university of kwazulu-natal faculty of science and agriculture school ...

The isolation of a pimarane in Plectranthus was an interesting find as pimarane-type
diterpenes are abundant in the Lamiaceae family (to which Plectranthus belongs) being
found predominantly in the Salvia and Nepeta genera and to a lesser extent in the
Rabdosia, Callicarpa, Premna, Amaracus, Orthosipho and Satureja genera (Alvarenga et
al., 2001). The finding of a pimarane from Plectranthus may now provide a biochemical
link between Plectranthus and these genera. Pimarane-type diterpenes have also been
isolated from the Erythroxylum genus (Ansell, 1989) which suggests a link between the
Lamiaceae family to which Plectranthus is a member of and the Erythroxylaceae to
which Erythroxylum belongs.
Compounds I and II showed good activity against Enterococcus faecalis and
Pseudomonas aeruginosa with MICs for compound I being 62.5µg/mL against both
bacterial strains and that of compound II being 31.30µg/mL and 7.8µg/mL for E. faecalis
and P. aeruginosa, respectively. This result is unexpected as royleanones having a highly
oxidised substituent at C-7 (such as the acetyl group in compound I) are usually more
active than those bearing a hydroxyl group at positions C-6 and C-7 (Teixeira et al.,
1997; Gaspar-Marques et al., 2006). The pimarane, ent-pimara-8(14),15-diene-3β,11α-
diol (III), although inactive against E. faecalis was very active against P. aeruginosa.
Based on criteria set by the CSIR where the total growth inhibition (TGI) value is used to
evaluate the anticancer activity of compounds, compounds I to III are considered to be
weakly active against breast (MCF-7), renal (TK-10) and melanoma (UACC-62) cell
lines. However when compared to the positive control, etoposide, compounds I to III
exhibit better antitumor activity against the breast cell line than the control. Compound
IV was inactive against all three cell lines.
Because compounds I to IV showed weak activity in the three cell prescreen against the
breast, renal and melanoma cell lines, there was no need to to pursue further anticancer
testing i.e. the 60-cell-line screen.
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