Tissue transport of anti-cancer drugs.

Blood-borne drug molecules are transported through as well as around cells in tissue. For small molecule drugs with a molar weight <1000, the wall of the capillary blood vessels in tumors usually is not a barrier. Just after a rise in the drug concentration in the blood, the cells closest to the microvessels are exposed to the highest drug concentrations. Short or long lasting concentration gradients away from the capillary vessels will develop. Since in a tumor the distance to the nearest blood vessel can be relatively large, inefficient transport of drugs to some cancer cells may limit drug efficacy. Studies on in vitro drug gradients have given insight into the factors determining this transport. Small intercellular distances, high cellular drug influx and low drug efflux rates, and high intracellular and extracellular drug binding favor the development of drug gradients. In the absence of drug metabolism, gradients "level out" over time and may reverse as the blood concentration drops. Understanding the drug transport process from the microvessels to every cancer cell will be important for optimizing cancer chemotherapy. Cancer cells that can "hide" for the drug may lead to regrowth of the tumor.