Targeting proton-sensing GPR68 as a ferroptosis-based therapy for glioblastoma

Glioblastoma (GBM) is the most common primary brain cancer in adults. Over 10,000 people are estimated to die yearly from GBM despite aggressive treatment, and there has been no substantial improvement in GBM patient survival for more than four decades (5-year survival rate: mid-1970s 4% vs. 7% today), emphasizing the need for new therapies. A hallmark of GBM is an acidic tumor microenvironment, which promotes tumor progression. GPR68 is a proton-sensing G-protein-coupled receptor activated in response to an acidic microenvironment and implicated in cancer-related processes. Proton Bio co-founders identified the first class of specific GPR68 inhibitors, the ogremorphins, that potently kill a diverse panel of GBM cells through ferroptosis, a novel form of iron-dependent regulated cell death, even those resistant to first-line therapy temozolomide, providing the foundation for the hypothesis that targeting GPR68 represents a novel therapeutic approach against GBM. While the parent ogremorphin had poor pharmacological properties that precluded in vivo validation of this hypothesis, structure-activity relationship analysis yielded improved analogs. Three analogs have been selected as lead candidates. The objectives of this Phase I project are to select a lead ogremorphin and to perform critical proof-of-concept testing in orthotopic GBM patient-derived xenografts (PDXs). To achieve these objectives, the following three specific aims will be pursued: 1) define the in vitro pharmacological properties of the ogremorphin lead candidates, 2) determine the detailed PK/brain accumulation profiles of ogremorphin lead candidates, and 3) demonstrate proof-of-concept that targeting GPR68 with the lead ogremorphin is efficacious against GBM in vivo. In Aim 1, the three lead candidates will be compared side-by- side for plasma protein binding, cytochrome P450 inhibition, microsomal stability, and hERG inhibition. In Aim 2, the pharmacokinetics and brain penetration of the lead candidates will be determined following oral administration in CD-1 mice. From the data generated in Aims 1 and 2, the lead ogremorphin will be selected. In Aim 3, critical proof-of-concept efficacy studies will be performed using orthotopic temozolomide-sensitive and temozolomide-resistant GBM PDX models. Treatment effects will be monitored by survival, MRI analysis of tumor growth, histology and immunohistochemistry. Successful establishment of proof-of-concept will validate GPR68 as a therapeutic target in GBM and move the lead ogremorphin into development. The development of GRP68 inhibitors as therapeutics could potentially revolutionize GBM treatment and finally bring about a welcome shift in the 5-year survival of GBM patients.