Pelin KAYA

Oncogenic RAS mutations are found in ~19% of cancer patients with K-Ras4B (hereafter K-Ras) being the most frequently mutated isoform and a driver of cancer cell stemness. Since Ras proteins are difficult to target directly, alternative approaches are investigated. PDE6D is a trafficking chaperone for prenylated proteins and has been nominated as a surrogate target for K-Ras. By binding the farnesyl-moiety at the C-terminus of K-Ras, PDE6D promotes K-Ras diffusion in the cytoplasm and helps maintain K-Ras plasma membrane localization and activity.
Several high-affinity PDE6D inhibitors were developed. However, their cellular efficacy remained below expectation, mostly because the inhibitors were ejected from the PDE6D binding pocket by a GTP-Arl2/3-dependent mechanism. Our group has previously developed novel PDE6D inhibitors, called Deltaflexins, which harbor a ‘chemical spring’ to resist this ejection mechanism.
Our best third generation Deltaflexin shows the same in vitro activity against PDE6D as the previously developed Deltazinone. On-target activity in cells was demonstrated using Bioluminescence resonance energy transfer (BRET) biosensors to assess K-Ras vs. H-Ras selectivity and disruption of the PDE6D/ K-Ras interaction. In addition, we developed BRET-biosensors to determine off-target activity against UNC119a, a trafficking chaperone for myristoylated proteins that is highly related to PDE6D. While neither Deltaflexins nor Deltazinone inhibited the UNC119a/Src-interaction, Deltarasin displayed significant off-target activity, which may in part explain its broader toxicity. Furthermore, the anti-proliferative activity of Deltaflexins on a panel of cancer cell lines reflects the KRAS vs. HRAS mutant selectivity. This study illustrates a significant potential for the improvement of PDE6D inhibitors.

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