Quantification of Protoporphyrin IX Accumulation in Glioblastoma Cells: A New Technique

Introduction. 5-Aminolevulinic Acid (5-ALA) is a precursor of heme synthesis. A metabolite, protoporphyrin IX (PpIX), selectively accumulates in neoplastic tissue including glioblastoma. Presurgical administration of 5-ALA forms the basis of fluorescence-guided resection (FGR) of glioblastoma (GBM) tumors. However, not all gliomas accumulate sufficient quantities of PpIX to fluoresce, thus limiting the utility of FGR. We therefore developed an assay to determine cellular and pharmacological factors that impact PpIX fluorescence in GBM. This assay takes advantage of a GBM cell line engineered to express yellow fluorescent protein. Methods. The human GBM cell line U87MG was transfected with a YFP expression vector. After treatment with a series of 5-ALA doses, both PpIX and YFP fluorescence were measured. The ratio of PpIX to YFP fluorescence was calculated. Results. YFP fluorescence permitted the quantification of cell numbers and did not interfere with 5-ALA metabolism. The PpIX/YFP fluorescence ratio provided accurate relative PpIX levels, allowing for the assessment of PpIX accumulation in tissue. Conclusion. Constitutive YFP expression strongly correlates with cell number and permits PpIX quantification. Absolute PpIX fluorescence alone does not provide information regarding PpIX accumulation within the cells. Our research indicates that our PpIX/YFP ratio assay may be a promising model for in vitro 5-ALA testing and its interactions with other compounds during FGR surgery. 1. Introduction Glioblastoma multiforme (GBM) is the most common and aggressive primary malignant brain tumor. With the current standard of care, recurrence is inevitable and less than 5% of GBM patients will survive five years after diagnosis [1]. Subtotal surgical resection may contribute to recurrence as tumor initiating cells are left in place. A valuable tool to achieve gross total resection and thereby increase progression-free survival is the use of 5-Aminolevulinic Acid (5-ALA) for fluorescence-guided resection (FGR) [2]. 5-ALA is a substrate of the heme biosynthesis pathway and is the source of carbon for porphyrin synthesis [3]. Malignant gliomas metabolize 5-ALA and accumulate the fluorescent compound, protoporphyrin IX (PpIX) [4]. The presence of PpIX fluorescence within the tumor bed allows for discrimination between neoplastic and nonneoplastic cells [4]. Yet, not all brain tumors accumulate sufficient quantities of PpIX to make the use of 5-ALA advantageous during surgery [5, 6], and it is not fully understood how 5-ALA might affect other physiological processes. At