BIOM-21. LIQUID BIOPSY IN SERA OF HIGH-GRADE GLIOMA PATIENTS: PROTOPORPHYRIN IX AS A BIOMARKER

High-grade gliomas (HGG) carry a dismal prognosis. Diagnosis comprises magnetic resonance imaging (MRI) followed by histopathological evaluation of tumor tissue after biopsy collection; no blood biomarker is currently available. Patients are subjected to repeated surgery and MRI for relapse monitoring. Protoporphyrin IX (PpIX) is already in routine use for selectively visualizing HGG tissue during fluorescence-guided resection (FGR). We investigated if it can also serve as serum HGG marker. Patients (HGG: n = 23 primary, pHGG; n = 5 recurrent, rHGG) undergoing FGR received 5-aminolevulinic acid (5-ALA) following standard clinical procedure. The control group of eight healthy volunteers (HCTR) also received 5-ALA. Serum was collected before and repeatedly up to 72 h after drug administration. PpIX was analyzed following liquid-liquid and solid-phase extraction using liquid chromatography coupled to ion trap mass spectrometry. The serum PpIX level before 5-ALA administration was similar in patient and control groups. Significant PpIX accumulation in HGG was observed after 5-ALA administration (ANOVA: p = 0.005, post-hoc: HCTR vs. pHGG p = 0.029, HCTR vs. rHGG p = 0.006). Separation of HCTR from pHGG was possible within CI_95% of t_Max (AUC: 0.943, CI_95% 0.884 - 1.000, p < 0.001); the optimal cut-off for diagnosis was 1275 pmol PpIX/ml serum, reaching 87.0 % accuracy, 90.5 % positive predictive value and 84.0 % negative predictive value. Distinguishing pHGG patients from healthy controls based on serum PpIX is possible. The prodrug 5-ALA is required for PpIX induction, which is safe for the standard clinical dosage. Thus, PpIX is a new target for liquid biopsy in glioma, but larger clinical studies are required to characterize its full potential.