We have gone on to demonstrate that co-administration of elacridar, a potent and specific third generation inhibitor of both efflux transporters25,27 can significantly enhance: i) brain concentrations of ispinesib, ii) the frequency of monopolar spindles—a surrogate marker for KIF11 inhibition; and iii) survival over ispinesib alone in an orthotopic xenograft model implanted with human GBM. Here, KIF11 is linked to glioblastoma.