Calphostin C is a metabolite of the fungus C. cladosporioides that specifically inhibits protein kinase C (PKC) (IC50 = 50 nM versus an IC50 > 50 μM for PKA) by competing at the binding site for diacylglycerol and phorbol esters. As a polycyclic hydrocarbon with strong absorbance in the visible and ultraviolet ranges, activation of this compound is strictly dependent on exposure to light. Independent of PKC, calphostin C also directly inhibits phospholipase D1 and D2 (IC50s = 100 nM). It demonstrates antitumor activity, inhibiting cell growth and promoting apoptosis in HeLa S3 and MCF-7 cells with IC50 values of 0.23 and 0.18 μM, respectively.
