Clerodane diterpenoids with anti-inflammatory and synergistic antibacterial activities from Tinospora crispa

Abstract

Ten new and four known clerodane diterpenoids were isolated from the vine and leaves of Tinospora crispa. The chemical structures and absolute stereochemical configurations of all the compounds were established by spectroscopic methods and single-crystal X-ray diffraction analysis combined with electronic circular dichroism (ECD) analysis. All these compounds possess a lactone ring system except compound 10 which has a rare bridge ring between C-2 and C-20. The lactone ring in these compounds is diversely constructed between C-1 and C-18, C-6 and C-18, C-6 and C-17, C-12 and C-17, or C-15 and C-16 in the clerodane diterpenoid skeleton. All the compounds were assayed for their anti-inflammatory activities on lipopolysaccharide (LPS)-activated microglial BV-2 cells. Compounds 5 and 7 exhibited nitric oxide (NO) release inhibitory activities with IC₅₀ values of 7.5 and 10.6 μM respectively. In particular, compound 7 exhibited better inhibitory activity and less cytotoxicity than the positive control minocycline. Moreover, compounds 9 and 14 were found to synergize with oxacillin (OXA) against methicillin-resistant Staphylococcus aureus (MRSA). This synergy was further confirmed by checkerboard and time-killing assays. Compounds 9 and 14 at the sub-MIC level significantly decreased the MIC of OXA from 32.0 to 1.0 μg mL⁻¹ and 0.5 μg mL⁻¹, respectively.