Pediatric Preclinical Testing Consortium Evaluation of the Anti-CD123 Antibody-Drug Conjugate, IMGN632, Against Patient-Derived Xenograft Models of Pediatric Acute Lymphoblastic Leukemia

Introduction: Cure rates for children with acute lymphoblastic leukemia (ALL) are close to 90%, however the outcome for high-risk subtypes remains poor. CD123, the alpha subunit of the IL-3 receptor, is overexpressed in many hematological malignancies, including ALL. CD123 is primarily expressed on B-ALL blasts and is associated with aggressive subtypes (Angelova et al, 104: 749-755, 2019). IMGN632 (pivekimab sunirine) is a CD123-targeting antibody-drug conjugate (ADC) comprised of the novel high-affinity anti-CD123 antibody G4723A, and the DNA alkylating payload DGN549, which is an indolindobenzodiazepine pseudodimer. IMGN632 is being studied in Phase I/II clinical trials in CD123-positive hematological malignancies as a monotherapy and in combination with standard of care drugs (ClinicalTrials.gov Identifiers NCT03386513, NCT04086264). Therefore, it was of interest for the Pediatric Preclinical Testing Consortium (PPTC) to test the in vivo activity of IMGN632 against patient-derived xenograft (PDX) models of pediatric ALL.

Methods: CD123 (IL3RA) mRNA expression was determined by RNA-seq on 90 ALL PDXs representing six broad subtypes (http://pedcbioportal.kidsfirstdrc.org). CD123 specific antibody-binding capacity (sABC) was quantified by flow cytometry. Two B-lineage ALL PDXs with moderate CD123 expression (one B-cell precursor, BCP; one mixed-lineage leukemia-rearranged, MLLr) were selected for a dose-response study, and 40 ALL PDXs (13 BCP, 3 Ph⁺, 8 Ph-like, 9 MLLr, 3 T-cell ALL, T-ALL; 4 early T-cell precursor, ETP-ALL) were selected for a single mouse trial (SMT). PDXs were grown orthotopically in NSG mice and engraftment was evaluated by enumerating the proportion of human versus mouse CD45⁺ cells (%huCD45⁺) in the peripheral blood. Treatment commenced when the %huCD45⁺ reached ≥1%, and events were defined as huCD45⁺ ≥25% or leukemia-related morbidity. For the dose-response study, mice received a control ADC or IMGN632 (60 µg/kg, 120 µg/kg, or 240 µg/kg IV weekly x 3). For the SMT, each PDX was inoculated into a single mouse and mice received IMGN632 (240 µg/kg IV weekly x 3). All mice were treated with an Fc receptor blocker (400 mg/kg, IP) and platelet-activating factor inhibitor (5 mg/kg, IP) prior to ADC treatment. Drug efficacy was assessed by mouse event-free survival (EFS) and stringent objective response measures (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). An objective response was defined as a decrease in the %huCD45⁺ to below 1% at any point after treatment initiation, with a Maintained Complete Response (MCR) indicated as the %huCD45⁺ <1% for at least 3 consecutive weeks at any point after the completion of treatment. Mice in which the %huCD45⁺ did not drop below 1% at any point after treatment initiation were not considered to have achieved an objective response, with Progressive Disease (PD) the worst response. IMGN632 was provided by ImmunoGen, Inc.

Results: CD123 mRNA expression ranged from 0.09-51.4 FPKM and was more highly expressed in B-lineage ALL PDXs (BCP, Ph⁺, Ph-like, MLLr; median 9.1, range 0.93-51.4 FPKM; n=65) than T-lineage PDXs (T-ALL; ETP; median 0.49, range 0.09-13.0 FPKM; n=25, p<0.0001). From the PDXs selected for the SMT, CD123 sABC ranged from 0 to 3,032 and CD123 mRNA expression correlated with CD123 sABC (n=30, p=0.0023, r=0.536). In the dose-response study IMGN632 elicited MCRs at all 3 doses for the MLLr-ALL PDX, with the high dose extending mouse median EFS by 362 days compared to the control (p=0.0004), and 233 days at the lowest dose (p=0.0002). The BCP-ALL PDX also scored MCRs at the two highest doses, and PD at the lowest dose. Of the 37 evaluable PDXs in the SMT, 29 scored objective responses (11/12 BCP, 2/3 Ph⁺, 8/8 Ph-like, 6/7 MLLr, 0/3 T-ALL, 2/4 ETP), with 26 PDXs attaining MCRs. Mouse EFS ranged from 6.8-379 days and was significantly greater in B-lineage PDXs (n=30) than T-lineage PDXs (n=7; p<0.0001).

Conclusions: IMGN632 exerted profound in vivo activity against pediatric ALL PDXs with varying CD123 expression levels, particularly those of B-lineage, inducing prolonged remissions at doses as low as 60 µg/kg. The significant difference in CD123 expression between B- and T-lineage ALL could have contributed to the difference in IMGN632 activity between the B-ALL and T-ALL models. These data strongly support the clinical evaluation of IMGN632 in B-lineage pediatric ALL. (Supported by NCI Grants CA199000, CA199222 and CA263963)

Watkins:ImmunoGen, Inc.: Current Employment. Sloss:ImmunoGen: Current Employment. Zweidler-McKay:ImmunoGen, Inc.: Current Employment.