In clinical trials for HGG, multiple agents targeting various oncogenic signaling pathways that play an important role in the biology of HGG have been studied, but unfortunately only a small number of patients seem to benefit from these treatment strategies. Whether these disappointing results are due to a restricted drug delivery through the blood-brain barrier, or due to differential biological characteristics of these HGGs, remains unknown. To better understand these clinical observations and to find potential insight how to overcome them, we intend to measure tumor concentrations of PKIs after approximately two weeks treatment and to determine whether these tumor concentrations correlate with plasma- and CSF concentrations of PKIs. Subsequently, we intend to determine the (phospho)proteomic profiles and kinase inhibitory activity in tumor tissue from these HGG patients after approximately two weeks of treatment with a PKI.