Relaxivities of Dendrons Based on a OEG-DTPA Architecture: Effect of Gd Placement and Dendron Functionalization

1 Institute for Research in Biomedicine, Baldiri Reixac 10, 08028 Barcelona, Spain 2Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Spain 3Combinatorial Chemistry Unit, Barcelona Science Park, Baldiri Reixac 10, 08028 Barcelona, Spain 4Departament de Bioquı́mica i Biologia Molecular, Universitat Autònoma de Barcelona, Unitat de Biociències, Edifici C, 08193 Cerdanyola del Vallès, Spain 5Institut de Biotecnologia i de Biomedicina, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain 6Department of Organic Chemistry, University of Barcelona, Mart́ı i Franquès 1-11, 08028 Barcelona, Spain 7School of Chemistry & Physics, University of KwaZulua-Natal, Durban 4001, South Africa


Introduction
Magnetic resonance imaging (MRI) is a widely used diagnostic tool to study the anatomy and function of the human body in both disease and health.Advantages of this imaging technique are that MRI is noninvasive, does not involve radiation, and has excellent spatial resolution [1].However, one of the drawbacks of MRI is its relatively low sensitivity.For this reason, there is an increasing demand for more effective and specific contrast agents that help to increase the contrast between pathological and healthy tissues.This can be enhanced by using contrast agents based on Gd 3+ that change the longitudinal relaxation rate ( 1 ) or by contrast agents based on superparamagnetic iron particles [2,3] that change the transverse relaxation rate ( 2 ).The main mode of imaging currently employed is positive mode imaging, based on  1 relaxivity.For bimodal imaging using both positive and negative modes (based on  2 relaxivity), versatile contrast agents, which can enhance both relaxivity constants, are required.Such bimodal contrast agents would provide more clinical information, by combining the unique strengths of each technology and, at the same time, by reducing the adverse effects of administration of multiple agents [4].The efficiency of both kinds of contrast agents is expressed in relaxivity constants  1 or  2 , depending on the type of relaxivity employed.These constants describe 2 Journal of Nanotechnology the relaxation rate enhancement of water protons per millimole of metal ion.The development of compounds with higher relaxivities could lead to contrast agents with higher sensitivity, which are detectable at lower doses and provide higher contrast at equivalent doses.The  1 relaxivity can be increased by enhancing the water-exchange rate [5,6] or by slowing down the tumbling rate of the contrast agent [7].Dendrimers, highly branched macromolecules with a welldefined architecture, are interesting platforms for increasing the relaxivity because the increased molecular size results in a decreased molecular rotation.Furthermore, one single dendrimer molecule can be functionalized with targeting moieties or outfitted with several gadolinium ions enhancing the relaxivity even more [8,9].Additionally, high molecular weight macromolecules have a longer blood circulation time as compared to low molecular weight (LWH) agents with a short residence time in the vascular system [10][11][12].
In our research group, we have worked extensively on the synthesis and biomedical applications of dendrons which consist of a DTPA derived core unit onto which monodisperse branches of oligoethylene glycol (OEG) are conjugated.The synthesis of diverse OEG-based dendrimers has been previously described by us [13][14][15].These dendrons can be grown up to generation 2 using amide bond formation or up to generation 3 by click chemistry using the copper catalyzed cycloaddition between azides and alkynes.The periphery of the dendrons consists of amine groups, which can be easily functionalized with diverse bioactive moieties such as peptides or diverse imaging agents.As was mentioned before, this type of molecules contains DTPA moieties within their structures, a widely known chelating agent [16], which can be placed in different positions of the dendron structure with diverse derivatizations.Taking into account the particular composition of these compounds, the aim of the present work was to study how the paramagnetic Gd 3+ ion could be entrapped inside the DTPA containing architecture of the dendrons and how this would affect the relaxivity of the chelate.

Synthesis Dendron D2 (DTPA-5OEG Ac
). DTPA bisanhydride (100 mg, 0.28 mmol) was dissolved in a mixture (7 : 3) of CH 2 Cl 2 /DMF (150 mL) and PyBOP (800 mg, 1.54 mmol) together with Boc-TOTA (492 mg, 1.54 mmol) was added.The basicity of the reaction mixture was adjusted to pH 8 by the addition of DIEA.The reaction was allowed to stir for 1 hour at room temperature, after which the solvent was concentrated under reduced pressure.The crude product was dissolved in 50 mL of CH 2 Cl 2 and washed three times with 5% NaHCO 3 (50 mL).The crude product was dissolved in 10 mL of CH 2 Cl 2 and transferred to a 50 mL falcon tube.Hexane (40 mL) was added and the falcon was vigorously shaken and centrifuged.The supernatant was discarded and the precipitated pellet corresponded to the crude product.
The crude was purified by flash chromatography over basic aluminum oxide eluting with 1% MeOH in DCM to yield the pure Boc-protected dendron (450 mg, 85%).The Boc-terminated dendron (180 mg, 0.095 mmol) was dissolved in 5 mL of TFA/H 2 O (95 : 5) and stirred for 1 hour at room temperature.Subsequently, the TFA was evaporated using a flow of N 2 and the product was precipitated in methyl tert-butyl ether in order to remove the carbocations generated during the deprotection.After decanting the methyl tertbutyl ether, the pellet containing the deprotected dendrimer was dissolved in 5 mL CH 2 Cl 2 and DIEA (0.63 mmol, 111 L) and Ac 2 O (0.52 mmol, 49 L) were added.The reaction was allowed to stir for 2 hours at room temperature, and then hexane (40 mL) was added.The mixture was stirred vigorously and centrifuged.The supernatant was discarded and the remaining oily precipitate corresponded to pure compound D2 (167 mg, 89%).

Synthesis Dendron D3 (DTPA-1OEG NH2-4OEG peptide).
The dendron DTPA-1coobn-4OEG Boc (synthesis reported in [12]) (600 mg, 0.24 mmol) was dissolved in 50 mL MeOH.The heterogenous catalyst 10% wt Pd/C was added (40 mg) and the flask was purged with N 2 and then with H 2 .The reaction was stirred for 2 hours at room temperature under H 2 atmosphere, and then the flask was purged again with N 2 .The catalyst was removed by filtration over Celite and washed with ethyl acetate (3 × 15 mL).The organic phase, consisting of MeOH and ethyl acetate, was evaporated to yield yellowish oil.This oil was dissolved with CH 2 Cl 2 /DMF (7 : 3, v/v; 100 mL) and PyBOP (135 mg, 0.26 mmol) together with 13-azido-4,7,10-trioxatridecaneamine (NH 2 -OEG-N 3 ; 65 mg, 0.26 mmol) was added.The basicity of the reaction mixture was adjusted to pH 8 by the addition of DIEA.The reaction was allowed to stir for 1 hour at room temperature, and then the solvent was concentrated under reduced pressure.The crude product was dissolved in 50 mL of CH 2 Cl 2 and washed three times with 5% NaHCO 3 (50 mL).The organic phase was dried with MgSO 4 , and the volume was reduced to 10 mL and transferred to a 50 mL falcon tube.Hexane (40 mL) was added and the falcon was vigorously shaken and centrifuged.The supernatant was discarded and the precipitated pellet corresponded to the crude product.The crude was purified by flash chromatography over basic aluminium oxide eluting with 1% MeOH in DCM to yield the pure dendron (389 mg, 91%).The dendron platform DTPA-1OEG N 3 -4OEG Boc (100 mg, 0.055 mmol) was dissolved in 5 mL 4 M HCl in dioxane and stirred at room temperature overnight.Then, dioxane was evaporated to dryness.The crude was dissolved in 100 mL of CH 2 Cl 2 /DMF (7 : 3, v/v) and PyBOP (124 mg, 0.24 mmol) together with Ac-NH-Asp(tBu)-Gly-Ser (tBu)-Arg(Pbf)-OH (201 mg, 0.24 mmol) was added.The basicity of the reaction mixture was adjusted to pH 8 by the addition of DIEA.The reaction was allowed to stir for 1 hour at room temperature, and then the solvent was concentrated under reduced pressure.The crude product was dissolved in 20 mL of CH 2 Cl 2 and washed three times with 5% NaHCO 3 (20 mL).The organic phase was dried with MgSO 4 and evaporated to yield the peptide functionalized dendron.This compound was dissolved in 5 mL of a mixture of EtOH/H 2 O (7 : 3, v/v) and NH 4 Cl (6 mg, 0.13 mmol) and fine zinc powder (5 mg, 0.072 mmol) were added.The suspension was magnetically stirred for several hours until the colour of the suspension turned to a lighter grey, indicating the oxidation of the zinc particles.Completion of the reaction was confirmed by HPLC-PDA and HPLC-MS.After completion, the EtOH was removed in vacuo.The crude was transferred to a 50 mL falcon tube using CH 2 Cl 2 (40 mL).The falcon was vigorously shaken and centrifuged, thereby precipitating the salts.The supernatant was collected and evaporated to yield the crude peptide functionalized dendron.The side-chain protecting groups were removed by dissolving the pellet in 4 mL of TFA/H 2 O/TIS (95 : 2.5 : 2.5).After 1 hour of reaction at room temperature, the TFA was removed using a flow of N 2 and the compound was precipitated in cold tert-butyl methyl ether.The precipitate was dissolved in 4 mL of H 2 O and dialyzed for several hours using a dialysis membrane with MWCO 1 kDa to yield the final product D3 (55 mg, 30%).

Formation of Gadolinium-Dendron Complexes.
The dendrons were dissolved in water and GdCl 3 was added (1.2 eq per chelating moiety).The pH of the solution was adjusted to 7 and the mixture was incubated overnight at room temperature.Then, the excess of Gd 3+ (nonchelated) was removed from the final samples by dialysis.The samples were dialyzed several times until Gd 3+ was not detected in the external dialysis media and dialyzed again one more time.The presence of Gd 3+ in the external dialysis media was checked by the use of xylenol orange, a widely sensitive indicator for heavy metals.The final complexes were lyophilized and analyzed HPLC-MS to verify full complex formation.

Data Processing.
For  1 and  2 calculations for each compound, three different regions of interest (ROIs) were manually defined inside each phantom vial, for each concentration point, after visual inspection of the images acquired for  1 and  2 measurements, in the coronal plane.Image analysis was carried out with Bruker software Paravision (version 4.0).The inverse of the calculated  1 and  2 values (1/ 1 and 1/ 2 ) was plotted against Gd concentration for each experimental point, and the slope of the line corresponded to the compound relaxivity (s −1 mM −1 ).

Statistical Analysis.
Normality of the data was first inspected in each group by the Kolmogorov-Smirnov test and variance homogeneity with the Levene test.Two-tailed student's -test for independent measurements was used for statistical analysis and the significance level for all tests was set to  < 0.05.

Results and Discussion
In light of the aforementioned objective, six different dendrons were synthesized (see Figure 1).Compound D1 is a first generation dendron, prepared from a bifunctional orthogonally protected DTPA core, which contains acetylated amines on its surface.Compound D2 is a first generation dendron with 5 OEG-Ac branches and it was synthesized using the commercially available DTPA bisanhydride.Compound D3 was prepared by conjugating a short DGSR peptide onto an orthogonally protected derivative of D2.Compound D3 was included in the study to see whether the functionalization of the periphery would influence the relaxivity.Compound D4 is a second generation dendron constructed by means of amide bonds [15].And finally compounds D5 and D6 are the second and third generation dendrons, respectively, constructed from the corresponding first generation dendron through a two-step process of diazo transfer and click chemistry reactions; thus, the dendrimer growth is achieved through triazole bonds [13].
The first step was to explore how the different DTPA containing dendritic structures were able to chelate gadolinium.The chelating capacity of the different dendritic structures was studied using HPLC-PDA and HPLC-MS after incubation of the dendrons in an aqueous solution of GdCl 3 at pH 7. Analysis by mass spectrometry clearly showed the mass of the dendron-metal complex and allowed us to distinguish between Gd 3+ -dendron complex and free dendron.After complete complex formation was observed, the excess of Gd 3+ was removed by several dialysis processes and final complex confirmed by HPLC-MS and HPLC-PDA.Using mass spectrometry technique, it was found that the chelation was influenced heavily by the derivation of the DTPA moiety (see Figure 2).For instance, the first generation dendron with a benzyl ester in the focal point (D1 derivative) was not able to chelate gadolinium.The term focal point refers to the functional group in the center of the DTPA moiety.However, when the benzyl group was replaced by an amide moiety (D2 and D3) or a free carboxylic acid (D1), the dendron was able to chelate gadolinium.
For the higher generation dendrons, their chelating capacity depends on the framework of the dendron, that is, triazole moiety (D5 and D6) or amide bond (D4).For example, in second generation dendrons, the four DTPA moieties in the apical positions (DTPA moieties of the branching units) did not chelate gadolinium when containing triazole rings (D5 and D6).Gadolinium chelation did occur when the second generation dendron framework was composed of amide bonds (D4) instead of the triazole rings.This means that this amide-type architecture has a higher gadolinium payload.
In order to see how the dendritic architecture of the dendron-gadolinium would influence the distribution of the paramagnetic effect of the gadolinium ion, we measured the water proton relaxation rates  1 and  2 at a concentration range of 0.125-2 mM of chelated Gd 3+ in water.The relaxivities  1 and  2 were calculated by determining the slope of the regression line of 1/ 1 and 1/ 2 versus the concentration with a least-squares method.The relaxivities  1 and  2 for the different types of dendron-metal complex are given in Table 1.The commercially available contrast agent DOTAREM was used as a reference.

𝑟 1 Relaxivity.
As can be seen in the measured relaxivities for D1, D5, and D6, containing the gadolinium ion in the barycenter of the OEG-based dendron indeed had a positive effect on the relaxation rate.As expected, this effect is a result of the increased size of the dendrons, which together with the placement of the metal in the barycenter decreased the overall tumbling rate of the dendron-metal complex.(ref [15])  The first generation dendron (D1) produced a nonsignificant increase ( > 0.05) as compared to DOTAREM, but the larger second (D5) and third generation (D6) dendrons gave a significant twofold and threefold increase, respectively, in the longitudinal relaxation rate ( 1 ).The increase in relaxivity (4.2-12.3mM −1 s −1 at 7) was not as high as compared to the values reported by Fulton and coworkers for their polysaccharide or oligoethylene glycol based dendritic structures in which the gadolinium ion was placed in the barycenter as well (ca.27 mM −1 s −1 at about 3) [17,18].Since the size of our higher generation dendrons is similar or even higher than that of the molecules described by Fulton and colleagues, other explanations should be considered to account for those differences.Then, the well-known decrease of  1 with field strength [19] could possibly explain most of the recorded differences.Indeed, a factor of ca.3-fold decrease in  1 has been described for nanoparticulated Gd quelates between 1.4 and 7 [20].Still, additional contributions could come from the fact that compounds described in [18] were glyconjugates, providing second-sphere water molecules close to the chelated Gd, with increased bulk relaxivity.This feature was absent in the dendron structures described in this work.
The second generation dendron based on an amide framework (D4) also exhibited a higher relaxation rate of 16.08 mmol −1 s −1 per gadolinium ion, four times greater than DOTAREM.This means that trapping the gadolinium ion in the amide apical centers, which are the branching units between the central core and the periphery of the dendron, also has a positive effect on the relaxivity.This effect seems to be even greater than when Gd 3+ is situated in the barycenter.Placement of a gadolinium ion in the apical centers has the advantage of an increased cargo load (4 Gd 3+ ions) compared to placement in the barycenter (1 Gd 3+ ) and a total relaxivity ( 1 ) of 64.40 mM −1 s −1 per dendron molecule.Compared to other dendrimer structures (i.e., PAMAM;  1 = 30 mM −1 s −1 per dendron molecule) which carry the gadolinium-ligand complex at the periphery [12], the gadolinium placed in the amide apical center is closer to the core of the dendrimer and its axis of motion.This helps to decrease the overall rotation of the gadolinium ion and then promote higher relaxivity, similarly as with placement in the barycenter.Therefore, placement in the amide apical center offers an elegant option to load a dendron with multiple gadolinium ions.At the same time, the periphery of the dendron is available for decoration with any desired functional moiety.
The most dramatic increase in the relaxivity constant  1 was caused by the dendron D3, the first generation dendron outfitted with four peptide DGSR sequences.The relaxivity was increased by a factor of 8-9 as compared to the other first generation dendrons, even though the molecular weight of the peptide-dendron conjugate was only 1.5 times higher.Therefore, the small increase in molecular weight could not be used to explain the large increase in relaxivity.A possible contribution to the relaxivity gain could arise from the formation of a hydrogen-bonded network of second-sphere water molecules attracted by the polar residues of the peptide sequences [18,21,22].An additional favorable aspect may be an  2 / 1 ratio within the expected for classical  1 agents (1.15), very similar to the one of DOTAREM (1.17).

𝑟 2
Relaxivity.Due to the decreased rotation of the gadolinium ion imposed by the dendrimer structure, the relaxivity constant  1 increased.But as can be seen in Table 1, there is the added feature of an increase in the  2 constant as well.The increase in the  2 constant follows a similar pattern as the increase of the  1 constant, with again compound D3 showing the most dramatic and statistically significant increase in comparison with DOTAREM relaxivity values.
In the literature, some research has been reported about simultaneous increases in both relaxivity constants  1 and  2 This was accomplished either by entrapping DOTAREM in hydrogel nanoparticles [23] or by outfitting polyamide dendrimers with hydroxypyridinone (HOPO) complexes [24,25].In the first report [23] related to hydrogel nanoparticles, the improvement in the relaxivity was attributed by the authors to both the restriction of the rotational motion of the gadolinium complex and the highly hydrated nature of the hydrogel network, which enhances the outer-sphere water molecule interactions and effectively transfers magnetic effect to the bulk water.This previous principle was also used to explain the higher  2 relaxivity of polyamide dendrimers with HOPO [24,25], where the outer-sphere water exchange plays a large role in  2 relaxation.Therefore, the increased  2 relaxivity induced by the OEG-based dendrons shown in this work might also be explained by the hydrophilic character of the molecules, which gives rise to a higher number of hydrogen bonded second and outer-sphere water molecules.
3.3.Phantom Studies at 7.0.In order to better visualize how the changes in relaxivity values were translated into contrast, both  1 and  2 weighted images of the phantom containing solutions of dendrons and DOTAREM acquired at 7 for relaxivity calculation are shown (Figure 3, A-D, DOTAREM versus dendron D3).In the  1 weighted images, signal enhancement clearly increased with the Gd 3+ concentration, whereas, in the  2 weighted images, a signal decrease (darkening) was observed with the increase of gadolinium concentration.The comparison with controls (no contrast agent added) confirmed that the enhancement of the signal, both positive and negative, was only observed in dendrons or DOTAREM solutions.Figure 3 (dendron D3) also shows clearly that the contrast enhancement obtained is better than the one obtained with DOTAREM, both for  1 and  2 weighted images.Overall, these studies corroborated the changes observed in relaxometric values and also pointed to the dual properties of the studied dendrons.

Conclusion
In this study, we examined how the architecture and functionalization of dendrons based on DTPA branching units influenced the relaxivity properties of the dendron-gadolinium complexes.It was found that, as expected, the increased size of the second and third generation dendrons significantly enhanced the relaxivity due to the decreased tumbling rate of the contrast agent complex when placing the paramagnetic gadolinium ion in the barycenter.An added effect of the OEG-based dendrons is their hydrophilic nature, which causes the relaxivity  2 to be increased as well.Furthermore, the periphery of the dendrons can be decorated with functional moieties of distinct natures, for instance peptides, which can be used for active targeting or even to enhance the relaxivity constants  1 and  2 even more.The simultaneous increase in the relaxivity constants  1 and  2 could be used to develop contrast agents for dual mode imaging, which can give complementary information to single contrast type agents.

Figure 1 : 3 )Figure 2 :
Figure 1: Molecular structures of the different OEG-DTPA dendrons used in this study.The central DTPA moiety is indicated in blue whereas the apical DTPA moieties are indicated in red.

Figure 3 :
Figure 3: MRI acquisitions at 7.0 of the phantom samples used for relaxivity calculation.Rows A and C show DOTAREM solutions ( 1 and  2 weighted MRI acquisitions, resp.) at increasing Gd 3+ concentration listed below the figure, while rows B and D show D3 solutions at the same concentrations/conditions as for DOTAREM, except by 0 mM (pure water).There was only one 0 mM (no CA) phantom tube.The faint image corresponding to 0 mM is outlined with a dotted white line in the  1 weighted acquisitions for better visualization.