Imetelstat (Rytelo®)

Author: Christina Bach, MBE, LCSW, OSW-C
Content Contributor: Allyson Van Horn, MPH, Katherine Okonak, LSW, and Niti Patel, PharmD 
Last Reviewed: June 11, 2026

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Pronounce: IM-e-TEL-stat

Classification: Telomerase Inhibitor

About Imetelstat (Rytelo®)

Imetelstat is a telomerase inhibitor. Telomerase inhibitors work by targeting the telomerase enzyme to stop uncontrolled cell division and do not grow. This is helpful in treating conditions like myelodysplastic syndrome (MDS). 

How To Take Imetelstat

Imetelstat is given intravenously (into a vein). The amount given and schedule are based on your size. You will have your labs checked before each dose. The results of your labs help guide your treatment plan. You will be given diphenhydramine (Benadryl) and hydrocortisone (steroid) before your infusion to help reduce an infusion-related reaction to the medication. Often, you will need to stay after your infusion is done to make sure you are not having a reaction.

Possible Side Effects

These are some of the most common or most serious side effects of this medication. Talk with your provider about the side effects you are having and how they can be managed.

Low White Blood Cell Count (Leukopenia or Neutropenia)

White blood cells (WBC) help your body fight infection. While you are having treatment for cancer, your WBC count can drop, putting you at a higher risk of an infection. Tell your provider right away if you have a fever (temperature greater than 100.4°F or 38°C), sore throat or cold, shortness of breath, cough, burning with urination, or a sore that doesn't heal.

Tips for preventing infection:

  • Hand washing: both yours and your visitors is the best way to prevent the spread of infection.
  • Avoid large crowds and people who are sick (those who have a cold, fever, or cough, or live with someone with these symptoms).
  • When working outside, wear clothes that protect you, like long pants and gloves.
  • Do not handle pet waste.
  • Keep all cuts and scratches clean.
  • Shower or bathe daily. 
  • Brush your teeth at least twice a day.
  • Do not cut cuticles or ingrown nails. You may wear nail polish, but not fake nails.
  • Ask your providers before scheduling dental appointments or procedures.
  • Ask your providers before you or someone you live with has any vaccines

Low Platelet Count (Thrombocytopenia)

Platelets are blood cells that help your blood clot. Thrombocytopenia is when your platelet count is lower than normal, and you are at a higher risk of bleeding. Tell your providers if you have any bruising or bleeding, including nose bleeds, bleeding gums, or blood in your urine or stool. If your count gets too low, you may receive platelets from a donor. 

  • Do not use a razor (an electric razor is fine).
  • Do not do activities that can cause injury or bleeding.
  • Talk to your provider before taking any non-steroidal, anti-inflammatory medications (NSAIDs) like Motrin/Advil (ibuprofen), Aleve (naproxen), Celebrex (celecoxib), and others. These can raise the risk of bleeding. 
  • Use a soft-bristle toothbrush to brush your teeth. Ask your provider about flossing or using toothpicks. 

Liver Toxicity

This medication can cause liver toxicity, or liver problems, which your provider may monitor for with blood tests called liver function tests. Symptoms may be:

  • Yellowing of your skin or eyes.
  • Dark or brown urine.
  • Pain in your abdomen (belly).

Tell your provider if you have any of these symptoms.

Fatigue

Fatigue can cause exhaustion, feeling worn out and that your body is "heavy" and hard to move, or being unable to focus. It is common when you have cancer, and it is not the same as feeling tired. You may need to adjust your schedule to manage fatigue. Plan times to rest during the day and save your energy for more important activities. Exercise can help lessen fatigue. Talk to your providers about ways to deal with this side effect.

Muscle or Joint Pain/Aches and Headache

You may have joint or muscle pain/aches and headaches while taking this medication. Your provider can recommend medications and other ways to help relieve your pain.

Fertility

This medication may affect your fertility. It can cause changes to your menstrual cycle or sperm. You may want to consider sperm banking or egg harvesting if you might wish to have a child in the future. Talk about your options with your provider before treatment.

Reproductive Concerns

There is a higher risk of health problems for your baby if you are pregnant, become pregnant, or get someone pregnant while on this medication. To lower the risk of birth defects, you should use birth control (contraception) to avoid pregnancy. Talk with your provider about what kind of birth control is safest for you and your partner.

If you can become pregnant, you will need to use effective birth control during treatment and for 1 week after the last dose of this medication, even if your menstrual cycle has stopped or changed.  You should not breastfeed during treatment and for 1 week after the last dose of this medication.

Important but Less Common Side Effects

Infusion-Related Side Effects

Some people may have a reaction during infusion of this medication. These can be:

  • Chills.
  • Fever.
  • Low blood pressure.
  • Nausea.
  • Vomiting.

Your provider will monitor you for symptoms of infusion-related reactions.