New developments: chloroquine-resistance in Plasmodium falciparum

Decrypting the complexity of the human malaria parasite biology through systems biology approaches

The human malaria parasite, Plasmodium falciparum, is a unicellular protozoan responsible for over half a million deaths annually. With a complex life cycle alternating between human and invertebrate hosts, this apicomplexan is notoriously adept at evading host immune responses and developing resistance to all clinically administered treatments. Advances in omics-based technologies, increased sensitivity of sequencing platforms and enhanced CRISPR based gene editing tools, have given researchers access to more in-depth and untapped information about this enigmatic micro-organism, a feat thought to be infeasible in the past decade. Here we discuss some of the most important scientific achievements made over the past few years with a focus on novel technologies and platforms that set the stage for subsequent discoveries. We also describe some of the systems-based methods applied to uncover gaps of knowledge left through single-omics applications with the hope that we will soon be able to overcome the spread of this life-threatening disease.

Deployment and utilization of next-generation sequencing of Plasmodium falciparum to guide anti-malarial drug policy decisions in sub-Saharan Africa: opportunities and challenges

Parasite resistance against anti-malarial drugs is a major threat to the ongoing malaria control and elimination strategies. This is especially true since resistance to the currently recommended artemisinins and partner drugs has been confirmed in South East Asia (SEA) and new anti-malarial compounds are not expected to be available in the near future. Spread from SEA or independent emergence of artemisinin resistance in sub-Saharan Africa (SSA) could reverse the achievements in malaria control that have been attained in the past two decades and derail the ongoing elimination strategies. The current surveillance of clinical efficacy and resistance to anti-malarial drugs is based on efficacy trials to assess the clinical performance of anti-malarials, in vivo/ex vivo assessment of parasite susceptibility to anti-malarials and prevalence of known molecular markers of drug resistance. Whereas clinical efficacy trials are restricted by cost and the complex logistics of patient follow-up, molecular detection of genetic mutations associated with resistance or reduced susceptibility to anti-malarials is by contrast a simple and powerful tool for early detection and monitoring of the prevalence of resistant parasites at population level. This provides needed information before clinical failure emerges, allowing policy makers to anticipate problems and respond. The various methods previously used in detection of molecular markers of drug resistance share some limitations: low-throughput, and high costs per sample and demanding infrastructure. However, recent technological advances including next-generation sequencing (NGS) methodologies promise greatly increased throughput and reduced costs, essentially providing unprecedented potential to address different research and operational questions of relevance for drug policy. This review assesses the potential role of NGS to provide comprehensive information that could guide drug policies in malaria endemic countries and looks at the foreseeable challenges facing the establishment of NGS approaches for routine surveillance of parasite resistance to anti-malarials in SSA.

Replacement of the 4'-hydroxy group of amodiaquine and amopyroquine by aromatic and aliphatic substituents: synthesis and antimalarial activity

The prophylactic administration of amodiaquine (AQ), a 4-aminoquinoline antimalarial drug, has been associated with side effects such as agranulocytosis and liver damage. The toxicity of this drug is mediated by amodiaquine quinone-imine, an electrophilic metabolite. Replacement of the 4'-hydroxy function of AQ with various alkyl, aryl, or heteroaryl substituents would provide analogues that avoid metabolism to potentially toxic derivatives. Following a multistep procedure, 33 compounds containing hydrophobic groups at the 4'-position were synthesized using Csp(2)-Csp(2) and Csp(2)-Csp(3) Suzuki-Miyaura cross-coupling reactions as the key step. The new derivatives were found to be active against both chloroquine (CQ)-sensitive and CQ-resistant strains of P. falciparum, with IC(50) values in the range of 7-200 nM. Alkyl analogues are more efficient than aryl or heteroaryl derivatives. All compounds were also assessed for their cytotoxicity and ability to inhibit beta-hematin formation in vitro. A detailed investigation of the structure-activity relationships for these new compounds was carried out; the 4'-methyl compound showed interesting in vivo antimalarial activity.

Quinolines and artemisinin: chemistry, biology and history

Plasmodium falciparum is the most important parasitic pathogen in humans, causing hundreds of millions of malaria infections and millions of deaths each year. At present there is no effective malaria vaccine and malaria therapy is totally reliant on the use of drugs. New drugs are urgently needed because of the rapid evolution and spread of parasite resistance to the current therapies. Drug resistance is one of the major factors contributing to the resurgence of malaria, especially resistance to the most affordable drugs such as chloroquine. We need to fully understand the antimalarial mode of action of the existing drugs and the way that the parasite becomes resistant to them in order to design and develop the new therapies that are so urgently needed. In respect of the quinolines and artemisinins, great progress has been made recently in studying the mechanisms of drug action and drug resistance in malaria parasites. Here we summarize from a historical, biological and chemical, perspective the exciting new advances that have been made in the study of these important antimalarial drugs.

MOLECULAR MECHANISMS OF RESISTANCE IN ANTIMALARIAL CHEMOTHERAPY: The Unmet Challenge

▪ Abstract 

The enormous public health problem posed by malaria has been substantially worsened in recent years by the emergence and worldwide spread of drug-resistant parasites. The utility of two major therapies, chloroquine and the synergistic combination of pyrimethamine/sulfadoxine, is now seriously compromised. Although several genetic mechanisms have been described, the major source of drug resistance appears to be point mutations in protein target genes. Clinically significant resistance to these agents requires the accumulation of multiple mutations, which genetic studies of parasite populations suggest arise focally and sweep through the population. Efforts to circumvent resistance range from the use of combination therapy with existing agents to laboratory studies directed toward discovering novel targets and therapies.

The prevention and management of drug resistance are among the most important practical problems of tropical medicine and public health.

Leonard J. Bruce-Chwatt, 1972

Reversal of mefloquine and quinine resistance in Plasmodium falciparum with NP30

Quinoline resistance in malaria is frequently compared with P-glycoprotein-mediated multidrug resistance (mdr) in mammalian cells. We have previously reported that nonylphenolethoxylates, such as NP30, are potential Plasmodium falciparum P-glycoprotein substrates and drug efflux inhibitors. We used in vitro assays to compare the ability of verapamil and NP30 to sensitize two parasite isolates to four quinolines: chloroquine (CQ), mefloquine (MF), quinine (QN), and quinidine (QD). NP30 was able to sensitize (reversal, >80%) P. falciparum to MF, QN, QD, and, to a lesser extent, CQ. The presence of 2 micro M verapamil had no effect on mefloquine resistance; however, the presence of verapamil modulated the activities of QN and QD in a manner parallel to that observed for CQ. Genetic analysis of putative quinoline resistance genes did not suggest an association between known point mutations in pfcrt and pfmdr1 and NP30 sensitization activity. We conclude that the sensitization action of NP30 is distinct both phenotypically and genotypically from that of verapamil.

Theories on malarial pigment formation and quinoline action

Haeme metabolism remains a vulnerable problem for the intraerythrocytic Plasmodium which catabolises haemoglobin as a source of amino acids in an acidic, oxygen-rich lysosome-like digestive vacuole. Haeme monomer, capable of generating oxygen radicals, transforms into an inert crystal named malarial pigment or haemozoin by forming unique dimers that then crystalise. Laveran first described pigmented bodies in humans to define a protozoan as the aetiologic agent of malaria. The trail of malaria pigment enabled Ross to implicate the mosquito in the life cycle of Plasmodium. In 1991, Slater and Cerami postulated a unique iron-carboxylate bond between two haemes in haemozoin crystals based on infrared and X-ray spectroscopy data. Additionally, parasite extracts were shown to possess a 'haeme polymerase' enzymatic activity as the process of crystal formation was then termed. Importantly, the quinolines, such as choloroquine, inhibit haemozoin formation. A Plasmodium falciparum derived histidine-rich protein II, which binds haeme and initiates haemozoin formation, is present in the digestive vacuole. Pfhistidine-rich protein II and Pfhistidine-rich protein III are sufficient, but not necessary for haemozoin formation as a laboratory clone lacking both still makes the haeme crystals. The reduvid bug, and the Schistosoma and Haemoproteus genera also make haemozoin. Recently, Bohle and coworkers used X-ray diffraction to document the iron-carboxylate bond in intact desiccated parasites and to show that a Fe1-O41 head to tail haeme dimer is the unit building block of haemozoin. The role of the Plasmodium histidine-rich protein family members, lipids or potential novel proteins in the exact molecular assembly of the large molecular weight haeme crystals in the protein rich digestive vacuole needs to be solved. Accurate experimental determination of the role of haemozoin formation and inhibition as the target of chloroquine is fundamental to determination of the mechanism of quinoline drug action and resistance. The enhanced understanding of the biosynthetic pathway leading to haemozoin formation using functional proteomic tools and the mechanisms through which existing antimalarial drugs affect Plasmodium haeme chemistry will help design improved chaemotherapeutic agents.