Chiang Mai J. Sci. 2014; 41(1) 117 Chiang Mai J. Sci. 2014; 41(1) : 117-127 http://epg.science.cmu.ac.th/ejournal/ Contributed Paper Antioxidant-guided Isolation of Rosmarinic Acid, a Major Constituent from Thunbergia laurifolia, and Its Use as a Bioactive Marker for Standardization Pipob Suwanchaikasem, Chaiyo Chaichantipyuth and Suchada Sukrong* Department of Pharmacognosy and Pharmaceutical Botany, Faculty of Pharmaceutical Sciences, Chulalongkorn University, Bangkok 10330, Thailand. *Author for correspondence; e-mail: suchada.su@chula.ac.th Received: 31 October 2012 Accepted: 19 March 2013 ABSTRACT Growing interest in the commercialization of Thunbergia laurifolia leaves as an herbal tea and the use of its extracts as a nutraceutical has led to increased research surrounding its bioactive components and the standardization of the plant. Guided isolation through thin-layer chromatography (TLC) bioautography using the 2, 2- diphenyl-1-picrylhydrazyl (DPPH) radical as a detection reagent led to the isolation of a potent antioxidant compound, rosmarinic acid, from the leaves of T. laurifolia. This study is the first report of the isolation of rosmarinic acid from T. laurifolia. Rosmarinic acid showed significant DPPH-scavenging capacity with an EC 50 value of 2.71 μg/ml. The high-performance liquid chromatography (HPLC) quantification of rosmarinic acid was accomplished. The developed method was applied to determine the amount of rosmarinic acid in T. laurifolia samples collected from different locations. The results demonstrated high variability in the amount of rosmarinic acid present in the samples, which correlated well with the observed antioxidant capacities detected by the free- radical scavenging assay. The HPLC method was also validated in terms of linearity, precision, and accuracy. The established HPLC method for the determination of rosmarinic acid, a bioactive principle, is a useful approach for standardization of the raw material and herbal preparations containing T. laurifolia. Keywords: Thunbergia laurifolia, rosmarinic acid, bioactivity-guided isolation, HPLC; DPPH-scavenging activity, Acanthaceae 1. INTRODUCTION Thunbergia laurifolia Lindl., commonly known in Thai as “Rang Chuet”, is a woody climbing plant belonging to the Acanthaceae family. In Thailand, the decoction of the leaves and roots of this plant is traditionally used as an antidote for poisoning caused by insecticides, ethyl alcohol, arsenic, and strychnine [1]. Currently, herbal teas, powders, and capsule preparations of T. laurifolia are commonly available in the herbal and nutraceutical markets. Despite its common use, no commercially available preparations with