Translational Theranostics in Glioblastoma: FAPI-46 as a Precision Medicine Tool

Glioblastoma represents the most lethal form of brain cancer, characterized by a 5-y survival rate of approximately 4%. Fibroblast activation protein (FAP), expressed within both the tumor microenvironment and glioblastoma cells, has been identified as a promising therapeutic target. This study explores the potential of FAP-targeted molecular probes as theranostic agents for glioblastoma. Methods: Human U-87 MG and murine SB28 glioblastoma-derived cell lines were used to establish immunodeficient and immunocompetent models, respectively. FAP inhibitor (FAPI)-46 was radiolabeled with ⁶⁸Ga for imaging and ²²⁵Ac or ¹⁷⁷Lu for therapy. Tumor uptake, therapeutic efficacy, immunomodulatory effects, and survival were evaluated for FAPI-46 alone or in combination with temozolomide in preclinical studies. [⁶⁸Ga]Ga-FAPI-46 tumor uptake was also evaluated in patients. Results: In subcutaneous xenografts, [⁶⁸Ga]Ga-FAPI-46 exhibited high tumor uptake with minimal background signal. In the immunocompromised model, a single dose of [²²⁵Ac]Ac-FAPI-46 combined with temozolomide was sufficient to increase median survival, whereas in the immunocompetent model, 3 consecutive daily doses were required to achieve a significant survival benefit. [¹⁷⁷Lu]Lu-FAPI-46 demonstrated lower therapeutic efficacy, as a monotherapy or in combination with temozolomide, than did [²²⁵Ac]Ac-FAPI-46. Immunomodulatory effects, assessed via flow cytometry, were insufficient to overcome the immunologically "cold" glioblastoma tumor microenvironment. In orthotopic xenografts and patients, [⁶⁸Ga]Ga-FAPI-46 showed low uptake, indicating limited blood-brain barrier penetration and resulting in tumor concentrations likely too low for therapeutic efficacy. Conclusion: FAPI-46 demonstrates significant potential as a theranostic agent for glioblastoma in peripheral models. However, its inability to cross the blood-brain barrier in orthotopic models highlights the need for future studies to improve its clinical applicability.