Herb pairs containing Angelicae Sinensis Radix (Danggui): A review of bio-active constituents and compatibility effects

Herb compatibility is one of the most important characteristics of traditional Chinese medicine (TCM). Rather than being used singly, Chinese herbs are often used in formulae to obtain synergistic effects or to diminish possible adverse reactions. Herb pair, the most fundamental and simplest form of multi-herb formulae, is a centralized representative of herb compatibility. Danggui (Angelicae Sinensis Radix), a widely used Chinese medicine, is usually combined with another herb to treat women's diseases in the clinic. A series of herb pairs containing Danggui have gradually become a focus of modern research, and they exhibit encouraging prospects for development.A systematic search for studies related to herb pairs containing Danggui was performed via a library search (books, theses, reports, newspapers, magazines, and conference proceedings) and an electronic search (Web of Science, PubMed, and Google Scholar). These sources were scrutinized for information on Danggui herb pairs.Based on a previous statistical analysis, a database containing 16,529 formulae of Danggui from the "Dictionary of Traditional Chinese Medicine Formulae" was reviewed. The results showed a high frequency of compatibility between Danggui and other 22 herbs. The most common ratio among these chosen herb pairs was 1:1, and a majority of the pairs were applied for the treatment of diseases in internal medicine. The present paper reviews ethnopharmacology and advances in variations of the bio-active components and compatibility effects of the herb pairs containing Danggui, especially Danggui-Huangqi, Danggui-Chuanxiong, and Danggui-Shaoyao, which are used at high frequency. It was also observed that there were fewer studies of Danggui-Fuzi, Danggui-Huanglian, Danggui-Gancao, Danggui-Fangfeng and Danggui-Ganjiang, although they have been recorded in classical books as commonly used herb pairs. Moreover, some herb pairs such as Danggui-Niuxi and Danggui-Chaihu have been used at high frequency according to the statistical analysis, however, they were not recognized as herb pairs in many relevant books.Recently, several TCM researchers have become interested in investigating the bio-active constituents and compatibility effects of herb pairs. Thus, some methods for in-depth study of herb pairs are essential to be established. The in vitro or in vivo bio-active constituents of herb pairs may differ from those of the single herbs. Additionally, comparative methods should be applied to study not only the bio-active constituents but also the effects of herb pairs. Study of component compatibility may be considered when the bio-active constituents and effects of an herb pair have been definitively demonstrated. Overall, the goal of our basic study of herb pairs should be their clinical application and the development of new drugs.