Gd-HPDO3A has been internalized into rat hepatocarcinoma cells in the cytoplasm (by electroporation) or in intracellular vesicles (by pinocytosis), respectively. In the former case, the observed relaxation rates are likely dependent upon the amount of internalized paramagnetic complex, whereas in the latter case the relaxation enhancement is "quenched" to a plateau value (about 3 s ؊1 ) when the entrapped amount of Gd-chelate is higher than 1 ؋ 10 10 Gd/cell. The observed behavior has been accounted in terms of a theoretical treatment based on equa

The glutamine transporting system is up-regulated in tumor cells because cell proliferation requires the uptake of large quantities of glutamine. It has been found that the paramagnetic magnetic resonance imaging (MRI) reporter Gd-DOTAMA-C6-Gln, where the glutamine residue is covalently bound to the Gd chelate through a C6 spacer, accumulates in tumor cells both "in vitro" and "in vivo" experiments. The observation that the relaxivity of cellular pellets does not increase with the increase in the amounts of entrapped Gd chelate is taken as an indication that the internalization has occurred through receptor mediated endocytosis. The iv administration of Gd-DOTAMA-C6-Gln allowed the MRI visualization of tumor masses in A/J mice grafted with the murine neuroblastoma cell line Neuro-2a and in Her-2/neu transgenic mice developing multiple mammary carcinoma, respectively.

Gd-DO3A-diph and Gd-AAZTAC17 are lipophilic magnetic resonance imaging (MRI) agents that display high affinity for low-density lipoprotein (LDL) particles. However, on binding to LDL, Gd-DO3A-diph shows a decreased hydration that results in a lower enhancement of water proton relaxation rate. Conversely, Gd-AAZTAC17 displays a strong relaxation enhancement at the imaging fields. Each LDL particle can load up to 100 and 400 UNITS of Gd-DO3A-diph and Gd-AAZTAC17, respectively. Their LDL adducts are taken up by human hepatoblastoma G2 (HepG2) and melanoma B16 tumor cells when added to the incubation medium. T(1) measurements of the labeled cells indicate that Gd-AAZTAC17 is significantly more efficient than Gd-DO3A-diph. Furthermore, it has been found that HepG2 hepatoma cells can internalize higher amounts of Gd-AAZTAC17 than B16 cells and the involvement of LDL receptors (LDLRs) has been demonstrated in competition assays with free LDL. Gd-AAZTAC17/LDL adduct proved to be an efficient probe in the magnetic resonance (MR) visualization of subcutaneous tumors in animal models obtained by injecting B16 melanoma cells into the right flank of mice. Finally, confocal microscopy validation of the distribution of LDL-based probes in the tumor has been obtained by doping the Gd-AAZTAC17/LDL adduct with a fluorescent phospholipid moiety.