, as it is not only essential for viral replication through cleavage of the viral poly-proteins pp1a and pp1ab, but also has de-ubiquitylation and de-ISGylation activities, which can affect innate immune responses. To understand the features of PLpro that dictate activity and anticipate how emerging PLpro variants will affect function, we employed Deep Mutational Scanning to evaluate the mutational effects on enzymatic activity and protein stability in mammalian cells. We confirm features of the active site and identify all mutations in neighboring residues that support or ablate activity. We characterize residues responsible for substrate binding and demonstrate that although the blocking loop is remarkably tolerant to nearly all mutations, its flexibility is important for enzymatic function. We additionally find a connected network of mutations affecting function but not structure that extends far from the active site. Using our DMS libraries we were able to identify drug-escape variants to a common PLpro inhibitor scaffold and predict that plasticity in both the S4 pocket and blocking loop sequence should be considered during the drug design process.

BAK and BAX execute intrinsic apoptosis by permeabilising the mitochondrial outer membrane. Their activity is regulated through interactions with pro-survival BCL-2 family proteins and with non-BCL-2 proteins including the mitochondrial porin VDAC2. VDAC2 is important for bringing both BAK and BAX to mitochondria where they execute their apoptotic function. Despite this important function in apoptosis, whilst interactions with pro-survival family members are well characterised and have culminated in the development of drugs that target these interfaces to induce cancer cell apoptosis, the interaction between BAK and VDAC2 remains largely undefined. Deep scanning mutagenesis coupled with cysteine linkage identified key residues in the interaction between BAK and VDAC2. Obstructive labelling of specific residues in the BH3 domain or hydrophobic groove of BAK disrupted this interaction. Conversely, mutating specific residues in a cytosol-exposed region of VDAC2 stabilised the interaction with BAK, and inhibited BAK apoptotic activity. Thus, this VDAC2–BAK interaction site can potentially be targeted to either inhibit BAK-mediated apoptosis in scenarios where excessive apoptosis contributes to disease, or to promote BAK-mediated apoptosis for cancer therapy.

Necroptosis is a mode of programmed, lytic cell death that is executed by the mixed lineage kinase domain-like (MLKL) pseudokinase following activation by the upstream kinases, receptor-interacting serine/threonine protein kinase (RIPK)-1 and RIPK3. Dysregulated necroptosis has been implicated in the pathophysiology of many human diseases, including inflammatory and degenerative conditions, infectious diseases and cancers, provoking interest in pharmacological targeting of the pathway. To identify small molecules impacting on the necroptotic machinery, we performed a phenotypic screen using a mouse cell line expressing an MLKL mutant that kills cells in the absence of upstream death or pathogen detector receptor activation. This screen identified the vascular endothelial growth factor receptor (VEGFR) and platelet-derived growth factor receptor (PDGFR) tyrosine kinase inhibitor, ABT-869 (Linifanib), as a small molecule inhibitor of necroptosis. We applied a suite of cellular, biochemical and biophysical analyses to pinpoint the apical necroptotic kinase, RIPK1, as the target of ABT-869 inhibition. Our study adds to the repertoire of established protein kinase inhibitors that additionally target RIPK1 and raises the prospect that serendipitous targeting of necroptosis signalling may contribute to their clinical efficacy in some settings.