A highly selective peptide agonist of the endothelin-B receptor. Endothelin B receptor agonist SPI-1620 binds to endothelin-B receptors on endothelial cells in tumor blood vessels, which, unlike the angioarchitecture of normal blood vessels, are relatively devoid of smooth muscle. This agent may induce a transient, selective increase in blood flow to a tumor, which may result in an increase in the delivery of anticancer agents to the tumor and, so, an increase in anticancer agent efficacy.