The activity of cGMP-dependent protein kinase (PKG) is controlled by factors that elevate cellular cGMP, like nitric oxide (NO), and by those that reduce cGMP levels, like certain phosphodiesterases. KT 5823 is a potent, selective inhibitor of cGMP-dependent protein kinase (PKG) (in vitro IC50 = 234 nM).1 KT 5823 is cell-permeable and is often used in intact cells to assess the role of PKG in signaling, although there are cases where it poorly inhibits PKG in cells.[2] KT 5823 is a weak inhibitor of PKC (Ki = 4 μM) and PKA (Ki >10 μM).[1]

Reference:
[1]. Hidaka, H., and Kobayashi, R. Pharmacology of protein kinase inhibitors. Annu. Rev. Pharmacol. Toxicol. 32, 377-397 (1992).
[2]. Burkhardt, M., Glazova, M., Gambaryan, S., et al. KT5823 inhibits cGMP-dependent protein kinase activity in vitro but not in intact human platelets and rat mesangial cells. J. Biol. Chem. 275(43), 33536-33541 (2000).