Children with Down Syndrome (DS) have an increased risk of developing leukemia, including both acute myeloid, as well as acute lymphoblastic leukemia (DS-ALL). DS ALL patients have a poorer tolerance for high-dose methotrexate (HD-MTX), which is one of the key components of ALL treatment. There are no specific dosing guidelines for HD-MTX in DS-ALL. As a result doses are frequently reduced. In this study the pharmacokinetics (PK) and pharmacodynamics of HD-MTX were studied retrospectively in DS-ALL patients in order to develop specific dosing guidelines. Only one study addressed MTX-pharmacokinetics (PK) in DS children (Garre et al, J Pediatrics 1987). They found that median MTX plasma concentrations, 42 hours after infusion, were significantly higher in 5 DS-ALL patients compared to 3 non-DS controls. A retrospective case-cohort study was performed with DS-ALL patients enrolled in treatment protocols DCOG ALL-8, -9 or -10 in 8 Dutch medical centers during the period Nov 1991-Dec 2006. MTX dosages varied from 2.0–5.0 gr/m2 in the different treatment protocols. Forty-four DS and 87 non DS-ALL patients were included. The latter were matched for treatment protocol, sex and body surface area. All DS-ALL patients had B-cell-precursor ALL, and the median diagnostic WBC was 8,8*109/L. MTX serum levels and toxicity data were collected from patient files. Toxicity was graded according to CTCAE v3.0. Population PK-models were fitted to data from all individuals simultaneously, using non-linear mixed effect modeling (NONMEM). The PK of MTX was described according to a two-compartment model with a first order elimination from the central compartment. A total of 468 HD-MTX courses were given to 44 DS and 87 non-DS children. In 20% of the DS-ALL patients doses were reduced, comprising 26/152 (17.1%) of MTX courses. In 18/26 courses, dose-reduction was electively initiated from the 1st course onwards, whereas in 8/26 courses dose-reductions were applied from the 2nd course onwards because of documented toxicity in the 1st course. Dose reductions did not occur in non DS-ALL patients (p<0.001). The cumulative frequency of grade 3/4 gastro-intestinal toxicity (mucositis) was significantly higher in DS versus non-DS children (27.6% of courses vs. 4.1%; p<0.001), including courses in which MTX was dose-reduced. Hemoglobin and WBC were significantly lower in DS-patients, but probably not clinically relevant, as the differences were small. The methotrexate clearance for DS-ALL patients was 5% lower than for non DS-ALL patients (p<0.001). Area under the curve (AUC) and MTX serum levels at various time-points after HD-MTX infusion were not different between DS and non-DS children. Moreover, there was no correlation between AUC (range 276–2603 μmol/L*hr) and grade 3 and 4 gastro-intestinal toxicity (rs 0.17; p=0.15). In fact grade 3/4 toxicity occurred at the lowest AUC of 276 μmol/L*hr. Hence no safe AUC could be defined. To summarize, DS-ALL patients suffer from increased risk of severe mucositis, which could not be explained by differences in PK, but are most likely due to pharmacodynamic effects of MTX. Given that only one patient needed a dose-reduction after severe toxicity when being treated with MTX dosages of 1–3 gr/m2, it seems safe to treat DS children with moderately high dosages of MTX and adjust dose in individual cases in case of severe toxicity. However, in case of 5 gr/m2, 8 patients needed dose-reduction, and hence this dose is probably too high for DS patients.

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