Comparison Study of Different Extracts of Plectranthus madagascariensis, P. neochilus and the Rare P. porcatus (Lamiaceae): Chemical Characterization, Antioxidant, Antimicrobial and Cytotoxic Activities

Medicinal plants of the Plectranthus genus (Lamiaceae) are known for their ethnopharmacological relevance, mainly against infectious, dermatologic and gastrointestinal pathologies. Three Plectranthus species originated from South Africa, namely P. madagascariensis, P. neochilus and the rare P. porcatus were hereby screened for their antimicrobial and cytotoxic activities related with their known and/or potential ethnomedicinal uses. Twenty-six extracts were prepared by the combination of extraction methods (infusion, decoction, microwave-assisted, ultrasound-assisted, maceration and supercritical fluid extraction) with different polarity solvents (water, methanol, acetone and supercritical CO(2)). The comparison study of these extracts was elucidated through the corresponding chemical characterization and cytotoxic activity data. Therefore, the acetone extract from P. madagascariensis prepared by ultrasound extraction method revealed potent antibacterial activity against Gram-positive bacteria (1.95 < minimum inhibitory concentration (MIC) < 7.81 μg/mL), including a methicillin-resistant Staphylococcus aureus (MRSA) strain. Additionally, acetone extracts from both P. madagascariensis and P. neochilus exhibited relevant antibacterial activity against Gram-negative Klebsiella pneumonia (0.48 < MIC < 3.91 μg/mL), validating the traditional uses of such plants as anti-infectious agents. All methanolic extracts showed potent antioxidant effects at 100 μg/mL measured as their radical scavenging activity (60.8–89.0%) in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The P. madagascariensis extract obtained by maceration in acetone showed moderate cytotoxic effects in the MDA-MB-231 cell line (triple negative human breast carcinoma). The extract concentration that caused a 50% inhibition in cell viability (IC(50)) was 64.52 μg/mL. All extracts in this comparative study were profiled by high-performance liquid chromatography-HPLC with a diode-array detector-DAD (HPLC-DAD) and the main known bioactive components were identified in each extract, which included polyphenols (caffeic 1, chlorogenic 2 and rosmarinic 3 acids), abietane diterpenes (7α-acetoxy-6β-hydroxyroyleanone 4 and coleon U 5) and flavone glycosides (rutin 6 and naringin 7).