The organometallic
anticancer agent titanocene dichloride, Cp(2)TiCl(2), is now in phase II clinical trials as an anticancer
drug, but its mechanism of action is poorly understood. We show here that the interactions of Cp(2)TiCl(2) with human serum
transferrin (hTF) and that of Ti(2)-hTF with
adenosine triphosphate (
ATP) have characteristics that could allow
transferrin to act as a mediator for
titanium delivery to
tumor cells. Such reactions may therefore be important to the anticancer activity of this new class of drugs. Cp(2)TiCl(2) reacts rapidly with human apo-
transferrin under physiological conditions (100 mM NaCl, 25 mM
bicarbonate, and 4 mM
phosphate, pH 7.4) with
carbonate as a synergistic
anion. The Cp
ligands are released from the
drug. Two-dimensional [(1)H, (13)C] NMR studies of epsilon-[(13)C]Met-hTF show that Ti(IV) loads the C-lobe first followed by the N-lobe and binds in the specific Fe(III) sites. The
protein conformational changes induced by Ti(IV) appear to be similar to those induced by Fe(III).
Carbonate can act as a synergistic
anion in Ti(2)-hTF but does not appear to be essential. A specific Ti(IV)-hTF adduct is formed even in the absence of
bicarbonate. When the pH of Ti(2)-hTF solutions is lowered, no Ti(IV) is released at the endosomal pH of ca. 5.0-5.5, but one Ti(IV) dissociates between pH 4.5-2.0. In contrast, in the presence of 1 mM
ATP, all Ti(IV) is readily released from both lobes when the pH is lowered from 7.0 to 4.5. Moreover, Fe(III) displaces Ti(IV) rapidly from the C-lobe of Ti(2)-hTF (<5 min) but only slowly (days) from the N-lobe. Thus, the species Fe(
C)Ti(N)-hTF might also provide a route for Ti(IV) entry into
tumor cells via the
transferrin receptor. Ti(2)-hTF effectively blocked cell uptake of radiolabeled (59)Fe-hTF into BeWo cells, a human placental
choriocarcinoma cell line in culture. These results imply that
titanium transferrin might be recognized by the
transferrin receptor and be taken up into
cancer cells.