A Spectrochemically Driven Study: Identifying Phenolic-Rich Extracts from Helichrysum stoechas, Lavandula pedunculata, and Thymus mastichina with Potential to Revert Skin Aging Effects

<b>Background/Objectives</b>: Plants inhabiting mediterranean-influenced climatic zones, like <i>Helichrysum stoechas</i> (L.) Moench subsp. <i>stoechas</i>, <i>Lavandula pedunculata</i> (Mill.) Cav., and <i>Thymus mastichina</i> (L.) L. subsp. <i>mastichina</i>, have been scarcely investigated regarding their richness in phenolic compounds, herein explored as sources of skin anti-aging compounds. <b>Methods</b>: In this investigation, Fourier transform infrared spectroscopy (FTIR) in attenuated total reflectance (ATR) mode and high-performance liquid chromatography coupled with diode-array detection and electrospray ionization tandem mass spectrometry (HPLC-DAD-ESI/MSⁿ) were employed to chemically characterize the hydroethanolic extracts (HEs), and their cell-free antioxidant potential was screened. Thereafter, non-toxic concentrations of HEs were determined in human skin cells using Alamar blue^® and Sulforhodamine B assays. The cytoprotective and antioxidant effects of HEs were assessed in <i>tert</i>-butyl hydroperoxide-stimulated fibroblasts, their anti-inflammatory potential was studied in lipopolysaccharide-injured macrophages, and enzymatic inhibition assays were performed. Notably, the irritant effects of HEs were tested according to Test Guideline No. 439 of the Organization for Economic Co-operation and Development (OECD). <b>Results</b>: The major compounds identified in the <i>T. mastichina</i> and <i>L. pedunculata</i> HEs were rosmarinic and salvianolic acid derivatives, while <i>H. stoechas</i> HE was mainly composed of caffeoyl and feruloyl derivatives, and <i>O</i>-glycosylated flavonoids. <i>T. mastichina</i> (≤0.4 mg/mL) exhibited significant cytoprotective, anti-inflammatory, and antioxidant effects, as well as remarkable anti-hyaluronidase activity. <b>Conclusions</b>: Shedding light on the quantitative and qualitative chemical picture of these HEs highlighted <i>T. mastichina</i> as a promising candidate to target skin aging effects, which correlates with its phenolic content. Further investigation is warranted regarding its anti-aging pharmacological activity, which could lead to the development of plant-based skin anti-aging products.