Magnetic microspheres as drug carriers: factors influencing localization at different anatomical sites in rats.

Factors of practical expediency influencing magnetic fixation of circulating microspheres, known to be capable of carrying therapeutically effective drugs, were studied in three different anatomic sites in rats: the tail, the hind leg, and the head (including both soft tissues and brain). Specific localization of magnetic microspheres was achieved at these three sites, but only with varying degrees of success. Specific targeting of the microspheres within brain tissue could not be accomplished. Factors influencing and tending to restrict specific retention of circulating microspheres by an external magnetic force include: complex vasculature networks, subsurface depth, and tissue-vascular barriers. Even with magnets producing high magnetic field gradients, effective localization of microspheres would appear to be restricted to tissues relatively close to body surfaces.