The present invention relates to efflux inhibitor compositions and methods of using these agents for treating conditions where the activity of efflux transporter proteins (e.g., Breast Cancer Resistance Protein (BCRP) and P-Glycoprotein (P-GP)) inhibit effective delivery of a therapeutic agent to a target tissue (e.g., brain, spinal cord, nerves, cerebrospinal fluid, testis, eyeballs, retina, inner ear, placenta, mammary gland, liver, biliary tract, kidney, intestines, lung, adrenal cortex, endometrium, hematopoietic cells, and/or stem cells).
