PREVYMIS (Letermovir): Uses, Dosage & Side Effects

What Is PREVYMIS and What Is It Used For?

PREVYMIS (letermovir) is an antiviral medication used to prevent cytomegalovirus (CMV) infection and disease in patients with weakened immune systems following stem cell (bone marrow) transplantation or kidney transplantation. It contains the active substance letermovir and works by blocking CMV replication.

PREVYMIS belongs to a new class of antiviral medications known as CMV terminase complex inhibitors. Approved by the European Medicines Agency (EMA) and the U.S. Food and Drug Administration (FDA), it represents a significant advancement in transplant medicine by offering a targeted approach to CMV prevention that differs fundamentally from older antiviral agents.

The medicine is specifically indicated for adults and children who have recently undergone certain types of organ or cell transplantation. For stem cell transplant (also called bone marrow transplant or hematopoietic stem cell transplant) recipients, PREVYMIS can be used in patients weighing at least 15 kg. For kidney transplant recipients, the medicine is approved for those weighing at least 40 kg. In both cases, the goal is to prevent CMV from causing illness during the critical post-transplant period when the immune system is significantly weakened by immunosuppressive therapy.

What is CMV and why is it dangerous after transplant?

Cytomegalovirus (CMV) is an extremely common virus that belongs to the herpesvirus family. According to the World Health Organization (WHO), an estimated 60–90% of adults worldwide have been infected with CMV at some point in their lives. In most healthy individuals, CMV causes no noticeable symptoms or only mild illness, as a competent immune system keeps the virus in check.

However, after a stem cell or kidney transplant, patients must take potent immunosuppressive medications to prevent their body from rejecting the transplant. This intentional suppression of the immune system creates a window of vulnerability during which CMV can reactivate from latency or establish a new infection. CMV disease in transplant recipients can manifest as pneumonitis (lung inflammation), hepatitis (liver inflammation), colitis (bowel inflammation), retinitis (eye disease), or widespread systemic infection – all of which carry significant morbidity and mortality risks.

How does PREVYMIS work?

Letermovir inhibits the CMV DNA terminase complex by targeting its pUL56 subunit. This enzyme complex is essential for cleaving and packaging newly formed CMV DNA into viral particles. By blocking this process, letermovir prevents the virus from completing its replication cycle and producing new infectious viral particles. Importantly, this mechanism is entirely distinct from all other approved anti-CMV agents (such as ganciclovir, valganciclovir, foscarnet, and cidofovir), which means there is no cross-resistance between PREVYMIS and these older drugs.

Unlike ganciclovir and valganciclovir, letermovir does not inhibit human DNA synthesis, which contributes to its favorable safety profile. The older anti-CMV drugs can cause significant bone marrow suppression (myelotoxicity), which is particularly problematic in stem cell transplant recipients who are already rebuilding their blood cell production. PREVYMIS avoids this toxicity, making it a preferred option for CMV prophylaxis in this vulnerable population.

What Should You Know Before Taking PREVYMIS?

Before taking PREVYMIS, inform your doctor about all medications you are taking, especially immunosuppressants, statins, and blood thinners. PREVYMIS must not be taken with pimozide, ergot alkaloids, or St. John's Wort. When combined with ciclosporin, additional medications are contraindicated.

PREVYMIS has significant interactions with many commonly used medications. It is critical that your healthcare team has a complete and accurate list of all medicines you take – including prescription drugs, over-the-counter medications, and herbal supplements – before starting PREVYMIS therapy. Your doctor will carefully evaluate potential interactions and may need to adjust doses of certain medications or choose alternative treatments.

Contraindications

You must not take PREVYMIS if any of the following apply:

• Allergy to letermovir or any of the other ingredients in the medicine
• You are taking pimozide (used to treat Tourette's syndrome)
• You are taking ergot alkaloids such as ergotamine or dihydroergotamine (used to treat migraines)
• You are taking St. John's Wort (Hypericum perforatum), a herbal supplement

Additionally, if you are taking PREVYMIS together with ciclosporin, the following medicines are also contraindicated:

• Dabigatran – a blood thinner used to prevent blood clots
• Atorvastatin, simvastatin, rosuvastatin, pitavastatin – cholesterol-lowering medications (statins)

Warnings and Precautions

Several important precautions should be observed while taking PREVYMIS. Your healthcare team will monitor you closely and may order additional blood tests to check levels of certain medications and assess organ function throughout your treatment.

Additional blood tests may be necessary to monitor levels of the following medications when taken with PREVYMIS:

• Ciclosporin, tacrolimus, sirolimus – immunosuppressants used to prevent transplant rejection
• Voriconazole – an antifungal medication

Because these immunosuppressants have narrow therapeutic windows (meaning the difference between an effective dose and a toxic dose is small), even modest changes in their blood levels can have significant clinical consequences. PREVYMIS can alter the metabolism of these drugs, potentially leading to either increased toxicity or reduced efficacy. Regular therapeutic drug monitoring is therefore essential.

Pregnancy and Breastfeeding

If you are pregnant, think you may be pregnant, or are planning to have a baby, consult your doctor before taking PREVYMIS. The medicine is not recommended during pregnancy because it has not been studied in pregnant women, and it is unknown whether letermovir could harm the developing fetus. Animal studies alone are insufficient to fully assess human reproductive risk.

Breastfeeding is not recommended during treatment with PREVYMIS. It is not known whether letermovir passes into breast milk and could affect the nursing infant. The potential risks to the baby must be weighed against the benefits of both the medicine and breastfeeding, and this decision should be made in consultation with your healthcare provider.

Children and Adolescents

PREVYMIS should not be used in children weighing less than 15 kg who have had a stem cell transplant, or in children weighing less than 40 kg who have had a kidney transplant, as the medicine has not been studied in these groups. For eligible pediatric patients, dosing is weight-based and determined by your doctor.

Driving and Operating Machinery

PREVYMIS may have a minor effect on your ability to drive or operate machinery. Some patients have reported fatigue or dizziness (a spinning sensation) during treatment. If you experience either of these effects, you should avoid driving or operating machinery until the symptoms resolve.

How Does PREVYMIS Interact with Other Drugs?

PREVYMIS has clinically significant interactions with many common medications including immunosuppressants (ciclosporin, tacrolimus, sirolimus), statins, blood thinners (dabigatran, warfarin), antiepileptics, HIV antivirals, and certain pain medications. Always inform your doctor about all medicines you take.

Drug interactions are a critical consideration with PREVYMIS because transplant patients typically take multiple medications simultaneously. Letermovir is both a substrate and an inhibitor/inducer of several drug-metabolizing enzymes and transport proteins. This means PREVYMIS can both be affected by other drugs and can alter the way your body processes other medications. Your doctor or pharmacist will carefully review all your medications to ensure they can be safely combined with PREVYMIS.

Major Interactions – Contraindicated Combinations

The following medications must never be taken together with PREVYMIS:

Additional Contraindications with Ciclosporin

When PREVYMIS is co-administered with ciclosporin, the following medications are also contraindicated due to significantly increased drug exposure:

Other Important Interactions

Inform your doctor if you are taking any of the following medications, as dose adjustments or additional monitoring may be required:

This is not a complete list of all possible drug interactions. Always ask your doctor or pharmacist for a comprehensive review of your medications before starting or stopping any treatment while on PREVYMIS.

What Is the Correct Dosage of PREVYMIS?

The standard adult dose of PREVYMIS is 480 mg once daily. When taken with ciclosporin, the dose is reduced to 240 mg once daily. Dosing in children is based on body weight. Tablets should be taken at the same time each day, swallowed whole, with or without food.

Always take PREVYMIS exactly as your doctor or pharmacist has instructed. The correct dose depends on your body weight and whether you are also taking ciclosporin, which is a commonly used immunosuppressant after transplantation. Your doctor will determine the appropriate dose for your individual situation.

Adults and Children (≥30 kg) – Without Ciclosporin

Adults and Children (≥30 kg) – With Ciclosporin

Pediatric Patients (15 kg to <30 kg) – Stem Cell Transplant

How to Take PREVYMIS

• Take PREVYMIS once daily as directed by your doctor
• Take it at the same time each day to maintain consistent blood levels
• PREVYMIS can be taken with or without food
• Swallow the tablet whole with water – do not split, crush, or chew the tablet, as these methods have not been studied

Missed Dose

It is very important that you do not miss or skip any doses of PREVYMIS. If you forget to take a dose, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and take the next one at the usual time. Do not take two doses at the same time to make up for a missed dose. Contact your doctor or pharmacist if you are unsure about what to do.

Overdose

If you have taken too much PREVYMIS, contact your doctor immediately. There is no specific antidote for letermovir overdose, and treatment will be supportive. Bring the medicine packaging with you so the healthcare team knows exactly what you have taken and the dose involved.

What Are the Side Effects of PREVYMIS?

The most common side effects of PREVYMIS are diarrhea, nausea, and vomiting (each affecting up to 1 in 10 patients). Less common side effects include allergic reactions, headache, dizziness, abnormal liver tests, and fatigue. Most side effects are mild to moderate and manageable.

Like all medicines, PREVYMIS can cause side effects, although not everybody gets them. Clinical trials and post-marketing surveillance have identified the following side effects, categorized by how frequently they occur. Most side effects observed with PREVYMIS are mild to moderate in severity and are generally manageable with supportive care or dose adjustments when necessary.

The frequency categories used below follow the standard medical convention recommended by the Council for International Organizations of Medical Sciences (CIOMS):

The gastrointestinal side effects (diarrhea, nausea, vomiting) are the most frequently reported and are typically mild and transient. They tend to occur early in treatment and often improve as your body adjusts to the medication. Staying well hydrated and eating small, frequent meals may help manage these symptoms.

If you experience any of these side effects and they persist or become bothersome, discuss them with your healthcare team. In rare cases, more serious reactions such as significant allergic responses may occur. Seek immediate medical attention if you develop difficulty breathing, severe skin rash, or significant facial swelling, as these could indicate a serious allergic reaction requiring emergency treatment.

How Should You Store PREVYMIS?

Store PREVYMIS at room temperature in its original packaging to protect from moisture. Keep out of the sight and reach of children. Do not use after the expiry date printed on the carton and blister pack.

Proper storage of PREVYMIS is essential to maintain the medicine's effectiveness and safety throughout its shelf life. Follow these storage guidelines carefully:

• Keep out of the sight and reach of children at all times
• No special temperature requirements – store at room temperature
• Store in the original packaging – the perforated blister packs protect the tablets from moisture
• Do not use after the expiry date (marked as "EXP" on the carton and blister pack) – the expiry date refers to the last day of that month
• Do not dispose of medicines via wastewater or household waste – return unused medicines to your pharmacist for safe disposal to protect the environment

PREVYMIS 240 mg tablets are packaged in cartons containing perforated unit-dose blisters made of polyamide/aluminium/PVC, with a total of 28 tablets (28×1). The 480 mg tablets are available in the same blister configuration. The blister packaging provides individual protection for each tablet, helping to maintain stability and prevent moisture damage.

What Does PREVYMIS Contain?

PREVYMIS contains letermovir as the active ingredient (240 mg or 480 mg per tablet). The tablets also contain lactose monohydrate, microcrystalline cellulose, and other excipients. The 240 mg tablet is yellow and oval; the 480 mg tablet is pink and oval.

Active Ingredient

Each film-coated tablet contains either 240 mg or 480 mg of letermovir as the active substance.

Inactive Ingredients (Excipients)

Tablet core: Microcrystalline cellulose (E460), croscarmellose sodium (E468), povidone (E1201), colloidal anhydrous silica (E551), magnesium stearate (E470b).

Film coating: Lactose monohydrate, hypromellose (E464), titanium dioxide (E171), triacetin, yellow iron oxide (E172), red iron oxide (480 mg tablets only) (E172), carnauba wax (E903).

Tablet Appearance

Frequently Asked Questions About PREVYMIS