Acute promyelocytic leukemia (APL), a specific subtype of acute myeloid leukemia (AML), accounts for about 10-15% of AML in adults, with a median age of onset of about 40 years, and with increasing age, the incidence of APL does not There is no significant increase in the incidence of APL with increasing age.
Whereas APL was once one of the most aggressive and lethal subtypes of leukemia, several clinical research advances have now made this disease curable in most cases. Also, APL has become the first leukemia subtype in AML to be successfully treated with targeted therapies.
I. Molecular biology pathogenesis
Although the etiology of most APL patients is not fully understood, with the development of research techniques in molecular biology and cytogenetics, there is sufficient understanding of the molecular biology pathogenesis of APL.
Currently, five types of RARα gene rearrangements have been identified in APL patients, which are t(15;17)(q22;q21), t(11;17)(q23;q21), t(5;17)(q35;q21), t(11;17)(q13;q21), and Statb5-RAR formed by intergenic chromosomal DNA deletions α fusion gene [1-3].
Of these, t(15;17) accounts for approximately 98% of APL, and the translocation causes an interactive rearrangement of the promyelocytic zinc finger (PML) gene on 15q22 with the retinoic acid receptor alpha gene (RARα) on 17q21 to form PML-RARα and RARα-PML fusion genes on chromosomes 15 and 17, respectively. All leukemic cells of patients expressed PML-RARα fusion gene, and only 70-80% of patients expressed both RARα-PMLL fusion gene, indicating that PML-RARα fusion gene was the key to the pathogenesis. The PML-RARα transcript has three isoforms, bcr1, bcr2 and bcr3, depending on the location of the PML breakpoint. the first two are similar in length and both are known as long type (L-type), but bcr2 is also known as variant type (V-type) and the latter is known as short type (S-type). the PML-RARα fusion protein, as a variant retinoid receptor, compared to the wild-type RARα protein has different DNA binding properties and is a repressor of RA signaling, becoming an intrinsic and effective repressor of RARα target gene transcription factors through different pathways.
Patients with variant t(11;17) APL, which accounts for approximately 0.8% of APL, is the most common type of variant translocation in APL. It results in a fusion of the PLZF and RARα genes, producing a PLZF-RARα fusion protein with co-repressor complex binding sites in both parts. Their specific structure increases their affinity for inhibitory factors and makes them less susceptible to dissociation under ATRA, thus exhibiting resistance to ATRA. In vitro tests have confirmed that histone deacetylation inhibitors are effective in restoring their sensitivity to ATRA [4].
In the remaining APL patients, it is seen that RARα forms different fusion genes with other non-PML, including the extremely rare nuclear phosphoprotein (NPM), nuclear matrix (NuMA) gene, and STAT5b gene.
II. Clinical features
In addition to the common manifestations of acute leukemia such as anemia, bleeding, infection, and leukemic cell infiltration, patients with APL also have some special manifestations with severe and obvious bleeding tendencies such as skin petechiae, epistaxis, gingival bleeding, hemoptysis, gastrointestinal bleeding, intracranial bleeding, and occasionally sudden blindness and vascular embolism manifested by thrombosis. Extramedullary infiltration of leukemic cells at the time of initial diagnosis is rare.
The peripheral blood leukocyte count of APL patients is often (3.0-15.0) × 109/L, mostly below 5.0 × 109/L while peripheral blood leukocyte count ≥ 10 × 109/L is called hyperleukocytosis, with high treatment risk and poor prognosis. High leukocyte count is mainly seen in patients with M3v type, which is usually (50.0~100.0) × 109/L.
The degree of bone marrow hyperplasia is often above active, with a consistent increase in abnormal promyelocytes, which usually account for more than 60%, and very few primitive granulocytes and intermediate granulocytes or below. Based on leukemic cell morphology, the FAB Working Group classifies it as acute myeloid leukemia type M3 – including both coarse granular APL (M3) and variant fine granular APL (M3v), with coarse granular APL (M3) accounting for approximately 75% of APL.
Patients with APL have unique and stable antigen expression characteristics – strong positive expression of CD33, positive expression of CD13 and CD117, occasional positive expression of HLA-DR and CD34, and negative expression of CD7, CD11a, CD11b, CD14 and CD18 [5-6].
III. Diagnosis
The more important laboratory test indicators for the diagnosis of APL are the following four.
1, bone marrow cytology examination of the bone marrow with increased granularity of abnormal early granulocytes, accounting for more than 30% of the non-red lineage. If t(15;17) or PML-RARα is present, the bone marrow can have less than 30% promyelocytes.
2. Immunophenotypic detection of leukemia cells mainly shows frequent expression of CD33, CD13 and other myeloid antigens, CD15, HLA-DR and CD34 are often negative, and there is often co-expression of CD2 and CD9, i.e. CD13(+), CD33(+), CD2/CD9(+), CD34-/+, HLA-DR(-) CD15(+), CD11b(-).
3, cytogenetic testing reveals specific chromosomal translocations or fusion genes, such as specific t(15;17)(q22;q21) or other variant abnormalities such as t(11;17)(q23;q21), t(11;17)(q13;q21), t(5;17)(q35;q21), der(17);.
4. molecular biology PML-RARα fusion gene (FISH), and its transcript (RT-PCR/Q-PCR) or fusion protein (diffuse microparticle fluorescence formed by direct immunofluorescence detection of PML oncogene structural domains performed by PML antibody), or can detect variant PLZF-RARα, NuMA-RARα, NPM-RARα, and STAT5b-RARα fusion genes.
APL can be diagnosed by meeting 1 + 3 or 1 + 4 of the above 4 indicators, with the immunophenotype serving as a secondary diagnostic criterion. However, it should be noted that cytogenetic examination is the key to definitive diagnosis, and RT-PCR may have false-positive or false-negative results, so it is better to apply several examination methods in combination.
Based on the FAB classification, the World Health Organization (WHO) suggested the classification of hematopoietic system and lymphoid tissue tumors based on cytomorphology, immunology, cytogenetics, and molecular biology, and classified all these diseases as AML with characteristic chromosomal alterations t(15;17)(q22;q21),(PML-RARα) and variants.
IV. Treatment
Currently, APL is potentially curable for most patients with APL. Standard regimens include ATRA + anthracycline-containing drugs for induction of remission and consolidation therapy, and ATRA-based maintenance therapy. Individualized treatment regimens based on treatment risk allow adjustment of treatment intensity to reduce treatment-related mortality while maintaining efficacy. In addition, the unique PML/RARa mutation can be used for rapid diagnosis and as a molecular marker for predicting the efficacy of ATRA as well as arsenical therapy.
(i) Induction protocol for patients with primary APL
According to the consensus of available research results, ATRA combined with anthracycline-based chemotherapy is currently the standard induction regimen for patients with newly diagnosed APL. This treatment helps to improve coagulation abnormalities in APL, control elevated white blood cell counts, reduce the incidence of severe bleeding and vincristine syndrome (RAS) and morbidity and mortality, and achieve a CR rate of 90% in APL.
There is some debate as to which anthracyclines to use and whether to use them in combination with other drugs. Regarding the choice of the type of anthracycline, only a slight survival advantage of desoxorubicin (idarubicin) over daunorubicin was found in combination with cytarabine in the treatment of young AML patients [7]. However, there are no prospective studies comparing these two anthracyclines in APL patients. The results of a randomized clinical trial by the European APL Collaborative Group showed an increase in relapse rates if agranulocytin was removed from a regimen with erythromycin as the anthracycline [8]. However, this study was not able to show a difference between the two in terms of CR rates and induction failure rates. In contrast, there are no reports of leukemia resistance with the combined use of ATRA and desoxorubicin.
After first reporting over 80% CR rates and high molecular remission rates in APL patients treated with arsenic trioxide (ATO) in China, investigators designed several clinical trials using ATRA as first-line therapy, in which results showed high CR rates (≥90%) in patients with induction-primary APL, either with ATRA or ATO alone or in combination with both drugs, but the combination of the two drugs achieved CR required significantly shorter time and better molecular biological assays to reduce tumor load [9]. Nevertheless, as there are no standard induction regimens and no results of randomized controlled studies with ATO as the main first-line treatment regimen, ATO is only indicated for patients with APL in whom chemotherapy is contraindicated.
In patients with primary APL, supportive and other relevant treatments are equally important for the outcome during the induction remission treatment phase [10]. Once APL is suspected according to myeloid morphological criteria, it should be treated as an emergency, even before the molecular diagnosis is available, and ATRA and supportive therapy can be started. This is due to the fact that a large proportion of patients with APL develop fatal bleeding during the diagnosis of the disease, before antileukemic therapy and during the early stages of induction therapy. Therefore, rapid administration of supportive therapy can reverse developing coagulation abnormalities and reduce the occurrence of bleeding. This consists mainly of massive transfusions of fresh frozen plasma and/or fibrinogen, and large amounts of platelets, allowing maintenance of fibrinogen levels at 1.5 g/L and platelets at 30-50 × 109/L until the clinical and laboratory manifestations of coagulation abnormalities disappear. In contrast, the role of heparin, tranexamic acid or other anticoagulant and antifibrinolytic treatments to reduce the risk of bleeding remains uncertain.
In addition, in patients treated with ATRA and ATO, care should be taken to prevent and treat the so-called “APL fractionation syndrome” by administering intravenous dexamethasone 10 mg twice daily at the early onset of symptoms. Temporary discontinuation of ATRA or ATO should be considered only when symptoms are severe.
(ii) Post-remission treatment
Induction therapy followed by at least two anthracycline-based chemotherapy sessions can result in a molecular remission rate of 90-99%, making this regimen the standard of care for consolidation [10]. Currently, it has been proposed to design individualized regimens to adjust the intensity of consolidation therapy according to the risk of recurrence, but this is still controversial [11].
Outside of chemotherapy consolidation, the results of a retrospective study by two independent collaborative groups, the Italian GIMEMA study group [12] and the Spanish PETHEMA study group [13], showed that the use of a standard dose of ATRA for 15 days during consolidation chemotherapy (45 mg/m2/d for adults and 25 mg/m2/d for children ), the efficacy was significantly improved, suggesting a synergistic effect of combination chemotherapy. Meanwhile, increasing the dose of anthracyclines such as IDA and adding ATRA during the consolidation phase for patients in the high-risk group was beneficial to further improve the overall survival rate. In addition, the results of the GIMEMA study group suggest that the use of agranulocytosis in the high-risk group is also beneficial for consolidation therapy.
The role of ATO in the post-remission treatment of patients with primary APL is not only to consolidate patients who have reached CR by induction with ATO and to reduce or even remove chemotherapy, but also to enhance the efficacy of ATRA-based combination chemotherapy. Although various studies have shown that ATO has a significant anti-leukemic effect, consolidation with ATO is still recommended to be limited to clinical studies and patients for whom chemotherapy is contraindicated, except in certain special cases, such as inability to afford standard chemotherapy [14].
(iii) Maintenance therapy
Maintenance therapy is necessary for APL. The European APL Group, the Medical Research Council (MRC) in the UK and several other experimental groups worldwide have shown that the use of ATRA in induction regimens is necessary for CR and long-term survival, especially for patients with APL with a primary white blood cell count less than 10 × 109/L, and that the continued use of ATRA until CR is necessary; at the same time, especially for older patients or those with a high white blood cell count at the time of primary diagnosis taking maintenance therapy containing Maintenance therapy with ATRA will help to reduce relapse and prolong survival. The currently recommended regimen for maintenance therapy is ATRA 45 mg/m2 for 15 days every 3 months, plus 6-MP 50 mg/m2?d and MTX 15 mg/m2?w for a total of 2 years [15].
(iv) Salvage therapy after relapse
Currently, ATO is considered the best regimen for relapse salvage therapy, but the best consolidation regimen after ATO-induced second remission is unclear and includes repeat ATO, combination chemotherapy, and hematopoietic stem cell transplantation (HSCT). hSCT is currently no longer used as first-line therapy, except in a few patients with persistent microscopic residual lesions at the end of consolidation therapy. However, HSCT is crucial for patients in second complete remission, i.e., still positive PCR after ATO salvage therapy, and allogeneic bone marrow transplantation is preferred if an HLA-locus compatible donor is available [16]. In addition, the efficacy of coupled anti-CD33 monoclonal antibody (gemtuzumabozogamicin) in the treatment of relapsed APL remains undetermined.