PGP and neoplasm: Liposomes overcome drug resistance due to endothelial P-gp efflux mechanism at blood-brain and blood-tumor barriers in brain tumors where the barriers allows extravasation of long circulating liposomes and circumvent drug resistance with stabilized liposomal Doxorubicin in rat intracranial sarcoma model (Siegal et al., 1995) and rat intracranial 9L gliosarcoma model (Zhou et al., 2002).