Acute promyelocytic leukemia (APL) is a blood cancer characterized by a marked increase in a type of white blood cells known as promyelocytes, a type of immature white blood cell. It develops in about 600 to 800 individuals each year in the United States, most often in adults around the age of 40. The characteristic symptom of APL is the associated bleeding disorder (coagulopathy), which can lead to excessive bleeding but also blood clot formation (thrombosis). In APL and other types of leukemia, the bone marrow is filled with malignant cells and is unable to produce functional cells. A decreased number of platelets (thrombocytopenia) is one of the contributing factors to the bleeding often present in APL. A decreased number of red blood cells (anemia) can lead to pallor and fatigue, while a decreased number of functional white blood cells (neutropenia) predispose affected individuals to infections. Fever, chills, night sweats, and weight loss, which are collectively known as constitutional or “B” symptoms, are also common in APL. The treatment of APL is centered elimination of the malignant cells and supportive care with transfusion of blood products to minimize the risk of bleeding or thrombosis, Medications, especially all-trans retinoic acid (ATRA; tretinoin) and arsenic trioxide allow malignant promyelocytes to mature into neutrophils, which are subsequently eliminated.
Circulating blood cells are formed in a region in the middle of large bones known as the bone marrow and are all derived from a primitive type of cell known as the hematopoietic stem cell. Hematopoiesis refers to the process of formation of blood cells from hematopoietic stem cells. The two principal lineages of blood cells are the myeloid and lymphoid cells. Cells derived from the myeloid lineage include red blood cells (erythrocytes), platelets (thrombocytes), and granulocytes and monocytes. A promyelocyte is a type of myeloid cell that normally matures to granulocytes. Eosinophils, neutrophils, and basophils are the three types of mature granulocytes.
Leukemia is defined as the uncontrolled proliferation of abnormal leukocytes in the blood and bone marrow. APL is a type of leukemia caused by the uncontrolled proliferation of promyelocytes that are arrested in their normal maturation process. It is a subtype of acute myeloid leukemia (AML), but it has its own uniquely different disease mechanism, clinical manifestations, and treatment. APL is a medical emergency, as treatment has to be initiated as soon as the disease is suspected to decrease the risk of complications associated with APL coagulopathy.
The development of therapy for APL is a success story in the realm of cancer and leukemia treatment. Before modern treatments were developed, the vast majority of affected individuals did not survive more than one month after diagnosis due to bleeding and/or infection. Research that elucidated the molecular mechanisms by which APL develops led to the development of ATRA, an oral medication that specifically targets the genetic defects in APL. Nowadays, with ATRA and other targeted therapies, such as arsenic trioxide (ATO), APL has shifted from an often fatal disease to a highly curable form of leukemia.
Causes
APL is caused by the uncontrolled proliferation of promyelocytes, a type of immature cell from the myeloid lineage of blood cells. The hallmark of APL is genetic alterations involving the retinoic acid receptor alpha (RARA) gene. Retinoic acid (a derivative of vitamin A) is critical in the process of cellular maturation and specialization (differentiation) of many cells, including myeloid precursors. In normal cells, the RARA protein is bound to proteins and forms a complex that prevents genes involved in cellular differentiation from being read; this is called transcriptional repression. When retinoic acid binds to this protein complex, transcriptional repression is relieved, genes involved in cellular differentiation can be read, and promyelocytes can continue their maturation and differentiation process into mature granulocytes. In the vast majority of cases of APL, RARA gene alterations occur due to the exchange of genetic material (translocation) between chromosomes 15 and 17, where the RARA and PML genes are located, respectively. This translocation results in a fusion gene termed PML/RARA, which leads to an abnormal retinoic acid receptor that blocks the differentiation process that is normally induced by retinoic acid. The myeloid precursors are, therefore “stuck” in the promyelocyte stage and accumulate in the bone marrow, and eventually the blood. The fusion gene PML/RARA that leads to APL is an acquired mutation and is not inherited. When patients enter remission, cells containing the PML/RARA fusion gene are no longer detectable.
The coagulopathy associated with APL is multifactorial. In addition to thrombocytopenia, which can lead to bleeding in many types of leukemia, other molecules present in promyelocytes contribute to the severity of the coagulopathy encountered in APL. Notably, tissue factor (TF), a molecule found on the surface of APL cells, activates the coagulation cascade. Annexin II is also present on the surface of APL cells and facilitates the activation of plasmin, a molecule that breaks down blood clots. Overall, the action of tissue factor and annexin II, in combination with other molecules, leads to excessive clotting (thrombosis), and excessive bleeding due to consumption of coagulation factors and excessive breakdown of clots.
The RAR-alpha (Retinoic acid-alpha) gene which encodes nuclear hormone receptor transcription factors is present on the long arm of chromosome 17 and is invariably involved in APL. It promotes the expression of various genes after binding to retinoic acid. In the majority (90% to 95%) of the cases, APL results from a t (15;17) (q22;q21) translocation resulting in the head to the tail fusion of promyelocytic leukemia (PML) gene to RAR-alpha to generate two fusion genes, PML–RARalpha and a reciprocal RAR-alpha-PML (80%) that encode a protein, which functions as an aberrant retinoid receptor. The other cytogenetic abnormalities associated with APL include t(5;17)(q35;q21), t(11;17)(q23;q21), t(11;17)(q13;q21), and t(17;17)(q11;q21) fuse RAR-alpha to the Nucleophosmin (NPM), Promyelocytic Leukemia Zinc Finger (PLZF), Nuclear Mitotic Apparatus (NuMA), and STAT5b genes, respectively, leading to expression of their fusion proteins. These translocations also have clinical significance due to their responsiveness (NPM/RAR-alpha, NuMA/RAR-alpha) or partial/complete refractoriness to retinoids (STAT5B/RAR-alpha, PLZF/RAR-alpha).[rx][rx]
Diagnosis
The diagnosis of APL is based on a combination of patient history, physical examination, and numerous laboratory tests. In cases where patients present with symptoms such as fever, fatigue, and bruising or bleeding, a complete blood count (CBC) is usually performed to evaluate the number of red blood cells (erythrocytes), and white blood cells (leukocytes), and platelets (thrombocytes). In APL, platelets and red blood cells are often low, and white blood cells might be low, normal, or elevated (however, the number of functional white blood cells is usually decreased). The combination of low platelets (thrombocytopenia), red blood cells (anemia), and white blood cells (leukopenia) is known as pancytopenia and is a warning sign that requires further speedy investigation. The cells taken from a blood sample can also be evaluated by a physician who specialized in disorders related to the blood (hematologist) to see if they are abnormal and potentially leukemic. Further evaluation usually requires bone marrow examination to assess for the presence of abnormal cells. Bone marrow samples are obtained by bone marrow aspiration and biopsy, which are respectively used to collect the liquid and solid portions of the bone marrow. The location of choice for bone marrow aspiration and biopsy is the hip bone (pelvic bone).
Once cells are obtained, they can be evaluated in numerous ways to confirm the diagnosis of APL and characterize the affected cells. Flow cytometry is a laboratory method where cells are suspended in fluid and processed into an instrument known as a flow cytometer. The cells flow one at a time through a laser, and the pattern of light scattering and cell fluorescence allows the identification of cells based on their size, shape, and the presence or absence of specific markers on the cell surface (immunophenotyping). The genes and chromosomes of the affected cells can also be evaluated. Karyotyping is a method where chromosomes are stained and visualized under a microscope during cell division. Fluorescence in situ hybridization (FISH) is a technique where selected chromosomal regions are stained to identify large genetic insertions, deletions, or translocations. Polymerase chain reaction (PCR) is a DNA sequencing technique that allows the detection of mutations and smaller insertions and deletions. Next-generation sequencing (NGS) is a method of evaluating multiple genes simultaneously for mutations.
When acute promyelocytic leukemia is suspected, evaluation of peripheral blood smear and FISH for the fusion of PML/RARA should be expedited for rapid diagnosis of this time-sensitive disease. A prompt coagulopathy workup including a platelet count, prothrombin time (PT), activated partial thromboplastin time (PTT), d-dimer or fibrin split products, and fibrinogen should also be performed. Bone marrow biopsy and immunophenotyping should also be performed. Conventional karyotyping should also be performed as a part of initial workup as it detects rare molecular subtypes of acute promyelocytic leukemia and other additional coexistent cytogenetic abnormalities- t(15:17). Reverse transcriptase-polymerase chain reaction (RT-PCR) for PML-RARA RNA is also used for confirming the diagnosis of acute promyelocytic leukemia and can also be used can for monitoring minimal residual disease.
Acute promyelocytic leukemia is classified into low-risk (white blood cell count (WBC) 10,000/microL or less and platelets 40,000/microL or more), intermediate (WBC 10,000/microL or less and platelets 40,000/microL or less), and high-risk (WBC more than 10,000/microL) to guide treatment.[rx][rx][rx][rx]
Lumbar puncture is done in high-risk patients with elevated WBC count if intrathecal therapy is contemplated. Further, a cardiac evaluation is necessary before administering anthracyclines.
In addition to tests used to diagnose and characterize APL, numerous ancillary tests are performed to evaluate the health of the patient and to assess for complications related to the disease. Especially in the case of APL, evaluation of coagulation parameters is crucial. Coagulation tests typically performed in the diagnostic evaluation of APL include prothrombin time (PT), activated partial thromboplastin time (aPTT), D-dimer (a product of clot breakdown), and fibrinogen levels. Other routine tests include measurement of levels of electrolytes, renal function tests, such as creatinine levels, cardiac function tests, and liver function tests.
Treatment
The treatment of APL is centered on all-trans retinoic acid (ATRA; tretinoin). The goal of this targeted therapy is to allow the differentiation of promyelocytes, which have been blocked by the PML/RARA fusion gene, into mature neutrophils. The treatment course is constituted of three phases: induction, consolidation, and maintenance. APL is a medical emergency, as treatment has to be initiated as soon as the disease is suspected to decrease the risk of bleeding complications associated with APL coagulopathy.
The first phase, induction, aims to put the patient in a state called complete remission (CR), where the majority of malignant cells from the blood and bone marrow will be eliminated and the production of normal blood cells (hematopoiesis) will be restored. Molecular complete remission is a more durable state of remission where the PML/RARA fusion gene is not detectable by polymerase chain reaction (PCR) testing. In the induction phase, oral ATRA is combined with different medications for as long as 60 days or until complete remission is achieved. Depending on the treatment regimen used, ATRA can be combined with idarubicin, daunorubicin, and cytarabine, or arsenic trioxide (ATO) with or without gemtuzumab ozogamicin (GO). Idarubicin and daunorubicin are chemotherapy agents that induce DNA damage and therefore kill malignant cells, while cytarabine prevents the synthesis of DNA in cells. As is the case for ATRA, ATO promotes the differentiation of promyelocytes into mature neutrophils, although it acts via a slightly different mechanism. GO is a medication that specifically targets CD33, a marker that is present on APL cells. ATRA and ATO are most often used together for treating APL, and GO are added for patients with high-risk APL due to elevated WBC counts.
After a complete remission is achieved, patients move to the consolidation phase, which aims to prevent relapse. In the United States, the most commonly prescribed consolidation regimen includes 4 cycles of ATO together with ATRA.
A third phase, maintenance, has been used to prevent relapse and usually involves less intensive therapy. Research now indicates that patients who have previously achieved complete molecular remission with ATRA and ATO do not require maintenance therapy. In other patients, ATRA alone is often used for maintenance, although it might be combined with some chemotherapy agents such as 6-mercaptopurine or methotrexate. During maintenance and follow-up, patients have to be monitored for possible disease relapse. Monitoring should be more frequent in the first year after remission is achieved since most relapses occur during that period. PCR can be performed on blood samples to see if there is sustained molecular complete remission. If this is not the case, a bone marrow biopsy should be performed for confirmation of relapse, and treatment for the relapsed disease can be initiated. In cases of relapse or treatment resistance, a possible treatment option is a prolonged infusion of intravenous ATO followed by allogeneic hematopoietic stem cell (HSC) transplantation. The rationale behind HSC transplantation is that it makes possible the pre-transplantation administration (preparative regimen) of very high doses of chemotherapy medications that are very effective to control cancer but also damage healthy hematopoietic stem cells. The patient’s HSCs are then replaced with healthy HSCs via transplantation. The stem cells can be obtained from the blood or bone marrow of a healthy donor (allogeneic transplantation) or the patient’s own body (autologous transplantation) before the preparative regimen is initiated if a molecular remission was achieved. Another goal of the preparative regimen is to suppress the immune system to decrease the risk of graft rejection. Although effective, HSC transplantation is a high-risk procedure and is associated with numerous short and long-term side effects. Patients, therefore, have to be chosen and followed carefully throughout the entire process.
Chemotherapy
Acute promyelocytic leukemia is a medical emergency with a very high pre-treatment mortality. All-Trans Retinoic Acid (ATRA) is the mainstay in the treatment of acute promyelocytic leukemia and used in all modern regimens. ATRA should be initiated without any delay even before cytogenetic confirmation is obtained. Before the introduction of ATRA in the 1980s, the prognosis of this disease was poor with chemotherapy alone. ATRA was then used in combination with anthracycline-based regimens with increased survival and cure rate. ATO (arsenic trioxide) also induces differentiation of the malignant myeloid clone by dissociating the PML/RAR-alpha-RXR complex from the target genes and is found to have a synergistic action with ATRA. ATRA-ATO was also shown to have comparatively lesser toxicities than ATRA-chemo. Hence, ATRA-ATO for induction and consolidation has emerged as the new standard of care for patients with low-(to-intermediate) risk acute promyelocytic leukemia. ATRA- Idarubicin or ATRA – ATO plus gemtuzumab ozogamicin (antibody-drug conjugate) are preferred in patients with high risk without cardiac dysfunction. ATRA-ATO therapy with or without gemtuzumab ozogamicin is also a reasonable choice for patients with severe comorbidities, older adults, and patients with cardiac dysfunction who cannot tolerate anthracycline-based regimens or overall poor functional status. Maintenance therapy after the initial consolidation is widely debated. Maintenance may not be necessary for patients receiving intensive induction/consolidation including ATO. Treatment and post-treatment monitoring for up to 2 years with PCR is recommended. Treatment of relapsed APL is beyond the scope of this article.
Supportive Therapy
Supportive therapy plays a very important role in the survival of patients with acute leukemia. Bleeding diathesis is a known complication, especially in patients receiving treatment, and platelets should be maintained above 30 to 50 × 10/l and fibrinogen above 100 mg/dl to 150 mg/dl, with aggressive blood product support. High suspicion should be maintained for systemic infections as the patients are routinely immunosuppressed. In granulocytopenia patients with fever, an empiric antibiotic regimen to treat gram-negative bacteria should be instituted. Vancomycin should be started, if there is suspicion of catheter-related infection or based on blood cultures or if there is a suspicion of severe unknown infection. Antifungals should be considered if fever persists 5 days after the initiation of empiric antibiotics with appropriate testing.
Because cure rates for APL are high, bone marrow transplantation is not the first option. It is only offered to patients who relapse or are resistant to therapy. Intrathecal therapy is done in high-risk patients.
In addition to therapy used to treat leukemia itself, other essential considerations specific to the management of APL include control of the associated coagulopathy and prevention or treatment of differentiation syndrome. Close monitoring of coagulation parameters with transfusion of blood products is necessary to minimize the risk of bleeding and thrombosis. Blood has four main components: plasma, which is the fluid that contains proteins and coagulation factors, red blood cells, white blood cells, and platelets. Commonly used blood products for the management of coagulopathies include platelets, fresh frozen plasma, and cryoprecipitate (a derivative of plasma proteins). Differentiation syndrome is treated with corticosteroids such as dexamethasone or prednisone. However, corticosteroids are sometimes given in advance with induction regimens in a preventive manner (prophylactically) to decrease the risk of differentiation syndrome.
References
