Chemotherapeutic drugs released from polymers: Distribution of 1,3-bis(2-chloroethyl)-1-nitrosourea in the rat brain

Purpose. The distribution of [³H]BCNU following release from polymer implants in the rat brain was measured and evaluated by using mathematical models. Methods. [³H]BCNU was loaded into p(CPP:SA) pellets, which were subsequently implanted intracerebrally in rats; [³H]BCNU was also directly injected into the brains of normal rats and rats with intracranially transplanted 9L gliomas. Concentrations of [³H]BCNU on coronal sections of the brain were measured by autoradiography and image processing. For comparison, the kinetics of [³H]BCNU release from the p(CPP:SA) polymer discs into phosphate-buffered saline were also measured. Results: High concentrations of BCNU (corresponding to ~1 mM) were measured near the polymer for the entire 30-day experiment. The penetration distance, defined as the distance from the polymer surface to the point where the concentration of [³H]BCNU in the tissue had dropped to 10% of the maximum value, was determined: penetration distance was ~5 mm at day 1 and ~1 mm at days 3 through 14. Local concentration profiles were compared with a mathematical model for estimation of the modulus φ², an indicator of the relative rate of elimination to diffusion in the brain. From day 3 to 14, φ² was ~7, indicating that BCNU elimination was rapid compared to the rate of diffusive penetration into tissue. The enhanced penetration observed on day 1 appears to be due to convection of extracellular fluid caused by transient, vasogenic edema, which disappears by day 3. Conclusions. Polymer implants produce very high levels of BCNU in the brain, but BCNU penetration into brain tissue is limited due to rapid elimination.