Delivery of drugs to solid tumors is often impeded by the elevation of the interstitial fluid pressure at the tumor core and the loss of plasma from the tumor to the surrounding normal tissue. Interstitial pressure in tumors is abnormally high because tumor blood vessels are highly permeable relative to their normal counterparts and tumors lack functional lymphatic vessels that can absorb the excess fluid loss [1]. As a result, there is a net outward flow of plasma-like fluid through the interstitial spaces of the tumor toward the exterior of the tumor where functional lymphatics may be found (Fig. 1).

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