Study design and patients
This study is a prospective, multicenter, phase III non-inferiority trial done at three hospitals in China. The initial full trial design was published in 2018 . Briefly, eligible patients were15 to 80 years old with newly diagnosed, low- or high-risk APL. Initial patient enrollment was solely based on cytomorphologic features according to the French-American-British (FAB) criteria . Genetic diagnosis of APL was confirmed by reverse-transcriptase–polymerase-chain-reaction (RT-PCR) for the presence of the PML-RARα fusion transcript [23,24] or the presence of t(15;17) via standard karyotyping or fluorescence in-situ hybridisation (FISH) . Additional inclusion criteria were a serum total bilirubin concentration of up to three times the maximum institutional upper limit of normal (ULN) and a serum creatinine concentration of up to 2.5 times the maximum ULN. Exclusion criteria were pregnancy, lactation, concomitant severe psychiatric disorder, significant arrhythmias, and other active malignancies.
This study was approved by the Ethics Committee of The First Affiliated Hospital of Xi'an Jiaotong University in Xi'an, China (Approval no. 2015‑012). The patients were enrolled from July 2015 to January 2021. Written informed consent was obtained from all patients before study entry. The trial was registered with the Chinese Clinical Trial Registry (ChiCTR-IPR-15006821) and conducted in accordance with the Declaration of Helsinki.
The treatment schedules for the ATRA-ATO and ATRA-ATO plus CHT regimens have been published in detail [21,26]. All patients were assigned to receive ATRA-ATO for induction, consolidation and maintenance or ATRA-ATO plus CHT induction therapy followed by three cycles of consolidation therapy with ATRA-ATO plus CHT, and maintenance therapy with ATRA-ATO. Synchronous administration of mannitol and ATO was to increase the permeability of the BBB, thereby improving the ATO concentration in the CSF and preventing CNSL in ATRA-ATO treated high-risk patients [21,26]. In induction therapy, a bone marrow examination to assess the response was performed when the absolute neutrophil count was more than 1.0 × 109/L and the platelet count was more than 100 × 109/L. Once in CR, the disease monitoring and management of minimal residual disease were performed as previously reported .
In this study, ATRA (Shandong Longfine Pharmaceutical Co., Ltd, China) was given at 60 mg/d (20-45 mg/m2/d) in divided doses since previous studies from the Hospital Saint Louis (French)  and the Shanghai Institute of Hematology (China)  reported that the lower ATRA dose had the same therapeutic effects as the conventional dose of 45 mg/m2/d. In previous trials, ATRA was used for 2 weeks every 4 weeks, and ATO was used for 4 weeks every 8 weeks in post-remission treatment [8,29]. To simplify the treatment strategy, both ATRA and ATO were administered for 2 weeks every 4 weeks in this study.
Supportive measures and management of complications
All patients received blood product transfusions to maintain platelet counts above 30 × 109/L and serum fibrinogen above 1.5 g/L. Hydroxyurea was the only cytotoxic agent allowed to control hyperleukocytosis in the non-CHT group. Patients received hydroxyurea at 1-4 g/d according to the initial study design, but after 2018, hydroxyurea was given at 4 g/d when WBC count > 10× 109/L . The maximum hydroxyurea dose could be up to 0.1 g/kg/d in high-risk patients if WBC count consistently increased.
No prophylaxis for differentiation syndrome (DS) was recommended, but dexamethasone was administered at a dose of 10 mg every 12 h intravenously at the earliest manifestations of suspected DS until the disappearance of signs and symptoms. Temporary discontinuation and dose adjustments were recommended to manage drug-related hematologic and nonhematologic toxicities.
Definitions and study end points
Patients were risk stratified based on the WBC count at diagnosis using a modified classification into a non-high-risk group (WBC < 10 × 109/L) and a high-risk group (WBC ≥ 10 × 109/L) . Leukocytosis was defined as a WBC count of more than 10 × 109/L during induction therapy.
The primary end point was event-free survival (EFS) and disease-free survival (DFS) at 2 years. EFS was defined as the time from diagnosis to first event, including death during inducting therapy, failure to achieve remission, relapse at any site, death during remission, or the development of second malignant neoplasm. DFS was defined as the time from Hematological complete remission to either hematological or molecular relapse or death from APL. The secondary end points included complete remission (CR), and overall survival (OS) at 2 years. Hematological complete remission (HCR) and relapse were defined according to criteria described by the National Cancer Institute (NCI) workshop . Molecular complete remission (MCR) was defined as the absence of detectable transcripts, and molecular relapse was defined as the reversion to positivity in two consecutive bone marrow samples performed at least 2 weeks apart . OS was defined as the time from diagnosis to death. Treatment-related toxicities were graded according to the NCI Common Terminology Criteria for Adverse Events, version 4.0 (CTCAE4). The monitoring of hematological and nonhematological adverse events details was performed as previously reported .
This trial was designed to test non-inferiority of DFS in the non-CHT group compared with the CHT group at 2 years after start of induction therapy. We chose 10% as the non-inferiority margin, a type I error probability of 5%, and a power of 80%. All patients enrolled in the study were analyzed following an intent-to-treat principle. Patient characteristics were summarized by crosstab. Chi‑square test or Fisher's exact test were used for comparison of categorical variables, and t test or Mann-Whitney U test for continuous variables. The survival distributions were estimated by the Kaplan-Meier method and compared with the use of log-rank test between the two groups. All statistical tests were two-sided with a significance level of 0.05, except the non-inferiority hypothesis. The data were analyzed using SPSS software (version 23.0) and R software (version 4.0.4).