Sotorasib: the fifth fragment-derived drug approved

Today the US FDA approved sotorasib (AMG 510) for a subset of patients with non-small cell lung cancer. This is the fifth fragment-derived drug approved.

 

Last year Practical Fragments outlined the discovery of sotorasib, tracing its origins to two independent fragment-based efforts. (Disclosure: I was part of this work at my previous company, Carmot Therapeutics.) Today's approval illustrates another aspect of the story: the speed with which the program progressed. From the Nature publication demonstrating that KRAS^G12C is ligandable it took just five years to deliver sotorasib to the clinic, and less than three to demonstrate sufficient safety and efficacy to win approval.

 

This is fast for any drug, and all the more so for a target previously deemed undruggable.

 

In addition to commending the scientists and clinicians, credit is also due to Amgen, which cleared all bureaucratic hurdles to push this program forward. Every company with even a passing interest in cancer saw the 2013 Nature paper, but many were put off by the covalent nature of the molecule. It took organizational vision - in addition to great science - to succeed.

 

It is also worth noting that of the five fragment-derived drugs now approved, two are for difficult targets; venetoclax blocks a protein-protein interaction.

 

Of course, the raison d'être for all these efforts is to help patients, and the emerging clinical data for sotorasib clearly demonstrate this. Of 126 patients with advanced, previously treated non-small cell lung cancer, 43 had partial responses, and 3 had complete responses. For those not familiar with cancer research this may not sound impressive, but these are patients with no other options. And this is just the beginning for sotorasib, the first drug to work via this long-sought mechanism. Combination therapies are already actively being pursued, and these often show dramatic improvements as physicians figure out how best to use new drugs.

 

Congratulations - and thanks - to everyone involved in bringing this new gift to humanity.

Sixteenth Annual Fragment-Based Drug Discovery Meeting

Last week Cambridge Healthtech Institute held its Sixteenth Drug Discovery Chemistry (DDC) meeting virtually for the second year in a row. The eight tracks, four in parallel, spanned three days. There were occasional kinks, and the 6:30 morning start time was painful for those of us on the Western edge of the US, but for the most part things went smoothly. Attendance was up 25% over last year, to 750 participants. Attendees from outside the US comprised 40%, similar to last year and up from 2019, while industry representation remained steady at 70%. As always I’ll just touch on a few broad themes – please feel free to add your impressions.

 

Methods

These meetings are always great venues to learn about new techniques, and one that stood out is “mass photometry” (MP). This label-free method allows visualization of single proteins in solution. The technology, which builds on interference reflection microscopy and interferometric scattering microscopy, rapidly provides information on the molecular weight of proteins to an accuracy of about 2 kDa. Stefan Geschwindner (AstraZeneca) used the method to follow a change in the monomer-dimer equilibrium of the protein PAD4 in response to ligand-binding. He noted that the instrument, sold by Refyn, provides measurements in about 10 seconds for proteins from 35 kDa to 4 MDa in size.

 

On the opposite end of reductionism, Bryan Roth (University of North Carolina, Chapel Hill) discussed customized high-throughput cell-based screens for 320 of the 358 GPCRs in the human genome. We’ve previously described how these assays have been used to screen molecules prioritized from massive computational screens. If you’d like to profile your own molecules, you can submit compounds here for free with no IP-entanglements.

 

Cryo-EM was the subject of our April 1 post last year, but as we discussed here the technique is no joke. Pamela Williams discussed how Astex has formed a consortium with several other companies to advance the technology. She noted that of the 270 membrane protein structures released last year with resolutions better than 3.5 Å, 167 came from cryo-EM while only 103 came from crystallography. Unlike in crystallography, resolution can vary over the protein; local resolution around the region of interest may be better or worse than global resolution. And there is still much to be done to truly industrialize the approach, particularly in sample preparation (specifically making the grids).

 

Speaking of crystallography, James Fraser (University of California, San Francisco) discussed a high-throughput crystallographic screen against the SARS-CoV-2 macrodomain protein, which we covered earlier this year. A fragment-linking program called “Fragmentstein” was used to generate molecules with low micromolar affinity.

 

At the last DDC meeting Frank von Delft (Diamond Light Source) described a high-throughput crystallographic screen of the SARS-CoV-2 M^Pro protein, and this year he provided an update on how the open-source COVID Moonshot effort has advanced hits to non-covalent 30 nanomolar binders with 100 nanomolar antiviral activity in cells. Current efforts are focused on improving metabolic stability and oral bioavailability, but Frank lamented the “astonishing inertia” on the part of funding agencies to support the effort. Although the spectacular success of vaccines should contain the outbreak, there are still occasional breakthrough infections, and not everyone can be vaccinated or mount a successful immune response. Small molecule drugs will be useful, and it would be nice if more people were working on them.

 

Success Stories

I’ll never turn down a crystal structure, but it is important to remember that fragments can be advanced even in the absence of structural information. As a case in point, Paolo di Fruscia (AstraZeneca) described the discovery of allosteric binders of MEK1; we wrote about these earlier this year.

 

On the other hand, Justin Dietrich described AbbVie’s discovery of TNFα inhibitors – an effort he said would not have been possible without structural information. As we discussed here, the team struggled with the lipophilic nature of some of their initial molecules; Justin noted that “if a protein wants grease you have to find a way to get it grease.” The physicochemical properties can then be improved during optimization.

 

Rod Hubbard (Vernalis) described the discovery of LpxC inhibitors. He emphasized the importance of using biophysical methods to carefully characterize and optimize the target protein, and noted that NMR can identify fragments that crystallography might miss. Also, as we noted when we wrote about this campaign last December, fragments can provide multiple starting points, and it can be useful to optimize fragments before embarking on a fragment-to-lead effort.

 

Jus Singh (Ankaa) provided a brief history of targeted covalent drugs. Some folks are still nervous about the potential for covalent molecules to cause idiosyncratic toxicity; indeed, this was one of the reasons Frank von Delft gave for pursuing non-covalent M^Pro inhibitors. However, Jus noted that five out of seven approved covalent kinase inhibitors are not associated with liver toxicity, while some non-covalent kinases inhibitors are. Jus also noted that all three FDA-approved BTK inhibitors are covalent, despite longstanding efforts to develop non-covalent inhibitors.

 

Finally, Micah Steffek (Genentech) described the discovery of LC3 binders, the first step in making an autophagy-based version of PROTACs. An NMR screen yielded hundreds of hits, but despite obtaining multiple crystal structures the team struggled to obtain molecules with better than mM potency. However, combining fragment information with results from a DEL-screen, akin to the story we described here, led to submicromolar inhibitors. Intriguingly, these covalently modified a lysine residue.

 

Odd and ends

Micah’s talk led to some discussion in a speaker panel as to the meaning of hit rates. Do you consider the primary hit rate or the confirmed hit rate? And what level of confirmation is required? As Rod Hubbard noted, different techniques may give different answers for good reasons, and you don’t want to throw away potential hits on the basis of the worst performing or least sensitive technique.

 

Mads Clausen (Danish Technical University) described his shapely library of fluorinated fragments (see here). Hit rates seem to be similar to flatter compounds, though with only four targets screened it is perhaps too early to draw conclusions.

 

But perhaps there is a limit to shapeliness: Justin Dietrich could not recall seeing any fully unsaturated Fsp³ = 1) fragments coming up as hits, despite being present in the AbbVie libraries. Andreas Lingel said he had occasionally seen these at Novartis, but they were never pursued. Similarly, no one could recall advancing any acyclic fragments. A quick glance at the 131 fragment-to-lead success stories captured in five annual reviews revealed that all of the fragments had at least one ring and one sp²-hybridized carbon.

 

And with that I’ll close. The Seventeenth installment of DDC is scheduled for April 18-21 of next year in San Diego, and the more biology-focused Discovery on Target is scheduled to take place both online and in-person in Boston from September 27-30. Hope to see you there!

Understanding fully-functionalized diazirine tags

Four years ago we highlighted work out of Ben Cravatt’s lab describing “fully functionalized fragments,” or FFFs (see here for a schematic). In addition to the variable fragment portion, FFFs contain a photoaffinity tag that can react with proteins plus an alkyne moiety that allows the labeled proteins to be captured for identification by Western blots or mass spectrometry. In the 2017 paper FFFs were used to identify fragments binding to proteins in cells, and more recently other researchers have used the same approach to find fragment hits against isolated proteins and even RNA. In all these cases the photoaffinity tag used has been a diazirine. But how do diazirines react with proteins?

 

As more researchers work with FFFs, it is important to consider what affects the reactivity of the probes. This question is addressed in two new papers.

 

The first, by Christina Woo and collaborators at Harvard, Dana-Farber Cancer Institute and Jnana appears in J. Am. Chem. Soc. The researchers looked at alkyl diazirines (exemplified by LD below – the most commonly used photoaffinity tag) as well as aryl fluorodiazirines (exemplified by Ar below). When treated with ultraviolet light, both moieties form diazo intermediates that can lose nitrogen to generate highly reactive carbenes. However, alkyl diazo intermediates can also react with carboxylic acids, while aryl diazo intermediates do not.

 

The researchers assessed the reactivity of alkyl and aryl diazirines with individual amino acids. In aqueous solution, the specific aryl diazirine tested did not react with any amino acid, while the alkyl diazirine reacted with Glu, Asp, and – at higher concentrations – Tyr and Cys. Moreover, reaction with Glu and Asp was pH-dependent; at higher pH, when the carboxylic acids were deprotonated, the reaction did not occur. A similar pH effect was observed with the model protein bovine serum albumin and the alkyl diazirine probe.

 

Next, the researchers tested a panel of 32 FFFs in intact cells or cell lysates (typically at 10 µM, with 60 s photoirradiation). Positively charged probes tended to give better labeling, suggesting that these FFFs were binding near acidic Glu and Asp residues on proteins. Positively charged fragments yielded an average of 50 unique binding sites, while neutral FFFs produced an average of 14 and negatively charged FFFs gave only 5. A closer look at some of the specific proteins revealed patches of electronegativity in the regions labeled. Interestingly, and consistent with earlier work, membrane proteins were particularly enriched, possibly because Glu and Asp residues in membrane proteins often have elevated pKa values and are thus likely to be more reactive with the diazo intermediates.

 

The second paper, in Chem Sci. by Christopher Parker and collaborators at Scripps Research Institute in Jupiter, FL and Bristol Myers Squibb, explores five different diazirine tags. In addition to the conventional LD probe, they examined an aryl and three alternative alkyl diazirines. All five (LD, Ar, BD, DF, and Tm) were appended to either a simple phenyl substituent (controls), a positively charged lipophilic fragment (FFFs), the promiscuous kinase inhibitor staurosporine, or the bromodomain ligand JQ1.

 

All the controls and FFFs could modify proteins in cells in a dose dependent manner after treatment with UV light. The Ar control appeared to give more non-specific binding, and the LD, Ar, and BD-tagged fragments produced more robust labeling than the Df and Tm fragments.

 

As for the target-specific probes, LD-, BD-, and Tm-tagged staurosporine each labeled between 529 and 836 proteins, but only 10-21 kinases – far fewer than the hundreds of kinases staurosporine inhibits. (To be fair, the photoaffinity tag did reduce the affinity of the probes for protein kinase A and abolished it entirely for the Ar probe.) Among the JQ1-derived probes, only the BD- and Tm-tags pulled down one or two bromodomains.

 

There are many critical but subtle details in both papers. For example, the DF and Tm tags require shorter wavelength illumination (300-310 nm) than the more conventional tags (~360 nm). Changing the wavelength can shift the balance between diazo intermediates versus carbenes. The diazirines can also react with water and buffers or generate olefins, though the latter reaction can be disfavored by deuterating the methylenes on either side of the diazirine.

 

So what does all this mean? To state the obvious, this is complex stuff, and small changes to the tag or conditions can completely change the outcome of the experiment. As the kinase and bromodomain examples show, a negative result does not mean your ligand does not bind to a given target, while a positive result says nothing about the strength of the interaction. Perhaps the “understanding” promised in the title of this post has some way to go. But photoaffinity tagging and chemoproteomics make a powerful combination, and these papers contribute to helping us figure out how to use this tool.

Fragments vs MAT2A: AstraZeneca’s turn

Last week we highlighted work out of Agios leading to a clinical compound for the oncology target MAT2a. But given the appeal of new cancer targets, Agios is not without competition. In a recent (open access) J. Med. Chem. paper, Claudia De Fusco, Marianne Schimple, and colleagues at AstraZeneca describe their efforts.

 

The researchers started with fragment screens using thermal shift, crystallography, and SPR, with the latter providing the most potent and efficient hits. In particular, compounds 1 and 2 stood out and were subsequently found crystallographically to bind at the same allosteric site discussed last week. Interestingly, despite its higher affinity, compound 2 was inactive in a functional assay while compound 1 had an IC₅₀ similar to its dissociation constant.

 

Initial efforts to improve the affinity of compound 1 were unsuccessful, but merging the two fragments yielded compound 5, which had comparable affinity to compound 2 and was also functionally active. Attempts to tweak the dimethylamine moiety were mostly unsuccessful, and a high-resolution (1.1 Å) crystal structure combined with quantum mechanical calculations revealed that the substituents around the aromatic quinazolinone ring system were twisted somewhat out of the plane.

 

Moving to the other side of the molecule, addition of the phenyl ring originally present in compound 2 gave a satisfying boost in affinity for compound 28, which was attributed in part to displacing two “unhappy” water molecules.

 

Compound 28 is active in cells and has good pharmacokinetics in rats. Unfortunately, it is cleared more rapidly in mice. Subcutaneous dosing in mouse xenograft models led to tumor stasis, though weight loss was also observed, suggesting toxicity. Though that likely precludes more intensive in vivo work, compound 28 could still be a useful chemical probe.

 

This paper is a nice example of fragment merging and growing. Notably, compound 28 is relatively small and very ligand efficient. Perhaps combining information from both this and the Agios series will lead to even better molecules.

Fragments in the clinic: AG-270

A promising oncology approach is to target “synthetic lethal” proteins that are required for cancer cells but not for ordinary cells. Methionine adenosyltransferase 2A (MAT2A) is a metabolic enzyme that produces S-adenosyl methionine (SAM). Cancer cells lacking another gene, methylthioadenosine phosphorylase (MTAP), appear particularly dependent on MAT2A. In a recent J. Med. Chem. paper, Zenon Konteatis and colleagues at Agios, Viva, and ChemPartner describe the first clinical inhibitor of MAT2A.

 

The researchers began by screening >2000 fragments in pools of 20, each at 50 µM, using ultrafiltration. The 31 hits were tested in enzymatic and SPR assays, and compound 1 confirmed in both. Testing 54 commercially available analogs led to compound 2, with low micromolar activity, and this molecule was profiled intensively.

 

Kinetic studies revealed compound 2 to be non-competitive with respect to substrates ATP and L-methionine, and crystallography confirmed that compound 2 binds in a previously discovered allosteric pocket. The molecule makes polar and hydrophobic contacts to MAT2A and also displaces several water molecules. SAR studies found a preference for aromatic moieties at the 2-position of the central core, but the phenyl off the 3-position could be substituted with more shapely moieties such as the piperidine in compound 9.

 

Closer examination of the structure of compound 2 bound to MAT2A revealed a protein-bound water molecule, and displacing this with the phenol in AGI-24512 led to a satisfying boost in biochemical potency as well as cell activity. However, the molecule has poor oral absorption and a short half-life in rats. Metabolite identification studies pinned the blame on the piperidine, with the phenol no doubt doing no favors. Medicinal chemistry ultimately led to AGI-25696, which despite its lower biochemical activity was active in cells, metabolically stable, and showed efficacy in a mouse xenograft model when dosed orally.

 

Despite these favorable properties, AGI-25696 has very high protein binding in human plasma (>99.9%) as well as high efflux, which would likely necessitate a high clinical dose. The researchers proposed that, due to the weakly acidic nature of the molecule, it could tautomerize, and each tautomer could bind to different plasma proteins. Simply methylating the N-H led to decreased plasma protein binding but also lower binding to MAT2A. Thus, the researchers sought to shield the N-H by forming an intramolecular hydrogen bond. After appending more than 70 heterocycles they eventually arrived at AG-270, which has a more respectable plasma protein binding of around 98.5%.

 

Extensive characterization of AG-270 revealed it to be potent with good pharmacokinetics and oral bioavailability. It is relatively clean in a panel of 95 potential off-targets and showed tumor growth reduction in xenograft models. But it is not without warts: low solubility necessitated a spray-dried dispersion for animal dosing, not surprising giving its high lipophilicity. Nonetheless, the molecule has entered a phase 1 clinical trial in patients with MTAP loss.

 

This is a lovely fragment-to-lead success story with several lessons. First, although enzymes are sometimes considered “easy,” this is not necessarily true. Indeed, at an ACS meeting in 2018 Anil Padyana mentioned that metabolic enzymes in particular often have shallow, polar active sites. Targeting allosteric sites, as done here, can be a useful alternative.

 

Second, it is striking how the core of the initial fragment remains intact in the clinical compound, a reminder of the power of fragments to efficiently explore chemical space.

 

Finally, this story is another important reminder that affinity is often just the beginning of a long journey. It took considerable effort to optimize the pharmaceutical properties from AGI-24512 to AG-270, including a decrease in ligand efficiency. In the end the team has succeeded, and Practical Fragments wishes them – and the patients – luck in the trials.