|Home | About | Journals | Submit | Contact Us | Français|
The prognosis after relapse of pediatric acute myeloid leukemia (AML) is poor and effective salvage regimens are urgently needed.
In Phase I and pilot studies, we evaluated the maximum tolerated dose (MTD) and dose-limiting toxicities (DLTs) of a 5-day course of cladribine (2-CDA) followed by topotecan in pediatric patients with relapsed/refractory AML. The 2-CDA dose was escalated as follows: 9.1, 13.6, 16.3, and 19.5 mg/m2 per day (8.9 mg/m2 per day in the pilot study). Outcome was analyzed according to the absence (stratum 1) vs. presence (stratum 2) of previous allogeneic hematopoietic stem cell transplantation. Twenty-six patients (20 in stratum 1, 6 in stratum 2) were treated.
The MTD was not reached in stratum 1, but DLT occurred at the lowest 2-CDA dosage (9.1 mg/m2 per day) in stratum 2. Febrile neutropenia was common in both strata. Nine (34.6%) of 26 patients experienced a complete response and 7 (30.4%) had a partial response; 5 (19.2%) are long-term survivors. Clinical outcome was not associated with 2-CDA or topotecan systemic exposure.
The combination was well tolerated in stratum 1, and the response rate is encouraging. This regimen offers a post-relapse treatment alternative for patients, especially those who have received anthracycline-containing chemotherapy.
Substantial progress in the management of childhood acute myeloid leukemia (AML) has been achieved over the past 30 years.1–3 However, about 30% to 40% of patients with AML experience relapse despite intensive chemotherapy and the use of allogeneic hematopoietic stem cell transplantation (HSCT). Patients with FLT3 internal tandem duplication, M6 or M7 FAB subtype, myelodysplastic syndrome (MDS)-related AML, monosomy 7 karyotype, or persistent disease after two courses of conventional induction chemotherapy have a particular poor outcome. Because most frontline AML protocols feature high cumulative doses of anthracyclines, treatment regimens for relapsed diseases should limit or avoid the use of this class of agents, particularly if salvage treatment includes HSCT.
Cladribine (2-CDA) is a deoxyadenosine analog. Intracellular 2-CDA is rapidly phosphorylated to the triphosphate (2-CDATP), which resists degradation by adenosine deaminase and therefore accumulates to cytotoxic levels.4 2-CDATP competes with dATP as a substrate of DNA polymerases, and its incorporation interferes with subsequent chain elongation. 2-CDA has been extensively evaluated at our institution in relapsed and refractory pediatric AML.5,6 In a Phase II study, a 5-day continuous infusion of single agent 2-CDA at 8.9/mg/m2 per day induced a high rate of responses in patients with relapsed AML.6 Subsequently, 2-CDA has been incorporated into upfront therapy for newly diagnosed patients with AML as a single agent or as combination therapy.7,8
Topotecan, a semisynthetic analog of camptothecin, is a specific inhibitor of topoisomerase I.9,10 Topotecan stabilizes the topoisomerase-I/DNA complex, causing DNA single-strand breaks and cell death, particularly during S-phase. Daily 30-minute infusions of topotecan induced significant responses in a Children’s Oncology Group’s Phase I study for children with refractory leukemia.11
The functional DNA interference caused by topotecan and by 2-CDA may result in enhanced cytotoxicity when these agents are given in combination. Because both agents can be administered as short bolus infusions,8,12 we studied various schedules of short exposure to these two drugs in vitro. Based on a preclinical trial showing inhibitory effect of 2-CDA followed by topotecan (given after a drug-free period) (unpublished data), we developed a Phase I study to determine the maximum tolerated dose (MTD) and dose-limiting toxicities (DLTs) of short infusions of escalating doses of 2-CDA followed by topotecan targeted to achieve a predetermined systemic exposure in pediatric patients with relapsed or refractory AML.
Eligible patients with relapsed or refractory AML, less than 21 years of age, were treated at St. Jude Children’s Research Hospital and at the University of Michigan Comprehensive Cancer Center from 1998 to 2003. They had a life expectancy ≥ 6 weeks, Eastern Cooperative Oncology Group (ECOG) performance status ≤ 3, and adequate renal (creatinine < 2 × normal) and hepatic (bilirubin ≤ 3 mg/dL; ALT and AST ≤ 500 U/dL) function. Three patients previously treated in a pilot study at St. Jude with the same combination were included. Patients were grouped for analysis as those who had not previously undergone allogeneic HSCT (stratum 1) and those who had (stratum 2). This study and the inclusion of the 3 additional patients were approved by Institutional Review Board. Signed informed consent was obtained from patients, parents, or legal guardians, with assent from the patients, if appropriate.
Both drugs were administered for five consecutive days per course. 2-CDA was given intravenously over 3 hours at a dosage of 9.1, 13.6, 16.3, or 19.5 mg/m2 per day; three patients treated in the pilot study received 8.9 mg/m2 per day. Topotecan was administered intravenously over 30 minutes, starting 8 hours after initiation of 2-CDA. The topotecan dosage was 4.0 mg/m2 on day 1 and was then individualized on the basis of topotecan clearance to target a systemic exposure of 140 ± 20 ng/mL·h. In each stratum the treatment and dose escalation were performed separately. All patients received filgrastim (G-CSF) 5 μg/kg/day for a minimum of 10 days, beginning 24 hours after completion of the last dose of topotecan.
A modified phase I design proposed by Storer13 was adopted. At least one patient was treated at a starting dose level of 2-CDA with topotecan for 5 days. If no dose-limiting toxicity (DLT) was observed, the next patient received the next higher 2-CDA dosage. If DLT occurred, the next patient was treated at the previous dose level and the traditional phase I design was adopted thereafter. Patients treated with this combination at the University of Michigan Comprehensive Cancer Center were evaluated for toxicity, response, and pharmacokinetics but did not participate in the dose escalation scheme.
Toxicity was graded according to the National Cancer Institute Common Toxicity Criteria (NCI-CTC) version 2.0. DLT was defined as any non-hematological toxicity ≥ grade 3, with the exception of nausea and vomiting and grade 3 febrile neutropenia. Any prolonged (> 35 days) hematologic toxicity that included neutropenia (ANC <300/μL) and thrombocytopenia (platelets <30,000/μL) in the absence of persistent or progressive disease was considered a DLT.
A bone marrow aspirate and core biopsy were assessed before and on day 15 after the start of the first course. Complete response (CR) was indicated by an aspirate containing <5% blast cells and partial response (PR), by 15% to 50% blast cells. Bone marrow studies were repeated after one week if the aspirate contained 5% to 15% blast cells. Patients were removed from the study if they had less than a PR in the first course or experienced unacceptable toxicity. Patients who had a CR or PR were eligible to receive additional courses at the same dosage level. There was no intrapatient dose escalation or de-escalation.
Each patient’s topotecan dosage was individualized to achieve a topotecan lactone systemic exposure of 140 ± 20 ng/ml·h.14 Briefly, plasma samples obtained before and 0.25, 0.5, 1, 3, and 6 hours after completion of the first topotecan infusion were immediately processed and analyzed. If the single-day topotecan lactone area under the concentration-time curve (AUC) was within the target range, no dose adjustment was required. Otherwise, the topotecan dosage was adjusted linearly on the basis of the patient’s topotecan lactone clearance estimated from the pharmacokinetic model, and repeat pharmacokinetic studies were performed on day 2. This procedure continued until the patient’s topotecan AUC was within the target range. A two-compartment pharmacokinetic model was fitted to the topotecan lactone plasma concentration as previously described.14
On day 1 of the first course of treatment, blood samples were obtained before the 2-CDA infusion; 30, 60, and 90 minutes after the start of the infusion; at the end of the infusion; and 2, 4, 10, and 24 hours after completion of the infusion. Plasma 2-CDA concentration was determined by using a sensitive and specific bi-gradient high-performance liquid chromatography assay with ultraviolet detection.7 At 264 nm, the range of the standard curve was 5 to 120 nM.
A two-compartment model was fit to the 2-CDA concentration-versus-time data by nonlinear regression with a Bayesian algorithm implemented in ADAPT II and using published values (mean and variance) as the Bayesian priors.7 Model parameters estimated for each patient included the volume of the central compartment, elimination rate constant, and intercompartment rate constants (kcp and kpc). These parameters were used to simulate each patient’s plasma concentration-time profile, from which the AUC from time zero to hour 24 (AUC0→24) was calculated.
Overall survival was defined as the time between study enrollment and death; data for survivors were censored at the time of last follow-up. Survival was estimated by the Kaplan-Meier method, and survival estimates were compared according to stratum and 2-CDA dose level by the exact log-rank test. Fisher’s exact test was used to examine the relation of clinical response to 2-CDA dose level, the proportion of topotecan AUC values within the target range after the initial dose, and the proportion of topotecan AUC values within the target range after dose adjustment. Logistic regression was used to assess the relation of clinical response to 2-CDA dose level, 2-CDA AUC, and topotecan AUC. The Kruskal-Wallis test was used to examine the association of 2-CDA AUC with 2-CDA dose. All tests were two-tailed. Results with p-value less than 0.05 were considered statistically significant. No adjustment was made for multiple tests.
The characteristics of the 26 patients are summarized in Table 1. Twenty-three patients (17 in stratum 1, 6 in stratum 2) were enrolled on the Phase I study. Three additional patients treated with 8.9 mg/m2 per day of 2-CDA during a pilot study were assigned to stratum 1. There were 4 cases of AML with prior MDS. Of the 20 patients in stratum 1, 14 patients had relapsed leukemia with a median initial remission duration of 11.4 months and 6 had refractory disease. All 6 patients in stratum 2 had relapsed disease with a median duration of initial remission of 18.5 months.
In stratum 1, the regimen was generally well tolerated (Tables 2 and and3).3). Febrile neutropenia was common (10 patients), and 2 patients had documented bacteremia (α-streptococcus and Pseudomonas aeruginosa, respectively). After a patient treated at 13.6 mg/m2 per day of 2-CDA had prolonged bone marrow suppression of undetermined etiology, 2-CDA dosage was assigned according to the traditional Phase I design. One patient receiving 19.5 mg/m2 per day of 2-CDA developed grade 4 renal failure and grade 4 respiratory failure with bilateral pulmonary effusions. Only this patient experienced DLT; the MTD was not reached in this stratum.
In stratum 2, five patients developed febrile neutropenia. Two patients receiving 9.1 mg/m2 per day of 2-CDA had grade 4 febrile neutropenia; one of these had sepsis (Enterobacter sp.) with an episode of hypotension, and one had documented disseminated aspergillosis complicated by severe hepatic veno-occlusive disease. One patient treated with 13.6 mg/m2 per day of 2-CDA had prolonged neutropenia and thrombocytopenia (>35 days) associated with fever. DLT was observed at the initial 2-CDA dose of 9.1 mg/m2 per day in stratum 2 and further patient enrollment was discontinued.
Nine (34.6%) of the 26 patients experienced CR and 7 (26.9%) PR; the overall response rate was 61.5% (Table 3). In stratum 1, 11 (55%) of the 20 patients had responses; 6 (30%) had CRs. Three (patients 1, 11 and 14 in Table 3) had one additional course of 2-CDA/topotecan prior to subsequent therapy. Eleven patients proceeded to allogeneic HSCT, and 4 were changed to gemtuzumab ozogamicin (n=2) or methotrexate and L-asparaginase (n=2) therapy. Five (83%) of the 6 patients in stratum 2 responded. The 3 who had complete responses subsequently received allogeneic HSCT, donor lymphocyte infusion, or interleukin-2 (IL-2).
Of the 16 responders overall, 9 (56.3%) remained in remission and received additional therapy (allogeneic HSCT, n=8; IL-2, n=1), but 4 of these died of treatment-related toxicity during remission. Five of the 26 patients (19.2%) survived in continuous complete remission at the last follow-up date (Fig. 1). We were unable to show a statistically significant difference in survival according to stratum (p=0.38), cytogenetics (t(8;21) or inv(16) vs. others; p=0.63), or duration of first remission (> 1 year vs. ≤ 1 year; p=0.84).
Table 4 summarizes the topotecan pharmacokinetic parameters derived from 54 pharmacokinetic studies in the 26 patients. Of the 26 studies performed after the initial topotecan dose of 4.0 mg/m2, only 6 (23%) showed topotecan AUC within the target range. Of the remaining 28 pharmacokinetic studies (those performed after targeted dose adjustment), 20 (72%) were within the target range. The median dosage achieving the target AUC in these patients was 4.0 mg/m2 (range, 1.7–6.0 mg/m2). In univariate and multivariate logistic regression models, clinical outcome was not found to be significantly associated with the proportion of topotecan AUC values within the target range after the initial dose or after dose adjustment.
2-CDA pharmacokinetics were studied in 16 of the 26 patients. The necessary blood samples for 3 patients treated before study initiation and for 7 other patients were not available. The median 2-CDA systemic clearance rate was 18.1 L/h·m2 (range; 10.6–27.5 L/h·m2). AUC varied widely at different 2-CDA dose levels, but the median systemic exposure was significantly associated with the dose level (Fig. 2A, p=0.031). Response occurred at all dose levels and was not significantly associated with 2-CDA AUC (Fig. 2B, p= 0.19).
Because of the risk of organ toxicity posed by frontline chemotherapy regimens for childhood AML, we sought to develop an anthracycline-free regimen for the treatment of relapsed or refractory AML. Our preclinical studies demonstrated that when 2-CDA is combined with topotecan its activity is schedule-dependent, especially at clinically relevant concentrations, i.e. 75 nM topotecan15 and 290 nM 2-CDA.16 In these experiments, inhibition of leukemia cell growth was optimal when 2-CDA was added to the culture first and topotecan was then added after a drug-free interval. It is possible that 2-CDA exacerbates the disruption of normal DNA replication caused by topotecan’s inhibition of topoisomerase, with a consequent increase in cytotoxicity. On the basis of these in vitro results, we designed the combination regimen to include a 5-hour delay between the end of the 2-CDA infusion and the start of the topotecan infusion. This interval allowed the plasma concentration of 2-CDA fall to ~40 nM, which was the optimal 2-CDA concentration in the preclinical studies.
The combination of 2-CDA and topotecan in our study was well tolerated by patients who had not undergone allogeneic HSCT; the MTD was not reached in this group. Because we found no association between efficacy and the 2-CDA dose or the 2-CDA/topotecan pharmacokinetics, we suggest that 2-CDA 9.1 mg/m2 per day and topotecan 4.0 mg/m2 per day are a reasonable dose regimen in this patient population.
Patients who had received allogeneic HSCT before relapse (stratum 2) had a high incidence of regimen-related toxicity in our study, with DLT observed at the initial dose of 2-CDA (9.1 mg/m2). However, 5 of 6 patients in this stratum had a response; therefore, lower doses of 2-CDA and/or topotecan may be considered for this patient population.
We would suggest to use prophylactic antibacterial and antifungal agents because fever with neutropenia was the most common toxicity of this regimen and we recently reported that prophylactic treatment during intensive treatment for pediatric AML reduced morbidity due to septicemia and dramatically decreased the incidence of septicemia and of inpatient days.17
After relapse of childhood AML, chemotherapy alone is unlikely to be curative due to drug resistance.18 Every effort should be made to induce remission before HSCT, but it is crucial to reduce the toxicity of reinduction therapy to a level that allows HSCT to proceed. Various regimens, including fludarabine plus cytarabine and cytarabine plus mitoxantrone/idarubicin, appear to give similar results.19,20 The addition of liposomal daunorubicin to fludarabine plus cytarabine is currently being tested in an effort to improve CR rates while minimizing cardiotoxicity. Our regimen induced an overall response rate of 61.5% and a CR rate of 34.6%. Nine (34.6%) patients remained in remission, although 4 died of toxicity associated with subsequent HSCT or IL-2 treatment. These patients are at high risk of transplant-related mortality and morbidity after second remission.18,21 Therefore, innovative HSCT approaches are needed. Donor-recipient mismatch of killer immunoglobulin-like receptors (KIR) and ligand in the presence of T-cell depletion can facilitate natural killer (NK) cell–mediated cytotoxicity to leukemia cells.22 Reduced-intensity regimens have been used in elderly adults or those with comorbidities and could be considered for this patient population.23
In summary, on the basis of our extensive preclinical data, we designed and conducted the first clinical trial of 2-CDA and topotecan in children with relapsed/refractory AML. The combination of short intravenous 2-CDA and topotecan infusions was safe and well tolerated among patients who had not undergone allogeneic HSCT. The response rate in this very drug-resistant group is encouraging. This regimen offers a treatment alternative for patients previously treated on anthracycline-containing regimens.
We thank Sharon Naron for editorial advice.
Supported in part by Cancer Center Core grant CA 21765 from the National Cancer Institute and by the American Lebanese Syrian Associated Charities. Dr. Ching-Hon Pui is an American Cancer Society Professor.