Bronchodilatory effects of S-isopetasin, an antimuscarinic sesquiterpene of Petasites formosanus, on obstructive airway hyperresponsiveness. Lin LH, Huang TJ, Wang SH, Lin YL, Wu SN, Ko WC. Eur J Pharmacol. 2008 Apr 28;584(2-3):398-404. doi: 10.1016/j.ejphar.2008.02.034. 

Differential inhibition of inflammatory effector functions by petasin,isopetasin and neopetasin in human eosinophils. Thomet OA, Wiesmann UN, Blaser K, Simon HU. Clin Exp Allergy. 2001 Aug;31(8):1310-20.