ABCB1 and glioblastoma: The unsatisfactory efficiency of anti-glioblastoma therapy is not only due to the restrictive nature of the BBB but also to additional factors such as the blood–tumor barrier (BTB), the development of multidrug resistance (MDR) mediated by P-glycoprotein (P-gp, also known as MDR1 or ABCB1), and the formation of vasculogenic mimicry [127,128,129].