Protoberberine-type Alkaloids as Lead Compounds for the Treatment of African Sleeping Sickness, Leishmaniasis, and Malaria

Natural products have played traditionally a pivotal role in the field of drug discovery. The impact of natural products in the biomedical sciences can be shown by a few distinct examples: The discovery of receptor sites (as in the case of nicotinic, muscarinic, and opioid receptors), the isolation of compounds with a unique activity for a disease for which there was no practical treatment (such as quinine in malaria and penicillin for bacterial infections), the identification of a novel mechanism of action in a known disease (such as the tubulin-stabilizing effect of paclitaxel in cancer), and the provision of lead molecules that can be further optimized (such as mefloquine, based on the lead compound quinine). The importance of natural products is also reflected when it is considered that over 30% of all the new small-molecule drugs approved for clinical use in Western countries and Japan during the twenty-five year period from 1981 to 2006 were either natural products or their derivatives, or synthesized molecules based on natural product pharmacophores.;In the present study, a library of 128 plants available at the College of Pharmacy, The Ohio State University, was screened for antileishmanial and antitrypanosomal activities, following a standardized solvent extraction scheme (Wall et al., 1996). Among the active extracts, the entire plant of Thalictrum lucidum L. (Ranunculaceae) was selected as a potential lead and was extracted in a larger scale, with a bioactivity-guided fractionation procedure employed to identify the active principle or principles. Bioactivity studies led to the identification of berberine (85) as the main active principle of T. lucidum, with two other protoberberine-type alkaloids, jatrorrhizine (87) and palmatine (89) isolated.;Following this lead, a small library of semisynthetic berberine derivatives was screened for antileishmanial and antitrypanosomal activities. Among the compounds evaluated in the in vitro test systems, samples that were labeled as 8,8-diethyldihydroberberine (HI salt) and 8,8-dimethyldihydroberberine (HI salt) were active against Leishmania donovani parasites with IC50 values of 12 and 15 ng/mL, respectively. However, both samples proved to be mixtures in NMR spectroscopic and chromatographic studies, possibly due to oxidation during prolonged storage. In an effort to identify the active principle, a semisynthetic study was carried out. Accordingly, the berberine analogue, 5,6-didehydro-8,8-diethyl-13-oxodihydroberberine chloride (129) was synthesized in sufficient amounts for further biological testing. 5,6-Didehydro-8,8-diethyl-13-oxodihydroberberine chloride s howed nanomolar level potency against the in vitro models of leishmaniasis, malaria, and trypanosomiasis used as well as activity in an in vivo model of visceral leishmaniasis. Therefore, this compound was elucidated as a bioactive oxidized form of the original 8,8-diethyldihydroberberine sample.;Overall, this study describes the discovery of a berberine analogue through a semisynthetic approach with an approximately thousand-fold improvement in activity against leishmaniasis, malaria, and trypanosomiasis as compared to the parent compound. Since the starting material, berberine, is relatively inexpensive, the discovery of 8,8,-dialkyldihydroberberine derivatives may lead to a new class of affordable drugs in the field of protozoal diseases.