The extraction studies and a one-step purification of the crude extract of Plumbago indica using silica-gel vacuum chromatography provided a plumbagin derivative-rich P. indica root extract (PPE). The PPE was standardised to contain total plumbagin derivatives not less than 13% w/w. Antibacterial activities of the standardised PPE and three naphthoquinones, plumbagin, elliptinone and 3,3'-biplumbagin, against Propionibacterium acnes, Staphylococcus aureus and Staphylococcus epidermidis were evaluated by using the microdilution assay. The bactericidal activities of the PPE against these bacteria were much stronger than those of elliptinone and 3,3'-biplumbagin and almost equal to those of plumbagin. Stability of the PPE was determined under various conditions through a period of four months. The PPE was stable over a period of four months when stored as a dried powder but only in a well-closed container protected from light under 4 ± 2°C.
Three naphthoquinones, plumbagin (1), 3,3'-biplumbagin (2) and elliptinone (3), isolated from Plumbago indica roots by antibacterial bioassay-guided isolation, were used as standard markers for quantitative determination. A reversed-phase HPLC method was established for the simultaneous determination of the naphthoquinones in P. indica root extracts. The method utilised a Phenomenex® ODS column (4.6 × 150 mm, 5 µm) at 25°C with a mixture of methanol and 5% aqueous acetic acid (80 : 20 v/v) as the mobile phase at a flow rate of 0.85 mL/min, and UV detection at 260 nm. The parameters of linearity, precision, accuracy specificity and sensitivity of the method were evaluated. The recovery of the method was 98.6-100.6% with good linearity (r (2 )≥ 0.9997) for all three naphthoquinones. A high degree of sensitivity, specificity as well as repeatability and reproducibility (R.S.D. values less than 5%) were also achieved.
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