A Cyclen‐Based Tetraphosphinate Chelator for the Preparation of Radiolabeled Tetrameric Bioconjugates

Abstract

The cyclen-based tetraphosphinate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis[methylene(2-carboxyethyl)phosphinic acid] (DOTPI) comprises four additional carboxylic acid moieties for bioconjugation. The thermodynamic stability constants (logK(ML)) of metal complexes, as determined by potentiometry, were 23.11 for Cu(II), 20.0 for Lu(III), 19.6 for Y(III), and 21.0 for Gd(III). DOTPI was functionalized with four cyclo(Arg-Gly-Asp-D-Phe-Lys) (RGD) peptides through polyethylene glycol (PEG4) linkers. The resulting tetrameric conjugate DOTPI(RGD)4 was radiolabeled with (177)Lu and (64)Cu and showed improved labeling efficiency compared with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The labeled compounds were fully stable in transchelation challenges against trisodium diethylenetriaminepentaacetate (DTPA) and disodium ethylenediaminetetraacetic acid (ETDA), in phosphate buffered saline (PBS), and human plasma. Integrin αvβ3 affinities of the non-radioactive Lu(III) and Cu(II) complexes of DOTPI(RGD)4 were 18 times higher (both IC50 about 70 picomolar) than that of the c(RGDfK) peptide (IC50 = 1.3 nanomolar). Facile access to tetrameric conjugates and the possibility of radiolabeling with therapeutic and diagnostic radionuclides render DOTPI suitable for application in peptide receptor radionuclide imaging (PRRI) and therapy (PRRT).

Aminoalkyl‐1,1‐bis(phosphinic acids): Stability, Acid–Base, and Coordination Properties

“…30 For H 4 L 4 , the next two protonations are for the carboxylate groups, and the last determined protonation constant belongs to a phosphinate group. The values are in the expected range for the –PO 2 – –CH 2 CH 2 CO 2 – fragment 31,32. The protonation constant of the second phosphinate group was not accessible as it is too low to be determined by potentiometry.…”

Dual‐Nuclide Radiopharmaceuticals for Positron Emission Tomography Based Dosimetry in Radiotherapy

“…To achieve regiospecific complexation of Ga III and Bi III , we decided to exploit the complementary metal ion selectivity of the homologous phosphinate chelators DOTPI (large trivalent cations) and TRAP (small trivalent cations, particularly Ga III ), which were linked through their N‐pendant arms (Scheme ). First, DOTPI was equipped with four terminal azide groups.…”

“…Subsequently, selective removal of M III from TRAP and Ga III from DOTPI, respectively, was achieved by exploiting the substantially different coordination behavior of the complexes. Because neither the [M III (TRAP)] nor the [Ga III (DOTPI)] system is particularly stable or kinetically inert, removal of metal ions from these chelates is feasible by means of transchelation with (aq.) Na 3 DTPA or Na 2 EDTA, respectively, under which conditions the respective other complexes, [M III (DOTPI)] and [Ga III (TRAP)], remain intact because of their high kinetic inertness.…”

In Vivo PET Imaging of the Cancer Integrin αvβ6 Using 68Ga-Labeled Cyclic RGD Nonapeptides

“…A pronounced hydrophilicity promotes a fast excretion from nontarget tissues via the kidneys and the urinary tract, which is desired for imaging probes. Concerning receptor affinities, it has been shown in numerous studies that tethering multiple copies of receptor ligands, for example, a v b 3 integrin-targeting peptides of the cyclo(RGDXK) type, to a given reporter (e.g., radionuclide or fluorophor) is a reliable method to obtain constructs with increased integrin activity as well as higher uptake in a v b 3 integrin-expressing tumors (36,(47)(48)(49)(50)(51)(52)(53)(54). The same was observed for a5b1 integrin tracers based on a a5b1-specific pseudopeptide (55)(56)(57), suggesting that integrin-targeted radiopharmaceuticals might generally benefit from multimerization.…”