Reduced In Vitro Doxycycline Susceptibility in Plasmodium falciparum Field Isolates from Kenya Is Associated with PfTetQ KYNNNN Sequence Polymorphism

Doxycycline 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 vitro doxycycline susceptibility profiles for 96 Plasmodium falciparum field isolates from Kenya were determined. Genetic polymorphisms were assessed in P. falciparum metabolite drug transporter (Pfmdt) and P. falciparum GTPase tetQ (PftetQ) genes. Copy number variation of the gene and the number of KYNNNN amino acid motif repeats within the protein encoded by PftetQ were determined. Reduced in vitro susceptibility 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; P value of <0.0002). Isolates with 1 copy of the Pfmdt gene had a median IC₅₀ of 6,971 nM, whereas those with a Pfmdt copy number of >1 had a median IC₅₀ of 9,912 nM (P = 0.0245). Isolates with 1 copy of PftetQ had a median IC₅₀ of 6,370 nM, whereas isolates with a PftetQ copy number of >1 had a median IC₅₀ of 3,422 nM (P < 0.0007). Isolates with 2 PfTetQ KYNNNN motif repeats had a median IC₅₀ of 26,165 nM, whereas isolates with 3 PfTetQ KYNNNN repeats had a median IC₅₀ of 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.