Anti-mycobacterial activity of labdane and halimane diterpenes obtained from Plectranthus ornatus Codd

Natural products are a unique source of lead compounds for medicinal chemistry. Several diterpenes isolated from Plectranthus spp. have been reported to have interesting tuberculostatic activity and P. ornatus Codd. has been used in some regions of Brazil as an anti-infective. Previously, diterpenes with halimane and labdane skeletons were isolated in large quantities from P. ornatus Codd. We assessed the anti-mycobacterial activity of these compounds, performing a preliminar assay with the non-virulent strain Mycobacterium smegmatis. The cytotoxicity of the diterpenes with halimane and labdane skeletons was tested with the lactate dehydrogenase assay, where no considerable cytotoxic effects were found up to 25 μg.mL-1. Subsequently, the microdilution method was used to determine the minimum inhibitory concentration (MIC) against M. smegmatis. The MIC that inhibited the growth of the non-virulent mycobacteria by ≥99% was 100 μg.mL-1 for the diterpene with halimane skeleton, whereas for the diterpene with a labdane skeleton was >100 μg.mL-1. To the best of our knowledge, this is the first report on diterpenes with halimane and labdane skeletons isolated from P. ornatus in macrophages cytotoxicity, and in a preliminar assay for anti-mycobacterial activity. Further studies are suggested on M. tuberculosis, particularly for the diterpenes with halimane skeleton.