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Severe, Malignant Osteopetrosis (SMO)

What is Severe, Malignant Osteopetrosis (SMO)?

There are several different forms of osteopetrosis (not to be confused with the more common osteoporosis, a very different condition), which are determined by their pattern of genetic inheritance and characteristics. All forms of osteopetrosis are characterized by an abnormal increase in bone density.1

Severe, malignant osteopetrosis is one form of osteopetrosis and is sometimes referred to as marble bone disease or malignant infantile osteopetrosis (MIOP) because it occurs in very young children. Severe, malignant osteopetrosis is a more severe form of malignant osteopetrosis.*1,2

While exact numbers are not known, it has been estimated that 1 out of 250,000 children are born with severe, malignant osteopetrosis.1

During normal bone development, existing bone material is constantly being replaced by new bone. Cells called osteoblasts cause new bone formation. Other cells called osteoclasts remove old bone through a process called resorption.1

In people with osteopetrosis, this balance is not maintained because their osteoclasts do not function properly. As a result, resorption of old bone material decreases while the formation of new bone continues. This leads to an abnormal increase in bone mass, which can make the bones more brittle. Because abnormal bone development affects many different systems in the body, osteopetrosis may cause problems such as1,2

  • Blood disorders
  • Decreased ability to fight infection
  • Bone fractures
  • Problems with vision and hearing
  • Abnormal appearance of the face and head

Common Characteristics

In SMO, the abnormal accumulation of bone material tends to narrow the space inside bones where bone marrow is formed. This can cause failure of the bone marrow, leading to a decrease in various blood cells such as red blood cells (anemia) and white blood cells (decreased ability to fight infection). Also, there can be narrowing of the “tunnels” within bones of the skull (called foramina) through which the nerves for vision and hearing pass. When these nerves are compressed by the overdevelopment of bones due to SMO, vision and hearing problems can result.2

Patients with SMO often suffer serious effects from the disease. These may include1,2

  • Failure to thrive, slow growth in childhood
  • Impaired vision or blindness
  • Hearing loss
  • Abnormal head shape
  • Bone marrow failure, anemia
  • Frequent and recurrent infections
  • Frequent and recurrent bone fractures

SMO is associated with diminished life expectancy, with most untreated children dying within the first decade of their life.1

Diagnosing SMO

Severe, malignant osteopetrosis is generally diagnosed in infants, often within the first year of life. The first signs of the disease commonly noticed by parents are vision problems and failure to thrive or slow growth. Other early signs include recurrent infections, bone fractures, nasal congestion, and unusual facial features.2

To diagnose that a person has severe, malignant osteopetrosis, the doctor may order an X-ray. By examining the X-ray images, doctors can look for the abnormal bone development that is characteristic of the disease. That information combined with the patient’s physical signs and symptoms usually leads to a firm diagnosis. This diagnosis can be definitively confirmed through genetic testing.1

Managing SMO

Therapies available for SMO can help to manage the complications that occur, delay progression of the disease, and prolong survival. Doctors may prescribe corticosteroid medications or high doses of calcitriol (a special form of vitamin D) in order to slow the progression of the disease. Problems caused by bone marrow failure may be treated with blood transfusions. Special treatments for vision, hearing, and dental problems may be required.1,2

ACTIMMUNE® (Interferon gamma-1b) is approved by the US Food and Drug Administration to slow the worsening of severe, malignant osteopetrosis (SMO). SMO is also a genetic disorder that affects normal bone formation.3

ACTIMMUNE plus calcitriol may provide benefits for patients with SMO by1,4

  • Increasing bone resorption
  • Stabilizing hemoglobin and platelet levels
  • Lowering the risk of serious bacterial infections associated with (or caused by) SMO

For some patients, HSCT (often referred to as BMT) may be considered. BMT provides a possible cure for CGD and SMO, but there are associated risks, including rejection of the transplanted cells. Ask your healthcare provider for more information.

Important Safety Information (ISI)

Approved Uses for ACTIMMUNE (Interferon gamma-1b)

Chronic Granulomatous Disease (CGD)
ACTIMMUNE is approved by the US Food and Drug Administration to reduce the frequency and severity of serious infections associated with Chronic Granulomatous Disease. CGD is a genetic disorder that affects the functioning of some cells of the immune system.

Severe, Malignant Osteopetrosis (SMO)
ACTIMMUNE is approved by the US Food and Drug Administration to slow the worsening of severe, malignant osteopetrosis. SMO is a genetic disorder that affects normal bone formation.

Important Safety Information (ISI)
The most common side effects with ACTIMMUNE are “flu-like” symptoms such as fever, headache, chills, myalgia (muscle pain), or fatigue, which may decrease in severity as treatment continues. Bedtime administration of ACTIMMUNE may minimize some of these symptoms. Acetaminophen may be helpful in preventing fever and headache.

ACTIMMUNE can cause severe allergic reactions and/or rash. Do not use ACTIMMUNE if you are allergic to interferon-gamma, E. coli-derived products, or any component of the product. (See Full Prescribing Information for a list of components.) If you develop a serious reaction to ACTIMMUNE discontinue it immediately and contact your doctor or seek medical help.

At high doses, ACTIMMUNE can cause (flu-like) symptoms, which may worsen some pre-existing heart conditions. Tell your doctor if you have a cardiac condition, such as irregular heartbeat, heart failure, or decreased blood flow to your heart.

ACTIMMUNE may cause reversible changes to your nervous system, including decreased mental status, walking disturbances, and dizziness. Tell your doctor if you have a history of seizures or other neurologic disorders.

Bone marrow function may be suppressed with ACTIMMUNE and decreased production of cells important to the body may occur. This effect, which can be severe, is usually reversible when the drug is discontinued or the dose is reduced. Tell your doctor if you have, or have had, reduced bone marrow function. Your doctor will monitor these cells with blood tests at the beginning of therapy and at 3 month intervals thereafter.

Taking ACTIMMUNE may cause reversible changes to your liver function, particularly in patients less than one year old. Your doctor will monitor your liver function with blood tests at the beginning of therapy and at 3 month intervals. If the patient is 1 year or less, monitoring will be done on a monthly basis.

If you are pregnant or plan to become pregnant or plan to nurse you should consult your physician.

If you are receiving ACTIMMUNE at home, your doctor will provide to you or your caregiver appropriate instructions on the administration of the drug and disposal of the container, needles, and syringes.

You are encouraged to report negative side effects of prescription drugs to the FDA. Visit www.fda.gov/medwatch, or call 1-800-FDA-1088.

This information is not intended to replace discussions with your doctor. For additional information about ACTIMMUNE, please consult the Full Prescribing Information and the Information for the Patient/Caregiver and talk to your doctor. ACTIMMUNE is available by prescription only.

Download a copy of the ACTIMMUNE Full Prescribing Information.


  1. Stark Z, Savarirayan R. Osteopetrosis. Orphanet J Rare Dis. 2009;4:5.
  2. Wilson CJ, Vellodi A. Autosomal recessive osteopetrosis: diagnosis, management, and outcome. Arch Dis Child. 2000;83(5):449-452.
  3. ACTIMMUNE (Interferon gamma-1b) Full Prescribing Information. Roswell, GA: Vidara Therapeutics Inc; 2013.
  4. Key LL Jr, Rodriguiz RM, Willi SM, et al. Long-term treatment of osteopetrosis with recombinant human interferon gamma. N Engl J Med. 1995;332(24):1594-1599.

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