The Y3+ complex of PCTMB, the tri-n-butyl phosphonate ester of pyclen (3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1(15),11,13-triene), was synthesized as well as its Ho3+ and Lu3+ analogues. X-ray diffraction analyses revealed isomorphous dimeric M-2(PCTMB)(2)center dot 9H(2)O (M = Y, Ho, Lu) structures that crystallize in the centrosymrnetric P (1) over bar triclinic space group. H-1 NMR and UV studies in aquecus solutions indicated that Y3+ complexation is fast, being quantitative in 167 min at pH 3.8 and in 13 min at pH 5.5 (25 degrees C, acetate buffer, I = 0.150 M, [Y3+] = [PCTMB] = 0.2 mM). H-1 NMR DOSY and photon correlation spectroscopy experiments evidenced the formation of aggregates in chloroform with a bimodal distribution that changes slightly with concentration (11-24 and 240-258 nm). The behavior of the acid-assisted dissociation of the complex of Y3+ with PCTMB was studied under pseudo-first-order conditions, and the half-life of the [Y(PCTMB)] complex in 0.5 M HCl at 25 degrees C was found to be 37 min, a value that decreases to 2.6 min in 5 M HCl. The Y3+ complex of PCTMB is thermodynamically very stable, with a stability constant of log KY-PCTMB = 19.49 and pY = 16.7 measured by potentiometry. Y-90 complexation studies revealed fast radiolabeling kinetics; optimal radiolabeling conditions were obtained for Y-90 in acetate medium, PCTMB at 10(-4) to 10(-2)M in acetate buffer pH = 4.75, 15 min at 45-60 degrees C. In vitro stability studies in human serum showed that [90Y(PCTMB)] is quite stable, with about 90% of the activity still in the form of the radiotracer at 24 h and 80% from 48 h to 72 h. A comparison with other ligands Such as PCTA, DOTA, and DTPA already used for in vivo application shows that [Y-90(PCTMB)] is an interesting lipophilic and neutral analogue of these reference Chelates for therapeutic applications in aqueous and nonaqueous media.