By Rich Soll, Senior Advisor, Strategic Initiatives, WuXi AppTec (@richsollwx)
Essential cellular processes, including proliferation, growth, survival, motility, autophagy as well as protein, lipid and nucleic acid synthesis, converge on a pathway called the mechanistic target of rapamycin (mTOR), which is widely regarded as the master regulator of growth and metabolism. mTOR kinase is the catalytic subunit of two distinct protein complexes, mTORC1 and mTORC2.
Although some functions of mTORC1 and mTORC2 overlap, the proteins have distinct roles: mTORC2 governs cell survival and cytoskeleton organization, whereas it is only recently appreciated that mTORC1 is a sensory hub for nutrients and energy utilization. mTORC1 dysregulation is thought to play a role in a number of diseases including depression fibrosis, diabetes, cancer and aging.
Navitor Pharmaceuticals is engaged in the discovery and development of selective mTORC1 modulators for treating a variety of conditions. Navitor’s Chief Research Officer, Eddine Saiah, shared the company’s mission, platform and pipeline with me.
WuXi: What is the significance of mTORC1 in human biology?
Saiah: Much of Navitor’s approach emanates from Navitor’s scientific founder, David Sabatini, of MIT’s biology department. Dr. Sabatini holds appointments as senior member of the Whitehead Institute, an Investigator at the Howard Hughes Medical Institute, member of the Koch Institute for Integrative Cancer Research, and is an American Cancer Society Research Professor.
Dr. Sabatini has been studying mTOR for over 25 years, since this pathway was first elucidated biochemically by him and others. His seminal work led to the findings that mTORC1 is a signaling hub that functions as a sensor for nutrients and for energy utilization from a host of stimuli, including growth factors, cytokines, and nutrients. As such, it regulates multiple cell functions such as proliferation, growth, autophagy and protein synthesis, as well as lipid and nucleic acid syntheses. In certain diseases such as depression, fibrosis, cancer, and diabetes, it has been shown that mTORC1 is dysregulated, meaning that the level of mTORC1 activity is too low or too high. Whereas mTORC1 controls key cell growth biosynthetic processes, and energy conservation, mTORC2 regulates cell survival.
Our goal is to modulate mTORC1 and not completely inhibit or hyperactivate this pathway. Our proprietary approach allows us to fine tune mTORC1 up or down, without impacting mTORC2. The drug rapamycin and related drugs called rapalogs, as well as mTOR kinase inhibitors, have been extensively characterized. These two classes of compounds lack selectivity for mTORC1 vs mTORC2, which leads to side effects attributable to mTORC2 inhibition.
WuXi: How then is mTORC1 regulated? Are there specific and selective mechanisms by which mTORC1 is activated or inhibited?
Saiah: When we started Navitor, we wanted to take a holistic approach to the modulation of this critical pathway. Historically, over-activation of the mTOR pathway contributed to the initiation and development of tumors across a broad range of cancers, including breast, prostate, lung, bladder, melanoma, brain and renal cancer.
Consistent with that view, we were able to inhibit mTORC1 by inhibiting the protein called Ras homolog enriched in the brain (RHEB), a GTPase that is known to activate mTORC1 signaling. In our Nature Communication paper in 2018, we showed that a compound we refer to as NR1 potently inhibits the mTORC1-driven phosphorylation of ribosomal protein S6 kinase, a downstream substrate of mTORC1, but did not inhibit AKT or ERK phosphorylation in contrast to the non-selective rapamycin upon prolonged treatment. The results were mirrored in PK/PD and functional in vivo studies. NR1 performed equally well to rapamycin and Torin-1 in the inhibition of protein synthesis. This was the first reported example of a small molecule inhibitor to RHEB, which in turn selectively inhibits mTORC1. Its identification, characterization and success was dependent on fragment screening, lots of structural biology and other biophysical techniques.
WuXi: What have we learned about mTORC1 activation?
Saiah: Over the last several years, we have gained a much deeper appreciation of the complexity by which mTORC1 serves as a sensory mechanism for cellular health. For example, protein production is regulated by the amount of amino acid available. Leucine is a key regulatory amino acid and can promote protein synthesis by regulating mTORC1 signaling in the following way: leucine binds to sestrin 1 and 2. This complex then binds to the regulatory multiprotein GATOR2, causing dissociation of GATOR2 from mTORC1 and leading to activation of mTORC1. Crystallography confirmed leucine binding to sestrin.
Analogously, mTORC1 can sense the amino acid arginine. Dr. Sabatini has shown that arginine disrupts the CASTOR1-GATOR2 complex by binding to CASTOR1. Beyond amino acids, SAMTOR was found to be an S-adenosylmethionine sensor for the mTORC1 pathway. In fact, more than 26 multiprotein complexes have been discovered to date, some of which have yet to be characterized.
WuXi: What diseases might be treated with an mTORC1 activator?
Saiah: There is evidence suggesting that mTORC1 activity is low in patients with depression, in Huntington’s disease and in patients suffering from cognitive decline. Treatment-resistant depression (TRD) is a subset of major depressive disorder (MDD) that refers to depressive episodes that are not adequately controlled by standard antidepressant therapy. Major depressive disorders affect 17% of the population and is one of the leading causes of disability worldwide.
Several studies including a postmortem analysis of healthy and severely depressed patients, as well as multiple pre-clinical studies have suggested an association between the activity of mTORC1 pathway signaling and depression. Standard antidepressant therapies, such as selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) are only modestly effective and may take weeks to produce therapeutic effects, and have low rates of full remission.
Newer drugs that modulate the presynaptic glutamate N-methyl-D-aspartic acid (NMDA) receptor, have demonstrated the potential for improved efficacy, with a rapid onset of antidepressant effects (days as opposed to weeks). Today there are several NMDA modulators in clinical development for depression, as well as Esketamine, which was recently approved by the FDA for the treatment of depression. Unfortunately, presynaptic NMDA receptor modulation can potentially cause side effects including dissociation (hallucination) and has abuse potential.
Although ketamine’s precise mechanism of action is not well understood, it has been shown that both mTORC1 signaling and increased synapse formation occurs in a portion of the brain known as the medial prefrontal cortex, an effect reversed by rapamycin.
We demonstrated that our small molecule mTORC1 activator NV-5138 was active in multiple pre-clinical models of depression. In order to achieve that, we needed to demonstrate the presence of sestrin1 and sestrin2 in neurons, we needed to design an analog that could fit the leucine binding pocket in sestrin, we needed a drug that would penetrate the brain as we were interested in brain disorders, we needed the drug to be stable to an enzyme known as Branched Chain Amino Transaminase (BCAT), which is responsible for leucine metabolism and we needed to make sure the drug was not a substrate in protein synthesis.
We discussed the discovery of NV-5138 in a recent Scientific Reports journal article. The compound met the criteria we were aiming for. Instrumental to the success was protein crystallography to confirm the binding mode and to inspire new designs and deep in house mTORC1 biology expertise. We further showed in PK/PD studies that this orally bioavailable drug, dose dependently, increased phospho-S240/244pS6 downstream of mTORC1 and saw synaptic protein synthesis and long-lived spine formation after a single dose of the drug. We then disclosed in a recent Journal of Clinical Investigation paper rapid synaptic changes in rodent mPFC and antidepressant behavioral responses similar to ketamine, but without NMDA modulation.
In essence, we have provided evidence for a key role of mTORC1 signaling in the CNS and as a potential novel target for antidepressant drug-development.
WuXi: How do your external partners help?
Saiah: As you pointed out, being a small biotech company, we extensively leverage external resources to complement our own internal resources. We have a lab here in Cambridge MA, but we also work with a large number of external partners in the U.S., Canada, Europe and China.
WuXi has been a fantastic partner for us. I would like specifically to single out the WuXi sites here in the U.S., New Jersey in particular, because that’s really opened new possibilities for us to do some very specific bioanalytical work on the PK/PD front that requires looking at tissues, whether it’s brain or kidneys and so forth.
It requires extremely sensitive quantification and also requires sharing tissues and other material. To minimize the risk of samples being delayed due to shipment or customs, we used the expertise of the New Jersey site. The WuXi team has been very responsive to our needs and delivered very high quality work to support our programs.
WuXi: Spectacular results! Where is the drug now?
Saiah: The drug is now in clinical studies for the treatment of major depressive disorder (MDD) with an initial focus on treatment-resistant depression (TRD).
WuXi: As a community, we are particularly indebted to Dr. Sabatini’s scientific efforts and very appreciative of Navitor’s translation of this very complex biology into differentiated products with a wide spectra of applications. What next?
Saiah: We are working on a class of agents we call Navalogs, which are selective mTORC1 inhibitors, for which we hope to be in the clinic by 2020. We are exploring the best opportunities for these compounds where dysregulated mTORC1 may be treated therapeutically, in kidney disease for example.
In addition, we continue to expand the potential of our mTORC1 activators by testing the compounds in preclinical models of cognition and Huntington’s disease.
When we started Navitor, depression was not necessarily an indication at the forefront of the list of indications we were looking at. But the way this work evolved, the growing understanding of the link between nutrients sensing and mTORC1, is really what led us down the path to pursuing depression. For our inhibitor program, our increased knowledge of mTORC1 is leading us to evaluate the potential for a truly selective mTORC1 inhibitor in the setting of chronic disease for the first time.
When you talk about mTORC1 most people think of oncology, but we’re thinking about chronic indications beyond cancer. So I personally believe this is potentially transformational work that combines very elegant and novel biology, creative problem solving and a very focused translational plan.