ReducedIn VitroDoxycycline Susceptibility in Plasmodium falciparum Field Isolates from Kenya Is Associated with PfTetQ KYNNNN Sequence Polymorphism

Abstract

ABSTRACTDoxycycline is widely used for malaria prophylaxis by international travelers. However, there is limited information on doxycycline efficacy in Kenya, and genetic polymorphisms associated with reduced efficacy are not well defined.In vitrodoxycycline susceptibility profiles for 96Plasmodium falciparumfield isolates from Kenya were determined. Genetic polymorphisms were assessed inP. falciparummetabolite drug transporter (Pfmdt) andP. falciparumGTPasetetQ(PftetQ) genes. Copy number variation of the gene and the number of KYNNNN amino acid motif repeats within the protein encoded by PftetQwere determined. Reducedin vitrosusceptibility to doxycycline was defined by 50% inhibitory concentrations (IC50s) of ≥35,000 nM. The odds ratio (OR) of having 2 PfTetQ KYNNNN amino acid repeats in isolates with IC50s of >35,000 nM relative to those with IC50s of <35,000 nM is 15 (95% confidence interval [CI], 3.0 to 74.3;Pvalue of <0.0002). Isolates with 1 copy of the Pfmdtgene had a median IC50of 6,971 nM, whereas those with a Pfmdtcopy number of >1 had a median IC50of 9,912 nM (P= 0.0245). Isolates with 1 copy of PftetQhad a median IC50of 6,370 nM, whereas isolates with a PftetQcopy number of >1 had a median IC50of 3,422 nM (P< 0.0007). Isolates with 2 PfTetQ KYNNNN motif repeats had a median IC50of 26,165 nM, whereas isolates with 3 PfTetQ KYNNNN repeats had a median IC50of 3,352 nM (P= 0.0023). PfTetQ sequence polymorphism is associated with a reduced doxycycline susceptibility phenotype in Kenyan isolates and is a potential marker for susceptibility testing.