The overall cure rate for childhood acute lymphoblastic leukemia (ALL) has reached eighty percent.1,2 However, for patients with higher risk (HR) features such as T-cell disease and white blood cell count (WBC) ≥50×109/L, the increase in event-free survival has been less impressive, and many of the treatment failures occur during therapy.1 The treatment of childhood ALL consists of 4 treatment elements: (a) remission induction, (b) consolidation and delayed intensification, (c) central nervous system (CNS)-directed treatment, and (d) maintenance therapy, which is continued 2.0-3.0 years from the time of diagnosis.3 The backbone of most maintenance therapy programs consists of oral daily 6-mercatopurine (6MP) and weekly methotrexate (MTX) with dosages adjusted to keep the WBC below 3.0-3.5×109/L.4 Many randomized studies have supported the necessity of maintenance therapy, at least for children with B-lineage ALL, although its mode of action is uncertain.5,6 Recently, reduction of maintenance therapy to only 6 months with a total duration of antileukemic therapy of 1 year was tested by the Tokyo ALL group.7 This strategy gave a 5-year event-free survival (EFS) for nonhigh risk patients (age <7.0 y and WBC <20×109/L) of only 60%, but a survival rate of 91%. The 5-year EFS for the highest risk patients (including T-ALL with WBC >50×109/L, and B-lineage ALL with a WBC >100×109/L) was 63%, which is rather similar to what has been obtained in historical controls or by other collaborative groups, who offered longer therapy for HR ALL.2,8–10 That study as well as other clinical observations have questioned whether a treatment duration of 2 years, and more specifically extended MTX/6MP maintenance therapy, is necessary for HR ALL patients.
On the basis of unsatisfactory results of the Nordic Society for Paediatric Haematology and Oncology (NOPHO) ALL-86 protocol for patients with HR ALL,2,11 the NOPHO tested in a nonrandomized study the efficacy of multidrug, cyclic LSA2L2 remission maintenance therapy12 compared with oral MTX/6MP maintenance therapy for patients with HR features. The present report analyses the outcome of patients with B-lineage ALL and a WBC ≥50×109/L or with T-lineage ALL, who received either of these 2 types of maintenance therapy. The overall results of Nordic HR-ALL patients treated according to the NOPHO ALL-92 protocol have previously been published.2,13
PATIENTS AND METHODS
From January 1992 to October 2001 1703 children of 1.0 to 14.9 years of age were diagnosed with B-cell precursor (pre-B) or T-cell ALL in the Nordic countries (Denmark, Finland, Iceland, Norway, and Sweden) (Fig. 1). Two patients with Down syndrome received no antileukemic therapy, 10 patients were treated according to non-NOPHO protocols, 1 patient was treated according to the previous NOPHO ALL-86 protocol, and 45 patients were treated according to the NOPHO ALL-2000 protocol, before it was officially opened. Of the remaining 1645 patients, who started therapy according to the NOPHO ALL-92 protocol, 22 patients died during induction therapy, and 9 patients had resistant disease and were excluded from further analysis. Finally, 2 patients with HR-ALL received neither MTX/6MP nor LSA2L2 maintenance therapy, and 3 HR-ALL patients shifted between these 2 maintenance therapies. None of the latter 5 patients have so far developed a relapse, and they were all excluded from this study.
Of the remaining 1609 patients, 325 patients had B-lineage ALL with a WBC ≥50×109/L or T-lineage ALL and were treated according the NOPHO HR protocols. Of these 84 patients did not receive maintenance therapy during their first remission owing to stem cell transplantation in first complete remission (CR1, n=32), death in CR1 (N=7), or a relapse before the scheduled start of maintenance therapy (n=45).
Of the remaining patients we furthermore excluded patients allocated to the HR-protocol owing to a t(4;11)(q21:2q23)-translocation (n=1), a t(9;22)(q34:q11)-translocation (n=2), or 5% or more leukemic blasts in the bone marrow day 29 (n=5).
The clinical characteristics and treatment assignment of the remaining eligible 152 boys and 81 girls with 98 cases of T-lineage and 135 B-lineage are given in Table 1. None of the patients had Down syndrome.
Risk Group Assignment
In the NOPHO ALL-92 protocol the HR groups were defined by WBC (≥50×109/L) at diagnosis, T-lineage ALL, the presence of CNS or testicular involvement, translocations t(9;22)(q34;q11) or t(4;11)(q21;q23), lymphomatous leukemia or mediastinal lymphoma, or a poor treatment response (M3 bone marrow at day 15 or M2/M3 at day 29).2,13 In the NOPHO ALL-92 protocol, patients who had HR features were to be assigned to the very high risk (VHR) treatment arm, if they were at least 5 years of age at diagnosis (owing to the use of cranial irradiation in that protocol arm) and in addition had (1) CNS leukemia, (2) lymphomatous leukemia, (3) HR ALL at diagnosis and a day 15 M3 or a day 29 M2/M3 bone marrow, and/or (4) T-cell disease with 1 or more additional HR-features. In addition to the inclusion of cranial irradiation, the primary therapeutic difference between the HR-protocol and VHR-protocol was the substitution of MTX/6MP (HR) with LSA2L2 maintenance therapy (VHR).2 Finally, to explore whether more intensive cyclic multi-drug maintenance therapy regimen could reduce the relapse rate for patients with HR features, Finnish patients with HR features received LSA2L2 maintenance therapy irrespective of whether their induction/consolidation/CNS-directed therapy had been according to the HR-ALL regimen or VHR-ALL regimen.14
Of the 233 patients, 131 patients had HR-ALL and 75 patients had VHR-ALL and were treated accordingly, and 22 Finnish HR-ALL patients were treated according to the HR-protocol, but received LSA2L2 maintenance therapy. The last 5 patients did not receive maintenance therapy according to the protocol stratification (Fig. 1). None of these 5 patients have developed an event, and they were included in survival analysis according to the maintenance therapy they actually received. Of the 22 Finnish HR-ALL patients, who were treated with LSA2L2 maintenance therapy, all were below 5.0 years of age at diagnosis, and 20 had B-lineage ALL.
On the basis of risk group assignments, patients were treated according to the HR or VHR treatment arms2,13 (Fig. 2).
For all patients this consisted of prednisolone (60 mg/m2/day on days 1 to 36, then tapered), weekly vincristine (VCR, 2.0 mg/m2 6 times), doxorubicin (40(mg/m2 4 times, asparaginase (30,000 IU/m2 daily on days 37 to 46), and intrathecal (IT) MTX on 4 occasions.
Consolidation therapy included alternating series of: (1) intravenous administration of cyclophosphamide (total cumulative dose: 3 g/m2) with low-dose cytarabine and either oral 6MP or oral 6-thioguanine; (2) HD-MTX 8 g/m2/24 hours with IT. MTX and leucovorin rescue 3 (VHR) or 4 (HR) times alternating with high-dose cytarabine [12 g/m2 times 3 (VHR) or 4 (HR)] with 1 (VHR) or 2 (HR) 2-months interval periods of oral weekly MTX and daily 6MP with 2 VCR/prednisolone reinductions per period; (3) 4 weeks of delayed intensification with dexamethasone (10 mg/m2/day for 3 weeks, then tapered), weekly VCR (2.0 mg/m2 4 times), weekly daunorubicine (30 mg/m2/day 3 times), and asparaginase (30,000 IU/m2 4 times).2
In total 76 nontransplanted patients on the VHR-ALL protocol received cranial irradiation in CR1, which included 2 T-ALL patients of 4.5 and 4.9 years at diagnosis of ALL, but above 5 years of age at the time of irradiation. Five patients with CNS leukemia at diagnosis and 4 additional patients with T-ALL (with 1 or more additional high risk features) received 24 Gy cranial irradiation, whereas the remaining 65 patients received 18 Gy. No spinal irradiation was given.
The 6MP/MTX maintenance therapy with starting oral 6MP doses of 75 mg/m2/day and oral MTX doses of 20 mg/m2/week was initiated at treatment week 63, titrated to a WBC between 1.5 and 3.5×109/L, and continued until 2.0 years after diagnosis. Every 8 weeks throughout maintenance therapy the patients received reinductions of VCR (1.5 mg/m2 once) and prednisolone (40 mg/m2/day for 1 week) with IT. MTX is given in age-adjusted doses.
LSA2L2 maintenance therapy was given to VHR-patients and to Finnish HR patients and discontinued 2 years from diagnosis irrespective of whether or not the patients had received all 6 scheduled LS2A2 courses. Each LS2A2 block consisted of 4 courses at 2 weeks intervals with alternating drug combinations (Fig. 2).2,12 Blood counts, treatment delays, and the number of LS2A2 blocks administered to each patient were not registered centrally.
Stem Cell Transplantation
None of the patients received stem cell transplantation in first complete remission after having initiated maintenance therapy.
Survival analyses were performed with a basic time scale defined by the date of diagnosis. As events in the EFS analyses, we included relapse, death in remission, or the diagnosis of an SMN, whichever occurred first. Patients who died in first remission or developed a SMN were censored at the time of these events in the analyses of risk factors for a leukemic relapse. Cox proportional hazard regression analyses were performed with the likelihood ratio test for differences in outcome.15,16 Nonparametric methods were applied to compare the distribution of parameters between subgroups.17 The Kaplan-Meier method was applied for estimation of remission duration and for the generation of survival curves.18 Subgroups were compared with the log-rank test,19 stratified where needed. Two-sided P values <0.05 were regarded as significant. Survival analyses were performed with the SAS statistical software. The median follow-up of the 172 patients who did not experience an event was 10.8 years (50% range: 8.2 to 12.5 y).
Of the 233 patients, 1 T-ALL patient on MTX/6MP maintenance therapy died in CR1, 1 T-ALL patient, who had received MTX/6MP maintenance therapy, developed a second malignant neoplasm (acute myeloid leukemia), and 58 patients developed a relapse of ALL 1.2-10.8 years from diagnosis (median: 2.3 y) with a cumulative incidence of relapse of 26%±3%. In total, 36 patients died 1.9 to 6.9 years from diagnosis (median: 3.0 y). The overall projected 11-year event-free survival (pEFS11y) and overall survival of the 234 patients was 0.74±0.03 and 0.85±0.02, respectively, with no significant difference between boys and girls.
Overall, patients with T-lineage ALL fared better than those with B-lineage (pEFS11y: 0.82±0.04 vs. 0.68±0.04; P=0.05) with a significant difference in risk of relapse (16%±4% vs. 32%±4%; P=0.02). For patients with B-lineage ALL, the 10 patients with a WBC ≥200×109/L fared significantly worse than the remaining patients (pEFS11y: 0.30±0.15 vs. 0.71±0.04; P=0.001), whereas the WBC at diagnosis did not significantly influence the event risk for those with T-lineage ALL.
Table 1 presents the impact of the 2 types of maintenance therapy by sex, age, immunophenotype, and WBC at diagnosis. In survival analysis stratified by immunophenotype, the patients who received oral MTX/6MP maintenance therapy had a significantly lower relapse risk than the patients who received LSA2L2 maintenance therapy (P=0.006), and that was true for both B-lineage ALL (n=135, pRelapse11y: 27%±5% vs. 45%±9%; P=0.02) and T-lineage ALL (n=98, pRelapse11y: 8%±5% vs. 21%±5%; P=0.12) (Fig. 3). As relapses occurring after only a few months of maintenance therapy may not be linked to the type of maintenance therapy given, survival analysis with stratification for lineage (B vs. T) were performed with exclusion of patients who had a relapse within 2.0 years from diagnosis. Still, those who received oral MTX/6MP maintenance therapy had a significantly lower relapse risk than the patients who received LSA2L2 maintenance therapy (P=0.01).
The 132 HR-ALL patients, who were treated with oral MTX/6MP maintenance therapy, did significantly better than the 23 HR-ALL patients, who were treated according to the LSA2L2 protocol (pEFS11y: 0.76±0.04 vs. 0.57±0.10; P=0.02). This difference in EFS was also significant for B-lineage HR-ALL patients (n=118; pEFS11y 0.74±0.05 vs. 0.50±0.11; P=0.01). The difference in pEFS11y between the 2 maintenance therapy groups was as pronounced, if only the 95 HR B-lineage ALL patients below 5.0 years of age were included, as this characterized the B-lineage HR-ALL patients in the Finnish LSA2L2 Maintenance Therapy Study (pEFS11y: 0.73±0.06 and 0.50±0.11, P=0.01) (Fig. 4). This difference in EFS stayed significant if the B-lineage ALL patients with WBC ≥200×109/L were excluded, or if the impact of the type of maintenance therapy was explored in Cox regression analysis with adjustments for the WBC (data not shown).
CNS relapse and CNS irradiation: of the 58 relapses, 48 involved the bone marrow and 14 involved the CNS of which 6 were isolated CNS relapses. The risk of any CNS-involving relapse was 7%±2% for the patients who received MTX/6MP maintenance therapy and 5%±2% for the patients who received LSA2L2 maintenance therapy (P=0.73). The risk of CNS relapse did not differ significantly between those that did and did not receive CNS irradiation in first remission (5%±3% and 7%±2%, respectively; P=0.80). Furthermore, the 76 patients who received CNS irradiation did not differ significantly in their overall risk of relapse from those, who were not irradiated (24%±5% vs. 26%±4%, P=0.90) or in the fraction of relapses that involved the CNS (4/18=22% vs. 10/40=25%). Among the 98 patients who received LSA2L2 maintenance therapy, only 18 out of the 76 patients that received cranial irradiation had a relapse compared with 11 out of the 24 patients who did not receive cranial irradiation (pRelapse11y: 24%±5% vs. 50%±11%; P=0.03). However, 21 out of the latter 24 patients who did not receive CNS irradiation had B-lineage ALL, compared with only 16 of the 60 patients that received CNS irradiation. After exclusion of the 76 patients who had received CNS-irradiation, those who received oral MTX/6MP maintenance therapy still had a significantly lower relapse risk than the patients who received LSA2L2 maintenance therapy (P=0.007).
Owing to the differences in the distribution of clinical characteristics among the patients that received the 2 types of maintenance therapy, we used Cox multivariate regression model with stratification for the immunophenotype to test the effect on event risk of sex, WBC at diagnosis, age at diagnosis, whether or not CNS irradiation was given, a day 15 bone marrow with ≥25% versus <25% lymphoblasts, and oral MTX/6MP (code=1) versus LSA2L2 maintenance therapy (code=0). Higher WBC (P=0.01) and administration of LSA2L2 maintenance therapy (P=0.04) were both related to an increased risk of an event (overall P value of the Cox model: 0.003). None of the other covariates reached a significance level <0.05 in any of the backward steps.
As the patients who received LSA2L2 maintenance therapy included relatively more boys and more T-lineage leukemias, and had a significantly higher age and a slightly higher WBC at diagnosis (Table 1), we did a Cox regression analysis with stratification by immunophenotype and with forced inclusion of sex, age, and WBC at diagnosis, presence of CNS-leukemia at diagnosis, administration of CNS irradiation, and the type of maintenance therapy given. Even with these adjustments in the Cox model for potential confounders, the patients who received LSA2L2 maintenance therapy had a significantly HR of relapse than those who received oral MTX/6MP maintenance therapy (P=0.01).
Today the most effective treatment protocols achieve 80% cure rates for children with ALL, which reflects an impressive development in the biologic understanding of the disease and risk grouping, in the intensity of the treatment programs, and in the supportive care.
The use of maintenance therapy is rather specific for ALL. The mode of action of MTX/6MP maintenance therapy could include elimination of persistent preleukemic cells and true leukemic cells,20 modulation of apoptotic pathways,21,22 direct antileukemic action,23,24 induction of differentiation,25 and/or changes in stromal support22,26,27 including antiangiogenic mechanisms.28,29 The metronomic structure of low-dose oral MTX/6MP maintenance therapy may be of significance for several of these modes of action.30
Even though maintenance therapy seem to be crucial for cure for a large proportion of childhood ALL cases,6,31,32 the optimal way to administer MTX/6MP maintenance therapy is yet to be determined, which can explain why the large collaborative groups differ in their guidelines for dose adjustments.4 Several studies have indicated that the treatment intensity of oral MTX/6MP maintenance therapy is important, whether measured by drug dosage,33,34 the degree of myelosuppression35 or hepatotoxicity,36 or by cytotoxic MTX/6MP metabolite levels,37,38 However, children with non-HR ALL have dominated most studies on maintenance therapy, and it has been unclear to what extent the conclusions are valid for T-lineage and HR B-lineage ALL. Furthermore, as the complexity of frontline ALL therapy increases, it becomes difficult to evaluate the efficacy and necessity of the different treatment phases outside randomized studies.
Thus, the results of this study should be interpreted cautiously, not least owing to unequal distribution of risk factors among patients on MTX/6MP and LSA2L2 maintenance therapy. However, even taking this into account, this nonrandomized study indicates that oral MTX/6MP maintenance therapy is important even for patients with HR features, although the results for T-lineage ALL did not reach statistical significance possibly owing to the lower number of T-ALL patients included. Others have similarly shown that the original United States Childrens' Cancer Group modified LSA2L2 treatment is inferior to a more conventional ALL treatment with delayed intensifications and oral MTX/6MP maintenance therapy for patients with poor prognostic features.12 However, in that study the LSA2L2 cycles made up most of the treatment, whereas in the NOPHO ALL-92 study the LSA2L2 maintenance therapy was initiated more than 1 year from diagnosis, that is, after the patients had received consolidation therapy, delayed intensification, and CNS-directed therapy with high-dose MTX and high-dose cytarabine with or without CNS irradiation.2,13 The fact that the relapse rate of the patients even at this late stage of their therapy was significantly influenced by the type of maintenance therapy, is in line with previous publications that demonstrate that the intensity of oral 6MP/MTX maintenance therapy significantly influences the risk of relapse even for patients with high risk features.35 Similar to the results published by the US Childrens Cancer Group, we found that patients who received both LSA2L2 maintenance therapy and cranial irradiation had a significantly lower probability of relapse than those patients who received LSA2L2 maintenance therapy but no irradiation.12
The reason for the inferiority of the LSA2L2 regimen is uncertain. Firstly, the LSA2L2 regimen may induce more cytopenic episodes than conventional oral MTX/6MP therapy, which was titrated to avoid both high WBC (target: <3.5×109/L) and low WBC (target: ≥1.5×109/L). Thus, LSA2L2 therapy carries a risk of treatment interruptions, which may allow the regrowth of leukemic cells.33,34 Whether that played a role for the relapse rate in this study is not known, as treatment delays were not routinely registered. Secondly, the LSA2L2 regimen used in the ALL-92 protocol included hydroxyurea and carmustine, which are anticancer agents with little antileukemic efficacy. It has previously been shown that substituting these agents with more efficacious drug combinations and pushing the doses intravenously. MTX to biologic tolerance during the cyclic LSA2L2 regimen may yield cure rates as good as, although not superior to, that achieved by MTX/6MP maintenance therapy.12 No published data support that any other pulsed regimens offer better cure rates than conventional MTX/6MP maintenance therapy. Finally, the patients on the VHR regimen received only 2 courses of high-dose methotrexate and of cytarabine, respectively, compared to 4 courses of high-dose methotrexate and of cytarabine for those 1 the HR regimen. However, this is unlikely to explain the differences in relapse rates, as the Finnish HR-ALL patients that received LSA2L2 maintenance therapy did significantly worse than the non-Finnish HR-ALL patients on MTX/6MP maintenance therapy even though these subsets had received the same consolidation and delayed intensification treatment. Noteworthy, when analysing all noninfant children with ALL in the Nordic countries, the Finnish patients had an EFS that did not differ significantly from that of the non-Finnish Nordic patients (data not shown).
In conclusion, these results indicate that oral MTX/6MP maintenance therapy administered after the first year of remission may improve the cure rates of children with T-lineage and with HR B-lineage ALL. Accordingly, in the NOPHO ALL-2008 protocol, oral MTX/6MP maintenance therapy will be given to all risk groups until 2.5 years from diagnosis.
The authors thank all the Nordic pediatric oncology centers that have supported this study with detailed registration of treatment data.
1. Schrappe M, Camitta B, Pui CH, et al. Long-term results of large prospective trials in childhood acute lymphoblastic leukemia
. Leukemia. 2000;14:2193–2194.
2. Gustafsson G, Schmiegelow K, Forestier E, et al. Improving outcome through two decades in childhood ALL in the Nordic countries: the impact of high-dose methotrexate in the reduction of CNS irradiation. Nordic Society of Pediatric Haematology and Oncology (NOPHO). Leukemia. 2000;14:2267–2275.
3. Schmiegelow K, Gustafsson G. Acute Lymphoblastic Leukemia
. In: Voute PA BASM, Caron M, eds. Cancer in Children. Oxford: Oxford University Press; 2005: 138–170.
4. Arico M, Baruchel A, Bertrand Y, et al. The seventh international childhood acute lymphoblastic leukemia
workshop report: Palermo, Italy, January 29-30, 2005. Leukemia. 2005;19:1145–1152.
5. Childhood ALL Collaborative Group. Duration and intensity of maintenance chemotherapy in acute lymphoblastic leukaemia: overview of 42 trials involving 12,000 randomised children. Lancet. 1996;347:1783–1788.
6. Riehm H, Gadner H, Henze G, et al. Results and significance of six randomised trials in four consecutive ALL-BFM studies. Hematol Blood Transf. 1990;33:439–450.
7. Toyoda Y, Manabe A, Tsuchida M, et al. Six months of maintenance chemotherapy after intensified treatment for acute lymphoblastic leukemia
of childhood. J Clin Oncol. 2000;18:1508–1516.
8. Schrappe M, Reiter A, Ludwig WD, et al, German-Austrian-Swiss ALL-BFM Study Group. Improved outcome in childhood acute lymphoblastic leukemia
despite reduced use of anthracyclines and cranial radiotherapy: results of trial ALL-BFM 90. Blood. 2000;95:3310–3322.
9. Pui CH, Boyett JM, Rivera GK, et al. Long-term results of Total Therapy studies 11, 12 and 13A for childhood acute lymphoblastic leukemia
at St Jude Children's Research Hospital. Leukemia. 2000;14:2286–2294.
10. Eden OB, Harrison G, Richards S, et al. Long-term follow-up of the United Kingdom Medical Research Council protocols for childhood acute lymphoblastic leukaemia, 1980-1997. Medical Research Council Childhood Leukaemia Working Party. Leukemia. 2000;14:2307–2320.
11. Gustafsson G, Kreuger A, Clausen N, et al. Intensified treatment of acute childhood lymphoblastic leukaemia has improved prognosis, especially in non-high-risk patients: the nordic experience of 2648 patients diagnosed between 1981 and 1996. Nordic Society of Paediatric Haematology and Oncology (NOPHO) (see comments). Acta Paediatr. 1998;87:1151–1161.
12. Steinherz PG, Gaynon PS, Breneman JC, et al. Treatment of patients with acute lymphoblastic leukemia
with bulky extramedullary disease and T-cell phenotype or other poor prognostic features: randomized controlled trial from the Children's Cancer Group. Cancer. 1998;82:600–612.
13. Saarinen-Pihkala UM, Gustafsson G, Carlsen N, et al. Outcome of children with high-risk acute lymphoblastic leukemia
(HR-ALL): nordic results on an intensive regimen with restricted central nervous system irradiation. Pediatr Blood Cancer. 2004;42:8–23.
14. Saarinen-Pihkala UM, Lanning M, Perkkio M, et al. Granulocyte-macrophage colony-stimulating factor support in therapy of high-risk acute lymphoblastic leukemia
in children. Med Pediatr Oncol. 2000;34:319–327.
15. Cox DR. Regression models and life-tables (with discussion). J R Stat Soc (B). 1972;34:187–220.
16. Andersen PK, Borgan Ø, et al. Statistical Models Based on Counting Processes. New York: Springer-Verlag; 1993.
17. Siegel S, Castellan NJ. Nonparametric statistics for the Behavioral Sciences. Singapore: McGrawHill; 1988.
18. Kaplan EJ, Meier P. Non-parametric estimation from incomplete observations. J Am Stat Assoc. 1958;53:457–481.
19. Mantel N. Evaluation of survival data and two new rank order statistics arising in its consideration. Cancer Chemother. 1966;50:163–170.
20. Zuna J, Ford AM, Peham M, et al. TEL deletion analysis supports a novel view of relapse in childhood acute lymphoblastic leukemia
. Clin Cancer Res. 2004;10:5355–5360.
21. Gale RP, Butturini A. Maintenance chemotherapy and cure of childhood acute lymphoblastic leukaemia. Lancet. 1991;338:1315–1318.
22. Kumagai M, Manabe A, Pui CH, et al. Stroma-supported culture in childhood B-lineage acute lymphoblastic leukemia
cells predicts treatment outcome. J Clin Invest. 1996;97:755–760.
23. Chabner BA, Allegra CJ, Curt GA, et al. Polyglutamation of methotrexate. Is methotrexate a prodrug? J Clin Invest. 1985;76:907–912.
24. Waters TR, Swann PF. Cytotoxic mechanism of 6-thioguanine: hMutSalpha, the human mismatch binding heterodimer, binds to DNA containing S6-methylthioguanine. Biochemistry. 1997;36:2501–2506.
25. Lin TL, Vala MS, Barber JP, et al. Induction of acute lymphocytic leukemia differentiation by maintenance therapy
. Leukemia. 2007;21:1915–1920.
26. Mudry RE, Fortney JE, York T, et al. Stromal cells regulate survival of B-lineage leukemic cells during chemotherapy. Blood. 2000;96:1926–1932.
27. Narendran A, Ganjavi H, Morson N, et al. Mutant p53 in bone marrow stromal cells increases VEGF expression and supports leukemia cell growth. Exp Hematol. 2003;31:693–701.
28. Keyhani A, Jendiroba DB, Freireich EJ. Angiogenesis and leukemia. Leuk Res. 2001;25:639–645.
29. Perez-Atayde AR, Sallan SE, Tedrow U, et al. Spectrum of tumor angiogenesis in the bone marrow of children with acute lymphoblastic leukemia
. Am J Pathol. 1997;150:815–821.
30. Kamen BA, Glod J, Cole PD. Metronomic therapy from a pharmacologist's view. J Pediatr Hematol Oncol. 2006;28:325–327.
31. Miller DR, Leikin SL, Albo VC, et al. Three versus five years of maintenance therapy
are equivalent in childhood acute lymphoblastic leukemia
: a report from the Childrens Cancer Study Group. J Clin Oncol. 1989;7:316–325.
32. Anonymous. Duration of chemotherapy in childhood acute lymphoblastic leukaemia. The medical research council's working party on leukaemia in childhood. Med Pediatr Oncol. 1982;10:511–520.
33. Schmiegelow K. Prognostic significance of methotrexate and 6-mercaptopurine dosage during maintenance chemotherapy for childhood acute lymphoblastic leukemia
. Pediatr Hematol Oncol. 1991;8:301–312.
34. Relling MV, Hancock ML, Boyett JM, et al. Prognostic importance of 6-mercaptopurine dose intensity in acute lymphoblastic leukemia
. Blood. 1999;93:2817–2823.
35. Schmiegelow K, Pulczynska MK. Maintenance chemotherapy for childhood acute lymphoblastic leukemia
: should dosage be guided by white blood cell counts? Am J Pediatr Hematol Oncol. 1990;12:462–467.
36. Schmiegelow K, Pulczynska M. Prognostic significance of hepatotoxicity during maintenance chemotherapy for childhood acute lymphoblastic leukaemia. Br J Cancer. 1990;61:767–772.
37. Schmiegelow K, Schroder H, Gustafsson G, et al. Risk of relapse in childhood acute lymphoblastic leukemia
is related to RBC methotrexate and mercaptopurine metabolites during maintenance chemotherapy. Nordic Society for Pediatric Hematology and Oncology. J Clin Oncol. 1995;13:345–351.
38. Lilleyman JS, Lennard L. Mercaptopurine metabolism and risk of relapse in childhood lymphoblastic leukaemia. Lancet. 1994;343:1188–1190.