Modulation of ion channels by natural products - identification of hERG channel inhibitors and GABAA receptor ligands from plant extracts

Ion channels are expressed in virtually all cell types in the human body and are involved in various physiological processes. Hence, it is not surprising that ion channels play an important role in modern drug discovery. Lead compounds are nowadays routinely tested against a panel of ion channels to evaluate selectivity and potential off-target activities.
The human ether-à-go-go-related gene (hERG) channel, a voltage-gated potassium channel, is the currently most critical antitarget with respect to cardiac safety. Inhibition of the hERG channel can prolong the QT interval on the electrocardiogram (ECG) and, as a consequence, lead to life-threatening arrhythmia. Considering the daily intake of plant-derived foods and herbal products, surprisingly few natural products have been tested for hERG blocking properties. In the course of an interdisciplinary hERG project, a selection of widely used herbal drugs and dietary plants was screened by means of a two-microelectrode voltage-clamp assay with Xenopus oocytes. Moderate hERG block was observed for the traditional Chinese herbal drug Coptidis rhizoma and black pepper fruits, and successfully tracked by HPLC-based activity profiling to dihydroberberine and piperine, respectively. The hERG blocking activity of cinnamon, guarana, and nutmeg, in contrast, was attributed to tannins. Our screening data suggest that major European medicinal plants and frequently consumed food plants are associated with a low risk for hERG inhibition. However, the case of Coptidis rhizoma pointed towards a need for a more thorough assessment of herbal drugs of the traditional Chinese medicine (TCM). Subsequent screening of a plant-derived alkaloid library led to the identification of several potent hERG blockers. Further investigations are certainly warranted to assess the cardiac safety profile of these alkaloids.
Dehydroevodiamine (DHE), a major bioactive constituent of the traditional Chinese herbal drug Evodiae fructus, has been previously shown to inhibit several cardiac ion currents in vitro. For further evaluation of its in vivo pharmacological and toxicological properties, gram amounts of DHE were needed. Since DHE is not commercially available, we developed an efficient method for its gram-scale isolation from a crude Evodia extract. Our approach is based on a combination of cation-exchange chromatography and preparative RP-HPLC. Moreover, the DHE content in commercially available Evodia products was assessed by HPLC-PDA analysis. A daily intake of up to mg amounts of DHE was calculated from recommended doses of these products. We also devised a procedure for the production of DHE-depleted Evodia products, should DHE indeed turn out to be toxicologically relevant.
The gamma-aminobutyric acid type A (GABAA) receptor, a ligand-gated chloride channel, mediates fast inhibitory neurotransmission in the central nervous system (CNS), and is thus a clinically important drug target. In the search for positive α1β2γ2S GABAA receptor modulators of plant origin, we investigated an extract of Curcuma kwangsiensis rhizomes. HPLC-based activity profiling was used in combination with a two-microelectrode voltage-clamp assay on Xenopus oocytes to identify the active constituents. Targeted isolation afforded a series of 11 structurally related labdane diterpenoids, including four new natural products. Structure elucidation was achieved by comprehensive analysis of HR-ESI-TOF-MS and NMR data. The absolute configuration of the compounds was assigned by electronic circular dichroism (ECD). The highest GABAA receptor modulating activity was observed for zerumin A.
From a more general perspective, this study demonstrates that HPLC-based activity profiling is an effective strategy to characterize bioactive compounds in crude natural extracts.