Rifapentine reaches breast milk at under 1% of the maternal dose

Design

Population pharmacokinetic modelling of a prospective study

Population

Pregnant and postpartum women on weekly isoniazid–rifapentine, and their infants

Primary outcome

Breast milk-to-plasma ratio and relative infant dose

Effect

Milk-to-plasma ratio 0.024 (rifapentine), 0.041 (metabolite); relative infant dose under 1%

IMPAACT 2001 studied once-weekly isoniazid and rifapentine (3HP) for TB prevention in pregnant and postpartum women. This analysis modelled rifapentine and its active metabolite in maternal plasma, breast milk, cord blood and infant plasma.

Breast milk concentrations were about 2.4% of maternal plasma for rifapentine and 4.1% for the metabolite, with rapid equilibration. The estimated maximum relative infant dose was below 1%. Rifapentine crossed the placenta and was measurable in infant plasma, at levels the authors judged unlikely to be clinically significant.

A relative infant dose under 10% is the usual threshold for compatibility with breastfeeding, so these figures are reassuring. In India, where 3HP is part of national TB preventive therapy, this supports offering it to breastfeeding women eligible for preventive treatment, without asking them to stop feeding. The study measured drug exposure, not infant outcomes.

• Do not stop breastfeeding for a mother on weekly isoniazid–rifapentine
• The infant receives under 1% of the maternal weight-adjusted dose via milk
• Remind mothers that rifamycins colour body fluids, including milk, orange
• Check for interactions — rifapentine induces enzymes and reduces hormonal contraceptive levels
• Consider pyridoxine for the mother and, if advised locally, the infant with isoniazid

Why it matters

It removes a reason women eligible for TB prevention are told to wait until they stop feeding.

Don't overread it

This study modelled drug levels; it did not measure infant clinical outcomes.

The statistics, in plain English

The relative infant dose compares the dose an infant receives through milk with the mother's dose by body weight. Under 10% is generally considered compatible with breastfeeding; this was under 1%.

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