The effect of lopinavir/ritonavir on the antimalarial activity of artemether or artemether/lumefantrine in a mouse model of Plasmodium berghei

Repurposing Drugs to Fight Hepatic Malaria Parasites

Malaria remains one of the most prevalent infectious diseases worldwide, primarily affecting some of the most vulnerable populations around the globe. Despite achievements in the treatment of this devastating disease, there is still an urgent need for the discovery of new drugs that tackle infection by Plasmodium parasites. However, de novo drug development is a costly and time-consuming process. An alternative strategy is to evaluate the anti-plasmodial activity of compounds that are already approved for other purposes, an approach known as drug repurposing. Here, we will review efforts to assess the anti-plasmodial activity of existing drugs, with an emphasis on the obligatory and clinically silent liver stage of infection. We will also review the current knowledge on the classes of compounds that might be therapeutically relevant against Plasmodium in the context of other communicable diseases that are prevalent in regions where malaria is endemic. Repositioning existing compounds may constitute a faster solution to the current gap of prophylactic and therapeutic drugs that act on Plasmodium parasites, overall contributing to the global effort of malaria eradication.

Pre-clinical evaluation of a P. berghei-based whole-sporozoite malaria vaccine candidate

Whole-sporozoite vaccination/immunization induces high levels of protective immunity in both rodent models of malaria and in humans. Recently, we generated a transgenic line of the rodent malaria parasite P. berghei (Pb) that expresses the P. falciparum (Pf) circumsporozoite protein (PfCS), and showed that this parasite line (PbVac) was capable of (1) infecting and developing in human hepatocytes but not in human erythrocytes, and (2) inducing neutralizing antibodies against the human Pf parasite. Here, we analyzed PbVac in detail and developed tools necessary for its use in clinical studies. A microbiological contaminant-free Master Cell Bank of PbVac parasites was generated through a process of cyclic propagation and clonal expansion in mice and mosquitoes and was genetically characterized. A highly sensitive qRT-PCR-based method was established that enables PbVac parasite detection and quantification at low parasite densities in vivo. This method was employed in a biodistribution study in a rabbit model, revealing that the parasite is only present at the site of administration and in the liver up to 48 h post infection and is no longer detectable at any site 10 days after administration. An extensive toxicology investigation carried out in rabbits further showed the absence of PbVac-related toxicity. In vivo drug sensitivity assays employing rodent models of infection showed that both the liver and the blood stage forms of PbVac were completely eliminated by Malarone^® treatment. Collectively, our pre-clinical safety assessment demonstrates that PbVac possesses all characteristics necessary to advance into clinical evaluation.

PbVac is a transgenic malaria parasite expressing circumsporozoite antigen from the human parasite Plasmodium falciparum. PbVac elicits neutralizing P. falciparum antibodies and can infect human hepatocytes but not erythrocytes, suggesting that humans would be non-permissive. Miguel Prudêncio and colleagues at the Institute of Molecular Medicine in Lisbon perform a detailed in vivo analysis and toxicology of PbVac. Extensive biodistribution analysis using a highly sensitive qPCR in non-permissive rabbit hosts shows PbVac are present at the initial bite site early on with later appearance in the liver, but by day 10 is undetectable. Importantly no PbVac could be detected in the blood at any time-point. PbVac was well tolerated with no apparent pathological signatures. In permissive mouse hosts PbVac could be effectively eliminated from both the blood and liver and could thereby act as a potential clinical ‘safety net’ in the event of an erythrocytic stage or persistence in the liver.

Inhibition of Plasmodium Hepatic Infection by Antiretroviral Compounds

Recent WHO guidelines on control of human immunodeficiency virus (HIV) call for the widespread use of antiretroviral (AR) therapy (ART) for people living with HIV. Given the considerable overlap between infections by HIV and Plasmodium, the causative agent of malaria, it is important to understand the impact of AR compounds and ART regimens on infections by malaria parasites. We undertook a systematic approach to identify AR drugs and ART drug combinations with inhibitory activity against the obligatory hepatic stage of Plasmodium infection. Our in vitro screen of a wide array of AR drugs identified the non-nucleoside reverse transcriptase inhibitors efavirenz and etravirine (ETV), and the protease inhibitor nelfinavir, as compounds that significantly impair the development of the rodent malaria parasite P. berghei in an hepatoma cell line. Furthermore, we show that WHO-recommended ART drug combinations currently employed in the field strongly inhibit Plasmodium liver infection in mice, an effect that may be significantly enhanced by the inclusion of ETV in the treatment. Our observations are the first report of ETV as an anti-Plasmodial drug, paving the way for further evaluation and potential use of ETV-containing ARTs in regions of geographical overlap between HIV and Plasmodium infections.

Lopinavir/ritonavir enhanced the antimalarial activity of amodiaquine and artesunate in a mouse model of Plasmodium berghei

Treatment of malaria and HIV in co-infected patients remains a challenge due to the limited information on interaction between drugs used for the treatment of the two infections. Thus, this study evaluated the interaction between lopinavir/ritonavir (LR) and artesunate (AS), amodiaquine (AQ) or a fixed dose of AS/AQ in a mouse model of chloroquine-resistant Plasmodium berghei. Combination of LR with graded doses of AS or AQ resulted in a significant reduced ED₅₀. In addition, parasites cleared completely from day 3 till day 21 post-infection in animals infected, treated with AS/AQ alone or AS/AQ with LR and all the animals survived till day 21 post-infection. In contrast, survival on day 21 in animals treated with AQ alone or AQ with LR was 20 and 60%, respectively. It appears that the protease inhibitor LR enhanced the antimalarial drugs AS and AQ.