Key Points Question How is natural variation in cytochrome P450 2D6 activity associated with therapeutic efficacy of primaquine phosphate against latent Plasmodium vivax malaria? Findings In this nested case-control study of 57 patients who had participated in a clinical trial of primaquine for radical cure of acute P vivax malaria, exposure to low levels of cytochrome P450 2D6 activity determined by genotype or measured by dextromethorphan metabolism phenotype was associated with a significantly increased likelihood of relapse of malaria in the year after directly observed high-dose primaquine therapy. Meaning Impaired cytochrome P450 2D6 activity was significantly associated with high risk of therapeutic failure of primaquine, and this finding suggests cytochrome P450 2D6 involvement in producing a therapeutically active metabolite.

BackgroundPrimaquine is the only drug available for preventing relapse following a primary attack by Plasmodium vivax malaria. This drug imposes several important problems: daily dosing over two weeks; toxicity in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency; partner blood schizontocides possibly impacting primaquine safety and efficacy; cytochrome P-450 abnormalities impairing metabolism and therapeutic activity; and some strains of parasite may be tolerant or resistant to primaquine. There are many possible causes of repeated relapses in a patient treated with primaquine.Case descriptionA 56-year-old Caucasian woman from New Zealand traveled to New Ireland, Papua New Guinea for two months in 2012. One month after returning home she stopped daily doxycycline prophylaxis against malaria, and one week later she became acutely ill and hospitalized with a diagnosis of Plasmodium vivax malaria. Over the ensuing year she suffered four more attacks of vivax malaria at approximately two-months intervals despite consuming primaquine daily for 14 days after each of those attacks, except the last. Genotype of the patient’s cytochrome P-450 2D6 alleles (*5/*41) corresponded with an intermediate metabolizer phenotype of predicted low activity.DiscussionMultiple relapses in patients taking primaquine as prescribed present a serious clinical problem, and understanding the basis of repeated therapeutic failure is a challenging technical problem. This case highlights these issues in a single traveler, but these problems will also arise as endemic nations approach elimination of malaria transmission.

Immune responses directed at glycosylphosphatidylinositol (GPI) anchors of Plasmodium falciparum may offer protection against symptomatic malaria. To independently explore the effect of age on generation of the anti-GPI IgG response, we measured serum anti-GPI IgGs in a longitudinal cohort of migrant Javanese children (6-12 years old) and adults (Ն20 years old) with equivalent numbers of exposures to P. falciparum in Papua, Indonesia. While the peak response in adults was achieved after a single infection, comparable responses in children required Ն3-4 infections. Significantly fewer children (16%) than adults (41%) showed a high (optical density > 0.44) anti-GPI IgG response (odds ratio [OR] ‫ס‬ 3.8, 95% confidence interval [CI] ‫ס‬ 2.3-6.3, P < 0.0001), and adults were more likely to show a persistently high response (OR ‫ס‬ 5.5, 95% CI ‫ס‬ 1.0-56.8, P ‫ס‬ 0.03). However, the minority of children showing a strong response were significantly less likely to experience symptoms with subsequent parasitemia compared with those with a weak response (OR ‫ס‬ 4.0, 95% CI ‫ס‬ 1.1-13.8, P ‫ס‬ 0.02). This effect was not seen among high-and low-responding adults (OR ‫ס‬ 1.2, 95% CI ‫ס‬ 0.5-2.8, P ‫ס‬ 0.60). Host age, independent of cumulative exposure, apparently represents a key determinant of the quantitative and qualitative nature of the IgG response to P. falciparum GPI.

Background Plasmodium vivax occurs as a latent infection of liver and a patent infection of red blood cells. Radical cure requires both blood schizontocidal and hypnozoitocidal chemotherapies. The hypnozoitocidal therapies available are primaquine and tafenoquine, 8-aminoquinoline drugs that can provoke threatening acute hemolytic anemia in patients having an X-linked G6PD-deficiency. Heterozygous females may screen as G6PD-normal prior to radical cure and go on to experience hemolytic crisis. Methods & findings This study examined G6PD phenotypes in 1928 female subjects living in malarious Sumba Island in eastern Indonesia to ascertain the prevalence of females vulnerable to diagnostic misclassification as G6PD-normal. All 367 (19%) females having <80% G6PD normal activity were genotyped. Among those, 103 (28%) were G6PD wild type, 251 (68·4%) were heterozygous, three (0·8%) were compound heterozygotes, and ten (2·7%) were homozygous deficient. The variants Vanua Lava, Viangchan, Coimbra, Chatham, and Kaiping occurred among them. Below the 70% of normal G6PD activity threshold, just 18 (8%) were G6PD-normal and 214 (92%) were G6PD-deficient. Among the 31 females with <30% G6PD normal activity were all ten homozygotes, all three compound heterozygotes, and just 18 were heterozygotes (7% of those). Conclusions In this population, most G6PD heterozygosity in females occurred between 30% and 70% of normal (69·3%; 183/264). The prevalence of females at risk of G6PD misclassification as normal by qualitative screening was 9·5% (183/1928). Qualitative G6PD screening prior to 8-aminoquinoline therapies against P. vivax may leave one in ten females at risk of hemolytic crisis, which may be remedied by point-of-care quantitative tests.

Genotyping Plasmodium vivax relapses can provide insights into hypnozoite biology. We performed targeted amplicon sequencing of 127 relapses occurring in Indonesian soldiers returning to malaria-free Java after yearlong deployment in malarious Eastern Indonesia. Hepatic carriage of multiple hypnozoite clones was evident in three-quarters of soldiers with two successive relapses, yet the majority of relapse episodes only displayed one clonal population. The number of clones detected in relapse episodes decreased over time and through successive relapses, especially in individuals who received hypnozoiticidal therapy. Interrogating the multiplicity of infection in this P. vivax relapse cohort reveals evidence of independent activation and slow depletion of hypnozoites over many months by multiple possible mechanisms, including parasite senescence and host immunity.