Treatment concepts of acute promyelocytic leukemia

Transcription

1 Critical Reviews in Oncology/Hematology 56 (2005) Treatment concepts of acute promyelocytic leukemia Eva Lengfelder a,, Susanne Saussele a, Andreas Weisser a, Thomas Büchner b, Rüdiger Hehlmann a a III. Medizinische Universitätsklinik, Klinikum Mannheim, Fakultät für Klinische Medizin Mannheim der Universität Heidelberg, Wiesbadener Str. 7-11, Mannheim, Germany b Medizinische Universitätsklinik A, Universtät Münster, Germany Accepted 6 August 2004 Contents 1. Introduction Disease description Treatment options in APL Chemotherapy Retinoids Arsenic compounds Allogeneic or autologous stem cell transplantation Experimental treatment approaches Results with chemotherapy in the pre-atra era Results with ATRA and chemotherapy Current treatment strategies ATRA in combination with standard dose chemotherapy Treatment intensification with high dose ara-c in combination with ATRA The role of maintenance therapy Prevention and management of problems during treatment with ATRA ATRA syndrome Bleeding and thrombosis Resistance against ATRA Prognostic factors and risk of relapse Prognostic significance of molecular monitoring of MRD Results with arsenic trioxide The role of stem cell transplantation Monoclonal antibodies Conclusions Reviewers References Biography Abstract In the past, acute promyelocytic leukemia (APL) was associated with a high risk of early mortality resulting from severe coagulopathy, frequently inducing fatal cerebral hemorrhage. With the introduction of the differentiating agent all-trans retinioc acid (ATRA) APL has changed to the best curable subtype of acute myeloid leukemia (AML). With ATRA and chemotherapy approximately 70 80% of patients with newly diagnosed APL achieve long-term remission and are probably cured. PML/RAR, the molecular fusion transcript of the specific translocation t(15;17) represents not only the target for ATRA but also permits a precise diagnosis and provides a marker for the identification Corresponding author. Tel.: ; fax: address: eva.lengfelder@med3.ma.uni-heidelberg.de (E. Lengfelder) /$ see front matter 2005 Elsevier Ireland Ltd. All rights reserved. doi: /j.critrevonc

2 262 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) of minimal residual or recurrent disease (MRD). During the last decade, substantial progress has been made with regard to the recognition of prognostic factors and the optimization of the combination of ATRA and chemotherapy. Remaining questions are the role of arsenic and of ara-c in first line therapy of APL as well as the indication of maintenance therapy in the individual patient. Several treatment options exist for patients with APL who have relapsed after ATRA and chemotherapy. Approximately 50% of the patients in first relapse can achieve long-lasting second remission and might be cured with salvage regimens. Currently, arsenic compounds and transplantation procedures seem to be the most promising options in relapsed disease. The role of CD33 antibodies has to be determined in future studies. Refining the molecular monitoring of MRD by quantitative RT-PCR, better elucidation of the biologic mechanisms, and the identification of prognostic factors might be helpful to make further progress in the treatment of APL Elsevier Ireland Ltd. All rights reserved. Keywords: Acute promyelocytic leukemia; All-trans retinoic acid; Anthracyclines; Cytosine arabinoside; Arsenic trioxide; Maintenance therapy; Prognostic factors 1. Introduction The treatment of acute promyelocytic leukemia (APL) with the differentiating agent all-trans retinoic acid (ATRA) represents the first paradigm of a specific therapy targeted against the molecular pathomechanism in treatment of acute leukemia. APL was once characterized by a high early death rate due to fatal bleeding. With the advent of ATRA, APL has now become the best curable subtype of acute myeloid leukemia (AML). The introduction of this novel treatment approach into clinical practice was possible for several reasons. First, APL is characterized by a unique genetic feature, i.e. the translocation t(15;17) and its molecular correlate PML/RAR which represents a specific target for therapy [1,2]. Second, the retinoids (Vitamin A derivatives) have been known to induce remission by induction of differentiation in sporadic cases of APL [3,4]. Third, ATRA was shown by in vitro experiments to have a considerably higher differentiating potential in APL blasts than other retinoids [5]. This corresponds to a high rate of clinical remissions and improvement of prognosis [6]. Furthermore, the specific fusion transcripts PML/RAR and RAR /PML permit not only a precise diagnosis but also provide markers for the identification of residual or recurrent disease [7,8]. This might allow further improvement of the therapy by the development of risk-adapted strategies. It is the aim of this review to give an overview on the current state of the art in the treatment of APL on the basis of established therapeutic concepts and to address further development and progress. 2. Disease description APL is a rare disease accounting for approximately 10% of AML. Mean age at diagnosis is approximately 40 years, which is considerably younger as compared to other subtypes of AML. The male to female ratio is balanced [9]. There appears to be an increased incidence among Hispanic patients (20 30%) of AML [10]. The disease was formerly associated with a high risk of early mortality before the onset of therapy or in the early treatment phase resulting from severe coagulopathy, frequently inducing fatal cerebral hemorrhage [9]. The process of leukemic transformation in APL is accompanied by a block in differentiation such that the marrow becomes replaced by abnormal promyelocytes (APL blasts) which are characterized by a typical morphology. Two morphological types of APL are distinguished, the hypergranular (or typical) form (AML FAB M3) and the microgranular (or hypogranular) variant (AML FAB M3v). The latter accounts for approximately 25% of APL cases. The hypergranular form is mostly associated with abnormally low WBC counts. In microgranular APL, the leukocyte count is high (often >10,000/ l) with a rapid doubling time [11,12]. The morphological diagnosis is confirmed by the APL specific translocation t(15;17) and its molecular correlates PML/RAR and RAR /PML [13,14]. Whereas PML/RAR is expressed regularly, the reciprocal transcript RAR /PML can be detected only in about 75% of patients [15]. Chromosomal abnormalities in addition to the translocation t(15;17) were detected in 31% of patients in a large series. The most frequent additional abnormality was trisomy 8 in 16% of patients [16]. Immunofluorescence using polyclonal or monoclonal PML antisera allows to confirm diagnosis of PML/RAR positive APL within a few hours through detection of a characteristic microparticulate nuclear staining pattern [17,18]. 3. Treatment options in APL An overview on the currently used treatment forms and their curative potential in newly diagnosed and relapsed APL is given in Table Chemotherapy Long-lasting remissions of APL were first achieved with the introduction of daunorubicin [19]. Patients with APL generally were included in treatment protocols for newly diagnosed AML consisting of daunorubicin and cytosine arabinoside (ara-c) in standard dose ( mg/(m 2 day) for 5 10 days). In several studies, high dose (HD)

3 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) Table 1 Treatment modalities in APL Treatment Induction of long-term remission Curative potential First line therapy Effectiveness in relapse Chemotherapy (anthracyclines and ara-c) Yes Yes Yes Yes Retinoids No No Yes Limited Arsenic compounds Yes Unknown To be investigated Yes Autologous stem cell transplantation Yes Probably No Yes Allogeneic stem cell transplantation Yes Yes No Yes Monoclonal antibodies (anti-cd33) To be investigated To be investigated To be investigated Yes (limited experience) ara-c ( mg/(m 2 12 h) for 3 6 days) was administered for induction or postremission therapy showing high antileukemic activity also in cases refractory against ara-c in standard dose [20,21]. Further treatment combinations given in AML as well as in APL include idarubicin, mitoxantrone, amsacrine, etoposide, methotrexate, 6-mercaptopurine and 6- thioguanine. As compared to anthraycyline/ara-c based regimens no benefit of these drugs has been found in APL [22] Retinoids In the past, 13-cis-retinoic acid has been known to induce remission by induction of differentiation in sporadic cases of APL [3,4]. First results with an other Vitamin A derivative, all-trans retinoic, acid was reported from China in 1988 [6]. By in vitro experiments ATRA was shown to have a 10-fold higher differentiating activity on APL blasts than the 13-cis-isomer [5]. ATRA-monotherapy (45 mg/(m 2 day)) induces complete hematological remission in over 90% of patients with newly diagnosed APL in approximately 4 5 weeks [6,23]. As ATRA alone cannot induce stable remissions, the combination with chemotherapy is necessary. The effectiveness of ATRA in relapsed APL is uncertain, due to the development of resistance in most cases [24]. Some years ago, the mode of action was partly elucidated. ATRA induces complete remission in APL by differentiation of the abnormal promyelocytes. The RAR -gene on chromosome 17 as well as the PML-gene on chromosome 15 play a role in cell differentiation. Due to the binding of corepressors on the pathologic fusion transcript PML/RAR, the translation of the DNA strand is blocked leading to a differentiation block and to an uncontrolled proliferation of the promyelocytic blasts. High doses of ATRA cause the release of the corepressors from the DNA and thereby allow the translation of the DNA enabling myeloid differentiation [25] Arsenic compounds In the past, arsenic compounds have already been used in traditional Chinese and Western medicine for medical purposes including some forms of leukemia. The outlook for patients with relapsed APL has been improved by the recent approval of arsenic trioxide As 2 O 3 (ATO, TRISENOX) for relapsed and refractory APL in the USA and in Europe [26]. With ATO monotherapy (0.15 mg/(kg day) for approximately days), over 80% of the relapsed patients achieved hematological and molecular remission which may persist for several years in single cases [26,27]. There are also some reports which point to a good effectiveness of ATO in first line therapy of APL. But the data are still preliminary due to the small number of patients and to the short follow up [27,28]. The mechanisms of action which could explain the antileukemic efficacy of ATO in APL are not yet completely elucidated. In vitro, ATO exerts a dose dependent dual effect on APL cells. At higher concentrations ( mol/l) preferentially apoptosis is triggered, at lower concentrations ( mol/l) partial differentiation of the leukemic cells is induced. ATO was shown to induce the degradation of the PML/RAR fusion protein, which may contribute to the differentiating process [29,30]. Besides these mechanisms, several other potential intracellular targets for apoptosis are discussed. ATO was also suggested to have antiangiogenic effects in solid tumors and possibly in leukemia [31]. Also, an other arsenic compound, tetra-arsenic tetrasulfide, has proven antileukemic activity in APL [32] Allogeneic or autologous stem cell transplantation Bone marrow (BMT) or peripheral stem cell transplantation (PBSCT) is not recommended for the routine use during the first CR of APL, as the great majority of patients with untreated APL have an excellent outcome with ATRA and chemotherapy. For patients with first or subsequent relapse of APL or those at high risk of relapse, allogeneic or autologous PBSCT or BMT may still provide a chance of cure [33 35] Experimental treatment approaches Further treatment approaches were mostly studied in patients with repeatedly relapsed APL and include treatment with monoclonal anti CD33 antibodies, liposomal ATRA or synthetic forms of ATRA as well as histone deacetylation inhibitors [36 39]. 4. Results with chemotherapy in the pre-atra era Before the introduction of ATRA, the routine therapy of newly diagnosed APL consisted of anthracyclines and standard dose ara-c. With these regimens complete remis-

4 264 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) sion rates of 50 80% were achieved. Failures to achieve CR were mainly due to fatal bleeding. Approximately 25 30% of APL patients reached long-lasting remission and were considered to be cured [22,40]. The high antileukemic efficacy of anthracyclines in APL was already suggested in the first publication in 1973 and was confirmed by the results of subsequent studies [19]. In a retrospective comparison of studies, performed in USA, the results of consecutive treatment protocols with increasing anthracycline doses were analyzed [41]. The results demonstrate that a higher cumulative anthracycline dose is associated with a higher rate of cure. In a prospective Italian study randomizing two different induction regimens, a higher induction dose of idarubicin was compared to a lower idarubicin dose combined with standard dose ara-c. In the treatment arm with intensified anthracycline monotherapy, a trend to a higher CR rate and a significant benefit in terms of event free survival (p = 0.035) was reached [42]. In the AML 86-protocol of the German AMLCG, a double induction therapy including daunorubicin and standard dose ara-c (TAD/TAD) was randomly compared to an intensified modification incorporating HD ara-c and mitoxantrone (TAD/HAM) in all subtypes of AML [43]. This strategy proved highly effective in the subgroup of patients with APL resulting in three and ten year relapse-free survival rates of 59 and 47%, respectively, with a benefit of HD ara-c. Relapse free survival at 9 years was 67% in patients randomized to HD ara-c as part of induction and 21% in those randomized to standard dose ara-c (p = 0.02) [44]. The application of maintenance chemotherapy was associated with a longer relapse free survival in patients with AML as well as with APL in comparison to no maintenance in two randomized studies [43,45]. In summary, these results indicate that a higher intensity and a longer duration of the chemotherapy is associated with a favorable influence on the cure rate. 5. Results with ATRA and chemotherapy 5.1. Current treatment strategies The combination of ATRA with anthracycline and ara-c based chemotherapy was associated with a significant improvement of survival and of remission duration in patients with newly diagnosed APL as compared to chemotherapy alone. This was first shown by historical comparison of APL studies with and without ATRA. The advantage associated with the administration of ATRA mostly results from a significant reduction of the relapse rate. An additional effect is the increase of the rate of complete remissions due to a reduction of the early death rate or of cases who are refractory to the induction chemotherapy [46 48]. Subsequent randomized studies uniformly demonstrate a significant benefit of the combination of ATRA with chemotherapy [49 51]. Based on these results, further trials in newly diagnosed APL were performed to optimize the combination of ATRA and chemotherapy involving three main treatment approaches. First, ATRA was combined with standard induction therapy for AML based on anthracyclines and standard dose ara-c followed by one or two consolidation courses (protocols of the French/European APL Group, USA Intergroup, British MRC) [46,49 53]. Second, in order to reduce the intensity of the induction therapy, ATRA was combined with anthracycline monotherapy followed by three consolidation courses (protocols of the Italian GIMEMA and Spanish PETHEMA) [54,55]. Third, the observation that cure cannot be reached by ATRA alone suggests that the antileukemic efficacy of the chemotherapy has influence on the outcome. For this reason, ATRA is combined with an intensified double induction chemotherapy including HD ara-c in the protocol of the German AMLCG [56]. In Table 2, the chemotherapy combined with ATRA for remission induction and the cumulative anthracycline and ara-c doses of larger studies are summarized [57]. In all listed studies, the cytogenetic or molecular proof of the translocation t(15;17) was mandatory for inclusion. The results are shown in Table 3. In summary, these studies demonstrate that approximately 70 80% of patients with newly diagnosed APL reach long-term remission and are probably cured with ATRA and chemotherapy ATRA in combination with standard dose chemotherapy In the Italian APL protocol (GIMEMA), the induction therapy with idarubicin and ATRA is followed by three consolidation courses combining anthracyclines (idarubicin, Table 2 Induction chemotherapy combined with ATRA and cumulative anthracycline and ara-c doses during induction and consolidation Chemotherapy combined with ATRA for remission induction Daunorubicin equivalent (mg/m 2 ) [51] Ara-C (g/m 2 ) AIDA [54] Idarubicin APL-93 [52] Daunorubicin, ara-c British MRC [53] Various anthracyclines, ara-c (additional substances >500 ca. 11 according to the design of consecutive protocols) PETHEMA [55,60] Idarubicin (LPA96); (LPA99) No ara-c AMLCG (25) [51,61] Daunorubicin, mitoxantrone, ara-c in standard and high dose 6-thioguanine 485 (during maintenance up to 665) 20.8 Equivalent dose of anthracyclines according to EORTC: daunorubicin 50 mg, mitoxantrone 12 mg and idarubicin 10 mg [51].

5 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) Table 3 Results with ATRA and chemotherapy Study (year) [Reference] No. of evaluable patients Age (years) CR after induction (%) OS/EFS at 4 years (%) Relapse at 4 years (%) Median follow up (inclusion of patients) a Hem. Mol. AIDA (1997) [54] b /79 b n.r. 12 months (7/1993 2/1996) APL-93 (1999 updated) [52] n.r. n.r. (1/1993 1/2000) 208 (WBC < 5000/ l) 122 ATRA CT /74 23 * 185 ATRA + CT /80 15 * 55 (WBC < 5000/ l) /60 9 * 214 (WBC > 5000/ l) /60 39 * British MRC (1999) [53] 239 n.r. n.r. (1/1993 1/1997) 119 (ATRA short) /n.r. 36 * 120 (ATRA until hem. CR) /n.r. 20 * PETHEMA (2003) [60] 426 (LPA96 and LPA99) n.r. (11/1996 4/2003) 83 low risk (LPA96 and n.r. 6.4 #,c n.r. (11/1996 4/2003) LPA99) 201 intermediate and 86 d /n.r. 8.7 #,d n.r. (10/1999 4/2003) high risk (LPA99) AMLCG (2003) [61] /75 9 * (11 # ) 4 years (12/1994 5/2003) 101 patients <60 years /81 6 * (9 # ) 32 patients >60 years /54 17 * (17 # ) CT: chemotherapy; CR: complete remission; hem.: hematological; mol.: molecular; OS/EFS: overall, event free survival; WBC: initial white blood cell count; n.r.: not reported. a Month/year. b At 2 years. c At 6 years. d At 3 years. * Probability of hematological relapse. # Probability of hematological and molecular relapse. mitoxantrone) with other cytostatics (etoposide, ara-c, 6-thioguanine) [54,58]. In the Spanish LPA96 study (PETHEMA), the identical dosage and time schedule of the anthracyclines and ATRA as in the Italian protocol is used, however all other chemotherapy are omitted [55]. The retrospective comparison of the data of both studies revealed that with ATRA and anthracycline monotherapy similar results were achieved as with the combination chemotherapy [59]. This has opened the discussion on the benefit of standard dose ara-c and other chemotherapy besides anthracyclines in combination with ATRA. Therefore, the French/European APL group is currently conducting a prospective trial in which patients with WBC count below 10,000/ l are randomly assigned to ATRA plus daunorubicin or ATRA plus daunorubicin and ara-c. Furthermore, in the joint study of the PETHEMA and GIMEMA, a subgroup of APL patients with a particularly favorable prognosis could be identified (white blood cell count < 10,000/ l and platelet count > 40,000/ l) [59]. The recently published results of the subsequent LPA99 study of the PETHEMA showed that a longer overall and disease free survival could be achieved in the poorer risk groups (intermediate and high risk) with an increased idarubicin dose and ATRA extended to the consolidation cycles [60]. In the French/European APL-93 study, the timing of ATRA and chemotherapy during induction therapy was investigated. By randomized comparison, ATRA and simultaneous chemotherapy proved significantly superior to ATRA followed by chemotherapy with regard to relapse rate [52]. In a randomized British multicenter trial (British MRC), two modifications of the application of ATRA were compared. Extended treatment with ATRA commencing with chemotherapy and continuing until CR was superior to a short course of ATRA (5 days) before the start of chemotherapy. With the extended duration of ATRA treatment, a higher CR rate and a lower relapse rate as well as a longer overall survival were achieved in patients with lower initial WBC counts. In patients with initially higher WBC counts, the modification of ATRA therapy had no prognostic influence [53] Treatment intensification with high dose ara-c in combination with ATRA In the treatment protocol of the German AMLCG for newly diagnosed APL, the induction chemotherapy combined with ATRA is intensified. A double induction therapy (TAD/HAM) incorporating HD ara-c is combined with ATRA. The second induction cycle is scheduled independent of the degree of cytopenia on day 21 after the first cycle. This is followed by consolidation and a 3 years cyclic monthly maintenance chemotherapy. In patients over 60 years, a second cycle with a reduced ara-c dose is only given if no

6 266 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) complete hematological remission is reached after the first cycle. This strategy includes the application of a substantially higher ara-c dose than in all other treatment protocols, whereas the anthracycline dose is comparable (Table 2). A recent update of this study with a median follow up of four years shows that a low relapse rate was reached in all risk groups of APL including patients with initially higher WBC counts and older patients (Table 3) [56,61]. However, the treatment intensification also includes the risks of increased additional toxicity and myelosuppression The role of maintenance therapy The possible benefit of maintenance therapy was investigated in two randomized studies. In the North American intergroup protocol, patients treated with ATRA maintenance therapy had an advantage in terms of relapse free survival as compared to no maintenance therapy [49,50]. In the French/European APL-93 protocol, patients were randomized into four arms of postconsolidation therapy: maintenance therapy with ATRA, chemotherapy (methotrexate and 6-mercaptopurine), combination of ATRA and chemotherapy, or no maintenance. Patients assigned to maintenance therapy (ATRA, chemotherapy or the combination of both) had a significantly lower relapse rate than without maintenance. Maintenance with chemotherapy but not with ATRA alone significantly improved the 5-year survival. In this study, no data of the PCR status after consolidation were reported [52,62]. In a recent Italian trial, patients were randomized to the identical maintenance arms as in the French/European protocol. Patients who were PCR-negative after consolidation did not benefit from any maintenance therapy in terms of disease free survival. The results of the Italian study raise the question whether maintenance is indicated in every patient with APL [63] Prevention and management of problems during treatment with ATRA ATRA syndrome The most relevant toxicity of ATRA is the ATRA syndrome, mainly manifested by fever, weight gain, respiratory distress, interstitial pulmonary infiltrates, pleural and pericardial effusion, hypotension and acute renal failure [64]. The diagnosis of ATRA syndrome is based on clinical symptoms which may occur solitary or in combination. Hence, a certain differentiation between the syndrome and complications of chemotherapy as congestive heart failure, pneumonia and sepsis may be difficult in the individual case. The syndrome is often associated with initially higher or rapidly increasing WBC counts or with the M3 variant, but it is also observed in patients with low WBC counts [65]. By now, there are no factors which are clearly predictive of the syndrome. The pathomechanism of the ATRA syndrome is not yet completely understood. Factors which might play a role are an altered expression of adhesion molecules on the surface of the differentiating APL blasts and of the endothelial cells which may facilitate the migration of the leukemic cells followed by damage of the tissue [66,67]. Possibly, also other mechanisms, e.g. the release of vasoactive substances and cytokines from the blast cells may contribute to the ATRA syndrome resembling a capillary leakage syndrome [64]. Although the manifestation of the syndrome may be influenced by several factors, the high range 4 26% reflects the lack of certain diagnostic criteria and of clearly established recommendations for prophylaxis and treatment [68,69]. Measures that reduce frequency and high mortality of the syndrome are the concurrent application of chemotherapy and high doses of corticosteroids at the first sign of the syndrome [64]. Prophylactic application of steroids has been tried, but the benefit has not been proven in any randomized study so far [70]. A number of reports have emerged suggesting that the development of the syndrome may be associated with a worse outcome. A detailed evaluation of patients included in the French APL 93-protocol showed that a shorter survival and a higher relapse rate were associated with the occurrence of the ATRA syndrome [68]. In this study, the incidence of the ATRA syndrome was significantly diminished among patients receiving concomitant ATRA and chemotherapy as compared to ATRA followed by chemotherapy (9.2% versus 18%; p = 0.035) [71]. The results of a comparison of patients included in the Italian studies before and after the advent of ATRA point to a higher rate of CNS relapses with ATRA and chemotherapy than with chemotherapy alone possibly related to modulation of the adhesions molecules by ATRA [72] Bleeding and thrombosis The coagulopathy in APL is characterized by the clinical and laboratory signs of hyperfibrinolysis, proteolysis and disseminated intravascular coagulation. The response to ATRA is associated with an improvement of the bleeding tendency within a few days [73]. In a report of the GIMEMA, the rates of bleeding during remission induction in studies before and after the introduction of ATRA were compared. Although the reduction of early fatal hemorrhage was not significant, a substantial clinical improvement was evident in terms of reduction of the severity of bleeding, blood product consumption and induction mortality when chemotherapy was combined with ATRA [74]. Several reports suggest that the parameters of hyperfibrinolysis resolve rapidly under treatment with ATRA in the majority of patients, whereas the laboratory signs of intravascular coagulation can persist over several weeks [75]. In this later phase of ATRA therapy, the hypercoagulability may be responsible for a higher risk of thromboembolic complications [76]. Although these observations may suggest treatment with heparin, the benefit of prophylactic anticoagulation is uncertain. There is also no proof that

7 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) inhibition of fibrinolysis with tranexamid acid can reduce the risk of fatal bleeding during induction [60]. Platelet transfusion to keep the platelets between 30,000 and 50,000/ l and support of fresh frozen plasma are recommended until the disappearance of coagulopathy Resistance against ATRA The phenomenon of ATRA resistance became evident soon after the availability of ATRA [24]. As there are only few reports of successful reinduction with ATRA monotherapy in relapse after pretreatment with ATRA, it is difficult to estimate the rate of patients which retain sensitivity to ATRA. According to the results of the French studies, no response to ATRA was seen in patients with a shorter interval than three months after ATRA was stopped [40]. Hence, the response to reinduction therapy with ATRA seems to depend on whether the patients are still receiving ATRA or on the length of the ATRA free interval. Several mechanisms are suggested to contribute to the development of ATRA resistance. A progressive diminution of the plasma ATRA concentration was observed 2 weeks after the start of ATRA therapy. This might be explained by a faster reduction of the intracellular ATRA concentration caused by an induction of metabolizing enzymes (cytochrome 450 enzyme system) or of the CRABP-protein which transports ATRA to the endoplasmatic reticulum where it is metabolized [77,78]. In vitro data indicate that an overexpression of the p-glycoprotein, which decreases intracellular drug concentration, might also be involved [79]. Mutations in the ATRA binding domain of the PML/RAR fusion gene may lead to a reduced binding of the ligand and altered regulation of gene expression associated with resistance to ATRA [80,81]. Despite several attempts to understand and overcome the mechanisms of ATRA resistance no approach has proven successful in clinical routine. Liposomal ATRA, an intravenous formulation of ATRA, may be an useful alternative for patients which are unable to tolerate oral medications [82]. Furthermore, it seems possible that some mechanisms of ATRA resistance might be circumvented, as intravenously administered liposomal ATRA bypasses the enterohepatic pathway thereby preventing the induction of metabolizing mechanisms in the liver. However, there are presently no data which allow to compare the liposomal and the conventional ATRA formulation in terms of antileukemic effectiveness and ATRA resistance. Of a number of agents with differentiating properties on APL blasts, the synthetic retinoid Am80 might have advantages over ATRA: a 10-fold higher potency in inducing differentiation and a lower rate of resistance [38]. But it is presently not available for the routine use. Modulation of histone deacetylation affects repression and derepression of gene transcription. The combination of ATRA with the histone deacetylase inhibitor phenylbutyrate restored ATRA sensitivity in a patient previously resistant to ATRA alone [39]. 6. Prognostic factors and risk of relapse Despite different treatment approaches, several prognostic factors have been identified. A high initial WBC count is associated with a worse prognosis due to a higher mortality during the induction phase [83]. This is important, as in approximately 20% of patients the presenting WBC count is >10,000/ l [53,56,60]. In studies using standard dose chemotherapy in combination with ATRA, the relapse rate was higher in patients with higher initial WBC counts. However, the threshold of the initial WBC count used to define the patients with poor risk varies between studies, ranging from 2000 to 10,000/ l [50,52,59]. Remarkably, in the study of the AMLCG incorporating HD ara-c in the induction therapy, the initial WBC count was not identified as a risk factor for relapse as compared to studies with less intensive chemotherapy [61]. These results indicate that the threshold of WBC count that separates different risk groups may be influenced by several factors including the modification of treatment. Older age is associated with a worse prognosis due to a higher rate of early death and of death in remission due to a worse tolerance of chemotherapy. Presently it is unclear, whether elderly patients will benefit from an attenuation of chemotherapy as addressed in a historical comparison of the GIMEMA of 134 elderly patients treated in two consecutive protocols with different intensities of chemotherapy [52,62,84]. In some reports, the V-transcript of PML/RAR and CD 56 expression on the blast cells are associated with a poor prognosis. CD 56 reflects the neural crest adhesion molecule believed to be involved in trafficking of leukemia cells [85 88]. Mutations of FLT3, which signalize a poor prognosis were frequently found in patients with a higher presenting WBC count [89]. An Italian study suggests that HLA-B13 is significantly associated with relapse [90]. As compared to chemotherapy alone, an increase of CNS relapses was observed since ATRA is part of the induction therapy [72,91]. This raises the question whether CNS prophylaxis should routinely be performed which is not established by now. The prognostic importance of other chromosomal abnormalities in addition to the t(15;17) is not completely established among patients treated either with chemotherapy alone or with the combination of ATRA and chemotherapy. In the majority of studies, no prognostic influence of additional chromosomal abnormalities was found [92 96]. Monitoring of the minimal residual disease (MRD) may provide information on the relapse risk in the individual case [97]. 7. Prognostic significance of molecular monitoring of MRD The clinical importance of molecular monitoring of MRD by RT-PCR of PML/RAR was demonstrated by several studies. Approximately 90 95% of patients test negative after intensive induction or consolidation therapy [54 56,60,61].

8 268 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) It has become evident that PML-RAR -positivity by qualitative nested RT-PCR after consolidation or reappearance of a positive PCR during follow up are associated with a subsequent overt hematologic relapse. However, a negative result does not guarantee persistence of remission [8,97]. These data have provided the rationale for monitoring of MRD by conventional qualitative RT-PCR assays during and after first-line or relapse treatment schedules including autologous and allogeneic stem cell transplantations. According to the results of the largest series reported by the GIMEMA group, 70% of hematological relapses could be successfully predicted on the basis of prior molecular relapse. Median time from molecular conversion to frank relapse was 3 months (range 1 14 months). However, achievement and persistence of PCR negativity (sensitivity of the assay 10 4 ) cannot be equated with cure, since 29% of the patients who ultimately relapsed tested PCR negative in the marrow at the end of consolidation and during follow up [97]. Although not yet proven in clinical studies, it is anticipated that treatment at the time of molecular relapse will be associated with a better prognosis than at the time of overt hematological relapse. This is supported be the observation that PCR negativity can be reestablished by ATRA alone or by less intensive chemotherapy [98]. According to the results of the British MRC trial, the time to molecular remission was identified as a prognostic factor with a benefit for patients who achieved PCR-negativity after a lower number of consolidation cycles (p = 0.006) [53]. This suggests that also the kinetics of the disease has prognostic significance. A quantification of the leukemic burden by quantitative RT-PCR of PML/RAR at diagnosis followed by a close monitoring after each treatment course and during follow up might improve the comparability of the different treatment approaches in APL and might give information on the individual prognosis [99,100]. 8. Results with arsenic trioxide Most APL patients who received arsenic compounds were treated in China and the results are only available in Chinese. In Table 4, an overview on the treatment with ATO in relapsed APL published in the English literature is given [26,27, ]. The sensitivity of ALP cells to ATO is reflected by the high rate of hematological CR of % after induction (Table 4) and by a molecular remission rate of 83% after consolidation with ATO [26]. The high antileukemic effectiveness of ATO is supported by the observation of long lasting remissions up to 41+ months on ATO monotherapy [27]. In most studies, the ATO dosage was 10 mg absolute/day or 0.15 mg/kg/day for up to 60 days given by intravenous infusion. But also lower doses are effective [103]. Postremission therapy after ATO was heterogeneous and included further courses of ATO, chemotherapy, the combination of both and/or allogeneic or autologous transplantation (Table 4). Table 4 Results with ATO in relapsed APL Postinduction therapy Long-term results Author (year) [Reference] n CR (%) NR (%) ED (%) ATO/day Induction with ATO (days) Shen et al. (1997) [101] mg Second ATO course Not reported Soignet et al. (1998, 2001) [26,102] mg Maximum 60 days Maximum 5-6 ATO-courses 22% DFS after ATO mono, 86% DFS auto/allo auto/allo PBSCT PBSCT, after 4.3 years Niu et al. (1999) [27] mg 42 (second course, ATO ± chemotherapy Median DFS 18 months, ATO mono 12/18 if no CR) relapses, ATO + chemo. 2/11 relapses Not reported RFS 49%, OS 61%, after 2 years Shen et al. (2001) [103] n.r. n.r mg/kg 28 (second course, if no CR) Kwong et al. (2001) [104] mg/kg Idarubicin 88% CR after a median follow up of 13 months Leoni et al. (2002) [105] mg HD ara-c, ATO auto/allo PBSCT 71% CR after a median follow up of 15 months Ohnishi et al. (2002) [106] mg MP ± MTX, ATRA, allo PBSCT CR duration 8 months (median) Lazo et al. (2003) [107] mg/kg ATO ± chemotherapy, allo PBSCT OS 67% after 22 months Total Up to 60 days Variable Consolidation with chemotherapy or PBSCT favorable OS, DFS, RFS: overall, disease free, relapse free survival; CR: complete hematological remission; auto, allo PBSCT: autologous, allogeneic peripheral blood stem cell transplantation; MTX: methotrexate; MP: 6-mercaptopurine; HD ara-c: high dose ara-c; mono: monotherapy; chemo: chemotherapy.

9 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) Unfortunately, the follow up of these studies is short and the number of patients small. Hence, the contribution of postremission strategies to the outcome is difficult to assess. In the longest published follow up of 7 48 months including 33 relapsed Chinese patients, the disease free survival rates for 1 and 2 years are 63.6 and 41.6%, respectively, and the actual median disease free survival is 17 months [27]. Importantly, factors with positive influence on remission duration were WBC counts lower than 10,000/ l atthe beginning of ATO treatment and the combination of ATO with chemotherapy. A recent update of American studies indicates that the relapse free survival after allogeneic or autologous stem cell transplantation is longer than after ATO monotherapy [26,102]. It was further shown that the survival was better in patients treated with ATO in first relapse than in subsequent relapses [108]. The benefit of combining ATO with ATRA is controversial and might be different in relapsed or newly diagnosed APL. Synergistic therapeutic effects between ATO and ATRA were described sporadically in advanced relapses of APL [ ]. Major side effects of ATO are the leukocyte activation syndrome observed in approximately 25% of patients and electrocardiogram abnormalities, in particular prolongation of the QT-interval sometimes leading to potentially fatal torsade de pointes-type ventricular tachycardia [112]. The leukocyte activation syndrome, similar to the ATRA syndrome, requires high dose steroids at the first suspicion and discontinuation of ATO, if the symptoms are threatening. Other less important side effects are hypokalemia, hyperglycemia, headache, insomnia, nausea, vomiting, diarrhea and neuropathy [113]. Guidelines for management of the side effects are reported in detail in the product information of ATO (TRISENOX Fa. Cell Therapeutics, USA). In summary, it should be emphasized that ATO was well tolerated inducing a molecular remission rate of 83% in relapsed APL patients that underlines the antileukemic activity of the drug. Recently, the results of a Chinese study using oral tetraarsenic tetra-sulfide in 129 patients at various stages of APL showed that also the oral compound is successful in the treatment of the disease [32]. 9. The role of stem cell transplantation The two largest series of transplanted APL patients were reported by the European Bone Marrow Transplantation (EBMT) Cooperative Group before the availability of ATRA and after the introduction of the drug (Table 5) [33,34]. Focusing on patients transplanted in second complete remission (CR2) in both series, the relapse rate was lower in patients who received an allogeneic transplantation as compared to patients who underwent autologous transplantation (Table 5). However, the treatment related mortality (TRM) was higher in the allografted than in the autografted group reducing the advantage in survival and leukemia free survival of the allogeneic approach. As it was not possible to estimate the relapse rate on the basis of the PCR status before transplantation, its influence on the results remains unclear and should be addressed in further studies. In a French study of 50 patients with first relapse of APL, second remission was uniformly induced with ATRA and a relatively intensive timed sequential chemotherapy consisting of mitoxantrone, intermediate dose ara-c and etoposide. In the 22 patients who were autografted, a 3-year relapse free survival rate of 77% was achieved. Eight of 11 patients who underwent allogeneic transplantation died in CR either from infection or from graft versus host disease leading to a median disease free survival of only 8.4 months. The authors concluded that the high toxicity of the chemotherapy was responsible for the poor outcome after allogeneic transplantation [114]. As suggested by the results of an Italian series of 15 patients transplanted in second CR, the presence of MRD in the bone marrow at the time of autologous transplantation seems to influence the outcome. In this report, all seven patients who tested PCR positive relapsed after a median time of 5 months after transplantation (range 2 9 months), whereas six of eight patients with PCR negativity were still in remission after a median time of 28 months (range months) [115]. However, as a long-lasting molecular remission was also observed in a PCR negative patient who received a PCR positive transplant, autologous transplantation may also be worth considering despite the presence of Table 5 Results of autologous and allogeneic transplantations in patients with APL Autologous transplantation Allogeneic transplantation n OS (%) LFS (%) RR (%) TRM (%) n OS (%) LFS (%) RR (%) TRM (%) [33] CR1 a 129 n.r. 48 ± 5 41 ± 5 18 ± n.r. 42 ± 6 28 ± 5 42 ± 8 CR2 a 58 n.r. 31 ± 7 54 ± 6 23 ± 9 33 n.r. 22 ± 8 64 ± ± 9 After 1993 [34] CR1 b n.r. 73 ± 9 70 ± 9 24 ± 9 12 ± 7 n.r. 77 ± 8 70 ± 9 15 ± 8 20 ± 8 CR2 b ± ± ± 9 25 ± ± ± ± 9 33 ± 9 CR1: first hematologic remission; CR2: second hematologic remission; n: number of patients; OS: overall survival; LFS: leukemia free survival; RR: relapse rate; TRM: treatment related mortality; n.r.: not reported. a At 7 years. b At 6 years.

10 270 E. Lengfelder et al. / Critical Reviews in Oncology/Hematology 56 (2005) positive PCR in the stem cells collected, if no other approach is possible [116]. In summary, younger patients in first or subsequent relapse who have a matched donor have a good chance to be cured by an allogeneic PBSCT or BMT from a family donor or a matched unrelated donor [34,35]. For patients without a donor or not suitable for an allogeneic transplantation, autologous PBSCT may represent an alternative, that can provide prolongation of remission and possibly cure if the harvested stem cells are PCR negative [34,115,116]. 10. Monoclonal antibodies Monoclonal antibodies have shown activity in relapsed and newly diagnosed APL and may complement the treatment strategies in the future. These antibodies are directed against CD33, a surface glycoprotein found on most myelocytic and monocytic leukemia cells including APL blasts. Early trials showed that the monoclonal mouse antibody M195 rapidly targets leukemic cells in patients and that 131 I- labeled M195 can eliminate large leukemic burdens. Lower doses of 131 I-labeled M195 had antileukemic activity in MRD in second remission of APL. The therapeutic use of this antibody was limited by the formation of human antimouse antibodies and by myelosuppression due to the cytotoxicity of 131 I [ ]. The humanised form of M195 (HuM195) can be administered repeatedly. It showed activity in MRD by inducing negativity of RT-PCR assays of PML/RAR in 11 of 22 evaluable patients in first hematological remission and in one of seven patients in second or later remission [30]. Using gemtuzumab ozogamicin ( Mylotarg ), a calicheamicin conjugated humanised anti-cd33 monoclonal antibody as postremission therapy, a molecular remission was observed in heavily pretreated patients [37,120,121]. Larger studies are required to define the role of this treatment form in APL. 11. Conclusions Currently, about 70 80% of patients with APL are expected to be cured by first line therapy consisting of ATRA and chemotherapy. The clinical course demonstrates that APL is heterogeneous. The individual prognosis is influenced by biologic factors and by therapy. More efforts are necessary to further increase the cure rate and to reduce the rate of early mortality, which is still approximately 10%. Several treatment options exist for patients with APL who have relapsed after ATRA and chemotherapy. Approximately 50% of the patients in first relapse presently have the chance to achieve long-lasting second remissions and probable cure with salvage regimens. Currently, arsenicals and transplantation procedures seem to be the most promising strategies. Their combination should be investigated in further studies. There is also need of a clearer assessment of the contribution of ara-c, of arsenicals and of maintenance therapy in first line treatment of APL. 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