To the editor,

Acute promyelocytic leukemia (APL) has now become a highly curable disease with all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) combined treatment. [1, 2] For low-risk patients, defined by a presenting white blood cell (WBC) count ≤ 10 × 109/L, a survival over 95% could be achieved using only ATRA and ATO without chemotherapy in most recent studies. [3,4,5,6,7,8,9,10]

For high-risk APL patients (WBC count > 10 × 109/L), two randomized controlled trial (RCT) studies have shown that ATRA + ATO + gemtuzumab ozogamicin/anthracycline as consolidation treatment had a high cure rate and possibly lower relapse rate than ATRA + chemotherapy. [9, 11] One RCT is ongoing to compare ATRA plus ATO vs ATRA plus chemotherapy as consolidation treatment in Europe (registered at www.clinicaltrials.gov as #NCT02688140), and the results have not been reported yet. To further simply the post-remission treatment, we design a protocol using oral arsenic and ATRA without chemotherapy as a first-line consolidation therapy and no maintenance treatment. In our pilot study, the 3-year estimated overall survival (OS) and event-free survival were 100% and 89.4% in 20 high-risk APL patients. [12]

From May 2019, we conducted a multicenter, single-arm, phase 2 study to assess the efficacy and safety of Realgar–Indigo naturalis formula (RIF) plus ATRA as consolidation therapy under an outpatient model for high-risk APL. This trial is ongoing and is registered with Chinese Clinical Trial Registry, ChiCTR1900023309. The protocol and detailed scheme are shown in supplemental material (Protocol, Additional file 1: Figure S1).

Fifty-four eligible patients were enrolled, and an intention-to-treat (ITT) analysis was performed. Excluding 15 patients who did not complete the entire consolidation therapy, 39 patients were included in the per-protocol (PP) analysis (Additional file 1: Figure S2). The clinical characteristics of patients in ITT and PP analysis are shown in Additional file 1: Table S1. There were 28 males and 26 females, and the median age was 40 years old (17–77 years). The median PML-RARA at enrollment was 0.19% (0–99%). Thirteen patients (24%) received ATRA + ATO/RIF without chemotherapy, and 41 patients (76%) received ATRA + ATO/RIF + chemotherapy as induction regimen. Chemotherapy drugs included anthracycline, cytarabine, and homoharringtonine. RIF and ATO were used in 42 (77.8%) and 12 (22.2%) patients in induction therapy, respectively. Five patients had cerebral hemorrhage in induction therapy, and one of them had brain surgery (Additional file 1: Table S2).

With a median follow-up of 13.8 months, the primary end point has been met as estimated 2-year disease-free survival (DFS) was 93.8% (95% CI: 76.9–98.4%) in ITT analysis. The estimated 2-year OS was 100% in ITT analysis. Estimated 2-year DFS was 93.1% (95% CI: 74.8–98.3%), and estimated 2-year OS was 100% in PP analysis (Fig. 1). These were consistent with the data from clinical trials which just use ATRA and ATO as consolidation therapy, with a DFS of 89% (5 years, n = 52), [3] 89.4% (3 years, n = 20), [12] and 96.4% (2 years, n = 56, Pediatric), [10], respectively. In addition, the survival was also comparable with the data from clinical trials which use ATRA + ATO + chemotherapy as consolidation therapy, with a DFS of 92% (2 years, n = 20), [7] 90.4% (3 years, n = 235), [11].

Fig. 1
figure 1

Survival among patients in ITT analysis and PP analysis. A DFS in ITT analysis; B DFS in PP analysis; C OS in ITT analysis; D OS in PP analysis; E Cumulative incidence of CMR from the beginning of the consolidation therapy in ITT analysis; and F Cumulative incidence of CMR in PP analysis. The ITT population comprised patients who enrolled and received at least one dose of study medication. The PP population comprised patients of a subgroup of ITT population who had completed the trial and underwent an end-of-therapy evaluation. Abbreviations: ITT: intention-to-treat; PP: per-protocol; DFS: disease-free survival; OS: overall survival; and CMR: complete molecular remission

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Except for 3 patients who were receiving consolidation therapy, the remaining 51 patients all achieved complete molecular remission (CMR), and nearly 90% of them achieved CMR at 4 months from the beginning of induction therapy (Fig. 1). The results of PML-RARA fusion gene test are shown in Additional file 1: Figure S3. Totally, only two patients occurred relapses with a cumulative incidence of relapse at 2 years of 6.2%. Detailed information of these two patients is shown in Additional file 1: Table S3.

Of the 52 participants who completed at least one cycle of consolidation therapy, 48 completed an adverse event survey. No grade 4–5 adverse events were observed. The most common non-hemopoietic toxic effects were headaches (15%), arthralgia and myalgia (19%), dry (19%), darkened skin (10%), and alopecia (10%). Adverse events such as infection, renal dysfunction, and cardiac toxicity were not observed. The majority of adverse events were grade 1–2, and only three grade 3 adverse events were observed. No patient died in the consolidation phase (Table 1).

Table 1 Adverse events
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In summary, this trial found that adult patients with high-risk APL could be successfully and safely treated with oral arsenic plus ATRA as consolidation therapy. A completely oral and chemo-free post-remission treatment without maintenance under an outpatient model will benefit more APL patients.