Abstract A recently resolved structure of Ti(IV) bound human serum transferrin (sTf) and cell viability studies reveal that the small molecule citrate and the iron(III) transport protein sTf work together to maintain Ti(IV) in a nontoxic speciation in the body. Identification of this biomolecular interaction elucidates important factors that must be considered when developing Ti(IV)-based anticancer agents. Chemical transferrin mimetic (cTfm) ligands are being employed in a Ti(IV) anticancer drug design strategy to take advantage of the structural features of the sTf metal binding site favorable for Ti(IV) coordination to facilitate Ti(IV) cytotoxicity. The very strong Fe(III) binding property of these ligands are exploited to create stable Ti(IV) complexes that are able to deliver Ti(IV) into cells but exhibit lability in the presence of Fe(III). These complexes are intended to operate by the dual function of iron depletion and Ti(IV) attack at intracellular sites. Work with the cTfm ligands N,N’-di(o-hydroxybenzyl)ethylenediamine-N,N’-diacetic acid (HBED) and deferasirox reveal the structural features of the cTfm ligands that maximize the stability of the Ti(IV) complex outside the cell but enable sufficient lability to trigger cytotoxicity within the cell. The features are those that minimize induced dissociation due to the presence of citrate and sTf. Preliminary results suggest that while Fe(III) depletion may play an important role in the activity of the Ti(IV) cTfm complexes, the two lead complexes may operate via different mechanisms of action. Citation Format: Arthur D. Tinoco, Sergio A. Loza-Rosas, Alexandra Vazquez, Kennett I. Rivero, Lenny M. Negron, Manoj Saxena, Shweta Sharma, Yamixa Delgado, Annelis Sanchez, Nicole Zambrana, Timothy B. Parks. Titanium(IV) regulation by serum transferrin and citrate sheds new insight into the use of chemical transferrin mimetics for Ti(IV) anticancer drug development. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3105A.