KS 502

Calmodulin plays an important role in cellular proliferation as part of a signal transduction pathway activated by phospholipase C. Drugs that block the ability of calmodulin to bind to and activate its target enzymes inhibit the growth of a wide variety of malignant cells. To identify more potent and selective inhibitors of this potential target for new drug development, we studied two recently synthesized compounds, KS-501 and KS-502, for their activity against calmodulin-sensitive enzymes and for their ability to block the growth of parental and multidrug-resistant leukemic cells. KS-501 and KS-502 inhibited the activation of a calmodulin-sensitive cyclic nucleotide phosphodiesterase. The mechanism of enzyme inhibition was through interfering with calmodulin activation rather than through a direct effect on the enzyme. KS-501 was more potent than KS-502 and was studied in greater detail. This compound inhibited the activation of calmodulin kinase I and II, but had less effect against cyclic adenosine 3',5'-monophosphate (cyclic AMP)-sensitive kinase. KS-501 was also more effective than KS-502 in inhibiting the growth of sensitive L1210 leukemic lymphocytes. Both compounds were less effective inhibitors of multidrug-resistant L1210 leukemia than of the parental line. These studies identify a new class of calmodulin inhibitor, with selectivity for calmodulin-dependent kinases over cyclic AMP-dependent protein kinase. Since the total synthesis of the KS-compounds has been accomplished, it should now be possible to develop derivatives with greater activity and selectivity.

New inhibitors of Ca2+ and calmodulin-dependent cyclic-nucleotide phosphodiesterase, KS-501 and KS-502 were isolated from a fungus, Sporothrix sp. KAC-1985. Inhibitory concentration causing 50% inhibition (IC50) values of KS-501 and KS-502 for bovine brain enzyme were 1.8 and 4.3 microM, respectively. The compounds exhibited no or weak inhibition for calmodulin-independent cyclic-nucleotide phosphodiesterases and protein kinase C.

The structures of KS-501 and KS-502, new inhibitors of Ca2+ and calmodulin-dependent cyclic nucleotide phosphodiesterase, were determined to be 2-(beta-D-galactofuranosyloxy)-6-heptyl-4-hydroxybenzoic acid 3-heptyl-5-hydroxyphenyl ester and 2-(beta-D-galactofuranosyloxy)6-heptyl-4-hydroxybenzoic acid 4-carboxy-3-heptyl-5-hydroxyphenyl ester, respectively, on the basis of chemical and physico-chemical evidences.