The doses of chemotherapy drugs that doctors can give to treat acute myeloid leukemia (AML) are limited by the serious side effects they can cause. Even though higher doses of these drugs might kill more cancer cells, they can’t be given because they could severely damage the bone marrow, which is where new blood cells are formed. This could lead to life-threatening infections, bleeding, and other problems caused by low blood cell counts.
Doctors can sometimes use a stem cell transplant (SCT), also called a bone marrow transplant, to give higher doses of chemotherapy than could normally be given. (Sometimes radiation therapy is given as well.) After the treatment is finished, the patient gets an infusion of blood-forming stem cells to restore their bone marrow.
The blood-forming stem cells used for a transplant can come either from blood or from bone marrow. Sometimes stem cells from a baby’s umbilical cord blood are used.
Types of SCT used for AML
Stem cell transplants differ based on whom the blood-forming stem cells come from.
Allogeneic stem cell transplant
This is the most common type of SCT used to treat AML. In an allogeneic SCT, the stem cells come from someone other than the patient – usually a donor whose tissue type (also known as the HLA type) closely matches the patient’s. Tissue type is based on certain substances on the surface of cells in the body. Differences in HLA types between the stem cell donor and recipient can cause the body's immune system to react against the cells. Therefore, the closer a tissue “match” is between the donor and the recipient, the better the chance the transplanted cells will “take” and begin making new blood cells.
The best donor is often a close relative, such as a brother or sister, if they are a good match. If no close relatives match, stem cells might be available from a matched unrelated donor (MUD), an unrelated volunteer whose tissue type matches that of the patient. But the use of stem cells from a MUD is linked to more complications. Sometimes umbilical cord stem cells are used. These stem cells come from blood drained from the umbilical cord and placenta after a baby is born and the umbilical cord is cut.
For most patients with AML, especially those at higher risk of having the leukemia return after treatment, using an allogeneic SCT is preferred over an autologous SCT (see below). Leukemia is a disease of the blood and bone marrow, so giving the patient their own cells back after treatment may mean giving them back some leukemia cells as well. Donor cells are also helpful because of the graft-versus-leukemia effect. When the donor immune cells are infused into the body, they may recognize any remaining leukemia cells as being foreign to them and attack them. This effect doesn’t happen with autologous stem cell transplants.
Allogeneic transplants can have serious risks and side effects, so patients typically need to be younger and relatively healthy to be good candidates. Another challenge is that it can sometimes be difficult to find a matched donor.
One of the most serious complications of allogeneic SCTs is known as graft-versus-host disease (GVHD). It happens when the patient’s immune system is taken over by that of the donor. When this happens, the donor immune system may see the patient’s own body tissues as foreign and attack them.
Symptoms can include severe skin rashes, itching, mouth sores (which can affect eating), nausea, and severe diarrhea. Liver damage can cause yellowing of the skin and eyes (jaundice). The lungs can also be damaged. The patient may also become easily fatigued and develop muscle aches. Sometimes GVHD can become disabling, and if it's severe enough, it can be life-threatening. Drugs that affect the immune system may be given to try to control it.
Non-myeloablative transplant (mini-transplant): Many older people can’t tolerate a standard allogeneic transplant that uses high doses of chemo. Some may still be able to get a non-myeloablative transplant (also known as a mini-transplant or reduced-intensity transplant), where they get lower doses of chemo and radiation that don’t completely destroy the cells in their bone marrow. They then get the allogeneic (donor) stem cells. These cells enter the body and establish a new immune system, which sees the leukemia cells as foreign and attacks them (a graft-versus-leukemia effect).
A non-myeloablative transplant can still sometimes work with much less toxicity. In fact, a patient can get the transplant as an outpatient. The major complication is graft-versus-host disease.
Many doctors still consider this an experimental procedure for AML, and it is being studied to determine how useful it may be.
Autologous stem cell transplant
In an autologous transplant, a patient’s own stem cells are removed from their bone marrow or blood. They are frozen and stored while the person gets treatment (high-dose chemotherapy and/or radiation). In the lab, a process called purging may be used to try to remove any leukemia cells in the samples. The stem cells are then put back (reinfused) into the patient’s blood after treatment.
Autologous transplants are sometimes used for people with AML who are in remission after initial treatment and who don’t have a matched donor for an allogeneic transplant. Some doctors feel that it is better than standard “consolidation” chemotherapy (see Typical Treatment of Acute Myeloid Leukemia (AML)) for these people, but not all doctors agree with this.
Autologous transplants are generally easier for patients to tolerate than allogeneic transplants, because they are getting their own cells back, which lowers the risk of some complications. But the high-dose chemo can still cause major side effects. This type of transplant can be done in any otherwise healthy person, although patients who are very old or have other health problems might not be suitable.
One problem with autologous transplants is that it’s hard to separate normal stem cells from leukemia cells in the bone marrow or blood samples. Even after purging (treating the stem cells in the lab to try to kill or remove any remaining leukemia cells), there is the risk of returning some leukemia cells with the stem cell transplant.