The B7-H3-Targeting Antibody-Drug Conjugate m276-SL-PBD Is Potently Effective Against Pediatric Cancer Preclinical Solid Tumor Models

Abstract

Purpose: Patients with relapsed pediatric solid malignancies have few therapeutic options, and many of these patients die of their disease. B7-H3 is an immune checkpoint protein encoded by the CD276 gene that is overexpressed in many pediatric cancers. Here, we investigate the activity of the B7-H3-targeting antibody-drug conjugate (ADC) m276-SL-PBD in pediatric solid malignancy patient-derived (PDX) and cell line-derived xenograft (CDX) models.

Experimental design: B7-H3 expression was quantified by RNA sequencing and by IHC on pediatric PDX microarrays. We tested the safety and efficacy of m276-SL-PBD in two stages. Randomized trials of m276-SL-PBD of 0.5 mg/kg on days 1, 8, and 15 compared with vehicle were performed in PDX or CDX models of Ewing sarcoma (N = 3), rhabdomyosarcoma (N = 4), Wilms tumors (N = 2), osteosarcoma (N = 5), and neuroblastoma (N = 12). We then performed a single mouse trial in 47 PDX or CDX models using a single 0.5 m/kg dose of m276-SL-PBD.

Results: The vast majority of PDX and CDX samples studied showed intense membranous B7-H3 expression (median H-score 177, SD 52). In the randomized trials, m276-SL-PBD showed a 92.3% response rate, with 61.5% of models showing a maintained complete response (MCR). These data were confirmed in the single mouse trial with an overall response rate of 91.5% and MCR rate of 64.4%. Treatment-related mortality rate was 5.5% with late weight loss observed in a subset of models dosed once a week for 3 weeks.

Conclusions: m276-SL-PBD has significant antitumor activity across a broad panel of pediatric solid tumor PDX models.

Figure 1.. CD276 expression in pediatric patient-derived and cell line-derived xenograft models.

A) RNAseq expression data for CD276 in pediatric solid tumors (TARGET, Treehouse) and normal tissue samples (GTEx database). B) CD276 mRNA expression from PPTC RNAseq data in nine solid pediatric tumor histologies present in this study (ATRT 6; Ewing sarcoma 9; extracranial Rhabdoid 4; embryonal rhabdomyosarcoma (RMS) 6; alveolar RMS 6; hepatoblastoma 1; neuroblastoma 33; osteosarcoma 30; Wilms tumor 12). Black points in panel B indicate models examined in N>1 or SMT study (of 62 unique models tested, 37 had RNA-sequencing data).

Figure 2.. CD276 protein expression is elevated in pediatric patient-derived and cell line-derived xenografts.

Immunohistochemistry was performed on 3 TMAs containing mixed PDX histologies from the SMT study (A), neuroblastoma models (B), and osteosarcoma models (C). Black points indicate models examined in N>1 or SMT study.

Figure 3.. Tumor volume and event-free survival with m276-SL-PBD administration.

A) Waterfall plots for Ewing sarcoma (10 mice per model), rhabdomyosarcoma (10 mice per model), osteosarcoma (10 mice per model), and neuroblastoma (2 mice per model) PDX/CDXs. Bars represent median percent change in minimum relative tumor volume in treated mice. B) Kaplan-Meier plot of all 47 single mouse experiments (N=1) with “+” representing right-censored data. Colors represent the location of tumor origin (bone includes osteosarcoma (2 models) and Ewing sarcoma (12 models); kidney/adrenal/liver includes neuroblastoma (2 models), extracranial Rhabdoid (5 model), hepatoblastoma (1 model), and Wilms tumor (5 models); RMS includes alveolar (6 models) and embryonal RMS (7 models); brain includes ATRT (5 models), PXA (1 model), and meningioma (1 model)). C) Waterfall plots for models in SMT. Bars represent median percent change in minimum relative tumor volume in treated mice.