Silybin A

Silybin A, aka silibinin A, is a natural product produced in the fruits of the two species of milk thistle: Silybum eburneum, which grows in Spain and northern Africa, and the more common S. marianum, which is native to many countries around the world. The molecule has a diastereomer, silybin B¹, that is formed in equal amounts by the plants; the combination of the two is simply called “silybin”. The isomers are contained in silymarin², the fruit extract that also contains other flavonolignans.

In 1965, H. Wagner, L. Hörhammer, and R. Münster at the University of Munich (West Germany) reported the structure of what they called “new flavonoids” from the fruit of S. marianum, one of which was silybin A. They also isolated the flavonolignan’s pentaacetate³. Three years later, Andrew Pelter at the University of Manchester (UK) and R. Hänsel at the Free University Berlin used NMR to reexamine and correct its structure and called it “the first flavonolignan”.

Silymarin has been used in traditional medicine for centuries; it is even alluded to in the Old Testament (Genesis 3:18). Ancient physicians used it to treat gallbladder disorders and liver dysfunction. In the 1970s, the World Health Organization classified silymarin as an official medicine for protecting the liver.

Since its discovery, researchers have investigated silybin for medicinal properties. In 2017, Michal Bijak at the University of Łódź (Poland) wrote an extensive review of the chemistry, bioavailability, and metabolism of silybin. The author concluded that “silybin is a very interesting chemical compound, the use of which as a dietary supplement is increasing all around the world, and this explains the recent studies aimed at increasing its oral bioavailability.”

It is noteworthy that Bijak referred to silybin as a supplement and not a pharmaceutical. The US Food and Drug Administration has not approved it for any medical use, but it has registered silymarin as a dietary supplement. Silybin and its more bioavailable derivatives are approved in Europe for treating liver poisoning, such as that caused by eating toxic mushrooms.

In 2022, Anton Gillessen at Herz-Jesu-Hospital (Münster, Germany), Xiaoyuan Xu at Peking University Health Science Center (Beijing), and 11 colleagues at multiple institutions summarized the international consensus recommendations for using silymarin to treat toxic liver disease (TLD). The authors found that “silymarin as an antioxidant and hepatoprotective agent is safe and has reasonable clinical efficacy in TLD.”

1. CAS Reg. No. 142797-34-0.
2. CAS Reg. No. 65666-07-1.
3. CAS Reg. No. 36804-18-9.