Methods and devices are disclosed for selective delivery of therapeutic substances to specific histologic or microanatomic areas of organs. Introduction of the therapeutic substance into a hollow organ space (such as an hepatobiliary duct or the gallbladder lumen) at a controlled pressure, volume or rate allows the substance to reach a predetermined cellular layer (such as the ephithelium or sub-epithelial space). The volume or flow rate of the substance can be controlled so that the intralumenal pressure reaches a predetermined threshold level beyond which subsequent subepithelial delivery of the substance occurs. Alternatively, a lower pressure is selected that does not exceed the threshold level, so that delivery occurs substantially only to the epithelial layer. Such site specific delivery of therapeutic agents permits localized delivery of substances (for example to the interstitial tissue of an organ) in concentrations that may otherwise produce systemic toxicity. Occlusion of venous or lymphatic drainage from the organ can also help prevent systemic administration of therapeutic substances, and increase selective delivery to superficial epithelial cellular layers. Delivery of genetic vectors can also be better targeted to cells where gene expression is desired. The access device comprises a cannula with a wall piercing tracar within the lumen. Two axially spaced inflatable balloons engage the wall securing the cannula and sealing the puncture site. A catheter equipped with an occlusion balloon is guided through the cannula to the location where the therapeutic substance is to be delivered.