Malaria Parasite Tackled in Mosquitoes

NEWS & VIEWS RESEARCH

N N F N Persistent sulfur-based radical – F BF4 O F S F O O Range of BF – groups Me 4 Me S Me F S F O H •+ O S Cross-coupling O + Thianthrenation O O F O F Figure 1 | A method for the site-selective modification of complex reactant. The products are compounds that bear a thianthrenium group molecules. Berger et al.1 report that a chemical species known as a persistent (blue; the dot on the lower sulfur atom represents an unpaired electron). sulfur-based radical, generated in situ, reacts with structurally complex A carbon–sulfur bond (green) in these products can take part in a variety molecules at just one carbon–hydrogen (C–H) bond (shown in red in this of metal-based cross-coupling reactions, thus allowing a wide range of example; other C–H bonds in the molecule are not shown). Which C–H chemical groups to be attached at the carbon atom in the original C–H bond. bond reacts depends on the electronic and structural properties of the Me, methyl group.

But despite impressive advances, metal- structurally complex molecules with different In the meantime, Berger et al. have established catalysed C–H functionalizations are not yet chemical groups is particularly useful for the foundations of a compelling new approach applicable to as broad a range of compounds synthesizing analogues of biologically active for cross-coupling that takes advantage of C–H as are cross-couplings, and they generally compounds in drug-discovery programmes8. functionalization. In effect, the authors have have site-selectivity issues similar to those of It is difficult to think of a comparable reaction developed an adhesive for decorating mol halogen-installing reactions. that works for this purpose as selectively and ecules that makes it much easier to generate Berger et al. now describe a development with as broad a range of substrates as the newly any structure that we might need. ■ that could help to address the issues of reported chemistry. molecular decoration. Inspired by research Nevertheless, these reactions might not be Eric M. Ferreira is in the Department of from almost 50 years ago4, the authors report applicable to all molecules. Some chemical Chemistry, University of Georgia, Athens, a highly selective method for replacing the groups are sensitive to the thianthrenation Georgia 30602, USA. hydrogen of a C–H bond with a group called a conditions (which are oxidizing), and might e-mail: [email protected] thianthrenium, thereby generating a new C–S therefore take part in oxidative side reactions. 1. Berger, F. et al. Nature 567, 223–228 (2019). bond (Fig. 1). The thianthrenium group then Moreover, the thianthrenated compounds 2. Wu, X.-F., Anbarasan, P., Neumann, H. & Beller, M. serves as a halogen proxy for a broad array of have not yet been shown to take part in certain Angew. Chem. Int. Edn 49, 9047–9050 (2010). cross-coupling reactions. widely used cross-coupling reactions (notably, 3. Chen, X., Engle, K. M., Wang, D.-H. & Yu, J.-Q. Angew. Chem. Int. Edn 48, 5094–5115 (2009). The first step of the process — the amination reactions, which generate carbon– 4. Shine, H. J. & Silber, J. J. J. Org. Chem. 36, thianthrenation step — is strikingly site- nitrogen bonds). More work is needed to 2923–2926 (1971). selective. This is because the thianthrenium develop such reactions. 5. Griller, D. & Ingold, K. U. Acc. Chem. Res. 9, 13–19 group derives from a species known as a per- Finally, the site selectivity of the thianthrena- (1976). 5,6 6. Romero, K. J., Galliher, M. S., Pratt, D. A. & sistent sulfur-based radical ; the use of this tion step is governed entirely by the electronic Stephenson, C. R. J. Chem. Soc. Rev. 47, 7851–7866 radical promotes a reaction that discriminates and structural properties of the reactant mol- (2018). 7. Srogl, J., Allred, G. D. & Liebeskind, L. S. J. Am. clearly between the many C–H bonds in a ecule. It will be interesting to see whether Chem. Soc. 119, 12376–12377 (1997). given molecule. Berger et al. demonstrate that alternative C–H sites on molecules can be selec- 8. Cernak, T., Dykstra, K. D., Tyagarajan, S., Vachal, P. & this selectivity holds across a wide range of tively thianthrenated using different protocols. Krska, S. W. Chem. Soc. Rev. 45, 546–576 (2016). molecules, and thereby provide data that will help chemists to work out which site is likely to react in complex molecules that have several MEDICAL RESEARCH potentially reactive C–H bonds. Essentially, in almost all of the cases evaluated, only one C–H bond reacts, with the selectivity dictated by the electronic and structural characteristics Malaria parasite of each molecule. Equally impressive is the breadth of the cross-coupling reactions that can be carried tackled in mosquitoes out using the thianthrenated compounds, in which the C–S bond acts as an ‘activated’ car- Insecticides that kill mosquitoes have helped to fight malaria, but insecticide bon–halogen bond7. A variety of palladium- resistance is rising. Treating mosquitoes with drugs that target the disease-causing catalysed cross-couplings are effective and, in parasite offers another way of tackling malaria. See Letter p.239 the most notable examples, the thianthrenium group reacts in preference to carbon–halogen bonds or other similarly reactive groups. The JANET HEMINGWAY insecticides is on the increase. On page 239, authors also find that their thianthrenium Paton et al.1 report a non-insecticidal interven- compounds work in other types of cross- ny long-term programme for disease tion that stops mosquitoes from transmitting coupling reaction, such as copper-mediated control that relies on one type of inter- malaria and that might offer a way to reduce processes and ‘photoredox’ couplings. vention has a high probability of fail- the reliance on insecticides alone as a means of About half of the molecules tested by Berger Aure owing to the development of treatment malaria prevention. and colleagues in their reactions are pharma- resistance. Efforts to limit malaria by using It is estimated2 that mosquito control using ceutical compounds, which demonstrates the insecticides to kill female mosquitoes, which bed nets impregnated with long-lasting insec- potential value of this chemistry to medici- transmit the disease-causing Plasmodium par- ticides called pyrethroids, and indoor insec- nal chemists. Having the ability to decorate asite, are no exception: mosquito resistance to ticide spraying, resulted in 1.3 billion fewer

RESEARCH NEWS & VIEWS malaria cases and 6.8 million fewer insecticide resistance7. This suggests deaths between 2000 and 2015. How- Treated that a multifaceted approach should be ever, despite these intensive efforts, bed net used, and progress will rely on efforts a 2018 report3 by the World Health to reduce the cost of bringing new Organization (WHO) states that, drugs to market and streamlining the between 2015 and 2017, global efforts Insecticide-resistant, associated development, policy and parasite-free mosquito to reduce the burden of malaria stalled. implementation steps8. The report highlights a funding gap Although Paton and colleagues’ that has resulted in a failure to achieve Midgut Mitochondrion approach holds promise, substantial universal coverage of bed nets, where hurdles must be overcome before a needed, and an increase in resistance product is generated that is recom- to antimalarial drugs and insecticides. Plasmodium mended and universally accepted by It also mentions the limited data avail- ookinete funders, countries and communities able on insecticide resistance, and for use in control programmes. Exten- Insecticide- states that the impact of pyrethroid sensitive sive work will be needed to optimize resistance on the effectiveness of cur- mosquito Cytochrome b drug development, and so produce rent pyrethroid-based interventions inhibitor long-lasting formulations. Moreover, is poorly understood. Although the assessments of the drug-manufacturing report says that such increasing resist- process should be undertaken, together ance will probably reduce the efficacy with a drug’s resistance potential and of current interventions, it downplays the cost and public acceptability of the the potential impact on the use of pyre- approach. throid-treated bed nets, stating that the The WHO has a system called nets, which have been responsible4 for prequalification, in which a product for approximately 68% of the estimated Figure 1 | How a non-insecticidal treatment might be used to tackling a health problem must meet reduction in malarial transmission block malaria transmission. Transmission of the Plasmodium certain specifications before it can be since 2000, continue to be effective. parasite to humans by mosquitoes causes malaria. A standard placed on a list for selection by donor Sadly, this optimism seems misplaced, disease-prevention approach is the treatment of bed nets with organizations. In addition, countries given the evidence that the increased insecticides that kill mosquitoes. However, mosquito resistance have product-recommendation sys- to insecticides is on the rise. Paton et al.1 report that exposing insecticide resistance is affecting the tems that consider proposed interven- mosquitoes to surfaces treated with antimalarial drugs targeting level of disease transmission. tions and establish the burden of proof 5 the parasite kills the parasite in its host, and they propose that Models had already predicted that bed-net treatment that combines both insecticides and such for whether a WHO recommendation insecticide resistance would affect parasite-targeting compounds might aid efforts to tackle malaria. is warranted for use there. Completing malaria transmission. Even low levels The authors studied drugs that inhibit the protein cytochrome b these steps is a difficult and lengthy of insecticide resistance would increase in mitochondria (the cell’s energy-generating organelles) in process. For example, the first PBO nets the incidence of malaria because of the Plasmodium. They report that these drugs block the development were a market-ready product bearing effect of reduced mosquito mortality of ookinete-stage Plasmodium parasites in the mosquito midgut. a WHO interim recommendation ten and lower overall community protec- Killing the parasite in this way would prevent insecticide-resistant years before the first WHO guidance tion during the projected three-year mosquitoes from transmitting disease. was issued on where and when this type lifespan of a bed net. A type of bed net of net should be used2. The combined called a PBO net that contains pyrethroid and with low doses of the drug was enough to have developmental, regulatory and other processes piperonyl butoxide, a compound that enhances an effect. This is within the time frame for involved can create a 20-year hiatus before a pyrethroid’s effectiveness, would be substan- which wild mosquitoes rest on bed nets. The new technology enters the marketplace and tially more effective than pyrethroid-only nets5. finding is important because, after more than is used at scale. Substantial effort and fund- It is estimated that switching from pyrethroid- a century of research, and small-scale projects ing will therefore be needed to ensure a rapid only bed nets to PBO nets would prevent up to involving biological or environmental control, evaluation of whether Paton and colleagues’ 0.5 clinical cases of malaria per person per year it is evident that non-insecticidal interven- suggested approach could have a role in in some scenarios5. Trials have demonstrated tions do not contribute substantially to efforts reversing the current trend of rising malaria that, in insecticide-resistant areas, PBO nets are to reduce malaria transmission. transmission. ■ more effective at reducing the transmission of Paton and colleagues suggest that malaria than are pyrethroid-only nets6. antimalarial drugs could be incorporated into Janet Hemingway is at the Liverpool School Given the urgent need to expand the range bed nets alongside insecticides (Fig. 1). Their of Tropical Medicine, Liverpool L3 5QA, UK. of effective interventions, Paton et al. suggest modelling predicts that this would reduce e-mail: [email protected] a way forward for the development of next- the prevalence of malaria, with the size of the 1. Paton, D. G. et al. Nature 567, 239–243 (2019). generation antimalarial bed nets. They tested impact being affected by the levels of disease 2. Killeen, G. & Ranson, H. Lancet 391, 1551–1552 whether Plasmodium falciparum, the major transmission, the number of people using nets (2018). malaria-causing parasite in Africa, could be and the degree of insecticide resistance. The 3. World Health Organization. World Malaria Report killed by exposing mosquitoes to the parasite- increased effect of adding the antimalarial 2018 (WHO, 2018); go.nature.com/2ssezu1 killing drugs that are used to prevent people drug to insecticide-treated nets is most strik- 4. Bhatt, S. et al. Nature 526, 207–211 (2015). 5. Churcher, T. S., Lissenden, N., Griffin, J. T., Worrall, E. from getting malaria. These work by targeting ing in the authors’ models when bed-net usage & Ranson, H. eLife 5, e16090 (2016). mitochondria — energy-generating organelles. is 70–100%, underscoring the need to strive for 6. Protopopoff, N. & Rowland, M. Lancet 391, The authors found that the antimalarial drug high levels of net usage even if the compounds 2415–2416 (2018). atovaquone, which inhibits the mitochondrial used to treat the nets change in the future. 7. World Health Organization. Global Plan for protein cytochrome b — as well as other types Malaria cannot be sustainably controlled or Insecticide Resistance Management in Malaria Vectors (WHO, 2012); go.nature.com/2tvtiyc of cytochrome b inhibitor drug — could kill eliminated without effective mosquito con- 8. Hemingway, J. et al. Lancet 387, 1785–1788 (2016). parasites in a mosquito host. Exposing mos- trol. Paton and colleagues’ approach fits well quitoes for six minutes to a glass surface coated with the WHO’s global plan for managing This article was published online on 27 February 2019.