Fragments in the clinic: PLX3397

Practical Fragments covers a wide variety of journals. J. Med. Chem., Bioorg. Med. Chem. Lett., Drug Disc. Today, and ACS Med. Chem. Lett. are all well-represented, but we also range further afield, from biggies such as Nature and Science to more niche titles such as ChemMedChem, Acta. Cryst. D., and Anal. Chim. Acta. The increasingly clinical relevance of fragment-based approaches is highlighted by a recent paper by William Tap and a large group of collaborators appearing in the New England Journal of Medicine. This reports on the results of the Daiichi Sankyo (née Plexxikon) drug PLX3397 in a phase I trial for tenosynovial giant-cell tumor, a rare but aggressive cancer of the tendon sheath.

The story actually starts with a 2013 paper by Chao Zhang and his Plexxikon colleagues in Proc. Nat. Acad. Sci. USA. The researchers were interested in inhibiting the enzymes CSF1R (or FMS) and KIT; both kinases are implicated in cancer as well as inflammatory diseases. The team started with 7-azaindole, the same fragment they used to discover vemurafenib. Structural studies of an early derivative, PLX070, revealed a hydrogen bond between the ligand oxygen and a conserved backbone amide. Further building led to PLX647, with good activity against both CSF1R and KIT. Selectivity profiling against a panel of 400 kinases revealed only two others with IC₅₀ values < 0.3 µM. The molecule was active in cell-based assays, had good pharmacokinetics in mice and rats, and was active in rodent models of inflammatory disease.

The new paper focuses on the results of a clinical trial with PLX3397, a derivative of PLX647. Despite its close structural similarity to PLX647, it binds to CSF1R in a slightly different manner. Both inhibitors bind to the inactive form of the kinase, but PLX3397 also recruits the so-called juxtamembrane domain of the kinase to stabilize this autoinhibited conformation. Pharmacokinetic and pharmacodynamics studies in animals were also positive.

Tenosynovial giant-cell tumor seems to be dependent on CSF1R, so the researchers performed a phase 1 dose-escalation study with an extension in which patients treated with the chosen phase 2 dose were treated longer. Of the 23 patients in this extension, 12 had a partial response and 7 had stable disease. A quick search of clinicaltrials.gov reveals that PLX3397 is currently in multiple trials for several indications, including a phase 3 trial for giant cell tumor of the tendon sheath.

Several lessons can be drawn from these studies. First, as the authors note, one fragment can give rise to multiple different clinical candidates. Indeed, in addition to vemurafenib, 7-azaindole was also the starting point for AZD5363. This is a good counterargument to those who believe that novelty is essential in fragments.

A second, related point is that selectivity is also not necessary for a fragment. The fact that 7-azaindole comes up so frequently as a kinase-binding fragment has not prevented researchers from growing it into remarkably selective inhibitors. An obvious corollary is that even subtle changes to a molecule can have dramatic effects: the added pyridyl nitrogen in PLX3397 is essential for stabilizing a unique conformation of the enzyme.

Finally, careful patient selection is critical to answering biological questions. I confess that I had never heard of tenosynovial giant-cell tumor, nor the role of CSF1R, but I’m glad others had. I look forward to seeing an increasing stream of fragment papers in clinical journals.