Thunbergia laurifolia (TL) has been traditionally used as an antidote and an antipyretic drug by folk healers for centuries in Thailand. Rosmarinic acid (RA) is major compound in TL extract and has attracted great interest due to its potential broad pharmacological effects. Herein, the permeability of RA was investigated in TL extract and as a pure compound in a Caco-2 cell model by using high-performance liquid chromatography with a photodiode array detector (HPLC-PDA). The results reveal that the apparent permeability coefficient (Papp) values of RA in TL extracts and pure RA significantly increased after deconjugation by β-glucuronidase/sulfatase enzymes. Our findings exhibit possible saturable biotransformation of RA and/or membrane transport while penetrated through Caco-2 cells. The cumulative amounts of RA as pure compounds and in TL extracts increased with the exposure time, and the efflux ratio (ER) was 0.27–1.14. RA in the TL extract has a similar absorption in the conjugated form and in the pure compound. The intestinal absorption of them is through passive diffusion. Therefore, our findings conclude that the intestinal transport of RA in TL extracts was mainly penetrated as conjugated forms with glucuronic acid and/or sulfate across Caco-2 cells and transported via passive diffusion.

Thunbergia laurifolia has been a popular herb used in Thai traditional medicine for detoxification and as antipyretic. It contains rosmarinic acid (RA), caffeic acid (CA) and vitexin as major compounds. In order to control the herbal quality, the stability indicating high-performance liquid chromatography (HPLC) was developed and validated. The stability study of compounds in T. laurifolia leaf extract was investigated. The chromatographic separation was performed using a reversed-phase C18 column and mobile phase consisted of 0.5% acetic acid and methanol using a gradient elution with 1.0 mL/min flow rate. The detection wavelength was set at 330 nm. The method was validated for its linearity, precision, accuracy, limit of detection and limit of quantitation. Forced degradation of three compounds in extract showed that they were stable in oxidative condition, but highly labile under alkaline hydrolytic conditions. All three compounds in T. laurifolia leaf extract were stable at room temperature at least 3 months while a remarkable decrease of RA, vitexin and CA in the extract were found in accelerated condition. This finding could be applied for predicting the storage recommendation and expiry of T. laurifolia extract and its related pharmaceutical products.