Phase I study of a bispecific ligand-directed toxin targeting CD22 and CD19 (DT2219) for refractory B-cell malignancies

Abstract

Purpose: The novel bispecific ligand-directed toxin (BLT) DT2219 consists of a recombinant fusion between the catalytic and translocation enhancing domain of diphtheria toxin (DT) and bispecific single-chain variable fragments (scFV) of antibodies targeting human CD19 and CD22. We conducted a phase I dose-escalation study to assess the safety, maximum tolerated dose, and preliminary efficacy of DT2219 in patients with relapsed/refractory B-cell lymphoma or leukemia.

Experimental design: DT2219 was administered intravenously over 2 hours every other day for 4 total doses. Dose was escalated from 0.5 μg/kg/day to 80 μg/kg/day in nine dose cohorts until a dose-limiting toxicity (DLT) was observed.

Results: Twenty-five patients with mature or precursor B-cell lymphoid malignancies expressing CD19 and/or CD22 enrolled to the study. Patients received median 3 prior lines of chemotherapy and 8 failed hematopoietic transplantation. All patients received a single course of DT2219; one patient was retreated. The most common adverse events, including weight gain, low albumin, transaminitis, and fever were transient grade 1-2 and occurred in patients in higher dose cohorts (≥40 μg/kg/day). Two subjects experienced DLT at dose levels 40 and 60 μg/kg. Durable objective responses occurred in 2 patients; one was complete remission after 2 cycles. Correlative studies showed a surprisingly low incidence of neutralizing antibody (30%).

Conclusions: We have determined the safety of a novel immunotoxin DT2219 and established its biologically active dose between 40 and 80 μg/kg/day ×4. A phase II study exploring repetitive courses of DT2219 is planned.

Figure 1. Immunologic and pharmacokinetic studies

A. Peripheral blood mononuclear cell (PBMC) analysis of a representative patient is shown. PBMCs were enriched from patient blood and collected at various times post-treatment. Flow cytometry was used to count cells expressing CD22, CD19, CD20, or CD3. B. A bioassay was used to determine the area under the curve (AUC) of serum DT2219 levels in serum by measuring the ability of diluted serum to inhibit proliferation of CD22+CD19+ Raji indicator cells. Drug serum levels at various times were analyzed using prism 5.0 software to calculate AUC. A concentration-time curve is shown for our second patient at 60ug/kg. T_1/2 was 59 minutes. C. DT2219 serum levels and neutralizing antibodies. Upper panels shows a patient treated at the 80 ug/kg dose level showing no evidence of DT2219 in serum (left) and high levels of neutralizing antibodies at day 8 through 22 (right). In contrast, lower panels shows that a patient treated at 60 ug/kg/day had a serum drug concentration (left) and no detectable neutralizing antibodies (right lower panel). DT2219 serum levels were calculated from assays in which various serum dilutions were tested for their ability to inhibit Raji cell proliferation. Serum collected prior to drug administration served as a negative control. Neutralization assays were performed based on the ability of undiluted patient serum samples to block the killing of a 99% inhibitory dose of DT2219. % neutralization was calculated

Figure 2. Imaging studies in patients attaining objective response on Phase 1 study

A. Abdominal CT imaging of 77-year-old patient with rituximab and chemotherapy-refractory CLL treated with DT2219 at dose level 40 μg/kg every other day × 4 doses before and at day 28 after therapy is shown. The 40% reduction in the abdominal tumor mass was observed after a single course of therapy. B. CT images of a 53-year-old female with CD22⁺CD19⁺ relapsed chemotherapy refractory marginal zone lymphoma. The patient was treated at dose level 60 μg/kg/day QODx4 and experienced a DLT of capillary leak syndrome and neutropenia. After regulatory approval, the patient received a second treatment course 8 weeks later at a reduced dose of 40 μg/kg/day QODx4 which resulted in complete resolution of the tumor mass. CT images were taken prior to therapy and after the second course of DT2219. Arrows indicate a tumor mass.