Acute Promyelocytic Leukemia (APL) is a unique subtype of AML. It accounts for 10-15% of all AML cases. In APL, promyelocytes (immature white cells) accumulate in the bone marrow. These cells stop maturing during the step in blood cell formation that comes after the development of myeloblasts. They also have a specific chromosome abnormality that involves a translocation of chromosome 15 and chromosome 17 (t15;17). This chromosome abnormality results in the abnormal fusion gene PML/RARα. This fusion gene is the key target of treatment.
Treatment for APL differs from all other AML treatments. Due to research advancements, APL has gone from the most fatal to the most curable form of acute leukemia in adults.
Signs and Symptoms of APL
The signs and symptoms of APL are similar to those of other types of AML. Patients often experience a general unwell or flu-like feeling due to the underproduction of blood cells.
Common signs and symptoms include:
- Pale skin
- Abnormal or excessive bleeding
- Bruising easily
- Small red spots on the skin
- Mild fever
- Loss of appetite
- Unintentional weight loss
- More frequent infection or difficulty recovering from illness
The initial work-up for APL is similar to the work-up done for other types of AML. The patient will undergo blood tests like a complete blood count (CBC) and a complete metabolic panel (CMP). If leukemia is suspected, a bone marrow aspiration and biopsy will be collected. Fluorescence in situ hybridization (FISH) will be performed on the sample to help identify certain changes in genes and chromosomes. If APL is present, the APL cells will have a very specific abnormality called a “balanced translocation,” in which parts of the chromosomes 15 and 17 are swapped. This results in the formation of an abnormal fusion gene known as “PML/RARα.” This mutated gene causes many of the features of the disease. A diagnosis of APL requires demonstration of the 15;17 translocation or of the PML/RARα gene.
An early diagnosis of APL is crucial to the treatment of the disease. Treatment must be started right away in order to avoid any serious or life-threatening complications.
The treatment a patient receives for APL depends on several factors such as age, health status and APL risk classification.
- Patients are considered low risk if they have a white blood cell count less than 10,000/μL at the time of diagnosis.
- Patients are considered high risk if they have a white blood cell count higher than 10,000/μL at the time of diagnosis.
Patients who are considered low risk are often treated with a less intensive treatment regimen. This risk classification is something that your doctor will take into consideration when discussing your treatment options.
APL treatment has three phases: induction, consolidation, and maintenance. All-trans retinoic acid (ATRA) is the main drug used in all three phases. It is a non-chemotherapy drug and is actually a derivative of vitamin A. ATRA is capable of eliminating the PML/RARα abnormality. This will allow the leukemic cells to develop into mature cells, and a remission often follows. However, ATRA cannot eradicate the leukemia cells by itself. In order to achieve a complete remission, treatment also requires the addition of arsenic trioxide (ATO) or chemotherapy. For some people at higher risk of APL coming back after treatment, the targeted drug gemtuzumab ozogamicin (Mylotarg) might be given as well.
The most common options for maintenance therapy are ATRA alone or ATRA along with chemotherapy (6-mercaptopurine (6-MP) and/or methotrexate). Maintenance therapy is typically given for about a year.