Category → Neurology
Largely because of that drug discovery relevance, however, Heptares is choosing to keep its structure somewhat close to the vest. Officials presented views of the structure, of a GPCR called Corticotropin Releasing Factor (CRF-1) receptor, at conferences on Friday and Monday. But Heptares CEO Malcolm Weir says his team has no immediate plans to publish the structure or to deposit coordinates into the repository known as the Protein Data Bank.
The structure, Weir says, is another success for Heptares’ GPCR stabilizing technology, StaR. The technique involves targeted mutations that help to trap a GPCR in a single biologically-relevant state. In the case of CRF-1, Weir says, the stabilized receptor is captured in the “off” state.
The structure itself, which is at a resolution of 3 Ångstroms, has the 7-helix membrane-spanning structure typical of GPCRs. However, CRF-1′s architecture is rather different from receptors in Family A, the only GPCR family for which X-ray structures had been available until now, Weir says. “The overall shape of the receptor looks different, the orientation of the helices looks different, and there are detailed differences within helices that are at analogous positions in Family A receptors,” he says. He notes that there are differences in helices 6 and 7, which undergo important motions during GPCR activation.
“This is an important breakthrough, although fine details of the structure and release of coordinates may still be some time away,” says Monash University’s Patrick Sexton, an expert in Family B GPCRs who was at Friday’s talk. The structure, he says, confirmed researchers’ expectations that the major differences in membrane-spanning helices between Family A and Family B receptors would occur on the extracellular side. “There was a very open and relatively deep extracellular binding pocket, with the receptor having a ‘V’ shaped appearance,” he says. This open pocket likely contributes to medicinal chemists’ difficulties obtaining high affinity small molecule ligands for Family B receptors, he suggests.
That open pocket might be involved in another Family B GPCR mystery, according to Roger Sunahara, also in attendance Friday, who studies GPCRs’ molecular mechanisms at the University of Michigan, Ann Arbor. All Family B GPCRs, including CRF-1, have a large domain at their amino-terminus that contains large portions of their ligand binding sites. That domain was not included in this structure, he says, but “it would appear that deleted globular N-terminal domain would fit quite nicely into the open pocket.”
The CRF-1 receptor is a drug target for depression and anxiety, but at least one CRF antagonist failed to show benefit compared to placebo in a clinical trial. Weir says the impact of the CRF-1 structure for drug discovery will not necessarily be in CRF-1 drug discovery per se, but in the ability to develop relevant computer models of related targets.
It hasn’t been possible to make accurate models of Family B receptors with Family A information, explains Ryan G. Coleman, a postdoctoral fellow at UCSF who develops GPCR models, but who was not in attendance at the talks. Quality models could streamline small molecule drug discovery for the entire family, he explains. Most of the natural ligands for Family B receptors are long peptides, which are notoriously tough to replace with small molecule drugs.
Experts like Coleman will have to wait for some time to learn about the structure for themselves, unless they happened to have a friend in the audience at Heptares’ talks. It’s not unheard of for there to be a gap of several months to two years between a structure’s announcement and publication.
“We’re delighted to have such an informative structure,” Weir says. “It’s very exciting.” He adds says Heptares is progressing toward a structure of the biggest fish in family B, GLP-1, in the “on” state.
The spectacular—and largely anticipated—failure of the Alzheimer’s treatment bapineuzumab has caused an outpouring of stories questioning what went wrong and what it means about pharma’s approach to R&D. Pfizer, Johnson & Johnson, and Elan, the developers of bapineuzumab, are taking a beating in the press for investing so heavily, not to mention raising the hope of so many patients, in a therapy that had not shown strong signs of efficacy in early trials.
Most stories are focused on the implications for Alzheimer’s research and, more generally, the pharma business model given the hundreds of millions of dollars the three companies sank into bapineuzumab. But news of its failure also resonated in research communities focused on other neurogenerative diseases, like Parkinson’s disease and Huntington’s disease, marked by protein aggregation.
I checked in with Todd Sherer, CEO of the Michael J. Fox Foundation to understand what Parkinson’s researchers might learn from the disappointing data from bapineuzumab. Sherer believes there are scientific and business ramifications of the results, both of which might have a chilling effect on neuroscience research.
From a scientific perspective, some are declaring the failure of bapineuzumab the nail in the coffin of the amyloid hypothesis, the theory that the beta-amyloid, the protein responsible for the plaque coating the brains of people with Alzheimer’s disease, is the primary cause of neuron death in the disease. Bapineuzumab, which blocks beta-amyloid, was one of a handful of treatments to test the hypothesis in the clinic. So far, every drug to reach late-stage trials has failed.
Sherer isn’t convinced bapineuzumab is the nail in the amyloid hypothesis coffin. “Obviously the results are very disappointing given the level of interest and investment that’s been put forward for this therapy,” Sherer says. “I don’ think that the result is a definitive answer to the amyloid hypothesis because there are many different ways to target amyloid aggregation therapeutically.”
Parkinson’s researchers are also trying to learn from the setbacks in Alzheimer’s and apply that to studies of drugs targeting alpha synuclein, the protein that clumps together in the brains of people with Parkinson’s disease. “One of the things that is a learning for us in Parkinson’s is really to try to be as smart and informative as we can be in the early clinical trials,” he says.
In Alzheimer’s, for example, the Alzheimer’s Disease Neuroimaging Initiative (ADNI), a collaboration between government, academic, and industry scientists, was formed in 2003 to identify biomarkers that can be used both in the diagnosis of the diseases and in the clinical development of Alzheimer’s drugs. However, Sherer points out that while progress in the ADNI initiative has been promising, it was started too late for many companies, which had already jumped into larger clinical trials of Alzheimer’s therapies.
The Fox Foundation already has a biomarker initiative for Parkinson’s ongoing. The goal is that when the first clinical trial for a vaccine alpha-synuclein, to be led by the Austrian biotech Affiris with support from the non-profit, starts later this year, the tools will be in place to conduct a highly informative study.
On the business side, Sherer worries about the impact of more bad news in Alzheimer’s at a time when many companies are already moving out of drug discovery in many areas of neuroscience. “One of the concerns I have is that investors like big pharma companies and others are already showing a trend towards risk aversion,” Sherer says. “That will just get reinforced by these large trials not succeeding.”
Although basic research is uncovering new therapeutic avenues in diseases like Alzheimer’s and Parkinson’s, companies may decide the bar for understanding the biological relevance for each drug target needs to be set much higher. But when it comes to Parkinson’s disease, he adds, “we are not going to have the luxury of knowing everything about the disease and the biochemical pathways before we need to push forward with therapies.”
One hope Sherer has is that companies will make much of the data from these failed trials available to the research community to try to understand what didn’t work, and what the results really mean. “It’ll be a goldmine of information for other Alzheimer’s trials, but also for other genetic diseases like Parkinson’s disease and Huntington’s disease.”
Gregory Petsko knows why he came to TEDMED. “I’m looking for Al Gore,” he told me flat-out over lunch. Folks who know Petskoknow the former Brandeis University biochemistry department chair isn’t one to mince words. And he’s nailed the reason why an academic might want to look outside traditional conferences and soak up some of the TEDMED aura. He’s looking for a charismatic champion to take up a biomedical cause: in Petsko’s case, it’s support for research in Alzheimer’s disease.
Petsko and Reisa Sperling, director of the Center for Alzheimer’s Research and Treatment at Brigham and Women’s Hospital, talked about Alzheimer’s at TEDMED on Wednesday. Both talks were cast as calls to action. Just consider the introduction Petsko got from TEDMED chair and Priceline.com founder Jay S. Walker: “This is a man who hears a bomb ticking.”
Alzheimer’s statistics are sobering and Petsko used them to dramatic effect. People who will reach 80 by the year 2050 have a 1 in 3 chance of developing the disease if nothing is done, he told the audience. “And yet I hear no clamor,” he said. “I hear no sense of urgency.”
Petsko shared some not-yet-published work with TEDMED’s audience. Continue reading →
Watch this space on Sunday as I cover the public unveiling of five drug candidates’ structures. I’ll be liveblogging the “First Disclosures of Clinical Candidates” symposium at the San Diego ACS National Meeting, which runs from 2PM to 5PM Pacific.
1:30PM It’s half an hour before the start of the session and the big ballroom is still pretty empty. Expect that to change in short order.
Company: Lexicon Pharmaceuticals
Meant to treat: type 2 diabetes
Mode of action: dual inhibitor of sodium glucose transporters 1 and 2, which play key roles in glucose absorption in the gastrointestinal tract and kidney
Medicinal chemistry tidbits: this drug candidate had Lexicon’s chemists refamiliarizing themselves with carbohydrate chemistry. Most inhibitors of sodium glucose transporters incorporate D-glucose in some way. Lexicon’s chemists realized they could try something different– inhibitors based on the scaffold of L-xylose, a non-natural sugar. The team has already published a J. Med. Chem paper (2009, 52, 6201–6204) explaining that strategy. LX4211 is a methyl thioglycoside-the team went with a methyl thioglycoside because upping the size too far beyond a methyl lost activity at SGLT1.
Status in the pipeline: LX4211 is currently completing Phase IIb trials.
Company: Bristol-Myers Squibb
Meant to treat: migraine
Mode of action: antagonist of the receptor for calcitonin gene-related peptide- increased levels of this peptide have been reported in cases of migraine
Medicinal chemistry tidbits: This team recently published an orally bioavailable CGRP inhibitor, BMS-846372 (ACS Med. Chem. Lett., DOI: 10.1021/ml300021s). However, BMS-846372 had limited aqueous solubility, something that might make its development challenging. To improve that solubility, the BMS team sought to add polar groups to their molecule, something that’s been tough to do with CGRP inhibitors historically. In the end, the team managed to add a primary amine to BMS-846372′s cycloheptane ring while maintaining CGRP activity, leading to BMS-927711.
Status in the pipeline: Phase II clinical trials
3:05 lots of questions from the audience for this talk! One questioner notes (as was noted in talk) that 4 CGRP inhibitors had gone before this drug in the clinic, and not made it through. Speaker notes that this candidate is more potent than others at CGRP (27 picomolar).
3:53 We’re a bit behind schedule but got plenty of good chemistry…
Meant to treat: tumors with loss-of-function in the tumor suppressor protein PTEN (phosphatase and tensin homolog)- 2nd most inactivated tumor suppressor after p53- cancers where this is often the case include prostate and endometrial
Mode of action: inhibitor of phosphoinositide 3-kinase-beta (PI3K-beta). Several lines of evidence suggest that proliferation in certain PTEN-deficient tumor cell lines is driven primarily by PI3K-beta.
Medicinal chemistry tidbits: The GSK team seemed boxed in because in 3 out of 4 animals used in preclinical testing, promising drug candidates had high clearance. It turned out that a carbonyl group that they thought was critical for interacting with the back pocket of the PI3K-beta enzyme wasn’t so critical after all. When they realized they could replace the carbonyl with a variety of functional groups, GSK2636771 eventually emerged. GSK2636771B (shown) is the tris salt of GSK2636771.
Status in the pipeline: Phase I clinical trials
Company: Gilead Sciences
Meant to treat: chronic infection with hepatitis B and C viruses
Mode of action: agonist of Toll-like receptor 7, which recognizes RNA from viral sources
Medicinal chemistry tidbits: The team paid a lot of attention to particular sidechain in their drug candidates– they examined a range of pKa’s from the acidic side of the scale to the basic side, and learned that a basic amine was important for agonist activity.
Status in the pipeline: Phase Ib clinical trials
Company: Bristol-Myers Squibb
Meant to treat: hepatitis C
Mode of action: inhibitor of viral NS5B replicase
Medicinal chemistry tidbits: This drug candidate is an allosteric inhibitor– early on in the program BMS researchers had evidence to suggest that allosteric inhibition of the replicase would be feasible, and would provide an alternative to the nucleoside analogs that constitute the vast majority of replicase inhibitors. The team started with fused indole lead structures which bound to the thumb site 1 allosteric site in the replicase (Bioorg. Med. Chem. Lett., DOI: 10.1016/j.bmcl.2011.03.067). Adding a morpholine amide enhanced potency, and adding substituents to it abrogated transactivation of the pregnane X receptor (PXR). Ultimately this group was replaced with a methylated piperazine, with substituents stitched together to give another ring. A cyclopropane adjusted the shape of the molecule to jibe with information gathered from an X-ray co-crystal structure.
Status in the pipeline: Phase II clinical trials
4:52 That’s it folks! Watch for additional coverage of these talks in an April issue of C&EN.
In this week’s issue, I look at the perceived exodus by pharma companies from neuroscience R&D. Between AstraZeneca’s recent cutbacks, the closure of Novartis’ neuroscience research facility in Basel, and earlier moves by GSK and Merck, industry watchers are understandably worried that the neuroscience pipeline will dry up.
One person who isn’t worried is Michael Ehlers, Pfizer’s chief scientific officer for neuroscience research. Ehlers came to Pfizer a year and a half ago from Duke, with the explicit mission to revamp how the company finds and develops drugs for brain diseases. The scientist is convinced that the field is ripe for new and better drugs, and that by staying in the game, Pfizer will be in a good position to capitalize on what he believes will be a healthy flow of new discoveries.
Many drug companies argue that the risk in neuroscience simply doesn’t justify the investment. The overarching sentiment is that the brain is still a black box: good targets are few and far between; clinical trials are long and unpredictable; regulatory approval is tough; and generic competition is plentiful. For many big pharma firms, the math just doesn’t add up.
“I personally don’t find that calculus to give you the total picture,” Ehlers says. Shifting resources away from neuroscience to focus on areas like oncology, where the environment looks favorable—clear clinical trial endpoints, the opportunity for fast-track approval, an easier chance for reimbursement from payors—only makes sense in the short term, Ehlers says. But that thinking “is short sighted as to where the fundamental state of biology is in neuroscience,” he says.
Why is Ehlers so encouraged about a field that so many are walking away from? He believes that neuroscience is poised to benefit from the kind of genetic links that generated so many targets—and eventually so many targeted-drugs—in oncology. “There is going to be kind of a revolution in the next five years—it’s not going to be tomorrow…but you have to think about that inflection of opportunity over the five-to-ten year time horizon.”
To take advantage of each new genetic clue, Ehlers has revamped Pfizer’s approach to neuroscience R&D. As this week’s story explains:
In the past, big pharma often gave its scientists a mandate to work in areas such as Alzheimer’s or schizophrenia, regardless of tractable drug targets. Now at Pfizer, Ehlers says, his team is “indication agnostic.” Any program that Pfizer undertakes must have a critical mass of biological knowledge—for example, human genetics, human phenotyping, and evidence of dysfunctional neurocircuits—to convince Ehlers it’s worth pursuing. “We start there,” he says. “That hasn’t always been the case.”
Moreover, Pfizer no longer relies on mouse models as predictors for responses in humans. “We’ve for the most part stopped all rodent behavior as a model for disease and are much more about what’s happening in the brain,” he says. Scientists measure human responses to prove experimentally that a drug works.
Pfizer’s goal, according to Ehlers, is to tackle fewer projects but have more confidence in their potential for success. The result should be a drug pipeline “rooted in something more than optimism.”
He cites Huntington’s disease as one area that, even before coming to Pfizer, he saw as a prime scientific opportunity. “You know the gene, you know a fair bit about what’s going on, you have a wealth of data, tons of models, a clear clinical course, and an identifiable patient population,” he says. “If we can’t deal with that, we’re in trouble.”
Biogen Idec made a splash last week when its oral medication for multiple sclerosis (MS), BG-12, was found to reduce relapses in 44-53% of nearly 3,800 patients in two separate Phase 3 clinical trials (CONFIRM and DEFINE, respectively). Continued hopes for an orally available, non-injectable MS treatment have created a race between Biogen Idec and several other firms, as C&EN’s Lisa Jarvis examines in a 2009 MS cover story. In fact, so much has changed in 2 years that two of the six Phase 3 drugs mentioned in that article – Teva’s laquinimod and Merck’s cladribine – have already been withdrawn from competition.
So what’s the secret sauce behind BG-12? Many pharmaceuticals are small molecules with multiple heteroatoms and aromatic rings, but not BG-12: it’s just dimethyl fumarate! A search for ‘fumarate’ on pubs.acs.org returned >4800 hits, which gives you an idea of its common use in several organic reactions: [3+2] cycloadditions, Diels-Alder reactions, and Michael additions. Interestingly, dimethyl fumarate is the all-E stereoisomer; the Z-configuration, where the two esters are on the same side of the central double bond, goes by the tagline ‘dimethyl maleate’ and does not seem to possess anti-MS effects.
Very small molecules such as BG-12 (molecular weight = 144) are notoriously tough to use as drugs: they hit lots of enzymatic targets, not just the intended ones, and tend to have unpredictable side effects (see Derek Lowe’s 2005 article regarding the FDA “approvability” of several common drugs today). Toss in BG-12’s alkylating behavior to boot (fumarates can interact with nucleophilic amines or sulfides at multiple sites, including enzyme active sites), and you have to wonder how it functions in the body. Well, so do scientists. A 2011 review implicates up to 3 potential biochemical mechanisms – the Nrf2 pathway Lisa mentioned in the 2009 piece, T-helper phenotype 2 interleukin upregulation (IL-4, IL-10, IL-5, which “change gears” for immune system functioning), and CD62E inhibition, which controls adhesion of blood cells to inflammation sites.
Side notes: Flavoring chemists have added fumaric acid, the parent diacid of BG-12, to industrially-prepared foodstuffs such as baking powder and fruit juices since the 1930s. A darker side of dimethyl fumarate emerges when you consider its non-medicinal use: certain furniture companies applied it to new upholstered chairs and sofas to stop mold growth. This unfortunately caused several cases of severe skin irritation, which a 2008 exposé in London’s Daily Mail likened to actual burns.
(An homage to Terra Sigillata; it might normally be covered on his beat.)
Astute readers of the New York Times may have noticed a front-page article from a few weeks back, highlighting a new late-night snack: Lazy Cakes. Taking a cue, perhaps, from the substance-laced brownies popular in the late ‘60s, these brownies pack a decidedly sleepy secret: each contains a “proprietary calming blend” of ingredients, chief among which is melatonin.
Melatonin is a hormone usually secreted by the pineal gland (a pinecone-shaped gland located just above the cerebellum) in humans and other mammals, in response to dark surroundings. (Note: Although they sound similar, melatonin should not be confused for melanin, the skin pigment formed by sunlight exposure) In mammals, melatonin induces the circadian rhythms associated with sleep, affects the onset of puberty and may help regulate DNA transcription.1,2 Biologically derived from tryptophan, the amino acid and purported suspect of the Thanksgiving “turkey coma”, melatonin has been shown clinically to have benefits for memory loss, in addition to antioxidant potential. Melatonin capsules have been sold over-the-counter for insomnia and jet lag since the 1980s.
Technically speaking, the product is labeled a dietary supplement, and as such skirts regulation by the FDA. One valid concern are possible interactions that melatonin, like other supplements, could have with prescription drugs, a topic addressed both by Terra Sig and C&E News. More controversy over the soporific snacks springs from their colored packaging and wide availability. This intrepid blogger ventured out into the wild to recover a sample for analysis. The packaging, upfront, has a distinctly comic-book appeal: purple and green swirls, a trippy logo evoking That ‘70s Show, and a cartoon brownie mascot leaned back for a snooze. The brownie itself is compact, and has quite a bit of heft for your average baked good. The back of the wrapper evokes language usually associated with cigarette labeling: multiple tiny lines of serious text stating Recommended for Adults Only, and Do Not Drive or Operate Heavy Machinery.
The “calming blend” also includes valerian root, which is commonly found in teas and herbal supplements. Containing sugar-decorated polycyclic lactones called iridoids, as well as valerenic acid3 derivatives, the extracts have been shown clinically to reduce anxiety and relieve insomnia.4 Passion flower extract brings a dose of alkaloids into the bedtime mix; well-known sleep inducers opium and morphine are part of this general molecular family. The other ingredients, however, seem to just be along for the ride: current “superfruits” goji berry and açai, with the old Vitamin C standby of rose hips.
For my part, I don’t believe that a baked good packing a pharmaceutical punch should be sold in colorful wrappers, next to the candy bars. However, having experienced my share of late-night grad school anxiety, I can’t blame someone for wanting a good solid nap, any way they can.
1. Fox, Stuart Ira. Human Physiology, Sixth Ed. Boston: WCB / McGraw Hill, 1999. pp. 289, 315.
2. Merck & Co., Inc. The Merck Index, 13th Ed. New Jersey, 2001. p. 5841
3. Ramharter, J.; Mulzer, J. Org. Lett. 2009, 11, 1151-1153.
4. NIH Dietary Supplement Fact Sheet: Valerian. 2008. Downloaded from http://ods.od.nih.gov
1PM Pacific: There’s one hour left before chemists will pack a ballroom in Anaheim to see potential new drugs’ structures unveiled for the first time. Watch this space for updates.
2:39PM Pacific: CEP-26401
This drug candidate now has a name: irdabisant
meant to treat: deficits in cognition and/or attention in diseases such as Alzheimer’s and schizophrenia
mode of action: inverse agonist of histamine H3 receptor, which regulates several neurotransmitter pathways involved in cognition, attention, memory
medicinal chemistry tidbits: Cephalon’s goal was to bring a high quality compound to the clinic to define the utility, if any, of H3 antagonists for these indications. The team studied compounds in this area that failed. Among the things they learned was that several adverse events could be tied to drug candidates’ lipophilicity. So the team prioritized lipophilicity and other such characteristics in its discovery workflow. status in the pipeline: completing Phase I in the beginning of April 2011, advancing to Phase II
structure coming soon!
UPDATED 3/29 with structure:
3:16PM Pacific: BMS-663068
company: Bristol-Myers Squibb
meant to treat: HIV
mode of action: inhibits HIV attachment to host cells by binding to the viral envelope gp120 protein and interfering with its attachment to host CD4 receptors
medicinal chemistry tidbits: potency and getting the drug candidates to reach the bloodstream efficiently were key. Replacing a methoxy group on with heterocycles, such as triazoles, gave a big boost in potency.
status in the pipeline: Completed Phase IIa clinical trials. Phase IIb studies are planned for later this year.
meant to treat: irritable bowel syndrome
mode of action: blocks a subtype of tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, in the gut.
medicinal chemistry tidbits: Lexicon started their medchem program with an open mind. They could have made a molecule that was exquisitely selective for the subtype of tryptophan hydroxylase in the gut, they could avoid hitting the other subtype by making their molecule stay out of the brain, or both. They ultimately ended up using the latter strategy, making molecules slightly on the heavy side (above 500 or 550 molecular weight) and adding groups like a carboxylic acid, that tend to keep things out of the brain.
status in the pipeline: Completed Phase IIa clinical trials.
5:30PM Pacific: MK-0893
meant to treat: type 2 diabetes
mode of action: blocks the receptor for the hormone glucagon. Glucagon is released by the pancreas in response to falling glucose levels.
medicinal chemistry tidbits: Merck kept several chemical scaffolds in play during this research program. But the team’s big breakthrough was adding a methyl group to the benzylic position of a promising compound, which greatly improved potency. This methyl group strategy hadn’t worked for previous compound series, but the team revisited it anyway.
status in the pipeline: Completed some Phase II trials, according to clinicaltrials.gov
meant to treat: Alzheimer’s disease
mode of action: blocks gamma-secretase, a key enzyme in the production of amyloid-beta, the peptide behind the plaques that mar Alzheimer’s patients’ brains.
medicinal chemistry tidbits: Adding a cyclopropyl group and a trifluoromethyl group enhanced molecules’ metabolic stability.
status in the pipeline: discontinued because of adverse liver side effects unrelated to its mode of action.
5:31PM Pacific: That’s all for now, folks. I hope to update with more structure information later. Watch for my full story on this symposium in a mid-April issue of C&EN.