Summation This update summarizes the growing application of “click” chemistry in diverse areas such as bioconjugation, drug discovery, materials science, and radiochemistry. This update also discusses click chemistry reactions that proceed rapidly with high selectivity, specificity and yield. Two important characteristics make click chemistry so attractive for assembling compounds, reagents, and biomolecules for pre-clinical and clinical applications. First, click reactions are bioorthogonal; neither the reactants nor their product’s functional groups interact with functionalized biomolecules. Second, the reactions proceed with ease under mild non-toxic conditions such as at room temperature and usually in water. The copper catalyzed Huisgen cycloaddition, azide-alkyne [3+2] dipolar cycloaddition, and Staudinger ligation, azide-phosphine ligation, each possess these unique qualities. These reactions can be used to modify one cellular component while leaving others unharmed or untouched. Click chemistry has found increasing applications in all aspects of drug discovery in medicinal chemistry such as for generating lead compounds through combinatorial methods. Bioconjugation via click chemistry is rigorously employed in proteomics and nucleic research. In radiochemistry, selective radiolabeling of biomolecules in cells and living organisms for imaging and therapy has been realized by this technology. Bifunctional chelating agents for several radionuclides useful for PET and SPECT imaging have also been prepared using click chemistry. This review concludes that click chemistry is not the perfect conjugation and assembly technology for all applications, but provides a powerful and attractive alternative to conventional chemistry. This chemistry has proven itself to be superior in satisfying many criteria (biocompatibility, selectivity, yield, stereospecificity, etc.); thus one can expect it will consequently become a more routine strategy in the near future for a wide range of applications.

Dinuclear europium(III) complexes of the macrocycles 1,3-bis[1-(4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane]-m-xylene (1), 1,4-bis[1-(4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane]-p-xylene (2), and mononuclear europium(III) complexes of macrocycles 1-methyl-,4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (3), 1-[3'-(N,N-diethylaminomethyl)benzyl]-4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (4), and 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (5) were prepared. Studies using direct excitation ((7)F0 --> (5)D0) europium(III) luminescence spectroscopy show that each Eu(III) center in the mononuclear and dinuclear complexes has two water ligands at pH 7.0, I = 0.10 M (NaNO3) and that there are no water ligand ionizations over the pH range of 7-9. All complexes promote cleavage of the RNA analogue 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) at 25 degrees C (I = 0.10 M (NaNO3), 20 mM buffer). Second-order rate constants for the cleavage of HpPNP by the catalysts increase linearly with pH in the pH range of 7-9. The second-order rate constant for HpPNP cleavage by the dinuclear Eu(III) complex (Eu2(1)) at pH 7 is 200 and 23-fold higher than that of Eu(5) and Eu(3), respectively, but only 7-fold higher than the mononuclear complex with an aryl pendent group, Eu(4). This shows that the macrocycle substituent modulates the efficiency of the Eu(III) catalysts. Eu2(1) promotes cleavage of a dinucleoside, uridylyl-3',5'-uridine (UpU) with a second-order rate constant at pH 7.6 (0.021 M(-1) s(-1)) that is 46-fold higher than that of the mononuclear Eu(5) complex. Methyl phosphate binding to the Eu(III) complexes is energetically most favorable for the best catalysts, and this supports an important role for the catalyst in stabilization of the developing negative charge on the phosphorane transition state. Despite the formation of a bridging phosphate ester between the two Eu(III) centers in Eu2(1) as shown by luminescence spectroscopy, the two metal ion centers are only weakly cooperative in cleavage of RNA and RNA analogues.

Gd-conjugated dendrimer nanoclusters (DNCs) are a promising platform for the early detection of disease; however, their clinical utility is potentially limited due to safety concerns related to nephrogenic systemic fibrosis (NSF). In this paper, biodegradable DNCs were prepared with polydisulfide linkages between the individual dendrimers to facilitate excretion. Further, DNCs were labeled with pre-metalated Gd chelates to eliminate the risk of free Gd becoming entrapped in dendrimer cavities. The biodegradable polydisulfide DNCs possessed a circulation half-life of > 1.6 h in mice and produced significant contrast enhancement in the abdominal aorta and kidneys for as long as 4 h. The DNCs were reduced in circulation as a result of thiol-disulfide exchange and the degradation products were rapidly excreted via renal filtration. These agents demonstrated effective and prolonged in vivo contrast enhancement and yet minimized Gd tissue retention. Biodegradable polydisulfide DNCs represent a promising biodegradable macromolecular MRI contrast agent for magnetic resonance angiography and can potentially be further developed into target specific MRI contrast agents.

This report presents the preparation and characterization of three [Gd-C-DOTA] −1 -dendrimer assemblies by way of analysis, NMRD spectroscopy and Photon Correlation Spectroscopy (PCS). The metal-ligand chelates were preformed in alcohol media prior to conjugation to generation 4, 5 and 6 PAMAM dendrimers. The dendrimer-based agents were purified by Sephadex ® G-25 column chromatography. The combustion analysis, SE-HPLC and UV-Vis data indicated chelate to dendrimer ratios of 28:1, 61:1 and 115:1 respectively. Molar relaxivity measured at pH 7.4, 22°C, and 3T (29.6, 49.8 and 89.1 mM −1 s −1 ) indicated the viability of conjugates as MRI contrast agents. 1/T1 NMRD profiles were measured at 23°C and indicated that at 22 MHz the 1/T1 reached a plateau at 60, 85 and 140 mM −1 s −1 for the generation 4, 5 and 6 dendrimer conjugates, respectively. The PCS data showed the respective size of 5.2, 6.5 and 7.8 nm for G-4, 5, and 6 conjugates.During the development of MRI, contrast agents have been employed to induce additional contrast and increase the sensitivity of MRI scans.(1,2) Increased usage of MRI, combined with the necessity for a contrast agent, prompts development of new efficient agents. Clinically used low molecular weight extracellular contrast agents, e.g. [Gd(DTPA) (H 2 O)] −2 (Magnevist ® ) suffer from rapid extravasation from blood vessels into interstitial space and fast decrease in concentration in blood vessels, combined with rapid whole body clearance.In recent years, dendrimers have become an exciting class of polymeric platforms for assembly of multifunctional macromolecular nanomaterials undergoing pre-clinical development as diagnostics and therapeutic delivery vehicles. There have been reports indicating that dendrimers are applicable as carriers for site-specific delivery of drugs and that they do not alter the function of the molecules attached.(3-8) It has also been discovered that dendrimers can act as drugs themselves.(9,10) In the MRI field, dendrimers have not only allowed molecules to retain their function, but have also greatly enhanced the signal to noise ratio of various imaging modalities; however, this application has never reached its full potential. (21) Herein, we report the preparation and characterization of three contrast agents with different generation (4, 5 and 6) of dendrimers. The C-DOTA (2-(4-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid) ligand is first used to sequester Gd(III) (Scheme 1) and thereafter the formed metal complex is covalently attached to the terminal −NH 2 groups of the dendrimer. We have recently reported that pre-metallation method to be significantly advantageous over conventional post-metallation methods, resulting in both a more directly characterizable product possessing an ~2-fold enhancement of molar relaxivity when using a bifunctional DTPA chelating agent while also decreasing the overall Gd(III) content by ~30%. (27) We have also reported the applicability of a C-DOTA derivative applied to complex Gd(III) as an agent...