Abstract
4050
Introduction: Glioblastomas (GBM) are grade IV gliomas and the most aggressive, infiltrative, and fatal type of brain cancer. GBM 2- and 5-year survival rates are 18% and 4%, respectively, after surgical resection combined with chemo- and radiotherapy. Despite intensive research efforts leading to various pharmacologic approaches, a cure has remained elusive. Indeed, no GBM-targeted agents have been approved by the FDA in the past decade. Recently, fibroblast activation protein (FAP) has emerged as a new therapeutic target. FAP is selectively expressed by cancer-associated fibroblasts (CAF). In GBM, FAP is expressed both by CAFs and tumor cells. Radiolabeled small molecules targeting FAP were recently investigated for their use as pan-cancer theranostics agents. The objective of this study was to define FAP expression in GBM tumors using RNA-Seq and immunohistochemistry as well as investigate the effects of FAP-targeted radioligand therapy in a murine model of GBM.
Methods: FAP expression was quantified using RNA-Seq and immunohistochemistry on purified-GBM samples acquired from patients admitted for surgery at UCLA-Ronald Reagan Medical Center, as well as matching patient-direct orthotopic xenograft and gliomaspheres. RNA-Seq results were correlated with data available from the TCGA GBM study. We used U87MG, a glioblastoma-derived cell line known to express FAP as a xenograft model for radioligand therapy. NSG mice were implanted with U87MG cells subcutaneously and tumors were allowed to grow. FAP expression by tumors was assessed by 68Ga-FAPi-46 imaging when tumors reached around 100 mm3. When tumors reached 206 ± 75 mm3, mice were randomized into four treatment groups and treated as follows: (1) vehicle, (2) 5 mg/kg temozolomide, (3) 55 MBq 177Lu-FAPi-46, and (4) 5 mg/kg temozolomide and 55 MBq 177Lu-FAPi-46.
Results: FAP expression showed a heterogeneous expression pattern across our purified-GBM samples that correlates with RNA-Seq data available in the TCGA database. Additionally, FAP expression was conserved in matching xenografts and gliomaspheres. A high expression of FAP was observed in 30-35% of patients. Immunohistochemistry staining for FAP in matching patient and PDX tissues showed an expression of FAP by tumor cells and in pericytes at the periphery of blood vessels as previously shown. In our model, we first confirmed that U87MG tumors expressed FAP by PET imaging with an average FAPi SUVmax of 5.05 ± 0.19. Mice treated with a single injection of 177Lu-FAPi-46 showed a delay in tumor growth compared to vehicle-treated animals. Treatment with temozolomide induced a delay in tumor growth of 38 days. The combination of 177Lu-FAPi-46 and temozolomide did not show a difference in tumor growth when compared to temozolomide alone. Finally, median survival was increased from 16 to 25 days after treatment with 177Lu-FAPi-46 alone (p < 0.05), and from 44 to 51 days for temozolomide and the combination of temozolomide and 177Lu-FAPi-46, respectively.
Conclusions: As FAP is expressed by both tumor cells and the tumor microenvironment in GBM, FAP-targeted radioligand therapy can act on these two tumor compartments. Our results suggest that 177Lu-FAPi-46 could be of potential interest for GBM theranostics and that the use of alpha emitters can further improve the effect of radioligands targeting FAP in GBM. The preliminary results of combining FAP radioligand therapy and temozolomide did not show added benefits and different treatment paradigms must be assessed. Finally, we confirmed that FAP expression is conserved from patient samples to PDX and gliomaspheres, allowing the use of these types of samples for preclinical evaluation of FAP radioligands in orthotopic GBM models.
