title: Plasmodium vivax trophozoites insensitive to chloroquine.
abstract: BACKGROUND
  Plasmodium vivax is a major cause of malaria and is still primarily treated with chloroquine. Chloroquine inhibits the polymerization of haem to inert haemozoin. Free haem monomers are thought to catalyze oxidative damage to the Plasmodium spp. trophozoite, the stage when haemoglobin catabolism is maximal. However preliminary in vitro observations on P. vivax clinical isolates suggest that only ring stages (early trophozoites) are sensitive to chloroquine. In this study, the stage specific action of chloroquine was investigated in synchronous cryopreserved isolates of P. vivax.
  
  
  METHODS
  The in vitro chloroquine sensitivity of paired ring and trophozoite stages from 11 cryopreserved P. vivax clinical isolates from Thailand and two Plasmodium falciparum clones (chloroquine resistant K1 and chloroquine sensitive FC27) was measured using a modified WHO microtest method and fluorometric SYBR Green I Assay. The time each stage was exposed to chloroquine treatment was controlled by washing the chloroquine off at 20 hours after the beginning of treatment.
  
  
  RESULTS
  Plasmodium vivax isolates added to the assay at ring stage had significantly lower median IC50s to chloroquine than the same isolates added at trophozoite stage (median IC50 12 nM vs 415 nM p < 0.01). Although only 36% (4/11) of the SYBR Green I assays for P. vivax were successful, both microscopy and SYBR Green I assays indicated that only P. vivax trophozoites were able to develop to schizonts at chloroquine concentrations above 100 nM.
  
  
  CONCLUSIONS
  Data from this study confirms the diminished sensitivity of P. vivax trophozoites to chloroquine, the stage thought to be the target of this drug. These results raise important questions about the pharmacodynamic action of chloroquine, and highlight a fundamental difference in the activity of chloroquine between P. vivax and P. falciparum.
authors.affiliation: International Health Program, Infectious Diseases Division, Menzies School of Health Research and Charles Darwin University, Darwin, Australia. wesley.sharrock@cdu.edu.au
authors.forename: Wesley W
authors.initials: WW
authors.lastname: Sharrock
authors.forename: Rossarin
authors.initials: R
authors.lastname: Suwanarusk
authors.forename: Usa
authors.initials: U
authors.lastname: Lek-Uthai
authors.forename: Michael D
authors.initials: MD
authors.lastname: Edstein
authors.forename: Varakorn
authors.initials: V
authors.lastname: Kosaisavee
authors.forename: Thomas
authors.initials: T
authors.lastname: Travers
authors.forename: Anchalee
authors.initials: A
authors.lastname: Jaidee
authors.forename: Kanlaya
authors.initials: K
authors.lastname: Sriprawat
authors.forename: Ric N
authors.initials: RN
authors.lastname: Price
authors.forename: François
authors.initials: F
authors.lastname: Nosten
authors.forename: Bruce
authors.initials: B
authors.lastname: Russell
mesh_terms: D000818:Animals; D000962:Antimalarials / Q000494:pharmacology*; D052160:Benzothiazoles; D002738:Chloroquine / Q000494:pharmacology*; D003959:Diamines; D004351:Drug Resistance; D006801:Humans; D020128:Inhibitory Concentration 50; D008853:Microscopy; D009930:Organic Chemicals / Q000378:metabolism; D021261:Parasitic Sensitivity Tests; D010963:Plasmodium falciparum / Q000166:cytology / Q000187:drug effects / Q000254:growth & development; D010966:Plasmodium vivax / Q000166:cytology / Q000187:drug effects* / Q000254:growth & development; D011804:Quinolines; D053058:Trophozoites / Q000187:drug effects*
chemical_list: D000962:Antimalarials; D052160:Benzothiazoles; D003959:Diamines; D009930:Organic Chemicals; D011804:Quinolines; C098022:SYBR Green I; D002738:Chloroquine