The present invention relates to pharmaceutical compositions containing targetable bioconjugates of hydralazine, a direct vasodilating agent previously shown to decrease tumor blood flow, oxygenation and interstitial fluid pressure in solid tumors. These bioconjugates are hydralazine prodrugs that contain hydralazine conjugated to biocompatible carrier molecules which specifically bind to sites that are expressed on a diverse variety of tumor cell types. These hydralazine prodrugs are preferably conjugated through an acid-labile hydrazone link that is designed to be stable in plasma and release hydralazine through acid-catalyzed hydrolysis in the acidic environment of the target tumor. Because these prodrugs are stable at physiological pH and in plasma, they are devoid of systemic vasoactive activity; however, they are acid-labile conjugates that can be hydrolyzed upon reaching the more acid environment of the tumor where the vasoactive activity of hydralazine is restored. These prodrugs selectively bind to tumor-specific receptors on tumor cells, and are degraded in the acidic tumor cell environment or the acidic lysosomal compartments after being internalized into the cell.