Non-Cytotoxic Inhibition of Macrophage Eicosanoid Biosynthesis and Effects on Leukocyte Functions and Reactive Oxygen Species of Two Novel Anti-Inflammatory Plant Diterpenoids
Two diterpenoids were prepared from hexane anti-inflammatory extracts of the Spanish herb Sideritis javalambrensis: ent-13-epi-12α-acetoxymanoyl oxide (= “manoyl oxide F1”) and ent-8α-hydroxy-labda-13(16),14-diene (= “labdane F2”). They were evaluated for possible anti-inflammatory actions in vitro in the concentration range 10-7 M to 10-4 M, and were compared with aspirin, sodium salicylate, and indomethacin. Neither compound affected superoxide generation or scavenging and they did not inhibit non-enzymatic lipid peroxidation. Neither compound affected azurophil granular enzyme secretion from activated human and rat neutrophils. The diterpenoids were not toxic to the leukocytes or to washed human erythrocytes up to 3 × 10-5 M but at 10-4 M some leakage of LDH or haemolysis was observed. However, both F1 and F2 inhibited prostaglandin E2 generation in cultured mouse peritoneal macrophages stimulated by zymosan, ionophore A23187, melittin, and PMA. Labdane F2 was more potent (approximate IC50 = 3 µM in zymosan-activated macrophages). We conclude that these two natural products interact with the eicosanoid system, but do not interfere with the other tested leukocyte functions or with reactive oxygen species, and are essentially non-toxic at sub-maximal doses. This biochemical profile distinguishes these diterpenoids from the anti-inflammatory poly-phenolics such as flavonoids obtained from the genus Sideritis, and suggests that medicinal decoctions of these plants are likely to owe any anti-inflammatory activity to more than one bioactive ingredient.