- Email: [email protected]
Artemisinin resistance needs to be defined rigorously to be understood: response to Dondorp and Ringwald
Letter Response
Artemisinin resistance needs to be defined rigorously to be understood: response to Dondorp and Ringwald Sanjeev Krishna1,2,3 and Peter Gottfried Kremsner2,3 1
Centre for Infection and Immunity, Division of Clinical Sciences, St George’s, University of London, Cranmer Terrace, London SW17 0RE, UK 2 Institut fu¨r Tropenmedizin, Universita¨t Tu¨bingen, Tu¨bingen, Germany 3 Centre de Recherches Me´dicales de Lambare´ne´, Lambare´ne´, Gabon
Dondorp and Ringwald are concerned that malaria control efforts in the Greater Mekong Region may be compromised if questions are raised about the definition of artemisinin resistance [1]. Surely, it is the very best interpretation of available evidence that should underpin these efforts? That is what we have attempted to provide in our opinion article [2]. Our point that slower parasite clearance is not necessarily artemisinin resistance goes beyond semantics because of its implications for research and perhaps policy. For example, studies on the mechanisms of resistance to partner drugs may be overshadowed, and artemisinins may be prematurely consigned to the scrap heap if they focus on what is rather exclusively defined as being ‘artemisinin resistance’. Why is delayed parasite clearance not an accurate marker for artemisinin resistance? Artemisinins need 7 days to cure malaria, whereas they are commonly given for 3 days in combination therapy. An incomplete treatment course with any antimicrobial can be associated with treatment failures without the need to invoke the specter of ‘resistance’. Furthermore, a slower clearance phenotype lacks diagnostic specificity (>50% of patients can have persistent parasitemia 72 h after artesunate monotherapy but achieve cure at 28 days in >94% of cases) [3]. Even when there is parasite persistence more than 3 days after treatment with an artemisinin, and this persistence is associated with treatment failures, for example, with dihydroartemisinin–piperaquine, we should be identifying and assessing markers for resistance to piperaquine. Patients with splenectomy have grossly prolonged parasite clearance times without this being caused by artemisinin resistant parasites, once again highlighting the complexities inherent in interpreting this phenotype. As parasites demonstrably circulate for longer than 72 h without increased risk of treatment failure, there are interesting questions that arise about the state of these parasites. The coincidence of prolonged clearance of parasites with failure to artesunate–mefloquine treatment is a correlative and not a causal observation, especially as we do not know the viability of these circulating parasites. The lack of conventional in vitro correlates in most (but not all [4]) studies of artemisinin resistance in this geographic area further supports the suggestion that artemisinin resistance may be misclassified. This may also be the Corresponding author: Krishna, S. ([email protected], [email protected]).
case because others have identified in vitro phenotypes [5], assessed in conventional assays in other regions, that are consistent with artemisinin resistance and that are associated with plausible mechanistic hypotheses [6]. Treatment failures may then arise with artemisinin combination therapies (TFACTs) because partner drugs are failing, and the accompanying artemisinin dosing regimen is insufficient to cure patients [2]. Genetic markers for resistance to some partner drugs [7,8] of artemisinins are already validated in clinical studies. Increased pfmdr1 copy number predicts failure with artesunate–mefloquine combination therapy [8]. However, these markers are not being described systematically in genome association studies such as the recently published one by Miotto and colleagues [9], particularly as gene dosage effects are important in affecting outcomes. Will these investigators provide in vitro assay results and molecular markers for resistance to partner drugs used in artemisinin combination therapies (ACTs) in these studies? Will correlations of treatment responses in individual patients also be assessed and presented? Without these full datasets being made available (thereby granting open access both in letter and spirit), we are left with a somewhat partial picture and interpretation. Consensus on resistance Consensus on a working definition of artemisinin resistance will be important to achieve in the future when genome studies are being implemented because robust phenotypes are essential for discerning useful associations. The identification of delayed clearance of parasites highlights a very interesting biological phenomenon [2], but its contribution to artemisinin resistance (sensu stricto) has yet to be established. As before, with regard to our studies aimed at advancing dosing regimens of artesunate [10], we look forward to debates [11,12] that enrich the process of reappraising and improving the management of malaria. References 1 Dondorp, A. and Ringwald, P. (2013) Artemisinin resistance is a clear and present danger. Trends Parasitol. http://dx.doi.org/10.1016/j.pt. 2013.05.005 2 Krishna, S. and Kremsner, P.G. (2013) Antidogmatic approaches to artemisinin resistance: reappraisal as treatment failure with artemisinin combination therapy. Trends Parasitol. http://dx.doi.org/ 10.1016/j.pt.2013.04.001 3 Saunders, D. et al. (2012) Pharmacokinetics and pharmacodynamics of oral artesunate monotherapy in patients with uncomplicated 361
Letter Response
4 5
6
7
362
Plasmodium falciparum malaria in Western Cambodia. Antimicrob. Agents Chemother. 56, 5484–5493 Noedl, H. et al. (2008) Evidence of artemisinin-resistant malaria in western Cambodia. N. Engl. J. Med. 359, 2619–2620 Jambou, R. et al. (2005) Resistance of Plasmodium falciparum field isolates to in-vitro artemether and point mutations of the SERCA-type PfATPase6. Lancet 366, 1960–1963 Pulcini, S. et al. (2013) Expression in yeast links field polymorphisms in PfATP6 to in vitro artemisinin resistance and identifies new inhibitor classes. J. Infect. Dis. http://dx.doi.org/10.1093/infdis/jit171 Malmberg, M. et al. (2013) Plasmodium falciparum drug resistance phenotype as assessed by patient antimalarial drug levels and its association with pfmdr1 polymorphisms. J. Infect. Dis. 207, 842–847
Trends in Parasitology August 2013, Vol. 29, No. 8
8 Price, R.N. et al. (2004) Mefloquine resistance in Plasmodium falciparum and increased pfmdr1 gene copy number. Lancet 364, 438–447 9 Miotto, O. et al. (2013) Multiple populations of artemisinin-resistant Plasmodium falciparum in Cambodia. Nat. Genet. 45, 648–655 10 Kremsner, P.G. et al. (2012) A simplified intravenous artesunate regimen for severe malaria. J. Infect. Dis. 205, 312–319 11 Dondorp, A.M. et al. (2012) Artesunate dosing in severe falciparum malaria. J. Infect. Dis. 206, 618–619 author reply 622 12 Kremsner, P.G. et al. (2012) A simplified intravenous artesunate regimen for severe malaria – reply. J. Infect. Dis. 206, 621–622 1471-4922/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pt.2013.05.006 Trends in Parasitology, August 2013, Vol. 29, No. 8
Artemisinin resistance needs to be defined rigorously to be understood: response to Dondorp and Ringwald Sanjeev Krishna1,2,3 and Peter Gottfried Kremsner2,3 1
Centre for Infection and Immunity, Division of Clinical Sciences, St George’s, University of London, Cranmer Terrace, London SW17 0RE, UK 2 Institut fu¨r Tropenmedizin, Universita¨t Tu¨bingen, Tu¨bingen, Germany 3 Centre de Recherches Me´dicales de Lambare´ne´, Lambare´ne´, Gabon
Dondorp and Ringwald are concerned that malaria control efforts in the Greater Mekong Region may be compromised if questions are raised about the definition of artemisinin resistance [1]. Surely, it is the very best interpretation of available evidence that should underpin these efforts? That is what we have attempted to provide in our opinion article [2]. Our point that slower parasite clearance is not necessarily artemisinin resistance goes beyond semantics because of its implications for research and perhaps policy. For example, studies on the mechanisms of resistance to partner drugs may be overshadowed, and artemisinins may be prematurely consigned to the scrap heap if they focus on what is rather exclusively defined as being ‘artemisinin resistance’. Why is delayed parasite clearance not an accurate marker for artemisinin resistance? Artemisinins need 7 days to cure malaria, whereas they are commonly given for 3 days in combination therapy. An incomplete treatment course with any antimicrobial can be associated with treatment failures without the need to invoke the specter of ‘resistance’. Furthermore, a slower clearance phenotype lacks diagnostic specificity (>50% of patients can have persistent parasitemia 72 h after artesunate monotherapy but achieve cure at 28 days in >94% of cases) [3]. Even when there is parasite persistence more than 3 days after treatment with an artemisinin, and this persistence is associated with treatment failures, for example, with dihydroartemisinin–piperaquine, we should be identifying and assessing markers for resistance to piperaquine. Patients with splenectomy have grossly prolonged parasite clearance times without this being caused by artemisinin resistant parasites, once again highlighting the complexities inherent in interpreting this phenotype. As parasites demonstrably circulate for longer than 72 h without increased risk of treatment failure, there are interesting questions that arise about the state of these parasites. The coincidence of prolonged clearance of parasites with failure to artesunate–mefloquine treatment is a correlative and not a causal observation, especially as we do not know the viability of these circulating parasites. The lack of conventional in vitro correlates in most (but not all [4]) studies of artemisinin resistance in this geographic area further supports the suggestion that artemisinin resistance may be misclassified. This may also be the Corresponding author: Krishna, S. ([email protected], [email protected]).
case because others have identified in vitro phenotypes [5], assessed in conventional assays in other regions, that are consistent with artemisinin resistance and that are associated with plausible mechanistic hypotheses [6]. Treatment failures may then arise with artemisinin combination therapies (TFACTs) because partner drugs are failing, and the accompanying artemisinin dosing regimen is insufficient to cure patients [2]. Genetic markers for resistance to some partner drugs [7,8] of artemisinins are already validated in clinical studies. Increased pfmdr1 copy number predicts failure with artesunate–mefloquine combination therapy [8]. However, these markers are not being described systematically in genome association studies such as the recently published one by Miotto and colleagues [9], particularly as gene dosage effects are important in affecting outcomes. Will these investigators provide in vitro assay results and molecular markers for resistance to partner drugs used in artemisinin combination therapies (ACTs) in these studies? Will correlations of treatment responses in individual patients also be assessed and presented? Without these full datasets being made available (thereby granting open access both in letter and spirit), we are left with a somewhat partial picture and interpretation. Consensus on resistance Consensus on a working definition of artemisinin resistance will be important to achieve in the future when genome studies are being implemented because robust phenotypes are essential for discerning useful associations. The identification of delayed clearance of parasites highlights a very interesting biological phenomenon [2], but its contribution to artemisinin resistance (sensu stricto) has yet to be established. As before, with regard to our studies aimed at advancing dosing regimens of artesunate [10], we look forward to debates [11,12] that enrich the process of reappraising and improving the management of malaria. References 1 Dondorp, A. and Ringwald, P. (2013) Artemisinin resistance is a clear and present danger. Trends Parasitol. http://dx.doi.org/10.1016/j.pt. 2013.05.005 2 Krishna, S. and Kremsner, P.G. (2013) Antidogmatic approaches to artemisinin resistance: reappraisal as treatment failure with artemisinin combination therapy. Trends Parasitol. http://dx.doi.org/ 10.1016/j.pt.2013.04.001 3 Saunders, D. et al. (2012) Pharmacokinetics and pharmacodynamics of oral artesunate monotherapy in patients with uncomplicated 361
Letter Response
4 5
6
7
362
Plasmodium falciparum malaria in Western Cambodia. Antimicrob. Agents Chemother. 56, 5484–5493 Noedl, H. et al. (2008) Evidence of artemisinin-resistant malaria in western Cambodia. N. Engl. J. Med. 359, 2619–2620 Jambou, R. et al. (2005) Resistance of Plasmodium falciparum field isolates to in-vitro artemether and point mutations of the SERCA-type PfATPase6. Lancet 366, 1960–1963 Pulcini, S. et al. (2013) Expression in yeast links field polymorphisms in PfATP6 to in vitro artemisinin resistance and identifies new inhibitor classes. J. Infect. Dis. http://dx.doi.org/10.1093/infdis/jit171 Malmberg, M. et al. (2013) Plasmodium falciparum drug resistance phenotype as assessed by patient antimalarial drug levels and its association with pfmdr1 polymorphisms. J. Infect. Dis. 207, 842–847
Trends in Parasitology August 2013, Vol. 29, No. 8
8 Price, R.N. et al. (2004) Mefloquine resistance in Plasmodium falciparum and increased pfmdr1 gene copy number. Lancet 364, 438–447 9 Miotto, O. et al. (2013) Multiple populations of artemisinin-resistant Plasmodium falciparum in Cambodia. Nat. Genet. 45, 648–655 10 Kremsner, P.G. et al. (2012) A simplified intravenous artesunate regimen for severe malaria. J. Infect. Dis. 205, 312–319 11 Dondorp, A.M. et al. (2012) Artesunate dosing in severe falciparum malaria. J. Infect. Dis. 206, 618–619 author reply 622 12 Kremsner, P.G. et al. (2012) A simplified intravenous artesunate regimen for severe malaria – reply. J. Infect. Dis. 206, 621–622 1471-4922/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pt.2013.05.006 Trends in Parasitology, August 2013, Vol. 29, No. 8