Anti-Leukemia Combinatorial Efficacy of ABBV155, a B7H3-BCL-XL Inhibitor Antibody-Drug Conjugate, in Combination with Venetoclax through BCL-XL/BIM and BCL-2/BIM Complex Dissociations

B7H3, an immune checkpoint protein, inhibits immune cell response and is overexpressed in various malignancies, correlating with poor prognosis. B7H3 is selectively highly expressed in acute myeloid leukemia (AML) blast but not in normal hematopoietic stem cells (Tyagi et al., Blood, 2022). BCL-XL has emerged as a promising target in AML, particularly in patients who have progressed from venetoclax (VEN)-based therapy, as their cancer cells shift dependency from BCL-2 to BCL-XL (Zhang et al. Nat Cancer, 2020).

Our study investigated the preclinical activity of ABBV-155 (mirzotamab clezutoclax; ABBV), a novel antibody-drug conjugate targeting B7H3 with a selective BCL-XL inhibitor payload. We hypothesized that ABBV could selectively deliver BCL-XL inhibitor to B7H3-expressing AML cells, inducing apoptosis alone or in combination with the BCL-2 inhibitor venetoclax.

We first tested B7H3 expression in four AML patient-derived xenograft (PDX) models and found high expression in all samples (56%-97%). Sample #3912018, with the highest B7H3 expression and multiple mutations (FLT3-ITD, DNMT3A, IDH1, KIT, NPM1), was selected for in vivotesting. NSG aged 6-8 weeks mice were intravenously injected with PDX cells (1e6 cells per mouse) following sublethal irradiation (250 cGy). Upon detecting circulating leukemia cells by hCD45 flow cytometry (41 days post-transplant), mice were randomized into four groups (8 mice per group) to receive vehicle, ABBV (10 mg/kg, i.p., weekly for 4 weeks), VEN (50 mg/kg, p.o., 5 days per week for 4 weeks), or a combination of ABBV and VEN. Tumor burden and treatment safety were monitored weekly via hCD45% in peripheral blood and mice body weight, respectively. After the last dose, 3 mice per group were euthanized for biomarker analysis (pharmacodynamic study), while the remaining 5 mice received a second treatment cycle after a 2-week break, continuing to monitor tumor burden and survival.

All treatments were well-tolerated, with less than 10% weight loss after 4 weeks. Compared to the vehicle group, both ABBV and VEN inhibited leukemia burden, with the combination achieving the best inhibition after 3 weeks of treatment (average circulating hCD45%: 19.4%, 11.1%, 10.2%, and 0.9% for vehicle, ABBV, VEN, and combination, respectively).

In the pharmacodynamic study, only the combination group significantly (p=0.0021) inhibited tumor burden, as reflected by spleen weight (average: 0.448, 0.600, 0.660, and 0.077 g for vehicle, ABBV, VEN, and combination). The combination largely eliminated leukemia in bone marrow, liver, and spleen, indicated by undetectable hCD45% (<1%) compared to leukemia infiltration (>95%) in other groups. Analysis of BCL-2 family complex showed that VEN dissociated BCL-2/BIM complexes, increasing BCL-XL/BIM and MCL-1/BIM complexes. ABBV dissociated BCL-XL/BIM complexes, increasing BCL-2/BIM complexes. The combination completely disrupted both BCL-2/BIM and BCL-XL/BIM complexes, supporting the hypothesis that maximal release of BH3-only proteins from two pro-survival proteins leads to highly efficacious AML clearance.

During the treatment break, leukemia burden increased rapidly in all groups except the combination. The second cycle treatment failed to inhibit progression in single-agent cohorts. On the contrary, mice in the combination treatment cohort survived, achieving significantly better survival (p=0.0382) with median overall survival of 122 days compared to 77, 69.5, and 77 days for vehicle, ABBV, VEN, respectively.

In summary, our study demonstrates the ABBV effectively dissociates the BCL-XL/BIM complex in a venetoclax-refractory AML PDX model. The combination targeted engagement of BCL-XL with B7H3-ADC and of BCL-2 with VEN significantly inhibits leukemia burden and prolongs survival in AML PDX models through synergistic release of BH3-only proteins. These findings suggest a promising therapeutic strategy for AML, particularly in cases resistant to venetoclax-based treatments.