PYC98

PYC98 is an 18–amino acid peptide inhibitor of c‑Jun N‑terminal kinase 1 (JNK1) that was designed as a substrate-based, retro‑inverso peptide to achieve isoform- and substrate-selective kinase inhibition. It preferentially inhibits JNK1-mediated phosphorylation of the transcription factor c‑Jun on Ser63 and, in its D‑amino-acid retro‑inverso form (D‑PYC98), also reduces phosphorylation of p38 and c‑Jun Ser63 during hyperosmotic stress, indicating broader modulation of stress-activated MAPK signaling.[3][5] PYC98 and D‑PYC98 are experimental research tools rather than approved therapeutics and are being explored preclinically as highly selective JNK1 pathway inhibitors with potential applications in oncology and other JNK‑driven diseases.[3][5]