From one Nobel Prize (P. Ehrlich) to another (Tu Youyou): 100 years of chemotherapy of infectious diseases
Authors: Z.I. Cabantchik, H. Drakesmith
Journal: Clinical Microbiology and Infection (2015)

The centenary of Paul Ehrlich's death was commemorated by scientific communities worldwide, including at the 2015 International BioIron Meeting held by the International BioIron Society in Hangzhou, China (http://www.bioiron.org/). Chemotherapy, founded by Ehrlich (1854–1915; 1908 Nobel laureate), received particular recognition in 2015 by the Nobel Prize in Medicine being awarded to Tu Youyou for leading the team that introduced the antimalarial artemisinin (qinghaosu). (The 2015 Nobel Prize in Medicine was also awarded jointly to Satushi Omura and William Campbell for the discovery of novel antihelminth antibiotics. These scientists followed drug-screening methods that had led earlier to the discovery of streptomycin and 17 other antibiotics by Selman A. Vaksman (1952 Nobel Prize), largely based on work schemes applied by Paul Ehrlich.) Notably, although over a century has elapsed, the approach to medical research introduced by Ehrlich is still rendering novel therapeutic tools. The concept of targeted chemotherapy, whereby a chemical agent designed by nature or made in the laboratory acts as a magic bullet (zauberkugel) against a specific pathogenic target, is attributed to Ehrlich [[1]Witkop B. Paul Ehrlich and his magic bullets, revisited.Proc Am Phil Soc. 1999; 143: 540-547PubMed Google Scholar]. He arrived at this idea by studying cells stained with dyes. It was with a variety of staining techniques and careful examination that Ehrlich revealed the potential of vital stains such as methylene blue and Trypan blue as tools to identify features associated with individual blood cell types [[2]Buchwalow I. Boecker W. Tiemann M. The contribution of Paul Ehrlich to histochemistry: a tribute on the occasion of the centenary of his death.Virchows Arch. 2015; 466: 111-116Crossref PubMed Scopus (14) Google Scholar]. Ehrlich then extended the idea of chemical staining of a specific tissue/cell into a chemical tool for targeting a pathological agent. His maxim Corpora non agunt nisi fixata (compounds will not act unless they bind) reflected his rationale for designing organotropic and parasitotropic chemicals as anti-infective agents. Those agents became the tools of his Therapia magna sterilisans as they were selected for their ability to affect the pathogen while sparing the host (ideally with a single dose). By experimenting with methylene blue, he discovered its antimalarial potential as well as its analgesic power. Those observations paved the road for introducing atebrin as a quinine substitute and chlorpromazine for the management of severe neuroses and psychoses. The initial studies with chemical dyes as potential therapeutics in animal models of infection were subsequently expanded to derivatives of the pentavalent arsenic atoxyl, a chemical previously shown to be toxic to parasites but also to humans. In collaboration with Alfred Bertheim he improved the therapeutic index of atoxyl by systematic modification and testing in trypanosome-infected animals. In collaboration with Sashiro Hata he extended the tests to an animal model of syphilis (Treponema pallidum), which culminated with the generation of arsphenamine (compound 608), the first synthetic anti-infective that spared the host. Following extensive animal and human testing, tens of thousands of arsphenamine samples (commercialized as Salvarsan®) were initially offered freely to doctors worldwide for clinical assessment. Within a few years Salvarsan was replaced by the more easily dispensable, clinically effective and less toxic analogue Neosalvarsan® (No. 914). The ultimate goal of Therapia magna sterilisans became a reality in Ehrlich's lifetime, as with a single Salvarsan delivery, patients with frambesia (yaws) were cured from Spirochaeta pertenuis infection. Moreover, his view of using a combination of drugs with diverse chemical structures to overcome pathogen resistance and minimize the toxic effects of single overdosing drugs, gained direct support by treating African trypanosomiasis with arsphenamine in combination with acriflavine and ochlororosaniline. Although Salvarsan gradually ceased to be used after the advent of penicillin, Ehrlich's approach to drug discovery became a pre-eminent model for future generations, as exemplified in the discovery of the antibacterials sulphonamide (Prontosil) by Gerhard Domagk and streptomycin by Selman Vaksman, and as recognized by this year’s Nobel Prizes for the work on antiparasitic drugs. The search for antimalarial magic bullets, including synthetic analogues of quinine and antifolates, comprised large screening studies sponsored by the US and UK governments, WHO and by private foundations, leading to a small number of new drugs such as atovaquone and proguanyl, for which parasite multi-drug resistance rapidly emerged and spread across continents [[3]White N.J. Antimalarial drug resistance.J Clin Inv. 2004; 113: 1084-1092Crossref PubMed Scopus (841) Google Scholar]. Antimalarial drug combinations proved beneficial, particularly when compliance/adherence to treatment were strictly implemented. However, resistance to novel agents continued to pose a major challenge [[3]White N.J. Antimalarial drug resistance.J Clin Inv. 2004; 113: 1084-1092Crossref PubMed Scopus (841) Google Scholar]. During the North–South Vietnam war, China set up a top-secret national military project (No. 453) aimed at identifying antimalarial drugs among Chinese traditional herbal medicines [[4]Cui L. Su X.Z. Discovery, mechanisms of action and combination therapy of artemisinin.Expert Rev Anti Infect Ther. 2009; 7: 999-1013Crossref PubMed Google Scholar]. Trained as a phytochemist and in traditional medicines, Tu Youyou assembled a team that screened thousands of boiled water extracts from dried plants on Plasmodium berghei-infected mice. Partial but inconsistent success with extracts from qinghao (Artemisia annua) led Tu Youyou to search in the Handbook of Prescriptions for Emergencies (Ge Hon, 284–363 ad) for clues that led to the successful extraction of the active component of qinghao. Cold extraction, originally with water but currently with ether, reproducibly generated active parasiticidal mixtures that were more potent than chloroquine, enabling collaborating groups in the Yunnan Institute of Materia Medica to isolate, crystallize and chemically characterize the active agent (qinghaosu—artemisinin) and in the Guangzhou University of Traditional Medicine to demonstrate its clinical efficacy (against Plasmodium falciparum and Plasmodium vivax) and adequate safety. The road was opened not only for mass production of the drug but also for the search for more potent analogues and for strategies to overcome recrudescence associated with monotherapy [[3]White N.J. Antimalarial drug resistance.J Clin Inv. 2004; 113: 1084-1092Crossref PubMed Scopus (841) Google Scholar]. These have largely been achieved by artemisinin-based combination therapy, leading to improved pharmacokinetics, fast and long-lasting antimalarial efficacy and acceptable tolerability [[4]Cui L. Su X.Z. Discovery, mechanisms of action and combination therapy of artemisinin.Expert Rev Anti Infect Ther. 2009; 7: 999-1013Crossref PubMed Google Scholar]. From the outset, the antimalarial action of artemisinins has been attributed to the pro-oxidant endoperoxide moiety capable of generating toxic reactive oxygen species by interacting with labile iron in heme or other complexes assumed to be generated in parasitized cells [[4]Cui L. Su X.Z. Discovery, mechanisms of action and combination therapy of artemisinin.Expert Rev Anti Infect Ther. 2009; 7: 999-1013Crossref PubMed Google Scholar]. This concept might explain the specific (‘magic bullet’) effect of artemisinin on infected erythrocytes and the antagonistic effects afforded by iron chelators on its antimalarial activity—although the structural resemblance between thapsigargin and artemisinin has led some investigators to suggest that artemisinin affects parasite calcium homeostasis [[4]Cui L. Su X.Z. Discovery, mechanisms of action and combination therapy of artemisinin.Expert Rev Anti Infect Ther. 2009; 7: 999-1013Crossref PubMed Google Scholar]. Artemisinin has provided enormous benefit for public health. However, Plasmodium has developed resistance to its effects and, alarmingly, resistant forms of the parasite are spreading [3White N.J. Antimalarial drug resistance.J Clin Inv. 2004; 113: 1084-1092Crossref PubMed Scopus (841) Google Scholar, 5Ashley E.A. Dhorda M. Fairhurst R.M. Amaratunga C. Lim P. Suon S. et al.Spread of artemisinin resistance in Plasmodium falciparum malaria.N Engl J Med. 2014; 371: 411-423Crossref PubMed Scopus (1442) Google Scholar]. New antimalarials are likely to be needed, and in this search to discover chemotherapeutic agents, the scientific community should bear in mind Paul Ehrlich's famous maxim about the key elements of research success known as the four Gs: Geld (money), Geduld (patience), Geschick (skill) and Glück (luck) [[1]Witkop B. Paul Ehrlich and his magic bullets, revisited.Proc Am Phil Soc. 1999; 143: 540-547PubMed Google Scholar]. All these elements contributed to the discovery of artemisinin and arsphenamine, and it is most probable that they will be needed again to develop new ‘magic bullets’ against malaria and other diseases.