Novel implantable and externally controllable bio-nanomachines-based treatment strategies for Glioblastoma brain cancer have been proposed recently to bring hope to patients who suffer from this devastating cancer type. The main challenges in developing such strategies lie in both crossing the stringent Blood-Brain Barrier and maximizing the drug concentration at particular sites rich in Glioblastoma cells within safety guidelines. Aiming to provide a first step towards the realization of such a novel treatment method, here we propose analytical models to characterize and analyze an exosome-mediated brain drug delivery molecular communication system. We consider biophysical models and derive the closed-form transfer functions for a communication system that comprises of the engineered neural stem cells that release exosomes into the extracellular space in the brain and Glioblastoma-like cells that collect exosomes from the extracellular space in the brain. The presented numerical results show a dependency of the exosome propagation on various hindrance sources in the extracellular space and a limited operation performance at high frequencies that refer to the exosome concentration dynamics. The collection of exosomes by Glioblastoma-like cells show a dependency on high and stable exosome concentration in the extracellular space and low-frequency operation for a reasonable performance output.