Primaquine Pharmacokinetics in Lactating Women and Their Infants

Background The weight of malaria falls most heavily on young children and pregnant women but studies of the safety of antimalarials in pregnancy and lactation are few. The only recommended medication used for radical treatment of P.vivax is primaquine. The 2010 WHO malaria guidelines recommend its use in all patients with P.vivax infection in areas of low transmission, in the absence of contraindications. Primaquine is contraindicated in pregnancy. The postpartum period presents a key opportunity to definitively treat women who suffer multiple malaria relapses during pregnancy. The 2010 WHO malaria treatment guidelines allow for primaquine use during lactation but there are no studies to date quantifying primaquine excretion in breast milk and the dose that breastfed infants would be exposed to is unknown. The investigators propose to study the pharmacokinetics of primaquine in maternal and infant plasma and in breast milk during a 14 day radical treatment of P.vivax.

Rationale

The global burden of malaria is great, with approximately 1-2 million deaths each year and many times that number of productive days lost to illness. Plasmodium vivax is the most prevalent malarial species, resulting in 132-391 million clinical infections each year and 80-90% of malaria in Asia, the Middle East, and the Western Pacific. Despite its prevalence, P.vivax has not been a focus of research until recently, and its effects and optimal treatments are still not fully understood. Though infrequently a fatal infection, P.vivax causes high levels of morbidity due to the chronic nature of the infection. Because of dormant liver stages (hypnozoites), illness can continue to recur for years, even after effective treatment has eliminated the parasites from the blood. In settings where there are hopes for malarial eradication, this means infected persons continue to be hosts even if vector-borne transmission is temporarily interrupted, and could potentially start a new cycle of transmission with each relapse. Each of these relapses causes a significant amount of morbidity and a small mortality risk for those infected.

These burdens are magnified during pregnancy when women experience relative immune suppression and can experience relapses every 4-8 weeks. P.vivax malaria in pregnancy is associated with low birth weight and anaemia; thrombocytopaenia; increased risk of fetal loss, premature delivery, neonatal and infant mortality; and, in some areas, risk for congenital malaria (a severe, sepsis-like illness). In Southeast Asia, where the investigators propose to conduct our study, the majority of cases of congenital malaria are from vivax infection. These women may have some respite during the postpartum period, with relapses spaced further apart, but frequently relapse again in subsequent pregnancies. New pregnancies during the end of lactation are common in many malaria endemic areas, including our setting, making definitive treatment of vivax infections in these women very challenging in the absence of data about the safety of primaquine for the breastfeeding infant.

Though the anti-malarial pharmacopeia has expanded over the past decade, primaquine remains the only definitive treatment for the hypnozoite forms of Plasmodium vivax and ovale. First synthesized in 1946, a 14 day treatment eradicates the parasites from the liver. However, this medication has been associated with a risk for methemoglobinemia, and acute intravascular haemolysis in glucose-6-phosphate-dehydrogenase (G6PD) deficient hosts (common in malaria-endemic areas). The elevations in methemoglobin are generally modest and asymptomatic, but occasionally significant methemoglobinemia can cause cyanosis and shortness of breath requiring discontinuation of the medication. Primaquine is not considered safe in pregnancy because of the possible risk of haemolysis and methaemoglobin in the fetus. The most recent WHO malaria guidelines (2010) have conflicting information about the its use during lactation. They cite breastfeeding as a contraindication for primaquine administration for P. falciparum malaria, but permit the use of primaquine for P. vivax during lactation if the breast fed infant is not G6PD deficient. No studies have ever been done in lactating women to determine the safety of primaquine in breastfeeding or the optimal time of administration, however, the relaxation of guidelines in infancy suggest potential for its safe use in lactation. Some authors note that primaquine's once daily dosing might allow for safe breastfeeding if the mother pumps the milk at the time when peak milk concentrations are expected and discards that milk.

Some inferences about the expected behavior of primaquine in lactation can be drawn from its known pharmacologic properties. Primaquine pharmacokinetics have been well characterized in healthy subjects and malaria patients after single and multiple oral dosing. Peak concentrations are reached within 2-3 hours after dosing and the plasma elimination half-life is ~7 hours. It is extensively distributed in the tissue and largely metabolized to inert carboxyprimaquine, the major plasma metabolite, which undergoes further biotransformation to unknown metabolites that are probably more toxic than the parent compound. The identification of other metabolites in humans has been difficult to pursue because the expected aminophenol metabolites are unstable. Until recently, techniques to separate positive and negative enantiomers for primaquine and carboxyprimaquine have not been possible, again obscuring the toxic and therapeutic mechanisms of the drug.

Primaquine has limited oral bioavailability and adult plasma concentrations are generally low, making dangerously high concentrations in maternal milk unlikely from a theoretical standpoint. However, recent data suggests that a previously unrecognized sex difference exists in the pharmacokinetics of the medication in men and women, with women accumulating higher drug concentrations than men. Since side effects of primaquine appear to be dose-related, this finding could be significant. The time to maximum concentration is about 2-3 hrs (longer for women than for men) and the investigators would expect to see a short lag in reaching peak breast milk concentrations. Primaquine has a low molecular weight (259) and slightly basic pH making it more likely to transfer into the breast milk.

Literature on medication pharmacokinetics in breast milk is sparse but has begun to capture the interest of researchers and human rights activists who note that women are often systematically excluded from medical research by virtue of pregnant or lactating status. As a result, during these two states the pharmacokinetic properties of drugs remain poorly understood, and much of our clinical therapeutic approaches are based on consensus and conjecture. No pharmacokinetic studies have been done to measure primaquine excretion in breast milk. A few studies have been done of other antimalarials during lactation and have shown low levels of drug in breast milk during treatment. The frequent venous sampling necessary for pharmacokinetic research is not ideal for participants, especially in pediatric populations. Recent efforts have been made to evaluate the usefulness of capillary or saliva samples as alternative sampling measures.