Malaria remains a public health problem in French Guiana despite measures to strengthen health protection and disease prevention. It occurs in geographical foci located along the rivers and near gold-mining sites in the Amazonian forest [
1], where the high number of illegal foreign workers unfortunately limits the impact of preventive measures. Self-medication using imported molecules with an unclear compliance contributes to the selection of drug-resistant parasites in the region [
2].
Plasmodium falciparum accounts for 30% of the 1,000 malaria cases reported each year by the health regional agency [
3]. As in the neighbouring countries of the Amazon basin, some strains of
P. falciparum are resistant to chloroquine, amodiaquine, sulfadoxine–pyrimethamine, chloroquine–proguanil, halofantrine, and even quinine [
4]. This justified the introduction of quinine–doxycycline combination therapy as a first-line treatment in 1995. It was replaced by the artemether–lumefantrine combination in 2002 but is still used to treat severe malaria in second-line treatment. Doxycycline is an antibiotic of the tetracycline family. Its anti-malarial activity has been known for 40 years following ex vivo [
5,
6] and clinical studies [
7‐
9]. The mode of action of this antibiotic on the
Plasmodium parasite is not fully understood. In bacteria, cyclines inhibit protein synthesis by binding to protein S7 of the small ribosomal subunit and to various ribonucleic acids of the 16S ribosomal RNA, preventing the binding of aminoacyl-transfer RNA to site A of the ribosome and thus blocking the elongation step of translation [
10]. In
P. falciparum, the doxycyline mechanism of action targets two organelles, the mitochondria and the apicoplast. Cyclines also decrease the activity of an enzyme, dihydroorotate dehydrogenase, involved in the de novo synthesis of pyrimidines [
11]. A related drug, minocycline, is also thought to decrease the transcription of mitochondrial genes (sub-unit I of cytochrome
c oxidase and apocytochrome
b) and plastid genes (sub-unit rpoB/C of RNA polymerase) [
12]. Doxycycline appears to principally target the apicoplast in
P.
falciparum, and may block translation by binding to the small ribosomal sub-unit, causing a delayed death [
13]. Doxycycline given on a daily basis has been shown to be an effective causal chemoprophylaxis [
14]. It is now recommended by the French health authorities for chemoprophylaxis in countries with high prevalence of chloroquine resistance or multidrug resistance (group 3 countries), as in French Guiana [
15]. French troops deployed in the illegal gold mines take 100 mg daily doses of doxycycline for prophylaxis. The ability of
P. falciparum to rapidly develop resistance and the use of doxycycline for both chemoprophylaxis and the treatment of malaria in French Guiana impose a close monitoring of resistance to this drug. No clinical treatment failure has been reported so far, but doxycycline is always used in combination for treatment. Although reported cases of malaria under doxycycline chemoprophylaxis are mostly believed to have resulted from poor compliance [
16], they could also be explained by resistance. It is critical to identify early signs of resistance before resistant strains become prevalent and compromise the clinical and prophylactic utility of the molecule. Indeed, Briolant et al. [
17] identify an association between the
P. falciparum metabolite drug transporter (
pfmdt; PFE0825w) and
P. falciparum GTPase TetQ (
pftetQ; PFL171c) gene copy numbers, the
pftetQ KYNNNN sequence polymorphism and a decreased ex vivo susceptibility to doxycycline in African
P. falciparum isolates. The threshold of decreased susceptibility to doxycycline was established ex vivo at 35 µM [
18].
This study first aimed to determine the distribution and range of 50% inhibitory concentrations (IC50) of doxycycline for 800 P. falciparum isolates assayed between 2000 and 2010 in French Guiana. In the second part, the association between the pftetQ, pfmdt copy numbers sequence polymorphisms of the pftetQ, pfssurRNA, pflssurRNA, pfrps7 and with decreased susceptibility to doxycycline has been evaluated.