The objective of this project is to design novel therapeutics targeting cancer metastasis, the most deadly aspect of epithelial cancers. Our previous studies have shown that Rac, a key signaling intermediate that regulates actin cytoskeletal changes during cell migration/invasion and cancer metastasis, is associated with high cancer metastatic efficiency. Since Rac proteins are activated by GDP/GTP exchange, inhibition of the interaction of Rac with its guanine nucleotide exchange factors (GEF) is expected to inhibit Rac activity. Since the known Rac inhibitor NSC-23766 is not effective at therapeutically useful concentrations, we synthesized novel NSC-23766 derivatives. Results show that the new inhibitor EHop-016 is 100-fold more efficient than the parent compound at inhibiting Rac activity of metastatic cancer cells without affecting the activity of the related GTPase, Rho. EHop-016 inhibited Rac activity of metastatic cancer cells with an IC50 of 0.78 microM. Cell viability assays, performed to test for the toxicity of this compound, demonstrated that at concentrations below 5 microM, EHop-016 did not affect mammary epithelial cell (MCF-10A) viability and decreased proliferation of metastatic cancer cells (MDA-MB-435) by only 20%. We conclude that EHop-16 is a potent Rac inhibitor with promise of further development as a small molecule inhibitor of cancer cell invasion, and thus, metastasis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3251. doi:10.1158/1538-7445.AM2011-3251