The present invention is concerned with delivering a pharmaceutical composition to a tissue target of a mammalian subject for treating brain diseases or disorders. The process includes the steps of: (i) providing a dispersion of the pharmaceutical composition as particles having an average particle size of from about 150 nm to about 100 microns, and (ii) administering the dispersion to the mammalian subject for delivery to the tissue target of a portion of the pharmaceutical composition by cells capable of reaching the tissue target. The dispersion of the pharmaceutical composition as particles, for example, can be phagocytized or adsorbed by the cells prior or subsequent to administration into the mammalian subject. The dispersion of the pharmaceutical composition can be administered to the central nervous system or the vascular system. After administration, the loaded cells transport the pharmaceutical composition as particles into the tissue target.