Background: HIV/AIDS remains a major health concern worldwide and the number of
people infected in Sub-Saharan Africa continues to increase. This despite increased
awareness and availability of HIV drugs in most countries. The success of current HIV-1
drugs is overshadowed by the emergence of drug resistant viral strains and the adverse
side-effects they may cause. It is these limitations and many more that drives the
continuous search for better HIV treatments. Research into drug discovery and
development using natural products is becoming better established. With natural
products, there are endless opportunities for discovering novel compounds which either
ends up as final drugs or as backbones of drug leads.
Methods: In this thesis, sixteen Lamiaceae (mint) plants were investigated for inhibitory
properties against HIV-1 as well as for beneficial immune enhancing effects. This family
of plants is commonly used in traditional medicine preparations for the treatment of
various ailments including those that are virus induced.
Cytotoxicity of the plant material was determined using tetrazolium dyes and the results
subsequently confirmed with flow cytometry and real-time cell analysis. Direct enzyme
assays were used to determine the inhibitory properties of the extracts and isolated
compounds against HIV-1 protease (PR), reverse transcriptase (RT) and integrase (IN).
The effect of the plant materials was also evaluated in an in vitro model of chronic and
latent infection by measuring HIV-1 p24 protein secretion of an infected cell line (U1)
following treatment. Most HIV-infected individuals only seek treatment during the
chronic stages of disease and latent reservoirs of the virus perpetuate treatment. The immune modulating properties were determined by quantitating the effects of plant
extracts/compounds on Th1/2/17 cytokine production in human mononuclear cells using
the cytometric bead array technology. Finally, anti-oxidant and anti-inflammatory
properties were also assessed using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and
nitric oxide colorimetric assays respectively.
Results and discussion: The 50% cytotoxic concentration (CC50) of the extracts was
between 4.2 and 100 μg/ml. Of the sixteen, extracts from six plants (Ocimum labiatum,
Ocimum serratum, Plectranthus barbatus, Plectranthus neochilus, Salvia apiana and
Stachys byzantina) were active against HIV-1. Four plants (P. neochilus, O. serratum, S. apiana and S. byzantina) demonstrated moderate inhibitory properties against HIV-1
PR, RT and IN (40-49%) and three of these plants (O. serratum, S. apiana and S.
byzantina) significantly (p<0.05) suppressed HIV-1 replication in U1 cells. The most
exciting data was obtained from extracts of P. barbatus and O. labiatum which
demonstrated inhibition classified as good (>50%) against HIV-1 PR (IC50s 62 ±0.2 and
49.8 ±0.4 μg/ml), reduced the production of pro-inflammatory cytokines at non-cytotoxic
concentrations and demonstrated strong antioxidant properties (IC50 values 13 ±0.8 and
15.8 ±0.3 μg/ml). O. labiatum extract also suppressed HIV-1 expression in U1 cells,
significantly (p<0.05). In addition, one of the extracts (P. ciliatus) had anti-cancer
potential with CC50 values <10 μg/ml.
O. labiatum extract was purified to yield a chlorophyll derivative, pheophytin-a
(phy-a); triterpene isomers (3-hydroxy-4,6a,6b,11,12,14b-hexamethyl-
acid), amyrin and a labdane diterpenoid (labda-8(17),12E,14-triene-2R,18-
diol). Phy-a inhibited HIV-1 PR with an IC50 value of 44.4 ±1.5 μg/ml. The triterpenes
activated latent HIV-1 (a serious obstacle in the eradication of the virus) while the
diterpenoid reduced the production of pro-inflammatory cytokines. These activities were
observed at non-toxic concentrations of these compounds. There is an ongoing search
for novel compounds that are able to activate latent HIV-1 to use in conjunction with
HAART. If the triterpenes were to progress to clinical use, their use would be in
activating latent virus for eradication by existing treatments.
Conclusion: The findings presented in this thesis provide some scientific explanation
for the anecdotal success of some Lamiaceae plants used traditionally to manage HIV/AIDS. The findings also conform to recommendations by the scientific community
regarding the validation of the beneficial effects of plant products used traditionally.