Abstract B025: How much is enough? BI-907828: a MDM2-p53 antagonist with limited BBB penetration but potent efficacy in glioblastoma
Authors: Zhang Wenjuan , Vaubel Rachael , Oh Juhee , Mlade Ann , Talele Surabhi , Zhang Wenqiu , Waller Katie , Burgenske Danielle , Sarkaria Jann , Elmquist William 
Published: March 2024

Abstract
Glioblastoma (GBM) is a highly aggressive and infiltrative primary malignant brain tumor. Current standard-of-care for newly diagnosed GBM patients includes maximal safe surgical resection, followed by concurrent radiation with temozolomide (TMZ), and adjuvant TMZ chemotherapy. However, despite aggressive treatments, GBM patients have a poor prognosis, with a median survival of 14.6 months and a 5-year survival rate of 6.8%. The p53 protein is a critical tumor suppressor in the cell cycle. The expression level of p53 is tightly controlled by its negative regulator, murine double minute 2 (MDM2). Amplification of the MDM2 gene, found in 14 % of GBMs, can inactivate p53, leading to loss of tumor suppressor functions. Therefore, we are investigating a potent MDM2 antagonist – BI-907828 in GBM to restore the p53 tumor suppressor function by inhibiting the p53/MDM2 interactions. We aim to evaluate the PK/PD of BI-907828 and demonstrate a new translational paradigm to rethink the “Delivery – Potency – Efficacy” relationship in drug discovery and development for central nervous system (CNS) tumors. BI-907828 was evaluated for cytotoxicity and the inhibition effect to MDM2-p53 pathway. Systemic PK and transport mechanisms controlling CNS distribution were evaluated in wild-type and transgenic mice that lack of efflux transporters P-gp and BCRP. BI-907828 unbound fractions in cell media, mouse and human specimens were determined by using rapid equilibrium dialysis to evaluate “active” concentrations. Efficacy measures, including survival and free drug distribution in tumor were assessed in mouse orthotopic GBM xenografts. BI-907828 exhibited potent inhibition of MDM2-p53 pathway and promoted cell death in GBM TP53 wild-type cells. MDM2-amplified cells are highly sensitive to BI-907828, with an effective unbound concentration of 0.1 nM. The CNS distribution of BI-907828 is limited by BBB efflux mediated by P-gp, resulting in a brain free partition coefficient Kp,uu of 0.002. Despite this seemingly “poor” BBB penetration, weekly administration of 10 mg/kg BI-907828 significantly extended median survival of orthotopic GBM108 xenografts to 218 days compared to 28 days with vehicle treatment (P &lt; 0.0001). These studies show that efficacy of BI-907828 in orthotopic models is related to high potency even though its CNS distribution is severely limited by BBB efflux. In another words, in this case, this little drug distribution to brain is “enough” for extending survival of orthotopic GBM108 xenografts. Therefore, a comprehensive understanding of drug “Delivery – Potency – Efficacy” relationship is warranted in drug discovery development, especially for treatment of CNS tumors.
Citation Format: Wenjuan Zhang, Rachael Vaubel, Juhee Oh, Ann Mlade, Surabhi Talele, Wenqiu Zhang, Katie Waller, Danielle Burgenske, Jann Sarkaria, William Elmquist. How much is enough? BI-907828: a MDM2-p53 antagonist with limited BBB penetration but potent efficacy in glioblastoma [abstract]. In: Proceedings of the AACR Special Conference on Brain Cancer; 2023 Oct 19-22; Minneapolis, Minnesota. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_1):Abstract nr B025.