A monoreactive NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) derived prochelator (1-(1-carboxy-3-carbo-tert-butoxypropyl)-4,7-(carbo-tert-butoxymethyl)-1,4,7-triazacyclononane (NODAGA(tBu)(3))) was synthesized in five steps with an overall yield of 21%. It is useful for the coupling to the N-terminus of peptides on solid phase and in solution; it was coupled to [Tyr3]-octreotide (TOC) on solid phase, and the resulting peptide, NODAGA-Tyr3-octreotide (NODAGATOC), was labeled with the radiometals 111In and 67Ga in high yields and good specific activities. [67Ga]- and [111In]-NODAGA-Tyr3-octreotide appear to be useful to visualize primary tumors and metastases which express somatostatin receptors subtype 2 (sstr2), such as neuroendocrine tumors, because of their high affinity to this receptor subtype with IC(50) = 3.5 +/- 1.6 nM and 1.7 +/- 0.2 nM, respectively. NODAGATOC could be used as a SPECT and PET tracer, when labeled with 111In, 67Ga, or 68Ga, and even for therapeutic applications. Surprisingly, [111In]-NODAGATOC shows 2 times higher binding affinity to sstr2, but also a factor of 4 higher affinity to sstr5 compared to [67Ga]-NODAGATOC. [67Ga]-NODAGATOC is very stable in serum and rat liver homogenate. There is no difference in the rate of internalization into AR4-2J rat pancreatic tumor cells; both radioligands are highly internalized, at 4 h a 3 times higher uptake compared to [111In]-DOTA-Tyr3-octreotide ([111In]-DOTATOC) was found. The biodistribution of [67Ga]-NODAGATOC in AR4-2J tumor bearing nude mice is very favorable at short times after injection; there is fast excretion from all nontarget organs except the kidneys and high uptake in sst receptor rich organs and in the AR4-2J tumor. Again it is superior to [111In]-DOTATOC in this respect. The results indicate an improved biological behavior which is likely due to the fact that an additional spacer group separates the chelate from the pharmacophoric part of the somatostatin analogue.

À ¼ GdDOTA). The metallostar was well tolerated by the animals at the concentrations of 0.0500 (high dose) and 0.0125 (low dose) mmol Gd kg À1 body weight; (BW). The signal enhancement in the inversion recovery fast low angle shot (IR FLASH) images after the high-dose metallostar injection was considerably higher than after GdDOTA injection (0.1 mmol Gd kg À1 BW), despite the higher dose of the latter. The high-dose metallostar injection resulted in a greater drop in the spin-lattice relaxation time (T 1 ), as calculated from the inversion recovery true fast imaging with steady-state precession (IR TrueFISP) data for various tissues, than the GdDOTA or the low dose metallostar injection. In summary, these studies have confirmed that the approximately four times higher relaxivity measured in vitro for the metallostar is retained under in vivo conditions. The pharmacokinetics of the metallostar was found to be similar to that of GdDOTA, involving fast renal clearance, a leakage to the extracellular space in the muscle tissue and no leakage to the brain. As expected on the basis of its moderate molecular weight, the metallostar does not function as a blood pool agent. The dynamic g scintigraphic studies performed in Wistar rats with the metallostar compound having 153 Sm enrichment also proved the renal elimination pathway. The biodistribution experiments are in full accordance with the MR and scintigraphic imaging. At 15 min post-injection the activity is primarily localized in the urine, while at 24 h post-injection almost all radioactivity is cleared from tissues and organs.

The detection of sialic acid in living systems is of importance for the diagnosis of several types of malignancy. We have designed and synthesized two new lanthanide ion ligands (L1 and L2) that are capable of molecular recognition of sialic acid residues. The basic structure of these ligands consists of a DTPA-bisamide (DTPA, diethylenetriamine pentaacetic acid) whose amide moieties each bear both a boronic function for interaction with the diol groups in the side chain of sialic acid, and a functional group that is positively charged at physiologic pH values and is designed to interact with the carboxylate anion of sialic acid. The relaxometric properties of the Gd3+ complexes of these two ligands were evaluated. The relaxivity of the GdL1 complex has a significant second-sphere contribution at pH values above the pKa of its phenylboronic acid moiety. The interaction of the Gd3+ complexes of L1 and L2 with each of several saccharides was investigated by means of a competitive fluorescent assay. The results show that both complexes recognize sialic acid with good selectivity in the presence of other sugars. The adduct formed by GdL2 with sialic acid has the higher conditional formation constant (50.43+/-4.61 M(-1) at pH 7.4). The ability of such complexes to recognize sialic acid was confirmed by the results of a study on the interaction of corresponding radiolabeled complexes (153SmL1 and 153SmL2) with C6 glioma rat cells. 153SmL2 in particular is retained on the cell surface in significant amounts.

The in vitro relaxivity of the Gd III -glycoconjugates was studied by 1 H nuclear magnetic relaxation dispersion (NMRD), yielding parameters close to those reported for other DOTA monoamides.The known recognition of sugars by lectins makes these glycoconjugates good candidates for medical imaging agents (MRI and gamma scintigraphy).3