A Class of Tricyclic Compounds Blocking Malaria Parasite Oocyst Development and Transmission

Abstract

Malaria is a deadly infectious disease in many tropical and subtropical countries. Previous efforts to eradicate malaria have failed, largely due to the emergence of drug-resistant parasites, insecticide-resistant mosquitoes and, in particular, the lack of drugs or vaccines to block parasite transmission. ATP-binding cassette (ABC) transporters are known to play a role in drug transport, metabolism, and resistance in many organisms, including malaria parasites. To investigate whether a Plasmodium falciparum ABC transporter (Pf14_0244 or PfABCG2) modulates parasite susceptibility to chemical compounds or plays a role in drug resistance, we disrupted the gene encoding PfABCG2, screened the recombinant and the wild-type 3D7 parasites against a library containing 2,816 drugs approved for human or animal use, and identified an antihistamine (ketotifen) that became less active against the PfABCG2-disrupted parasite in culture. In addition to some activity against asexual stages and gametocytes, ketotifen was highly potent in blocking oocyst development of P. falciparum and the rodent parasite Plasmodium yoelii in mosquitoes. Tests of structurally related tricyclic compounds identified additional compounds with similar activities in inhibiting transmission. Additionally, ketotifen appeared to have some activity against relapse of Plasmodium cynomolgi infection in rhesus monkeys. Further clinical evaluation of ketotifen and related compounds, including synthetic new derivatives, in blocking malaria transmission may provide new weapons for the current effort of malaria eradication.

A female gametocyte-specific ABC transporter plays a role in lipid metabolism in the malaria parasite

“…In a recent publication, Eastman et al 54 investigated the involvement of gABCG2 in drug transport and drug resistance in gametocytes. In this study, the gene for gABCG2 was also deleted, but in contrast to our findings, there was a decrease in gametocyte production following removal of gABCG2.…”

“…In this earlier study, the two gene-deletion clones analysed were derived from a single transfection, and in their discussion the authors pointed out that a secondary off-target effect responsible for the decrease in gametocyte numbers cannot be excluded. Several attempts to restore gametocyte production by complementing the gene were unsuccessful 54 . Hence, the two principal criteria for an experimental genetics analysis 55 were not met, making it difficult to reconcile the apparent discrepancies to our findings.…”

Characterization of native PfABCG protein in Plasmodium falciparum

“…Using high throughput drug screening of gene knockouts in P. falciparum, Eastman et al [36] have shown recently that knockdown of PfABCG in 3D7 decreased its sensitivity to Ketotifen, a noncompetitive H1-antihistamine drug, relative to wild type parasite. Given the differential expression of PfABCG in different strains of P. falciparum (Fig.…”

“…4). Moreover, it is not clear why knockdown of PfABCG expression in 3D7 [36], which is presumably expressed only in gametocytes and not in the asexual stages of the parasite, could affect the sensitivity of the asexual stages of the parasite to ketotifen. The fact that knockdown of PfABCG in 3D7 leads to decreased sensitivity to ketotifen suggests that the protein (PfABCG) is functionally expressed in the various asexual stages of the parasite.…”

“…Although this possibility cannot be entirely ruled out at this stage, it is highly unlikely for the following reasons: (a) the specificity of the PfABCG antiserum towards the recombinant N-terminal fragment of PfABCG and the absence of other reactive polypeptides in both erythrocytes and P. falciparum lysate, save the 70 kDa polypeptide reactivity which could not be competed with excess antigen, (b) the correlation between PfABCG protein and RNA expression levels in HB3, 7G8 and 3D7 strains of P. falciparum (Fig. 4), and (c) the differential sensitivity of the three chloroquine sensitive strains (e.g., HB3, D10 and 3D7) to ketotifen, a PfABCG associated drug [36]. Another possibility is that the 65kDa polypeptide is a proteolytic fragment of the native polypeptide or a modified PfABCG.…”