The gradient of doxorubicin fluorescence intensity in relation to distance from the nearest blood vessel and a model of doxorubicin distribution in solid tumours. Mice-bearing AR1 tumors (A) or BC19 xenografts (B) were treated with either doxorubicin alone, or pretreated with verapamil or PSC 833 and doxorubicin. Tumors were resected, sectioned and imaged. Image analysis was undertaken using customized algorithms. Values represent mean ± standard error. In panel A, 6, 10 and 9 tumors were analyzed, respectively. In panel B, 6, 7 and 6 tumors were analyzed, respectively. (C) represents a model of doxorubicin distribution without fluouresence interference from neighboring out-of-section blood vessels.