Fragments vs JAK – but phototoxicity

The four members of the JAK family of kinases have received plenty of attention due to their role in inflammation. Two drugs that inhibit these targets, tofacitinib and ruxolitinib, have been approved for rheumatoid arthritis and myelofibrosis, respectively. Psoriasis is another possible indication, but for this disease a topical drug might be useful, particularly one with low systemic bioavailability. The search for such molecules is the subject of a paper recently published in ACS Med. Chem. Lett. by Andrea Ritźen and colleagues at LEO Pharma.

The researchers screened 500 fragments at 100 µM each using surface plasmon resonance (SPR) against JAK2. Hits were then tested in a biochemical assay against JAK1; in general, there was good correlation, suggesting a (desirable) lack of selectivity between the two family members. One of the more attractive hits was compound 1, which was characterized crystallographically bound to JAK2.

Compound 1 is an indazole, which is often seen in kinase inhibitors binding to the so-called hinge binding site where the adenine of ATP binds. Other indazoles have previously been reported as JAK2 inhibitors. Nonetheless, the sulfonamide moiety of compound 1 provides an interesting new vector. Moreover, a search of published structures suggested that adding a phenol moiety could make additional contacts to the proteins. This led to the design and synthesis of compound 2, which showed a dramatic boost in potency as well as measurable cell activity. Further optimization led ultimately to compound 34, with low nanomolar biochemical activity and mid-nanomolar cell activity. Compound 34 was also reasonably selective in a panel of 20 kinases.

A topical drug needs to be stable in sunlight, but unfortunately compound 34 showed phototoxicity. This led to the testing of a few other compounds, revealing that even the initial fragment 1 is unstable over a period of a few hours in simulated outdoor light. Indazole itself seems reasonably stable, suggesting that perhaps adding different substituents could fix the problem.

This is a brief but satisfying example of using published information as well as medicinal chemistry to advance a fragment hit. Although the program does not appear to have led to a drug lead, it is laudable that the researchers describe the photoinstability of the indazoles. With these moieties appearing so frequently in campaigns against kinases, this could be a valuable cautionary tale to others pursuing similar scaffolds.