Wide Variability in the Time Required for Immunotoxins to Kill B Lineage Acute Lymphoblastic Leukemia Cells: Implications for Trial Design

Abstract

Purpose Recombinant immunotoxins (rITs) targeting CD22 are highly active in hairy cell leukemia, but less so in acute lymphoblastic leukemia (ALL). This study aims to understand the variable activity of a rIT against ALL towards improving responses in clinical application. Experimental Design We determined in vitro activity of rITs by WST-8 assays and the time needed to kill ALL cell lines and patient-derived ALL blasts by flow cytometry. The findings were translated into two systemic ALL xenograft models. Differences in time needed to kill KOPN-8 cells for distinct rITs were addressed biochemically. Results In vitro activity (IC50) of anti-CD22 rIT varied 210-fold from 0.02 to 4.6 ng/ml. Activity also varied greatly depending on the time ALL cells were exposed to immunotoxin from <30 min to >4 days. For KOPN-8, the difference in exposure time was related to intracellular rIT-processing. We showed in newly developed ALL xenograft models, where immunotoxins have a short half-life, that the needed exposure time in vitro predicted the responses in vivo. By replacing bolus dose with small doses at frequent intervals or with continuous infusion, responses were substantially improved. We confirmed exposure time variability on patient-derived ALL samples and showed a correlation between exposure time needed to reach maximal cytotoxicity in vitro and their clinical response. Conclusion The exposure time needed for rITs targeting CD22 to kill ALL cells varies widely. Our results suggest that ALL patients would have a better response rate to anti-CD22 immunotoxins if treated by continuous infusion rather than by bolus injections.

Phase 1 study of the anti-CD22 immunotoxin moxetumomab pasudotox for childhood acute lymphoblastic leukemia

“…Finally, we recently demonstrated wide interpatient variability in the exposure time required to induce cytotoxicity of ALL blasts by moxetumomab pasudotox. 26 Thus, the activity of moxetumomab pasudotox in patients with ALL might be improved by changing administration from bolus to continuous infusion, especially given the short elimination half-life observed in this study (supplemental Table 3). Importantly, dexamethasone cotreatment is unlikely to have contributed to the antileukemia activity observed in this trial.…”

Paclitaxel synergizes with exposure time adjusted CD22-targeting immunotoxins against B-cell malignancies

“…To search for combination therapies which enhance the in vivo efficacy of CD22-targeting rITs, we used our recently developed KOPN-8 xenograft model [ 24 ]. After i.v.…”

“…We recently showed that the activity of CD22-targeting rIT against ALL cells in vitro and in vivo improved substantially when the cells were exposed longer to CD22-targeting rIT [ 24 ]. Because rITs have a short plasma half-life in mice [ 8 ] and men [ 7 ], blood levels fall quickly after a bolus dose.…”

“…Because rITs have a short plasma half-life in mice [ 8 ] and men [ 7 ], blood levels fall quickly after a bolus dose. The rIT exposure time of ALL cells growing in murine bone marrow (BM) achieved by bolus doses was too short for them to die; when high rIT blood levels were maintained by repeated injections, in vivo efficacy was greatly improved [ 24 ].…”

“…Here, we tested whether the addition of paclitaxel would improve treatment outcome in the KOPN-8 xenograft model [ 24 ]. To extend our studies toward MCL, various cell lines were tested in vitro and LR was found to be more active than Moxe.…”