Immunosuppressant flavonoids from Scutellaria baicalensis

Some plants used in Traditional Chinese Medicine serve as treatment for disease states where a suppression of the cellular immune response is desired. However, the compounds responsible for the immunosuppressant effects of these plants are not necessarily known. The immunosuppressant compounds in the roots of Scutellaria baicalensis, one of the most promising plants identified in a previous screening, were tracked by HPLC activity profiling and concomitant on-line spectroscopic analysis. Compounds were then isolated by preparative chromatography, and structures elucidated by spectroscopic methods. Twelve flavonoids (5-16) were identified from the active time windows, and structurally related flavones 2, 4, and 17, and flavanones 1 and 3 were isolated from adjacent fractions. All flavonoids possessed an unusual substitution pattern on the B-ring, with an absence of substituents at C-3 and C-4. Compounds 11, 13, 14, and 16 inhibited T-cell proliferation (IC₅₀ values at 12.1-39 μM) at non-cytotoxic concentrations. The findings may support the use of S. baicalensis in disorders where a modulation of the cellular immune response is desirable.