Development of PDE6D and CK1α Degraders through Chemical Derivatization of FPFT-2216

Immunomodulatory drugs are a class of drugs approved for the treatment of multiple myeloma. These compounds exert their clinical effects by inducing interactions between the CRL4^CRBN E3 ubiquitin ligase and a C2H2 zinc finger degron motif, resulting in degradation of degron-containing targets. However, although many cellular proteins feature the degron motif, only a subset of those are degradable via this strategy. Here, we demonstrated that FPFT-2216, a previously reported "molecular glue" compound, degrades PDE6D, in addition to IKZF1, IKZF3, and CK1α. We used FPFT-2216 as a starting point for a focused medicinal chemistry campaign and developed TMX-4100 and TMX-4116, which exhibit greater selectivity for degrading PDE6D and CK1α, respectively. We also showed that the region in PDE6D that interacts with the FPFT-2216 derivatives is not the previously pursued prenyl-binding pocket. Moreover, we found that PDE6D depletion by FPFT-2216 does not impede the growth of KRAS^G12C-dependent MIA PaCa-2 cells, highlighting the challenges of drugging PDE6D-KRAS. Taken together, the approach we described here represents a general scheme to rapidly develop selective degraders by reprogramming E3 ubiquitin ligase substrate specificity.