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).
