Leucas zeylanica (Family: Lamiaceae) and Tagetes erecta (Family: Asteraceae) are annual herbs traditionally used in home remedies and Ayurvedic medicine. Although both plants have been individually studied for their pharmacological properties, their synergistic effects and combined use in cosmetic formulations for sun protection have not been previously reported. This study aimed to evaluate the synergistic antioxidant and photoprotective activities of 95% ethanol extracts of the aerial parts of L. zeylanica, flower heads of T. erecta, and their combinations (1:1, 1:2, and 2:1). Antioxidant activity was assessed using the DPPH radical scavenging assay, with ascorbic acid as the positive control, and results were expressed as EC50 values. Sun protection factor (SPF) was determined using the Mansur equation by measuring absorbance in the range of 290–320 nm. The EC50 values for L. zeylanica, T. erecta, and their combinations (1:1, 1:2, and 2:1) were 104.12, 80.54, 82.01, 84.95, and 55.26 μg/ml, respectively. Corresponding SPF values were 18.93, 6.78, 18.33, 9.40, and 19.03. The 2:1 combination exhibited the strongest antioxidant activity and highest SPF, indicating a synergistic effect. Based on these findings, an emulgel formulation was developed using the most potent extract ratio. Stability and toxicity evaluations of the individual extracts, their combination, and the formulated emulgel confirmed suitability and safety for topical application.
Wanniarachchi et al. (Mon,) studied this question.