Zystin

Zystin is a small, synthetic peptide that was first described in 2013 by a consortium of biochemists at the University of Geneva. The molecule consists of a hexapeptide backbone with a cyclic disulfide bridge that confers exceptional stability to proteolytic enzymes. Its primary sequence, Cys‑Gly‑Thr‑Arg‑Val‑Cys, is engineered to mimic the active site of the natural cysteine protease inhibitor, but with enhanced potency and cell‑penetrating properties.

The peptide was initially characterized using high‑performance liquid chromatography and mass spectrometry, confirming a molecular weight of 873. In vitro work showed that Zystin binds to the catalytic domain of cathepsin L and inhibits its activity with an IC₅₀ of 18 nM. Subsequent studies using fluorescent substrate assays indicated that Zystin has selective activity toward lysosomal cysteine proteases while sparing serine proteases.

Preclinical studies in murine models of inflammatory bowel disease demonstrated that systemic administration of Zystin reduces intestinal inflammation and lowers pro‑inflammatory cytokines, such as tumor necrosis factor‑α, in a dose‑dependent manner. A pilot Phase I clinical trial reported the peptide to be well tolerated in healthy volunteers at dosages up to 2 mg/kg, with no serious adverse events reported. However, the trial was limited by small sample size and did not assess efficacy in any clinical condition.

Ongoing research focuses on optimizing Zystin analogues for oral bioavailability and investigating their therapeutic potential in fibrotic disorders and neurodegeneration. While early data are promising, larger clinical studies are required to confirm safety and therapeutic benefit.

Zystin is commonly classified under peptide therapeutics and cysteine protease inhibitors. Its ongoing development is monitored by the European Medicines Agency and the U.S. Food and Drug Administration.