Antimalarial effects of human immunodeficiency virus type 1 protease inhibitors differ from those of the aspartic protease inhibitor pepstatin

Malaria in HIV-Infected Children Receiving HIV Protease-Inhibitor- Compared with Non-Nucleoside Reverse Transcriptase Inhibitor-Based Antiretroviral Therapy, IMPAACT P1068s, Substudy to P1060

Background

HIV and malaria geographically overlap. HIV protease inhibitors kill malaria parasites in vitro and in vivo, but further evaluation in clinical studies is needed.

Methods

Thirty-one children from Malawi aged 4–62 months were followed every 3 months and at intercurrent illness visits for ≤47 months (September 2009-December 2011). We compared malaria parasite carriage by blood smear microscopy (BS) and confirmed clinical malaria incidence (CCM, or positive BS with malaria symptoms) in children initiated on HIV antiretroviral therapy (ART) with zidovudine, lamivudine, and either nevirapine (NVP), a non-nucleoside reverse transcriptase inhibitor, or lopinavir-ritonavir (LPV-rtv), a protease inhibitor.

Results

We found an association between increased time to recurrent positive BS, but not CCM, when anti-malarial treatment and LPV-rtv based ART were used concurrently and when accounting for a LPV-rtv and antimalarial treatment interaction (adjusted HR 0.39; 95% CI (0.17,0.89); p = 0.03).

Conclusions

LPV-rtv in combination with malaria treatment was associated with lower risk of recurrent positive BS, but not CCM, in HIV-infected children. Larger, randomized studies are needed to confirm these findings which may permit ART optimization for malaria-endemic settings.

Trial Registration

ClinicalTrials.gov NCT00719602

Comprehensive review on various strategies for antimalarial drug discovery

The resistance of malaria parasites to existing drugs carries on growing and progressively limiting our ability to manage this severe disease and finally lead to a massive global health burden. Till now, malaria control has relied upon the traditional quinoline, antifolate and artemisinin compounds. Very few new antimalarials were developed in the past 50 years. Among recent approaches, identification of novel chemotherapeutic targets, exploration of natural products with medicinal significance, covalent bitherapy having a dual mode of action into a single hybrid molecule and malaria vaccine development are explored heavily. The proper execution of these approaches and proper investment from international agencies will accelerate the discovery of drugs that provide new hope for the control or eventual eradication of this global infectious disease. This review explores various strategies for assessment and development of new antimalarial drugs. Current status and scientific value of previous approaches are systematically reviewed and new approaches provide a pragmatic forecast for future developments are introduced as well.

The Glucose Transporter PfHT1 Is an Antimalarial Target of the HIV Protease Inhibitor Lopinavir

Malaria and HIV infection are coendemic in a large portion of the world and remain a major cause of morbidity and mortality. Growing resistance of Plasmodium species to existing therapies has increased the need for new therapeutic approaches. The Plasmodium glucose transporter PfHT is known to be essential for parasite growth and survival. We have previously shown that HIV protease inhibitors (PIs) act as antagonists of mammalian glucose transporters. While the PI lopinavir is known to have antimalarial activity, the mechanism of action is unknown. We report here that lopinavir blocks glucose uptake into isolated malaria parasites at therapeutically relevant drug levels. Malaria parasites depend on a constant supply of glucose as their primary source of energy, and decreasing the available concentration of glucose leads to parasite death. We identified the malarial glucose transporter PfHT as a target for inhibition by lopinavir that leads to parasite death. This discovery provides a mechanistic basis for the antimalarial effect of lopinavir and provides a direct target for novel drug design with utility beyond the HIV-infected population.

Lopinavir/ritonavir-based antiretroviral treatment (ART) versus efavirenz-based ART for the prevention of malaria among HIV-infected pregnant women

BACKGROUND

Human immunodeficiency virus (HIV)-infected pregnant women are at increased risk of malaria and its complications. In vitro and in vivo data suggest that the HIV protease inhibitors lopinavir/ritonavir may have potent antimalarial activity. We sought to evaluate whether lopinavir/ritonavir-based antiretroviral therapy (ART) reduced the risk of placental malaria.

METHODS

HIV-infected, ART-naive pregnant women were enrolled between gestational weeks 12 and 28 and randomly assigned to receive lopinavir/ritonavir-based or efavirenz-based ART. Women received daily trimethoprim-sulfamethoxazole prophylaxis and insecticide-treated bed nets at enrollment and were followed up to 1 year after delivery. The primary outcome was placental malaria, defined by the detection of malaria parasites, using microscopy or polymerase chain reaction (PCR) analysis of placental blood specimens. Secondary outcomes included placental malaria, defined by histopathologic results; adverse birth outcomes; incidence of malaria; and prevalence of asymptomatic parasitemia. Analyses were done using an intention-to-treat approach.

RESULTS

Of 389 subjects randomly assigned to a treatment group, 377 were followed through to delivery. There was no significant difference in the risk of placental malaria, as defined by thick smear or PCR findings, between the lopinavir/ritonavir-based and efavirenz-based ART arms (7.4% vs 9.8%; P = .45). Similarly, there were no differences in secondary outcomes between the 2 treatment arms.

CONCLUSIONS

Lopinavir/ritonavir-based ART did not reduce the risk of placental or maternal malaria or improve birth outcomes, compared with efavirenz-based ART.

CLINICAL TRIALS REGISTRATION

NCT00993031.

The effect of antiretrovirals on Plasmodium falciparum liver stages

HIV and malaria overlap geographically, but the full impact of different antiretrovirals on malaria remains poorly understood. We examined the antimalarial activity of the HIV protease inhibitors lopinavir and saquinavir and the non-nucleoside reverse transcriptase inhibitor nevirapine on Plasmodium falciparum liver stages. Our results demonstrate that the HIV PI lopinavir inhibits liver stage parasites at clinically relevant concentrations, that is, at drug levels achieved in HIV-infected patients on standard dosing regimens. Because drugs that inhibit liver stages target parasites when they are present in lower numbers, these results might have implications for eradication efforts.

HIV treatments have malaria gametocyte killing and transmission blocking activity

BACKGROUND

 Millions of individuals being treated for human immunodeficiency virus (HIV) live in malaria-endemic areas, but the effects of these treatments on malaria transmission are unknown. While drugs like HIV protease inhibitors (PIs) and trimethoprim-sulfamethoxazole (TMP-SMX) have known activity against parasites during liver or asexual blood stages, their effects on transmission stages require further study.

METHODS

 The HIV PIs lopinavir and saquinavir, the nonnucleoside reverse-transcriptase inhibitor nevirapine, and the antibiotic TMP-SMX were assessed for activity against Plasmodium falciparum transmission stages. The alamarBlue assay was used to determine the effects of drugs on gametocyte viability, and exflagellation was assessed to determine the effects of drugs on gametocyte maturation. The effects of drug on transmission were assessed by calculating the mosquito oocyst count as a marker for infectivity, using standard membrane feeding assays.

RESULTS

 Lopinavir and saquinavir have gametocytocidal and transmission blocking activities at or approaching clinically relevant treatment levels, while nevirapine does not. TMP-SMX is not gametocytocidal, but at prophylactic levels it blocks transmission.

CONCLUSIONS

 Specific HIV treatments have gametocyte killing and transmission-blocking effects. Clinical studies are warranted to evaluate these findings and their potential impact on eradication efforts.

HIV nonnucleoside reverse transcriptase inhibitors and trimethoprim-sulfamethoxazole inhibit plasmodium liver stages

BACKGROUND

Although nonnucleoside reverse transcriptase inhibitors (NNRTIs) are usually part of first-line treatment regimens for human immunodeficiency virus (HIV), their activity on Plasmodium liver stages remains unexplored. Additionally, trimethoprim-sulfamethoxazole (TMP-SMX), used for opportunistic infection prophylaxis in HIV-exposed infants and HIV-infected patients, reduces clinical episodes of malaria; however, TMP-SMX effect on Plasmodium liver stages requires further study.

METHODS

We characterized NNRTI and TMP-SMX effects on Plasmodium liver stages in vivo using Plasmodium yoelii. On the basis of these results, we conducted in vitro studies assessing TMP-SMX effects on the rodent parasites P. yoelii and Plasmodium berghei and on the human malaria parasite Plasmodium falciparum.

RESULTS

Our data showed NNRTI treatment modestly reduced P. yoelii liver stage parasite burden and minimally extended prepatent period. TMP-SMX administration significantly reduced liver stage parasite burden, preventing development of patent parasitemia in vivo. TMP-SMX inhibited development of rodent and P. falciparum liver stage parasites in vitro.

CONCLUSIONS

NNRTIs modestly affect liver stage Plasmodium parasites, whereas TMP-SMX prevents patent parasitemia. Because drugs that inhibit liver stages target parasites when they are present in lower numbers, these results may have implications for eradication efforts. Understanding HIV drug effects on Plasmodium liver stages will aid in optimizing treatment regimens for HIV-exposed and HIV-infected infected patients in malaria-endemic areas.

Linear and cyclic dipeptides with antimalarial activity

Several natural and synthetic polypeptides possess important antimalarial activity. Shorter peptides with potent antimalarial activity have also been described, among them linear di-, tri-, tetra- and pentapeptides and their cyclic analogs. In an attempt to find dipeptides with antimalarial activities we show that linear and cyclic dipeptides, the latter known as diketopiperazines, still retain the fundamental core to preserve antimalarial activity. Thirteen linear dipeptides and ten diketopiperazines were investigated. Eight linear dipeptides showed IC(50) values between 2.78 and 7.07 μM, while eight diketopiperazines were also active with IC(50) values between 2.26 and 4.26 μM on Plasmodium berghei schizont cultures.

Crystal structures of multidrug-resistant HIV-1 protease in complex with two potent anti-malarial compounds

Two potent inhibitors (compounds 1 and 2) of malarial aspartyl protease, plasmepsin-II, were evaluated against wild type (NL4-3) and multidrug-resistant clinical isolate 769 (MDR) variants of human immunodeficiency virus type-1 (HIV-1) aspartyl protease. Enzyme inhibition assays showed that both 1 and 2 have better potency against NL4-3 than against MDR protease. Crystal structures of MDR protease in complex with 1 and 2 were solved and analyzed. Crystallographic analysis revealed that the MDR protease exhibits a typical wide-open conformation of the flaps (Gly48 to Gly52) causing an overall expansion in the active site cavity, which, in turn caused unstable binding of the inhibitors. Due to the expansion of the active site cavity, both compounds showed loss of direct contacts with the MDR protease compared to the docking models of NL4-3. Multiple water molecules showed a rich network of hydrogen bonds contributing to the stability of the ligand binding in the distorted binding pockets of the MDR protease in both crystal structures. Docking analysis of 1 and 2 showed a decrease in the binding affinity for both compounds against MDR supporting our structure-function studies. Thus, compounds 1 and 2 show promising inhibitory activity against HIV-1 protease variants and hence are good candidates for further development to enhance their potency against NL4-3 as well as MDR HIV-1 protease variants.

In vitro activity of antiretroviral drugs against Plasmodium falciparum

Malaria and HIV infection are both very common in many developing countries. With the increasing availability of therapy for HIV infection, it was of interest to determine whether antiretroviral drugs exert antimalarial effects. We therefore tested the in vitro activity of 19 antiretroviral drugs against the W2 and 3D7 strains of Plasmodium falciparum at concentrations up to 50 μM. None of 5 tested nucleoside reverse transcriptase inhibitors demonstrated activity. Two nonnucleoside reverse transcriptase inhibitors, efavirenz (mean 50% inhibitory concentration [IC(50)] of 22 to 30 μM against the two strains) and etravirine (3.1 to 3.4 μM), were active; nevirapine was not active. Also active were the fusion inhibitor enfuvirtide (6.2 to 7.9 μM) and the entry inhibitor maraviroc (15 to 21 μM). Raltegravir was not active. However, for all active drugs mentioned above, the IC(50)s were considerably greater than the concentrations achieved with standard dosing. The effects most likely to be clinically relevant were with HIV protease inhibitors. Of the tested compounds, activity was seen with lopinavir (2.7 to 2.9 μM), atazanavir (3.3 to 13.0 μM), saquinavir (5.0 to 12.1 μM), nelfinavir (6.5 to 12.1 μM), ritonavir (9.5 to 10.9 μM), tipranavir (15.5 to 22.3 μM), and amprenavir (28.1 to 40.8) but not darunavir. Lopinavir was active at levels well below those achieved with standard dosing of coformulated lopinavir-ritonavir. Lopinavir also demonstrated modest synergy with the antimalarial lumefantrine (mean fractional inhibitory concentration index of 0.66 for W2 and 0.53 for 3D7). Prior data showed that lopinavir-ritonavir also extends the pharmacokinetic exposure of lumefantrine. Thus, when used to treat HIV infection, lopinavir-ritonavir may have clinically relevant antimalarial activity and also enhance the activity of antimalarials.

Expanding the Antimalarial Drug Arsenal-Now, But How?

The number of available and effective antimalarial drugs is quickly dwindling. This is mainly because a number of drug resistance-associated mutations in malaria parasite genes, such as crt, mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarial compounds. Unfortunately, malaria parasites have started to exhibit some level of resistance in Southeast Asia even to the most recently introduced class of drugs, artemisinins. While there is much need, the antimalarial drug development pipeline remains woefully thin, with little chemical diversity, and there is currently no alternative to the precious artemisinins. It is difficult to predict where the next generation of antimalarial drugs will come from; however, there are six major approaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation or combination approach; (ii) repurposing drugs that are currently used to treat other infections or diseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring natural sources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based ("targeted") discoveries. When any newly discovered effective antimalarial treatment is used by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive nor conclusive. Our purpose is to provide an overview of antimalarial drug resistance, associated parasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum), and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarial drugs), and highlight some examples of the aforementioned approaches to future antimalarial treatment. These approaches can be categorized into "short term" (4. Feasible options for now) and "long term" (5. Next generation of antimalarial treatment-Approaches and candidates). However, these two categories are interrelated, and the approaches in both should be implemented in parallel with focus on developing a successful, long-lasting antimalarial chemotherapy.

HIV proteinase inhibitors target the Ddi1-like protein of Leishmania parasites

HIV proteinase inhibitors reduce the levels of Leishmania parasites in vivo and in vitro, but their biochemical target is unknown. We have identified an ortholog of the yeast Ddi1 protein as the only member of the aspartic proteinase family in Leishmania parasites, and in this study we investigate this protein as a potential target for the drugs. To date, no enzyme assay has been developed for the Ddi1 proteins, but Saccharomyces cerevisiae lacking the DDI1 gene secrete high levels of protein into the medium. We developed an assay in which these knockout yeast were functionally complemented to low secretion by introduction of genes encoding Ddi1 orthologs from Leishmania major or humans. Plasmid alone controls gave no complementation. Treatment of the Ddi1 transformants with HIV proteinase inhibitors showed differential effects dependent on the origin of the Ddi1. Dose responses allowed calculation of IC₅₀ values; e.g., for nelfinavir, of 3.4 μM (human Ddi1) and 0.44 μM (Leishmania Ddi1). IC₅₀ values with Leishmania constructs mirror the potency of inhibitors against parasites. Our results show that Ddi1 proteins are targets of HIV proteinase inhibitors and indicates the Leishmania Ddi1 as the likely target for these drugs and a potential target for antiparasitic therapy.—White, R. E., Powell, D. J., Berry, C. HIV proteinase inhibitors target the Ddi1-Like protein of Leishmania parasites.

Antimalarial activity of ultra-short peptides

Ultra-short peptides 1-9 were designed and synthesized with phenylalanine, ornithine and proline amino acid residues and their effect on antimalarial activity was analyzed. On the basis of the IC₅₀ data for these compounds, the effects of nature, polarity, and amino acid sequence on Plasmodium berghei schizont cultures were analyzed too. Tetrapeptides Phe-Orn-Phe-Orn (4) and Lys-Phe-Phe-Orn (5) showed a very important activity with IC₅₀ values of 3.31 and 2.57 μM, respectively. These two tetrapeptides are candidates for subsequent in vivo assays and SARS investigations.

HIV protease inhibitors inhibit the development of preerythrocytic-stage plasmodium parasites

Recent studies have demonstrated that human immunodeficiency virus (HIV) protease inhibitors (PIs) exert inhibitory effects on erythrocytic stages of the human-malaria parasite Plasmodium falciparum in vitro and on erythrocytic stages of the rodent-malaria parasite Plasmodium chabaudi in vivo. Although it remains unclear how HIV PIs inhibit the parasite, the effect seen on parasite development in the erythrocytic stages is potent. The effect on preerythrocytic stages has not yet been investigated. Using the rodent parasite Plasmodium berghei, we screened a panel of HIV PIs in vitro for effects on the preerythrocytic stages. Our data indicated that the HIV PIs lopinavir and saquinavir affect preerythrocytic-stage parasite development in vitro. We then evaluated the effect of HIV PIs on preerythrocytic stages in vivo using the rodent parasite Plasmodium yoelii. We found that lopinavir/ritonavir had a dose-dependent effect on liver-stage parasite development. Given that sub-Saharan Africa is where the HIV/AIDS pandemic intersects with malaria, these results merit analysis in clinical settings.

Stronger activity of human immunodeficiency virus type 1 protease inhibitors against clinical isolates of Plasmodium vivax than against those of P. falciparum

Recent studies using laboratory clones have demonstrated that several antiretroviral protease inhibitors (PIs) inhibit the growth of Plasmodium falciparum at concentrations that may be of clinical significance, especially during human immunodeficiency virus type 1 (HIV-1) and malaria coinfection. Using clinical isolates, we now demonstrate the in vitro effectiveness of two HIV-1 aspartic PIs, saquinavir (SQV) and ritonavir (RTV), against P. vivax (n = 30) and P. falciparum (n = 20) from populations subjected to high levels of mefloquine and artesunate pressure on the Thailand-Myanmar border. The median 50% inhibitory concentration values of P. vivax to RTV and SQV were 2,233 nM (range, 732 to 7,738 nM) and 4,230 nM (range, 1,326 to 8,452 nM), respectively, both within the therapeutic concentration range commonly found for patients treated with these PIs. RTV was fourfold more effective at inhibiting P. vivax than it was at inhibiting P. falciparum, compared to a twofold difference in SQV sensitivity. An increased P. falciparum mdr1 copy number was present in 33% (3/9) of isolates and that of P. vivax mdr1 was present in 9% of isolates (2/22), but neither was associated with PI sensitivity. The inter-Plasmodium sp. variations in PI sensitivity indicate key differences between P. vivax and P. falciparum. PI-containing antiretroviral regimens may demonstrate prophylactic activity against both vivax and falciparum malaria in HIV-infected patients who reside in areas where multidrug-resistant P. vivax or P. falciparum is found.

HIV and malaria co-infection: interactions and consequences of chemotherapy

The global epidemiology of HIV/AIDS and malaria overlap because a significant number of HIV-infected individuals live in regions with different levels of malaria transmission. Although the consequences of co-infection with HIV and malaria parasites are not fully understood, available evidence suggests that the infections act synergistically and together result in worse outcomes. The importance of understanding chemotherapeutic interactions during malaria and HIV co-infection is now being recognized. We know that some antimalarial drugs have weak antiretroviral effects; however, recent studies have also demonstrated that certain antiretroviral agents can inhibit malaria-parasite growth. Here, we discuss recent findings on the impact of HIV/AIDS and malaria co-infection and the possible roles of chemotherapy in improving the treatment of these diseases.

Critical roles for the digestive vacuole plasmepsins ofPlasmodium falciparumin vacuolar function

Knockout mutants of Plasmodium falciparum lacking pfpm1, pfpm2 and pfhap (triple-PM KO), and mutants lacking all four digestive vacuole (DV) plasmepsins (pfpm4, pfpm1, pfpm2 and pfhap; quadruple-PM KO), were prepared by double cross-over integration effecting chromosomal deletions of up to 14.6 kb. The triple-PM KO was similar to the parental line (3D7) in growth rate, morphology and sensitivity to proteinase inhibitors. The quadruple-PM KO showed a significantly slower rate of growth in standard medium, which manifested as delayed schizont maturation accompanied by reduced formation of haemozoin. In amino acid-limited medium, the reduction in growth rate of the quadruple-PM KO was pronounced. The sensitivity of both the triple- and quadruple-PM KOs to six different HIV aspartic proteinase inhibitors was comparable to that of 3D7, thus establishing that the DV plasmepsins were not the primary targets of the antimalarial activity of these clinically important compounds. Electron microscopic analysis revealed the presence of multilamellar bodies resembling ceroid in the DV of the quadruple-PM KO, and intermediates of the autophagic pathway accumulated as determined by Western blot analysis. Thus, the DV plasmepsins, although not essential, contribute significantly to the fitness of the parasite and are required for efficient degradation of endosomal vesicles delivered to the DV.

Synergistic interactions of the antiretroviral protease inhibitors saquinavir and ritonavir with chloroquine and mefloquine against Plasmodium falciparum in vitro

The antimalarial activity of several antiretroviral protease inhibitor combinations was investigated. Data demonstrate that ritonavir and saquinavir behave synergistically with chloroquine and mefloquine. These data, and interactions with pepstatin-A, E-64, and bestatin, suggest that human immunodeficiency virus protease inhibitors do not target digestive-vacuole plasmepsins.