Drug resistance in African trypanosomiasis: the melarsoprol and pentamidine story

Abstract

Melarsoprol and pentamidine represent the two main classes of drugs, the arsenicals and diamidines, historically used to treat the diseases caused by African trypanosomes: sleeping sickness in humans and Nagana in livestock. Cross-resistance to these drugs was first observed over 60 years ago and remains the only example of cross-resistance among sleeping sickness therapies. A Trypanosoma brucei adenosine transporter is well known for its role in the uptake of both drugs. More recently, aquaglyceroporin 2 (AQP2) loss of function was linked to melarsoprol-pentamidine cross-resistance. AQP2, a channel that appears to facilitate drug accumulation, may also be linked to clinical cases of resistance. Here, we review these findings and consider some new questions as well as future prospects for tackling the devastating diseases caused by these parasites.

Figure 1. Timeline of arsenical and diamidine therapies against HAT

The timeline indicates selected key developments relating to these therapies and in our understanding of drug-uptake and resistance.

Figure 2. Schematic model of melarsoprol and pentamidine transport in T. brucei

The schematic shows known and putative mechanisms. Both drugs are thought to enter trypanosomes via the P2 and AQP2 transporters; the weight of the arrows reflects relative contribution to uptake. The presence of the AQP2 gene appears to correlate with HAPT1 activity but it is as yet not certain that AQP2 codes for this activity. The P-type H+ ATPases (HA1-3) may provide a proton motive force that drives pentamidine uptake. Melarsen oxide forms a toxic adduct with trypanothione (TSH), known as Mel T, which inhibits TSH synthesis and is also removed from the cell via the ABC transporter, MRPA. An additional low affinity pentamidine transport activity (not shown) may contribute to transport at pentamidine at concentrations above 0.1 μM [29].

Figure 3. P2 and AQP2 substrates

Substrates known to transit through each transporter are shown. Inorganic arsenite and antimonite are known to be transported via aquaglyceroporins, but an outstanding question is how AQP2 facilitates the transit of the much larger pentamidine and melarsoprol structures. In the case of pentamidine, competition studies suggest that a single benzene ring with the amidine in the fourth position is sufficient for recognition [75].

Figure 4. The two branches of trypanosomatid aquaporins

The AQPs from T. brucei, T. cruzi and L. major (n=12, blue) are shown in the context of AQP diversity of Arabidopsis thaliana (n=35, green) and Homo sapiens (n=13, black). The Neighbour-Joining tree was built from a MUSCLE alignment of the amino acid sequences [101] and drawn with MEGA5 [102]. Evolutionary distances were computed using the Poisson correction and are in number of amino acid substitutions per site (scale bar). The grey numbers on the main branches indicate percent positives of 2000 rounds of bootstrapping. Aquaglyceroporins linked to drug transport are within the red dashed oval: NIP, PIP, SIP and TIP; Nodulin-26 like, Plasma-membrane, Small basic and Tonoplast Intrinsic Proteins. Trypanosomatid accession numbers: LmAQP1, LmjF.31.0020; TbAQP1, Tb927.6.1520; TbAQP2, Tb927.10.14170, TbAQP3, Tb927.10.14160.