Susceptibility of Plasmodium falciparum Isolates to Doxycycline Is Associated with pftetQ Sequence Polymorphisms and pftetQ and pfmdt Copy Numbers

Background. Doxycycline is used in combination with quinine for malaria treatment or alone for malaria chemoprophylaxis. However, the occurrence of malaria after doxycycline chemoprophylaxis has been reported. Identification of genetic determinants that contribute to the susceptibility of Plasmodium falciparum to doxycycline will be important for the detection and surveillance of doxycycline resistance. Methods. Sequence analysis of 11 genes (pftufA, pfEF-TS, pfmdt, pftetQ, pfrps3, pfrps7, pfrps8, pfrps9, pfrps11, pfrps14, and pfrps17) and evaluation of pfmdt and pftetQ copy numbers by quantitative real-time polymerase chain reaction were conducted in 90 African P. falciparum isolates that were obtained from 14 countries and that belonged to phenotypic groups differing in their doxycycline median inhibitory concentrations. Results. We found that pfmdt copy number of >1 (adjusted odds ratio [OR], 7.09 [95% confidence interval {CI}, 1.58-31.82]; P = .011), pftetQ copy number of >1 (adjusted OR, 5.23 [95% CI, 1.06-25.77]; P = .042), and KYNNNN amino acid motif repeats of <3 (adjusted OR, 3.00 [95% CI, 1.02-8.86]; P = .046) were independently associated with decreased susceptibility to doxycycline. Conclusions. Our findings suggest that pfmdt and pftetQ copy numbers and pftetQ sequence polymorphisms are potential molecular markers of decreased in vitro susceptibility to doxycycline in African P. falciparum isolates.