Zhang, Wenjuan

Abstract Radioresistance of melanoma brain metastases limits the clinical utility of conventionally fractionated brain radiation in this disease, and strategies to improve radiation response could have significant clinical impact. The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is critical for repair of radiation-induced DNA damage, and inhibitors of this kinase can have potent effects on radiation sensitivity. In this study, the radiosensitizing effects of the DNA-PKcs inhibitor peposertib were evaluated in patient-derived xenografts of melanoma brain metastases (M12, M15, M27). In clonogenic survival assays, peposertib augmented radiation-induced killing of M12 cells at concentrations ≥100 nmol/L, and a minimum of 16 hours exposure allowed maximal sensitization. This information was integrated with pharmacokinetic modeling to define an optimal dosing regimen for peposertib of 125 mpk dosed just prior to and 7 hours after irradiation. Using this drug dosing regimen in combination with 2.5 Gy × 5 fractions of radiation, significant prolongation in median survival was observed in M12-eGFP (104%; P = 0.0015) and M15 (50%; P = 0.03), while more limited effects were seen in M27 (16%, P = 0.04). These data support the concept of developing peposertib as a radiosensitizer for brain metastases and provide a paradigm for integrating in vitro and pharmacokinetic data to define an optimal radiosensitizing regimen for potent DNA repair inhibitors.

Abstract MDM2–p53 inhibition may be effective in glioblastoma (GBM). This study evaluates the pharmacokinetics/pharmacodynamics of BI-907828, a potent antagonist of MDM2, in GBM, and demonstrates a translational paradigm with a focus on a unified “Delivery – Potency – Efficacy” relationship in drug development for central nervous system(CNS) tumors. BI-907828 was tested for cytotoxicity and MDM2–p53 pathway inhibition. Systemic pharmacokinetics and transport mechanisms controlling CNS distribution were evaluated in mice. BI-907828 free fractions in cell media, mouse and human specimens were measured to determine “active” unbound concentrations. Efficacy measures, including overall survival and target expression 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 nmol/L. The CNS distribution of BI-907828 is limited by blood–brain barrier (BBB) efflux mediated by P-gp, resulting in a Kp,uu_brain of 0.002. Despite this seemingly “poor” BBB penetration, weekly administration of 10 mg/kg BI-907828 extended median survival of orthotopic GBM108 xenografts from 28 to 218 days (P < 0.0001). This excellent efficacy can be attributed to high potency, resulting in a limited, yet effective, exposure in the CNS. These studies show that efficacy of BI-907828 in orthotopic models is related to high potency even though its CNS distribution is limited by BBB efflux. Therefore, a comprehensive understanding of all aspects of the “Delivery – Potency – Efficacy” relationship is warranted in drug discovery and development, especially for treatment of CNS tumors.

AbstractBrain tumours have a poor prognosis and lack effective treatments. The blood–brain barrier (BBB) represents a major hurdle to drug delivery to brain tumours. In some locations in the tumour, the BBB may be disrupted to form the blood–brain tumour barrier (BBTB). This leaky BBTB enables diagnosis of brain tumours by contrast enhanced magnetic resonance imaging; however, this disruption is heterogeneous throughout the tumour. Thus, relying on the disrupted BBTB for achieving effective drug concentrations in brain tumours has met with little clinical success. Because of this, it would be beneficial to design drugs and drug delivery strategies to overcome the ‘normal’ BBB to effectively treat the brain tumours. In this review, we discuss the role of BBB/BBTB in brain tumour diagnosis and treatment highlighting the heterogeneity of the BBTB. We also discuss various strategies to improve drug delivery across the BBB/BBTB to treat both primary and metastatic brain tumours. Recognizing that the BBB represents a critical determinant of drug efficacy in central nervous system tumours will allow a more rapid translation from basic science to clinical application. A more complete understanding of the factors, such as BBB–limited drug delivery, that have hindered progress in treating both primary and metastatic brain tumours, is necessary to develop more effective therapies.

Effective treatments for brain tumors remain one of the most urgent and unmet needs in modern oncology. This is due not only to the presence of the neurovascular unit/blood–brain barrier (NVU/BBB) but also to the heterogeneity of barrier alteration in the case of brain tumors, which results in what is referred to as the blood–tumor barrier (BTB). Herein, we discuss this heterogeneity, how it contributes to the failure of novel pharmaceutical treatment strategies, and why a “whole brain” approach to the treatment of brain tumors might be beneficial. We discuss various methods by which these obstacles might be overcome and assess how these strategies are progressing in the clinic. We believe that by approaching brain tumor treatment from this perspective, a new paradigm for drug delivery to brain tumors might be established.

Abstract Radio-resistance mechanisms limit the benefit of radiation therapy (RT) for melanoma brain metastases. A key pathway for radiation-induced DNA double-strand break repair is non-homologous end joining, which is critically mediated by DNA-dependent protein kinase (DNA-PKcs). Here we evaluated radio-sensitizing effects of M3814, a selective oral inhibitor of DNA-PKcs, in patient-derived xenografts (PDXs) of melanoma brain metastases. In a clonogenic survival assay, M3841 augmented RT-induced killing of M12 cells at concentrations of ≥300 nM, and a minimum of 16 h exposure with ~300 nM M3814 was required for effective sensitization. M3814 inhibited RT-induced (5 Gy) auto-phosphorylation of serine-2056 of DNA-PKcs in primary cultures of M12, M15 and M27 PDX lines. Interestingly, inhibition of RT-induced DNA-PKcs by M3814 coincided with increased KAP1 phosphorylation, a DNA damage signaling regulated via ATM. Persistent γH2AX foci were observed in 28% M12 cells at 24 hours after co-treatment with M3814 and RT as compared to 12% cells following RT alone. In vivo pharmacokinetic analysis after single oral dose of 20 mg/kg M3814, showed reasonably short half-life (~2.44 hours) and poor brain distribution in wild-type FBV mice (Kpuu, 0.027). Consistent with an efflux liability, brain distribution of M3814 in triple knockout mice for BCRP/MDR1A/B was ~11 fold higher (Kpuu, 0.215). Compared to normal brain, much higher M3814 concentrations were detected in intracranially implanted M12 tumors (~23 fold and ~20 fold) 2 and 6 hours after a single oral dose of 50mg/kg respectively. The relative exclusion of M3814 from normal brain as compared to brain metastases suggests that this drug may have a favorable toxicity profile when combined with radiation for treatment of melanoma brain metastases, and this hypothesis is being tested in ongoing efficacy studies.