Prostaglandin E2 (PGE2), the most relevant eicosanoid promoting inflammation and
tumorigenesis, is formed by cyclooxygenases (COXs) and PGE2 synthases from free arachidonic
acid. Preparations of the leaves of Salvia officinalis are commonly used in folk medicine as an
effective antiseptic and anti-inflammatory remedy and possess anticancer activity. Here, we demonstrate that a standard ethyl acetate extract of S. officinalis efficiently suppresses the
formation of PGE2 in a cell-free assay by direct interference with microsomal PGE2 synthase
(mPGES)-1. Bioactivity-guided fractionation of the extract yielded closely related fractions that
potently suppressed mPGES-1 with IC50 values between 1.9 and 3.5 μg/ml. Component analysis
of these fractions revealed the diterpenes carnosol and carnosic acid as potential bioactive
principles inhibiting mPGES-1 activity with IC50 values of 5.0 μM. Using a human whole-blood
assay as a robust cell-based model, carnosic acid, but not carnosol, blocked PGE2 generation upon
stimulation with lipopolysaccharide (IC50 = 9.3 μM). Carnosic acid neither inhibited the
concomitant biosynthesis of other prostanoids [6-keto PGF1α, 12(S)-hydroxy-5-cis-8,10-transheptadecatrienoic
acid, and thromboxane B2] in human whole blood nor affected the activities of
COX-1/2 in a cell-free assay. Together, S. officinalis extracts and its ingredients carnosol and
carnosic acid inhibit PGE2 formation by selectively targeting mPGES-1. We conclude that the
inhibitory effect of carnosic acid on PGE2 formation, observed in the physiologically relevant
whole-blood model, may critically contribute to the anti-inflammatory and anticarcinogenic
properties of S. officinalis.