Oseltamivir and zanamivir are competitive inhibitors for the neuraminidase enzyme for the influenza virus. They block the surface receptor enzyme and prevent release of virus from the host cell, thus limiting propagation of the infection. These medications can be given as prophylaxis after exposure to influenza or can be given therapeutically for a suspected or confirmed infection. Oseltamivir is recommended for treatment of suspected or confirmed influenza infection in the special population of pregnant women, as the risk for complications of influenza is increased in this group.

Safety evidence

However, there are limited data on the safety and efficacy of the neuraminidase inhibitors in pregnancy. With respect to safety, there have been seven publications in the literature addressing the risk for major birth defects following treatment or prophylaxis with one or both of these products, with the majority of the published data relating to oseltamivir exposure.

In a review by Tanaka et al. in 2009, 90 pregnancies treated therapeutically with oseltamivir in the first trimester were reported to two teratogen information services in Japan; one major birth defect (1.1%) was reported ( CMAJ. 2009 Jul 7;181[1-2]:55-8 ). A year later, Greer et al. published a retrospective chart review at a Texas hospital between 2003 and 2008. During that period, 137 pregnancies that involved a pharmacy record of dispensing of oseltamivir were identified. Of these, 18 were dispensed in the first trimester, and none were linked to a major birth defect outcome ( Obstet Gynecol. 2010 Apr;115[4]:711-6 ).

A 2011 record linkage study in Sweden identified 86 pregnant women for whom oseltamivir (n=81) or zanamivir had been prescribed. Of these, four were linked to a major birth defect in the infant; however, only one of the four prescriptions had been filled in the first trimester ( Pharmacoepidemiol Drug Saf. 2011 Oct;20[10]:1030-4 ). In 2013, Saito et al. reported on a case series gathered from 157 obstetric facilities in Japan. Among 156 infants born to women exposed to oseltamivir in the first trimester, 2 (1.3%) were reported to have a major congenital anomaly; there were no congenital malformations reported in the 15 first-trimester exposures to zanamivir ( Am J Obstet Gynecol. 2013 Aug;209[2[:130.e1-9 ).

In 2014, a teratogen information service in the United Kingdom reported on eight first-trimester exposures to oseltamivir and 37 to zanamivir, with no major birth defects noted in either group ( BJOG. 2014 Jun;121[7]:901-6 ). Additionally, a French prescription database study identified 49 pregnancies thought to be exposed to oseltamivir in the first trimester with one reported congenital anomaly ( BJOG. 2014 Jun;121[7]:895-900 ).

Finally, the manufacturer of oseltamivir published a summary of pregnancies from global pharmacovigilance data accumulated through spontaneous reports and other studies between 2000 and 2012 ( Pharmacoepidemiol Drug Saf. 2014 Oct;23[10]:1035-42 ). Outcomes were available for 1,875 infants. Among these, 81 (4.3%) had major birth defects. However, following case review, the authors indicated that only 11 of the defects (occurring in 9 infants) were biologically plausible based on the timing of the exposure to oseltamivir.

Efficacy examined

With respect to efficacy, two small studies have addressed the pharmacokinetics of oseltamivir in pregnancy to determine if the recommended dosages for nonpregnant individuals are appropriate for pregnancy.

In the earlier of the two studies, Greer et al. looked at the pharmacokinetics of oseltamivir in 30 pregnant women, 10 in each of the three trimesters, who were taking 75 mg of the drug either once or twice daily. Maternal samples were drawn before and after the first dose of oseltamivir. They found little evidence of differences across the three trimesters and concluded that the parent drug values were in the pharmacologic range for clinical efficacy ( Am J Obstet Gynecol. 2011 Jun;204[6 Suppl 1]:S89-93 ).

In contrast, Pillai et al. enrolled a small sample of women being treated with oseltamivir; they evaluated pharmacokinetics for the active metabolite of oseltamivir following 48 or more hours of treatment in 29 pregnant and 35 nonpregnant women ( Br J Clin Pharmacol. 2015 Nov;80[5]:1042-50 ). Significantly lower levels of the active metabolite were noted in the pregnant women, compared with nonpregnant women. The authors suggested that the physiologic changes of pregnancy, correlated with increased renal clearance, produced an approximate 30% lower exposure to the drug in the pregnant state. While they were not able to relate this to maternal or infant outcomes, this finding suggested that further work is needed to determine if dosing recommendations should be adjusted in pregnancy.

The current recommendation is that pregnant women or women within 2 weeks post partum be given oseltamivir for treatment of suspected or confirmed influenza regardless of trimester of pregnancy. The limited safety data that are currently available have not suggested an increased risk for major birth defects following treatment with this product. However, the data are sparse for oseltamivir and even more so for zanamivir. Larger studies focused on these treatments are needed.

Dr. Chambers is professor of pediatrics and director of clinical research at Rady Children’s Hospital, and associate director of the Clinical and Translational Research Institute at the University of California, San Diego. She is director of MotherToBaby California, past president of the Organization of Teratology Information Specialists, and past president of the Teratology Society. She reported having no financial disclosures relevant to this column, but has received research funding Roche-Genentech and GlaxoSmithKline unrelated to antiviral medications. Email her at obnews@frontlinemedcom.com.