Pharmacokinetic Results Of a Feasibility and Phase II Study (ITCC 021) On Bortezomib (BTZ) In Pediatric Relapsed ALL: Lack Of Significant Penetration Of BTZ In The Cerebrospinal Fluid

Prognosis of refractory and relapsed ALL is poor and improvement requires the introduction of agents with a new mechanism of action. Bortezomib (BTZ) as a proteasome inhibitor is such an agent, and was safe as single agent in phase I studies in children (Blaney 2004; Horton 2007). Messinger et al. (2012) reported a single-arm study that BTZ can be combined safely with conventional drugs; the combination was remarkably effective. BTZ results in sensitization of malignant cells to anticancer agents, both in vitro for leukemias as well as for multiple myeloma patients. In patients with ALL, this effect regarding glucocorticoids has not been addressed yet. It also has not been studied whether BTZ reaches the cerebrospinal fluid (CSF), which is relevant in pediatric leukemias in view of the frequent leptomeningeal involvement. In the setting of the lack of clinical experience with BTZ in children in Europe, we developed a European multicentre feasibility/phase II study in refractory or relapsed ALL, in which all patients get BTZ (NTR 1881, EUDRaCT 2009-014037-25, ITCC 021). A randomisation is done for BTZ to start “early”, on day 1 of treatment, or “late”, on day 8 of treatment. Bortezomib is given as iv push for 4 doses at 1.3 mg/m2/dose, thus in group “early” on days 1, 4, 8 and 11 and in group “late” on days 8, 11, 15 and 18. In addition, all patients receive dexamethasone (10 mg/m2/day in 3 doses for 2 weeks, orally or iv) and vincristine (1.5 mg/m2/dose with a maximum of 2 mg as 1-hour infusion on days 8 and 15), and one intrathecal administration of methotrexate (dose age-adjusted) on day 1. Eligible patients have 2^nd or greater relapsed ALL, 1^st relapsed ALL after allogeneic stem cell transplantation (allo-SCT) in 1st complete remission (CR1), or refractory 1st relapsed ALL, bone marrow involvement and at least 100 leukemic cells per ul blood. Exclusion criteria includes symptomatic CNS leukemia, among other factors. It is planned to have 24 fully evaluable patients. This interim analysis is limited to a description of pharmacokinetic (PK) data, especially concerning the CSF.

As per June 1, 2013 a total of 14 patients has been enrolled, 9 boys and 5 girls, median 8.7 years of age (range, 1.6-16.2). Most had 2nd relapsed ALL (n=9), others 1st relapsed ALL following allo-SCT in CR1 (n=3) or refractory 1st relapsed ALL (n=2).

Regarding PK in the peripheral blood, there was remarkable intra- and inter-individual variability in peak plasma concentrations, between patients ranging from 4.7 to 2920 ng/ml 15 minutes after the first administration of BTZ, median 12.4 ng/ml (18.7 ng/ml in the group with BTZ “early”, 12.1 ng/ml in the group with BTZ “late”). Peak levels after the fourth administration were higher, median 41.1 ng/ml (29.5 ng/ml in the group with BTZ “early” and 158.9 ng/ml in the group with BTZ “late”). There was a 10-fold interindividual variation in the area-under the concentration versus time curve until 72 h (AUC[0-72h]) after administration. Median ratio of AUC[0-72h] fourth dose / AUC[0-72h] first dose was 2.7 (range 0.9 – 9.3), which is indicative of accumulation.

In all patients, PK of BTZ was studied in the CSF 15 minutes after administration of the first and third dose (group “early” only) of BTZ, as well as 4 days (group “late”) or one week (group “early”) after the last administration. In general, no BTZ was detected with a lower detection limit of 0.1 ng/ml. In 4 patients some BTZ was detected in CSF, at 0.2 – 0.4 – 1.7 - 5.2 ng/ml. Of potential interest, the latter patient also had the highest peak plasma level of BTZ (2920 ng/ml) and the highest AUC.

Future analyses in the complete cohort of 24 randomised patients will focus on more extensive population PK and pharmacodynamic analyses, as well as on efficacy of BTZ. The higher peak plasma levels after the fourth dose suggest decreased clearance (especially in the group which received bortezomib “late”), which indeed has been reported in adults and which requires careful monitoring of toxicity over time. Mean peak plasma concentrations in adults were reported to be 173 ng/ml, and thus seem higher. Meanwhile, BTZ does not or hardly penetrate the cerebrospinal fluid and is unlikely to be a drug that significantly adds to the treatment of leptomeningeal involvement in leukemia.

Financially supported by the Dutch Foundation Children Cancer-free.