Phytochemical Analysis and Anti-Ulcer Potential of Phenolic Compounds of Inonotus nidus-pici Pilát

Background/Objectives: Fungotherapy has long been recognized as a therapeutic approach for treating and preventing various diseases. As an important representative of the so-called functional mushrooms, Chaga plays a crucial role in this system. Since this species is of limited distribution in Bulgaria, we are interested in studying a related but different species, Inonotus nidus-pici Pilát, with potential benefits for human health. Methods: The phytochemical composition of phenolic compounds in the studied species was analyzed using spectrophotometric methods and high-performance liquid chromatography (HPLC). Additionally, antioxidant activity was assessed using various assays, and the gastroprotective effect was evaluated in experimental rat models with indomethacin-induced gastric damage. Results: The quantities of the main classes of phenolic compounds in the studied object were determined, and an enriched phenolic extract (EPE) was obtained. The amount of phenolic compounds, in decreasing order, is as follows: tannins (1.67 ± 0.02%), phenolic acids (1.50 ± 0.09%), and flavonoids (1.24 ± 0.04%). Quercetin was the most present flavonoid (15.95 ± 0.05 mg/g DWE), followed by (+)-catechin (9.86 ± 0.15 mg/g DWE) and kaempferol (1.67 ± 0.09 mg/g DWE) in the enriched phenolic extract. The quantity of other established compounds was significantly lower. Of all ten phenolic acids identified in the same extract, the highest concentration was found only for rosmarinic acid (6.41 ± 0.08 mg/g DWE) and somewhat for p-coumaric acid (2.13 ± 0.12 mg/g DWE). Among all the applied methods regarding antioxidant activity, the highest potential of the extract for reducing copper ions was the most pronounced (1506.93 μM TE/g DWE), and the ability of the extract to reduce iron ions was almost the same (1354.05 μM TE/g DWE). In the experimental indomethacin-induced gastric ulcer rat model, EPE (25 mg/kg and 10 mg/kg) demonstrated a dose-dependent gastroprotective effect. Conclusions: The results of the experiments confirm the potential of the wood fungus species as a source of valuable biologically active compounds with beneficial and pharmacological effects. However, further studies are needed to fully determine its chemical composition and the biological activities related to it.