A comprehensive review of acute promyelocytic leukemia in children

The outcome of patients with acute promyelocytic leukemia (APL) has substantially improved since the successful introduction of tretinoin, and nowadays combining tretinoin with chemotherapy is potentially curative for at least 70-75% of patients with newly diagnosed APL. In most pediatric series, APL represents < or = 10% of childhood acute myelogenous leukemia. APL in children is more common in girls and in obese children. It is characterized by a higher incidence of hyperleukocytosis, an increased incidence of microgranular morphology and by more frequent occurrence of the PML/RARalpha isoforms bcr 2 and bcr 3 compared to adults. Tretinoin-based therapy is curative for the majority of children with APL. Recent data indicate that > or = 2 negative RT-PCR assays for PML/RARalpha on bone marrow performed at least 1 month apart after completing therapy are strongly associated with long-term remissions, while conversion to PCR positivity for PML/RARalpha during remission is highly predictive of impending relapse. Data from recent studies in adults and limited data from children show that arsenic trioxide is the single most effective agent in APL and deserves immediate study in newly diagnosed children in an effort to further improve prognosis and to limit exposure to conventional cytotoxic chemotherapy.     

The recent JALSG study for newly diagnosed patients with acute promyelocytic leukemia (APL)

Based on the prognostic factors obtained from our previous APL92 study, in the JALSG APL97 study, we intensified chemotherapy for patients with leukocyte counts > or = 3,000/microL and > or = 10,000/microL, also intensified consolidation chemotherapy, and then tested whether further chemotherapy is required in patients with negative RT-PCR for PML/RARalpha after the completion of consolidation therapy. Of 256 presently evaluable patients, 244 (95%) achieved CR. Predicted 5-year EFS is 67% and predicted 5-year overall survival 84%.     

Long-term follow up of re-induction with a new synthetic retinoid, Am-80, for relapse of acute promyelocytic leukemia previously treated with all-trans retinoic acid: results of 7 cases from a single institute

Seven patients experiencing first (n = 5) or second (n = 2) relapse of acute promyelocytic leukemia (APL) were treated with a new synthetic retinoid, Am-80. All 7 patients were previously treated with all-trans retinoic acid (ATRA). Am-80 was orally administered at a dose of 6 mg/m2 daily. Chemotherapy was combined in 3 patients because of leukocytosis. All 7 patients achieved a complete remission (CR) during periods ranging from 36-56 days (median 52 days). Adverse effects such as hyperlipidemia and skin lesions, were tolerable. After achieving CR, 3 patients underwent allogeneic bone marrow transplantation and 4 patients received only consolidation chemotherapy. In 2 of 3 patients who received allogeneic transplantation, relapse free survival has lasted for 9.7 and 28.3 months. Furthermore, in 2 of 4 patients who received only chemotherapy, relapse free survival has lasted for 84.7 and 90.1 months. Am-80 is an active agent for APL patients who have relapsed from ATRA-induced remission.     

Two Patients with All -trans Retinoic Acid-Resistant Acute Promyelocytic Leukemia Treated Successfully with Gemtuzumab Ozogamicin as a Single Agent

Acute promyelocytic leukemia (APL) cells express a considerable level of CD33, which is the target of gemtuzumab ozogamicin (GO), and a significantly lower level of P-glycoprotein (P-gp). Therefore, GO is predicted to be a successful treatment for APL. In this article, we report on the GO treatment of 2 patients with APL, who had fully relapsed after induction therapy with all-trans retinoic acid (ATRA) following chemotherapy. Both patients had relapsed 3 times and were resistant to reinduction therapy with ATRA. GO (9 mg/m2) was administered on days 1 and 15. After GO treatment, both patients achieved complete hematologic and molecular remission. GO may be another promising agent for the treatment of ATRA-resistant relapsed APL when given as salvage chemotherapy.     

Alterations in tretinoin pharmacokinetics following administration of liposomal all-trans retinoic acid

We administered liposome-encapsulated all-trans retinoic acid (L-ATRA) to 48 patients with refractory hematologic malignancies using an every- other-day schedule for 28 days and doses of 15 to 175 mg/m2. In 19 patients, pharmacology studies were conducted after the first (day 1) and seventh (day 15) doses. In contrast to the decline in tretinoin concentration seen within 3 to 4 days of administration of daily oral ATRA, there were no differences between the area under the curve (AUC) of tretinoin concentration versus time on day 1 and day 15 (P = .98, Wilcoxon signed-rank test). Peak day 1 concentrations after 15 mg/m2 were higher than those reported after 45 mg/m2 oral ATRA. Six patients with relapsed acute promyelocytic leukemia (APL) were treated. Three, each in first relapse and at least year from the last exposure to oral ATRA, achieved a complete response (CR). Disease recurred in two (one at 3 months despite maintenance L-ATRA and similarity in tretinoin AUC on days 1 and 85, and the other at 5 months, 2 months after discontinuation of L-ATRA) and the third was transplanted 1 month into CR. The three nonresponders were in at least a second relapse and failed to respond to oral ATRA before or immediately after receiving L- ATRA. Severe toxicity developed in three of eight patients treated at 175 mg/m2 (joint pains in two, skin in one). The maximum tolerated dose (MTD) was determined to be 140 mg/m2, at which dose grade 2 toxicity (primarily headache and skin) occurred in eight of eight patients, but grade 3 to 4 toxicity in none. Compared with oral ATRA, L-ATRA apparently results in greater exposure to tretinoin and for a longer time.     

Prolonged treatment with arsenic trioxide (ATO) and all-trans-retinoic acid (ATRA) for relapsed acute promyelocytic leukemia previously treated with ATRA and chemotherapy

Prolonged therapy with all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO) is highly effective in newly diagnosed acute promyelocytic leukemia (APL) but there is limited data on the efficacy of this regimen in the relapse setting. We report here on 22 APL patients treated with prolonged ATRA-ATO therapy at the time of disease relapse. Twenty patients obtained molecular complete remission (CRm) after 2 cycles (90%). Of these, two patients underwent hematopoietic stem cell transplant (HSCT) while the remaining proceeded to receive additional cycles (up to a total of 5) of ATRA-ATO. With a median follow-up of 58 months from the time of relapse (range: 21–128 months), the 4-year OS probability was 0.85 (95% CI 0.61–0.94), DFS was 0.74 (95% CI 0.49–0.88), and EFS 0.68 (95% CI 0.45–0.83). Two patients were resistant to ATRA-ATO salvage and five relapsed at a median of 19 months. Of these, four died due to progressive disease while three relapsed achieved a new CRm after further salvage therapy. This experience confirms the potentially curative effect of prolonged ATRA-ATO therapy in relapsed APL, especially in patients with long first complete remission.     

Arsenic trioxide therapy for relapsed acute promyelocytic leukemia: a bridge to transplantation

BACKGROUND AND OBJECTIVES: Arsenic trioxide (ATO) has been reported to be a safe and effective treatment for relapsed acute promyelocytic leukemia (APL). The aim of this study was to evaluate the efficacy and toxicity as well as the eligibility to stem cell transplantation (SCT) in a series of 7 patients with relapsing APL, managed with ATO. DESIGN AND METHODS: Seven patients with relapsing APL while on maintenance treatment with all-trans-retinoic acid (ATRA) or who were ATRA refractory-received ATO at a dose of 10 mg daily by 2-hour intravenous infusion until complete remission (CR). After consolidation chemotherapy, patients were programmed to receive autologous or allogeneic stem cell transplantation (SCT) according to donor availability. The median age of the patients was 55 (21-71) years: 2 patients presented with concomitant extramedullary relapse. RESULTS: Six patients (86%) achieved CR after a median of 35 ATO doses (20-49) with negligible toxicity; one patient died from pneumonia. After consolidation with a four-day course of cytarabine at 1 g/m2 and mitoxantrone 6 mg/m2, two patients underwent allogeneic SCT, two received PML/RARa negative autologous peripheral blood stem cells collected after consolidation plus granulocyte colony-stimulating factor, one failed mobilization and received a second consolidation course. One elderly patient refused further treatment and relapsed 6 months later. After a median follow-up of 15 months from CR2 achievement, 5 patients are alive in continuous CR. INTERPRETATION AND CONCLUSIONS: The high CR rate and the mild toxicity confirm that ATO represents a valid alternative to salvage chemotherapy for patients relapsing while on ATRA treatment or who are ATRA-refractory. Allogeneic or autologous SCT after ATO-induced CR is feasible in the majority of patients.     

Constitutive degradation of PML/RARalpha through the proteasome pathway mediates retinoic acid resistance

PML/RARalpha is the leukemogenetic protein of acute promyelocytic leukemia (APL). Treatment with retinoic acid (RA) induces degradation of PML/RARalpha, differentiation of leukaemic blasts, and disease remission. However, RA resistance arises during RA treatment of APL patients. To investigate the phenomenon of RA resistance in APL, we generated RA-resistant sublines from APL-derived NB4 cells. The NB4.007/6 RA-resistant subline does not express the PML/RARalpha protein, although its mRNA is detectable at levels comparable to those of the parental cell line. In vitro degradation assays showed that the half-life of PML/RARalpha is less than 30 minutes in NB4.007/6 and longer than 3 hours in NB4. Treatment of NB4.007/6 cells with the proteasome inhibitors LLnL and lactacystin partially restored PML/RARalpha protein expression and resulted in a partial release of the RA-resistant phenotype. Similarly, forced expression of PML/RARalpha, but not RARalpha, into the NB4/007.6 cells restored sensitivity to RA treatment to levels comparable to those of the NB4 cells. These results indicate that constitutive degradation of PML/RARalpha protein may lead to RA resistance and that PML/RARalpha expression is crucial to convey RA sensitivity to APL cells.     

Identification of the t(15;17) in AML FAB types other than M3: evaluation of the role of molecular screening for the PML/RARalpha rearrangement in newly diagnosed AML. The Medical Research Council (MRC) Adult Leukaemia Working Party

Acute promyelocytic leukaemia (APL) is characterized by the t(15;17) leading to the formation of PML-RARalpha and RARalpha-PML fusion genes; this rearrangement has been considered both diagnostic for, and restricted to, this subtype of acute myeloid leukaemia (AML FAB M3). We describe two cases of AML with the t(15;17) associated with a PML/RARalpha rearrangement which lacked typical APL morphology, classified as FAB M1 and M2 respectively. In both cases morphological review revealed small populations of cells which exhibited some features associated with APL. In the case classified as M1, PML immunofluorescence studies revealed the classic microparticulate nuclear staining pattern as observed in typical cases of APL with the t(15;17). Similarly, blasts from this case were found to be sensitive to ATRA in vitro as determined by NBT reduction test and by normalization of the PML nuclear body staining pattern. To determine the frequency of PML/RARalpha rearrangements in FAB subtypes other than M3, 530 patients from the MRC AML trials were screened using nested RT-PCR. Only one individual, initially classified as M5 with a normal karyotype, was found to have a PML/RARalpha rearrangement. The diagnosis was revised to M3 variant on subsequent morphological review. In conclusion, this study demonstrates that, in rare cases, the t(15;17) is not restricted to patients with M3 morphology as defined by current FAB criteria. Therefore, although we consider cytogenetic analysis of newly diagnosed cases of AML to be mandatory, our data suggests that routine molecular screening for PML/RARalpha rearrangements is not justified and should be reserved for those cases displaying features which may be suspicious of APL even if such cells comprise only a minority of the total population.     

Novel mutation in the PML/RARalpha chimeric gene exhibits dramatically decreased ligand-binding activity and confers acquired resistance to retinoic acid in acute promyelocytic leukemia.

OBJECTIVE: All-trans retinoic acid (RA) resistance in acute promyelocytic leukemia (APL) has been a serious clinical problem in differentiation-inducing therapy. However, the mechanisms underlying acquired RA resistance in APL patients are not well understood. MATERIALS AND METHODS: We recently established a spontaneous RA-resistant APL cell line (UF-1) from a patient and used this cell line as an excellent in vitro model for RA-resistant clinical situations. We investigated the structural and functional abnormalities of chimeric PML/RARalpha gene in UF-1 cells and preserved materials from the original patient. RESULTS: A novel point mutation was detected in the ligand-binding (E) domain of the RARalpha portion of the PML/RARalpha gene in UF-1 cells. This mutation resulted in amino acid substitution of Arg611 (CGG) for Trp611 (TGG) in the short-form PML/RARalpha protein, which corresponded to Arg276 in wild-type RARalpha. Importantly, the same mutation was also detected in the preserved materials from the original patient. COS-1 cells were transiently transfected with cDNA encoding wild-type and mutant PML/RARalpha constructed by site-directed mutagenesis and performed RA-binding assay. Interestingly, RA-binding activity was dramatically decreased in the mutant PML/RARalpha compared with that of the wild-type chimeric protein, suggesting that this single amino acid substitution is critical for RA binding. CONCLUSIONS: These results strongly suggest that a novel point mutation in the ligand-binding domain of the RARalpha portion (Arg611) of the chimeric PML/RARalpha gene decreased sensitivity to all-trans RA. We conclude that acquisition of the PML/RARalpha mutation is one possible mechanism for development of RA resistance in patients with APL in vivo.