Imatinib is the early poster child of personalized medicine. The drug famously works by binding to the mutant kinase BCR-ABL1, and its approval by the FDA for chronic myelogenous leukemia in 2001 arguably launched hundreds of programs targeting kinases. Although imatininb is remarkably effective, resistance sometimes develops, usually caused by mutations that lead to loss of affinity for the drug. Imatinib and other approved drugs that target BCR-ABL1 all bind in the active (ATP-binding) site of the kinase, and they all have various off-targets that can lead to toxicity. To sidestep these issues, researchers at Novartis have developed an allosteric inhibitor, as described by Andreas Marzinzik and colleagues in a new paper in J. Med. Chem.
The ABL1 kinase is naturally autoinhibited by the binding of a myristoyl group to an allosteric pocket. Although the pocket exists in BCR-ABL1, the site that is normally myristoylated is lost. The researchers wanted to create a molecule that would mimic the function of the myristoyl group and exert its inhibitory effect within the allosteric pocket.
An NMR-based screen of 500 fragments yielded 30 hits – perhaps not a surprisingly high hit rate given the lipophilicity of the pocket. Compound 2, with low micromolar affinity, had a high ligand efficiency. Unfortunately, it and similarly high-affinity fragments showed no cell-based activity. A crystal structure of compound 2 bound to the protein revealed that, although the fragment binds in the myristate pocket, its binding mode would actually prevent the conformational change necessary for allosteric inhibition. Tweaking and growing the fragment led to molecules such as compound 4, which were still inactive.
To determine why, the researchers developed a clever NMR assay based on a specific valine residue located in a disordered region of the protein that becomes helical in the allosterically inhibited state of the protein. This assay allowed them to distinguish which protein conformation molecules bound and revealed that, contrary to design, compound 4 did not in fact bind to the inhibited form of the protein. Other researchers had found a different series of molecules that also bind in the myristate pocket, and these all contained a trifluoromethoxy group. When this moiety was grafted onto compound 4, the resulting compound 5 showed cell-based activity.
Now the medicinal chemistry began in earnest. Crystallography revealed a lipophilic cleft in the allosterically inhibited form of the protein which could be filled with a pyrimidine, and the cationic solubilizing group in compound 5 was replaced by the neutral moiety in compound 7. This compound showed some hERG channel inhibition, which could be fixed by replacing the pyrimidine with a pyrazole. Also, crystallography revealed that there was a little extra space near one of the fluorine atoms, which could be replaced with a chlorine in the clinical compound asciminib (ABL001). A crystal structure of this molecule shows it binding to the inactive conformation of the protein (the helix that forms is in the upper right).
Asciminib effectively inhibits proliferation of cells containing either wild-type or T315I BCR-ABL1, the latter being one of the more pernicious resistance mutations. The compound is also highly selective against > 60 other kinases, and is only active against CML cell lines in a panel of 546 cancer cell lines, suggesting that it should be well tolerated. Mouse xenograft models were also impressive, and the compound is currently in a phase 3 clinical trial.
This is a thorough, clearly written account combining biophysics, modeling, chemistry, and biology to discover a first-in-class drug. It is also a useful reminder that binding alone may not be sufficient to cause desired effects. As with all the clinical-stage programs, Practical Fragments wishes everyone involved the best of luck!