S-trityl-L-Cysteine is a non-natural amino acid and an inhibitor of Eg5, also known as KSP and Kif11, a mitotic kinesin necessary for mitotic spindle formation. S-trityl-L-Cysteine inhibits the ATPase activity of Eg5 in basal and microtubule-stimulated states (IC50s = 1,000 and 140 nM, respectively). It is selective for Eg5 over nine other human kinesins in an enzyme-coupled assay. It reversibly inhibits Eg5-driven microtubule sliding velocity with an IC50 value of 500 nM using X. laevis recombinant Eg5. It induces cell cycle arrest in HeLa cells (IC50 = 700 nM), reversibly halting the cell cycle in the mitotic phase by inhibiting the separation of duplicated chromosomes and preventing bipolar spindle formation. S-trityl-L-Cysteine inhibits the growth of cancer cells in vitro when tested against the National Cancer Institute (NCI) 60 human cancer cell line panel (average GI50 = 1.3 μM) and in mouse xenograft models.

In vitro, S-trityl-L-Cysteine targets the catalytic domain of Eg5 and inhibits Eg5 basal and microtubule-activated ATPase activity as well as mant-ADP release. S-trityl-L-Cysteine is a tight binding inhibitor (estimation of K(i,app) <150 nm at 300 mm NaCl and 600 nm at 25 mm KCl). S-trityl-L-Cysteine binds more tightly than monastrol because it has both an approximately 8-fold faster association rate and approximately 4-fold slower release rate (6.1/microM/s and 3.6/s for S-trityl-L-Cysteine versus 0.78 /microM/s and 15/s for monastrol). S-trityl-L-Cysteine inhibits Eg5-driven microtubule sliding velocity in a reversible fashion with an IC50 of 500 nm[2].