Preventing relapses during pregnancy
It is generally preferable to avoid taking medications during pregnancy to avoid exposing the developing fetus to a drug. Neurologists generally advise women with multiple sclerosis to stop their disease-modifying therapy either while they are trying to become pregnant or soon after pregnancy (depending on the medication they are taking).
In many medications cases (e.g. Gilenya, Tecfidera), the drug must be present in the body for it to have a beneficial effect. In other cases, the medication induces long-lasting effects that persist long after the drug is gone from the body. So in those cases, the drug’s effect (rather than the amount of drug in the body) is what is important. For example, Tysabri blocks the passage of immune cells into the central nervous system and the amount of this blockade is the key consideration. The drug is gone from the body in about six weeks, but the drug is dosed every four weeks to maintain the level of blockade. In the case of Ocrevus, it is gone from the body in about 20 weeks, but its effect on suppressing B cells (a type of immune cell that causes damage in MS) is a little longer, so it is typically dosed every 26 weeks. (Studies are now looking at whether these drugs can be dosed less frequently.)
What this means is that when a pregnant woman stops these long-lasting medications, she will still obtain some carry-over benefits during the pregnancy itself. However, there is an important difference between these two treatments. When Tysabri is stopped, a person can experience a rebound effect in which there is a worsening of MS symptoms unless treatment is re-started. With Ocrevus (as well as other long-acting treatment such as Kesimpta and Mavenclad), there is no rebound effect.
This is an important distinction, as shown in a recent study. Researchers looked at women with highly active MS who stopped Tysabri or Ocrevus (or a similar medication, Rituxan) when they became pregnant (Demortiere and colleagues. Neurol Neuroimmunol Neuroinflamm 2023;10:e200161). With Tysabri, they stopped treatment during the first trimester; with Ocrevus/Rituxan, they stopped it in the year before they became pregnant. In the Tysabri group, about 7% of women had an MS relapse during their pregnancy compared to 3% in the Ocrevus/Rituxan group.
After the baby was born, about 21% of women who had been on Tysabri had a relapse – even though most of the women who relapsed restarted Tysabri within a month of giving birth. About one-third also had new lesions on their MRI. In contrast, no one in the Ocrevus/Rituxan group had a postpartum relapse and only one person had new disease activity on their MRI.
The researchers concluded that Ocrevus/Rituxan provides better protection against MS disease activity during and after pregnancy than Tysabri. They noted that Tysabri can be continued during pregnancy if circumstances warrant it, but many people would be uncomfortable with this option because of concerns about exposing the developing fetus to a medication. Even if this were done, Tysabri is typically stopped during nursing because the drug does get into breast milk.
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