EC1456 is a folate-tubulysin conjugate constructed with an all-D enantiomeric spacer/linker configuration. When tested against folate receptor (FR)-positive cells, EC1456 demonstrated dose-responsive activity with an approximate 1000-fold level of specificity. Treatment of nude mice bearing FR-positive human xenografts (as large as 800 mm3) with non-toxic doses of EC1456 led to cures in 100% of the mice. Combinations of low dose EC1456 with standard of care agents such as platins, taxanes, topotecan and bevacizumab, safely and significantly augmented the growth inhibitory effects of these commonly used agents. When tested against FR-positive human tumor xenograft models having confirmed resistance to a folate-vinca alkaloid (vintafolide), cisplatin or paclitaxel, EC1456 was found to generate partial to curative responses. Taken together, these studies demonstrate that EC1456 has significant anti-proliferative activity against FR-positive tumors, including models which were anticancer drug resistant, thereby justifying a Phase 1 trial of this agent for the treatment of advanced human cancers.