- Polysaccharide-K (PSK), also known as Krestin, is a clinically significant protein-bound β-glucan complex extracted from the fruiting body of the mushroom Trametes versicolor (commonly called turkey tail).
- It is one of the most extensively studied mushroom-derived biological response modifiers (BRMs) and has been developed into an approved anticancer adjuvant therapy in Japan since the 1970s. Chemically, PSK is a polysaccharide-protein complex, consisting mainly of β-(1→4)- and β-(1→6)-linked glucose residues with additional mannose, galactose, and fucose residues, bound to about 25–38% protein. This unique molecular structure stabilizes the β-glucan conformation, enhances solubility, and is believed to be essential for its bioactivity.
- The primary mechanism of PSK lies in its immunomodulatory effects. It activates both the innate and adaptive immune systems by stimulating macrophages, dendritic cells, T lymphocytes, and natural killer (NK) cells. These immune responses are mediated through recognition of the β-glucan components by pattern recognition receptors (PRRs) such as Dectin-1, Toll-like receptors (TLRs), and complement receptors. Activation of these pathways enhances cytokine production, promotes antigen presentation, and increases cytotoxic activity against tumor cells. Rather than acting as a cytotoxic drug, PSK functions by boosting the host’s immune defense, improving the body’s ability to control tumor growth and resist infections.
- Clinically, PSK has been widely used as an adjunct therapy in cancer treatment, particularly in Japan and other parts of East Asia. It is prescribed alongside chemotherapy and radiotherapy for a range of cancers, including gastric, colorectal, esophageal, lung, and breast cancers. Numerous clinical trials have demonstrated that patients receiving PSK in combination with standard therapies have improved survival rates, reduced recurrence, and enhanced quality of life compared to those receiving conventional therapy alone. It has also been reported to reduce chemotherapy side effects such as immune suppression and fatigue, making it valuable in supportive care.
- Beyond oncology, PSK exhibits a wide range of biological activities, including antiviral, antibacterial, antioxidant, and hepatoprotective effects. It has shown potential in enhancing resistance to infections, regulating gut microbiota, and protecting against organ damage caused by oxidative stress or toxins. These broad pharmacological actions have reinforced PSK’s role as a multifunctional therapeutic agent and functional food component.
- From a pharmaceutical perspective, PSK is noteworthy for being one of the few mushroom-derived compounds formally integrated into modern medicine. While many mushroom polysaccharides remain in the domain of nutraceuticals and dietary supplements, PSK has undergone rigorous clinical testing and regulatory approval. Its success has inspired further research into mushroom-derived polysaccharide-protein complexes, such as PSP (polysaccharopeptide), which is chemically similar but distinct in composition.
