[intro music]
Host – Dan Keller
Hello, and welcome to Episode Fifty-One of Multiple Sclerosis Discovery, the podcast of the MS Discovery Forum. I’m your host, Dan Keller.
This week’s podcast features Dr. Luke Lairson of Scripps Research Institute, who discusses discovery of small molecules to induce remyelination and, in particular, some muscarinic receptor antagonists currently approved for other indications. But first, here are some new items in the MS Discovery Forum.
According to our curated list of the latest scientific articles related to MS, 114 such articles were published in the first two weeks of August. We selected a few of these articles as our Editor’s Picks. One is a longitudinal study of gray matter lesions and cortical atrophy in MS published in PLOS ONE. The investigators obtained MRIs at baseline and five years later from subjects with clinically isolated syndrome, early and late relapsing-remitting MS, and secondary progressive MS, and examined lesion placement and cortical thinning in the different disease subtypes. To see this publication and the other articles we selected, go to msdiscovery.org and click on Papers.
Our Drug-Development Pipeline includes continually updated information on 44 investigational agents for MS. During the past week, we added 1 new trial, we updated information on 2 other trials, and we added 12 other pieces of information. The drugs with important additions and changes are ATX-MS-1467, daclizumab, dimethyl fumarate, fingolimod, glatiramer acetate, interferon beta-1a, and interferon beta-1b. To find information on all 44 compounds, visit msdiscovery.org and click first on Research Resources and then on Drug-Development Pipeline.
[transition music]
And now to the interview. Dr. Luke Lairson is an assistant professor at the Scripps Research Institute and a principal investigator at Calibr, the California Institute for Biomedical Research, in La Jolla. We spoke at the research institute.
Interviewer – Dan Keller
Dr. Lairson, we're talking about the potential for remyelination. And your institution is taking a very systematic and maybe novel approach. Can you describe how you're going about looking at possible ways to induce remyelination?
Interviewee – Luke Lairson
Sure. So we're using a phenotypic assays to look for small molecules that selectively induce the differentiation of the precursor cell population which is required for remyelination, which are the so called oligodendrocyte precursor cells or OPCs. We developed imaging-based assays where we could look for small molecules that selectively induce that differentiation phenotype.
MSDF
And how are you going about screening compounds, and what is your institute set up to do?
Dr. Lairson
So at Scripps and Calibr we have the capacity to screen on a million compound scale capacity to look at molecules. In this particular assay, we do it in a 3D four-well format, which limits us to screening collections on the scale of hundreds of thousands. And to date, we've screened about 200,000 compounds with this assay.
MSDF
In terms of multiple sclerosis, what are you looking at now?
Dr. Lairson
From our preliminary screen of our collection of bioactive compounds, including FDA-approved drugs and drugs in late-stage clinical development, we identified a series of compounds for which OPC differentiation had not been previously reported, which are muscarinic receptor antagonists, which are clinically approved drugs and which work in the central nervous system. And we demonstrated a number of these compounds work in two different rodent models of remyelination in MS. And we're currently developing these as a lead class of compounds as a combination therapy when combined with existing immunosuppressant drugs, including Gilenya.
MSDF
What compounds have you focused on most? There's a multitude of approved antimuscarinic agents.
Dr. Lairson
Right. So this is actually a a critical point. So we identified a number of compounds which had antimuscarinic activity, which were active in our OPC differentiation assay. We used pharmacology to demonstrate that the antagonism of M1 or M3 receptor subtype is a required component of the mechanism of these drugs. However, we think that there's a second target, and it's a dual mechanism action through which these drugs are acting, which we're currently trying to elucidate what that second target is to fully characterize the mechanism. That second target actually provides the opportunity to identify compounds that have a potential to have a better therapeutic index in vivo. So the lead compound we published was benztropine. Jonah Chan's lab at UCSF later showed that clemastine also works, which was in our paper, as well. So these are drugs that have been demonstrated to work in vivo.
What we did after we published that is we then looked at every compound that we could get our hands on that had antimuscarinic activity – specifically targeting M1/M3 receptor subtypes – and then characterized their activity in the OPC differentiation assay and, as well as profiling their potency on the M1/M3 receptor subtypes, with the goal of looking for compounds that have an optimal therapeutic index in terms of on-target toxicity. So benztropine induces OPC differentiation in the low micromolar range, but it antagonizes M1 and M3 receptors in the low nanomolar range so there's a discrepancy there. And we think that the on-target toxicity of antimuscarinic activity is going to limit the therapeutic potential of these drugs. We've since identified other FDA-approved drugs for which that index is improved and we compounds with approximately 100-fold improvement in therapeutic index, which we've demonstrated in the EAE model are active. And we're currently evaluating them in combination with immunosuppressant drugs to identify an optimal combination, which could well be benztropine or clemastine but may be another FDA-approved drug.
MSDF
You're at very early stage in terms of clinical utility of these things in MS. But is there any way to separate out the negative antimuscarinic effects that affect people taking drugs for overactive bladder and various other things from their therapeutic effect? Or is it really intrinsic to attacking that receptor?
Dr. Lairson
That's the key point. So we do think that it's that on-target toxicity which is going to potentially limit this class of compound, which is why we're looking for these other compounds and where we have an improved therapeutic index of inducing remyelination versus antagonizing those receptor subtypes. And likely this class of drugs – and any class of drugs that induces remyelination – is going to have to be used in combination with immunosuppressant drugs. It will require a careful clinical evaluation to figure out which combination will be the most effective and what doses will be safe.
MSDF
But you also have been doing T-cell assays I take it in looking at benztropine in your work. So what goes on with immune modulation? Is there any effect there?
Dr. Lairson
So we did extensive studies with benztropine to evaluate its activity in not just T-cell biology but also macrophage biology both in vitro and in vivo. And we found benztropine had no affect on in vitro or in vivo T cell numbers or activity in terms of cytokine production. It has no affect on macrophage polarization in vitro or in vivo, including looking at spinal cords of animals. We don't think that it's acting through a peripheral immune system effect. We can't rule out an important concept that came out at a recent meeting was we need to look at the affect of these compounds on microglial cell activation in the brain and also in astrocyte activation.
MSDF
So it looks like it's a pure remyelination effect at this point and not really an immunosuppressive effect, which would argue for having to use it in conjunction with today's drugs for MS.
Dr. Lairson
Correct, that's our current reasoning, yeah.
MSDF
Have you looked at other models other than cuprizone?
Dr. Lairson
Yeah, we looked at the EAE…PLP-induced EAE model of relapsing-remitting MS, and we've looked at the MOG model of progressive MS and the cuprizone model. Yeah, those are the models we've looked at to date.
MSDF
With similar results?
Dr. Lairson
The compounds we've evaluated have all been active in in those models.
MSDF
Do you have an idea of the mechanism of action how this is actually working in the oligodendrocyte precursor cells?
Dr. Lairson
Downstream of the muscarinic receptor…so as I said, based on pharmacology, these classes of compounds – these neurotransmitter receptor modulating agents – are notoriously pleiotropic in that they had multiple receptor subtypes in the brain. So benztropine, for example, it hits nicotine and histamine receptors in this dopamine reuptake inhibitor in addition to being an antimuscarinic. We've shown that those activities are not responsible for inducing OPC differentiation. However, as I said, we've identified multiple compounds that do inhibit muscarinic receptors – specifically receptor subtypes 1 and 3 – that do not induce OPC differentiation. So we think there's a second target; we're actively trying to identify what that second target and downstream mechanism is.
MSDF
Do you think the same compound would attack both targets, or are you going to need to give multiple compounds to hit multiple targets very selectively as I would think would be the hope?
Dr. Lairson
The existing compounds we have the argument is that they are hitting both of these targets to induce the differentiation. In that, there's a number of compounds that do hit the M1/M3 receptors that do not induce differentiation, which argue that you need both. The compounds we've identified fortuitously hit both of the necessary targets.
MSDF
In the antimuscarinic field, often the goal is to be very selective and limit activity at different receptors, but it sounds like you want some overlap here.
Dr. Lairson
Exactly. We've also initiated some medicinal chemistry where we're trying to see if we can dial in potency for the second target. So we know if benztropine is active on muscarinic and low nanomolar can we improve its potency in the OPC assay by dialing in potency on that second target?
MSDF
By modifying the molecule?
Dr. Lairson
Yeah, so making analogs of existing active antimuscarinic agents and then evaluating their activity in the OPC differentiation assay, as well as evaluate their antimuscarinic activity.
MSDF
Are you hoping that the same active part of the molecule hits both receptors, or have you ever considered making a bifunctional molecule that would be best at both receptors?
Dr. Lairson
If we knew what the other receptor was, we could potentially address that, or you could argue a bifunctional versus having two unique compounds so it would be you'd have to evaluate that in vivo I think, yeah. The other argument for moving away from this is that as soon as you make a change to that compound it's no longer an approved drug, and you have to go through the rigor of bringing that to the clinic.
MSDF
Now in your screening, you're using a lot of drugs – a lot of compounds at this point – that have already passed phase 1 screening or phase 1 clinical testing, and this has shown safety. Does that speed up do you think the approval process if these things look active?
Dr. Lairson
It does. So UCSF has actually initiated a phase 2 trial to evaluate clemastine in MS. So they were able to immediately proceed from a screening result to a clinical trial because it's an approved drug.
MSDF
And just about Calibr, your institute. You have full facilities for taking this from screening up to what stage?
Dr. Lairson
Up to the rodent proof of concept stage. So we have a high throughput screening facility, as I mentioned, and then we have a medicinal chemistry group, a pharmacology group where we can do pharmacokinetics in-house and then in core biology. So we take it to the rodent proof of concept.
MSDF
Do you do any synthetic chemistry, or this is all screening of existing molecules?
Dr. Lairson
Yeah, we do significant amount of synthetic chemistry so we have a group of 20 chemists – medicinal chemists – that are making analogs. And we do a significant amount of contract research to get compounds and analogs made.
MSDF
What have we missed, or what do you think is important to add, if anything?
Dr. Lairson
The other aspect of our program is we've identified novel compounds for which their mechanism of action is unclear. So we've identified multiple scaffolds. We've focused on three of those, which have been subjected to medicinal chemistry optimization. So we identified screening hits, which we were unable to evaluate in the rodent models due to their pharmacokinetic property. But we've now identified analogs of those compounds which are potent in the OPC assay, in which we can achieve reasonable levels in the brain. So we're currently evaluating those in these preclinical rodent models. And once we demonstrate efficacy there, we'll then go and evaluate their mechanism of action.
MSDF
You test these compounds first in a phenotypic sense to see if they actually do something that you want done. And then you trace it back to mechanism of action?
Dr. Lairson
Correct. That's our general approach for a lot of assays. So rather than looking at a validated, biochemical target, typically we'll just look for small molecules that induce the cell fate decision that we're interested in. For me, personally, it's the most interesting part of the project is I’m figuring out how those compounds that we rule out known mechanisms how they're actually acting. So we do the mass spec-based proteomics to figure out the specific protein target. And then we use standard cellular molecular biology techniques to elucidate the downstream mechanism of action.
MSDF
So I suppose the phenotype you're looking for in the case of this research we've been discussing is remyelination. How do you look for that activity?
Dr. Lairson
The remyelination activity in vitro? We use a co-culture assay, which we collaborate with Rusty Gage at Salk where we pre-differentiate neurons in a dish and then co-culture with our oligodendrocyte precursors plus or minus drug and then look at the ability of those drugs to enhance the rate of myelination of co-cultured axons.
MSDF
And you just stain the cells in vitro looking for myelin production or myelin basic protein?
Dr. Lairson
Exactly, yeah. So we just look at myelin basic protein co-localization with axon.
MSDF
Very good. I appreciate it, thanks.
Dr. Lairson
Absolutely. Thanks very much for your interest.
[transition music]
MSDF
Thank you for listening to Episode Fifty-One of Multiple Sclerosis Discovery. This podcast was produced by the MS Discovery Forum, MSDF, the premier source of independent news and information on MS research. Msdiscovery.org is part of the non-profit Accelerated Cure Project for Multiple Sclerosis. Robert McBurney is our President and CEO, and Hollie Schmidt is vice president of scientific operations.
Msdiscovery.org aims to focus attention on what is known and not yet known about the causes of MS and related conditions, their pathological mechanisms, and potential ways to intervene. By communicating this information in a way that builds bridges among different disciplines, we hope to open new routes toward significant clinical advances.
We’re interested in your opinions. Please join the discussion on one of our online forums or send comments, criticisms, and suggestions to editor@msdiscovery.org.
[outro music]
