Retevmo (Selpercatinib): What It Is and What to Expect

RET is a gene that helps nerves and other tissues develop. In some cancers it goes wrong in one of two ways.

Sometimes a piece of RET breaks off and joins a different gene. That is called a fusion, and the joined gene makes a protein that is stuck in the on position. Sometimes a single letter of RET changes instead, which is called a mutation and has a similar effect.

Selpercatinib was built to fit that RET protein and block it. Retevmo is the brand name; selpercatinib is the generic name.

The label covers four groups.

1. Adults with non-small cell lung cancer that has spread or is locally advanced and has a RET fusion.
2. Adults and children from 2 years old with advanced or spread medullary thyroid cancer carrying a RET mutation, who need drug treatment.
3. Adults and children from 2 years old with advanced or spread thyroid cancer carrying a RET fusion, who need drug treatment and whose cancer no longer responds to radioactive iodine.
4. Adults and children from 2 years old with any other solid tumor carrying a RET fusion, once it has grown despite earlier treatment or there is no good alternative.

That fourth group is unusual. It does not name an organ at all. The RET fusion itself is the ticket.

You cannot tell from a scan whether a cancer has a RET change. It takes a laboratory test on the tumor tissue, or in some cases on blood, and the label requires an FDA-approved one.

RET changes are uncommon, so most people tested will not have one. A negative result is the ordinary result. It closes off this drug, not your treatment options.

High blood pressure occurred in 43% of people taking selpercatinib. In 20% it was severe.

That makes it the side effect most likely to change your daily routine. Your team will want your blood pressure controlled before you start, and checked after the first week, then regularly. Many people are given a blood pressure medicine alongside.

A home monitor is worth asking about. Bring the readings to appointments.

Liver enzymes rise in more than half of people on this drug. Most of the time there are no symptoms at all, which is exactly why blood is tested on a schedule rather than when you feel unwell.

Selpercatinib can also stretch the heart's electrical cycle, seen on an ECG as a long QT interval. Expect tracings before and during treatment, and tell your team about every other medicine you take, because some add to the same effect.

Serious bleeding has occurred, and some cases were fatal. Coughing up blood, blood in stool or urine, or a bad headache with confusion needs urgent care.

This drug can slow wound healing. The label says to stop it at least 7 days before planned surgery, and not to restart it for at least 2 weeks afterwards, and until the wound has healed.

That includes dental surgery and other procedures that may not feel like a big deal. Mention selpercatinib to anyone booking one.

Some people develop a reaction with fever, rash and aching joints, alongside falling platelets or rising liver enzymes. It shows up around two weeks in. Report a rash rather than assuming it is a virus, and if a temperature comes with it, phone at once rather than waiting.

In medullary thyroid cancer with a large amount of disease, breaking cancer cells down quickly can overwhelm the kidneys. This is tumor lysis syndrome. Extra fluids and blood tests in the first weeks are how it is prevented.

• Call 911 or go to an emergency department if you cough up blood, pass blood in your stool or urine, or get a sudden severe headache with confusion. Go too if you faint or your heart pounds and will not settle.
• Call 911 or go to an emergency department if your lips or tongue swell, your throat tightens, or you struggle to breathe after a dose.
• Ring your cancer team at once, day or night, if your temperature reaches 100.4°F (38°C) or higher. While you are on this treatment a fever counts as a medical emergency, whether or not chills come with it and whether or not anyone has told you your white cells are low. Selpercatinib can lower those cells, and you cannot tell from how you feel. If nobody answers within minutes, go to an emergency department and say straight away that you are on cancer treatment.
• Go to an emergency department the same day if diarrhoea is frequent and will not stop, you cannot keep fluids down, or you feel faint standing up. Losing that much fluid can injure the kidneys, and drips may be needed.
• Call your care team the same day if a rash and aching joints appear, especially around two weeks in. Do not put that down to a virus. If a temperature comes with them, use the emergency bullet above and call at once instead. Ring the same day as well for a home blood pressure reading well above the number your team set for you, for a headache with blurred vision, or if you pass much less urine than usual.
• Call your care team within a day or two if your readings drift up over several days, if looser stools are becoming a daily pattern, or if you have a procedure booked, including dental work, so the 7-day stop before surgery can be arranged.

• https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7fa848ba-a59c-4144-9f52-64d090f4d828
• https://www.cancer.gov/about-cancer/treatment/drugs/selpercatinib
• https://www.cdc.gov/cancer-preventing-infections/patients/fever.html