5 research outputs found

    Cu-Free 1,3-Dipolar Cycloaddition Click Reactions To Form Isoxazole Linkers in Chelating Ligands for <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> Centers (M = Re, <sup>99m</sup>Tc)

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    Isoxazole ring formation was examined as a potential Cu-free alternative click reaction to Cu<sup>I</sup>-catalyzed alkyne/azide cycloaddition. The isoxazole reaction was explored at macroscopic and radiotracer concentrations with the <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re, <sup>99m</sup>Tc) core for use as a noncoordinating linker strategy between covalently linked molecules. Two click assembly methods (<i>click, then chelate</i> and <i>chelate, then click</i>) were examined to determine the feasibility of isoxazole ring formation with either alkyne-functionalized tridentate chelates or their respective <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexes with a model nitrile oxide generator. Macroscale experiments, alkyne-functionalized chelates, or Re complexes indicate facile formation of the isoxazole ring. <sup>99m</sup>Tc experiments demonstrate efficient radiolabeling with <i>click, then chelate</i>; however, the <i>chelate, then click</i> approach led to faster product formation, but lower yields compared to the Re analogues

    pH-Controlled Coordination Mode Rearrangements of “Clickable” Huisgen-Based Multidentate Ligands with [M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re, <sup>99m</sup>Tc)

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    The viability of the Huisgen cycloaddition reaction for clickable radiopharmaceutical probes was explored with an alkyne-functionalized 2-[(pyridin-2-ylmethyl)­amino]­acetic acid (PMAA) ligand system, <b>3</b>, and <i>fac</i>-[M<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup> (M = Re, <sup>99m</sup>Tc). Two synthetic strategies, (1) <i>click, then chelate</i> and (2) <i>chelate, then click</i>, were investigated to determine the impact of assembly order on the reactivity of the system. In the c<i>lick, then chelate</i> approach, <i>fac</i>-[M<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup> was reacted with the PMAA ligand “clicked” to the benzyl azide, <b>5</b>, to yield two unique coordination species, <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(O,N<sub>amine</sub>,N<sub>py</sub><b>-5</b>)], M = Re (<b>8</b>), <sup>99m</sup>Tc (<b>8A</b>), and <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub><b>-5</b>)], M = Re (<b>9</b>), <sup>99m</sup>Tc (<b>9A</b>), where coordination is through the triazole (N<sub>tri</sub>), central amine (N<sub>amine</sub>), pyridine (N<sub>py</sub>), or carboxylate (O). Depending on the reaction pH, different ratios of complexes <b>8</b>(<b>A</b>) and <b>9</b>(<b>A</b>) were observed, but single species were obtained of (O,N<sub>amine</sub>,N<sub>py</sub>) coordination, <b>8</b>(<b>A</b>), in basic pHs (>9) and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) coordination, <b>9</b>(<b>A</b>), in slightly acidic pHs (<4). In the <i>chelate, then click</i> approach, the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination of [M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> was preorganized in the alkyne-functionalized <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(O,N<sub>amine</sub>,N<sub>py</sub><b>-3</b>)], M = Re (<b>6</b>), <sup>99m</sup>Tc (<b>6A</b>), followed by standard Cu<sup>I</sup>-catalyzed Huisgen “<i>click</i>” conditions at pH ≈ 7.4, where the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination mode remained unchanged upon formation of the triazole product in the clicked molecule. Despite the slow substitution kinetics of the low-spin d<sup>6</sup> metal, the coordination modes (O,N<sub>amine</sub>,N<sub>py</sub>) and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) were found to reversibly intraconvert between <b>8</b>(<b>A</b>) and <b>9</b>(<b>A</b>) based upon changes in pH that mirrored the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination in basic pHs and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) coordination in acidic pHs. Comparison of the Re and <sup>99m</sup>Tc analogs also revealed faster intraconversion between the coordination modes for <sup>99m</sup>Tc

    pH-Controlled Coordination Mode Rearrangements of “Clickable” Huisgen-Based Multidentate Ligands with [M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re, <sup>99m</sup>Tc)

    No full text
    The viability of the Huisgen cycloaddition reaction for clickable radiopharmaceutical probes was explored with an alkyne-functionalized 2-[(pyridin-2-ylmethyl)­amino]­acetic acid (PMAA) ligand system, <b>3</b>, and <i>fac</i>-[M<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup> (M = Re, <sup>99m</sup>Tc). Two synthetic strategies, (1) <i>click, then chelate</i> and (2) <i>chelate, then click</i>, were investigated to determine the impact of assembly order on the reactivity of the system. In the c<i>lick, then chelate</i> approach, <i>fac</i>-[M<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup> was reacted with the PMAA ligand “clicked” to the benzyl azide, <b>5</b>, to yield two unique coordination species, <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(O,N<sub>amine</sub>,N<sub>py</sub><b>-5</b>)], M = Re (<b>8</b>), <sup>99m</sup>Tc (<b>8A</b>), and <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub><b>-5</b>)], M = Re (<b>9</b>), <sup>99m</sup>Tc (<b>9A</b>), where coordination is through the triazole (N<sub>tri</sub>), central amine (N<sub>amine</sub>), pyridine (N<sub>py</sub>), or carboxylate (O). Depending on the reaction pH, different ratios of complexes <b>8</b>(<b>A</b>) and <b>9</b>(<b>A</b>) were observed, but single species were obtained of (O,N<sub>amine</sub>,N<sub>py</sub>) coordination, <b>8</b>(<b>A</b>), in basic pHs (>9) and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) coordination, <b>9</b>(<b>A</b>), in slightly acidic pHs (<4). In the <i>chelate, then click</i> approach, the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination of [M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> was preorganized in the alkyne-functionalized <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>(O,N<sub>amine</sub>,N<sub>py</sub><b>-3</b>)], M = Re (<b>6</b>), <sup>99m</sup>Tc (<b>6A</b>), followed by standard Cu<sup>I</sup>-catalyzed Huisgen “<i>click</i>” conditions at pH ≈ 7.4, where the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination mode remained unchanged upon formation of the triazole product in the clicked molecule. Despite the slow substitution kinetics of the low-spin d<sup>6</sup> metal, the coordination modes (O,N<sub>amine</sub>,N<sub>py</sub>) and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) were found to reversibly intraconvert between <b>8</b>(<b>A</b>) and <b>9</b>(<b>A</b>) based upon changes in pH that mirrored the (O,N<sub>amine</sub>,N<sub>py</sub>) coordination in basic pHs and (N<sub>tri</sub>,N<sub>amine</sub>,N<sub>py</sub>) coordination in acidic pHs. Comparison of the Re and <sup>99m</sup>Tc analogs also revealed faster intraconversion between the coordination modes for <sup>99m</sup>Tc

    Clickable, Hydrophilic Ligand for <i>fac-</i>[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re/<sup>99m</sup>Tc) Applied in an <i>S</i>‑Functionalized α‑MSH Peptide

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    The copper­(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (<b>1</b> and <b>3</b>) for <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re/<sup>99m</sup>Tc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (<b>3</b>) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of <i>fac</i>-[<sup>99m</sup>Tc<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The <i>fac</i>-[Re<sup>I</sup>(CO)<sub>3</sub>(<b>3</b>)] complex (<b>4</b>) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between <b>3</b> and <i>fac</i>-[<sup>99m</sup>Tc<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup>. The corresponding <sup>99m</sup>Tc complex (<b>4a</b>) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log <i>P</i> analysis compared to an analogous complex with nonfunctionalized pyridine rings (<b>2a</b>). An α-MSH peptide functionalized with an azide was labeled with <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> using both <i>click, then chelate</i> (CuAAC reaction with <b>1</b> or <b>3</b> followed by metal complexation) and <i>chelate, then click</i> (metal complexation of <b>1</b> and <b>3</b> followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexation within a peptide framework. The peptides from the <i>click, then chelate</i> strategy had different HPLC <i>t</i><sub>R</sub>’s and in vitro stabilities compared to those from the <i>chelate, then click</i> strategy, suggesting nonspecific coordination of <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> using this synthetic route. The <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup>-complexed peptides from the <i>chelate, then click</i> strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC<sub>50</sub> analyses, and led to moderately high uptake in B16F10 melanoma cells. Log <i>P</i> analysis of the <sup>99m</sup>Tc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (<b>10a′</b>) compared to the peptide with the unmodified DPA chelate (<b>9a′</b>). In vivo biodistribution analysis of <b>9a′</b> and <b>10a′</b> showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for <b>10a′</b> compared to predominantly hepatic accumulation for <b>9a′</b>. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexes in future studies

    Clickable, Hydrophilic Ligand for <i>fac-</i>[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re/<sup>99m</sup>Tc) Applied in an <i>S</i>‑Functionalized α‑MSH Peptide

    No full text
    The copper­(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (<b>1</b> and <b>3</b>) for <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> (M = Re/<sup>99m</sup>Tc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (<b>3</b>) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of <i>fac</i>-[<sup>99m</sup>Tc<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The <i>fac</i>-[Re<sup>I</sup>(CO)<sub>3</sub>(<b>3</b>)] complex (<b>4</b>) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between <b>3</b> and <i>fac</i>-[<sup>99m</sup>Tc<sup>I</sup>(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup>. The corresponding <sup>99m</sup>Tc complex (<b>4a</b>) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log <i>P</i> analysis compared to an analogous complex with nonfunctionalized pyridine rings (<b>2a</b>). An α-MSH peptide functionalized with an azide was labeled with <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> using both <i>click, then chelate</i> (CuAAC reaction with <b>1</b> or <b>3</b> followed by metal complexation) and <i>chelate, then click</i> (metal complexation of <b>1</b> and <b>3</b> followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexation within a peptide framework. The peptides from the <i>click, then chelate</i> strategy had different HPLC <i>t</i><sub>R</sub>’s and in vitro stabilities compared to those from the <i>chelate, then click</i> strategy, suggesting nonspecific coordination of <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> using this synthetic route. The <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup>-complexed peptides from the <i>chelate, then click</i> strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC<sub>50</sub> analyses, and led to moderately high uptake in B16F10 melanoma cells. Log <i>P</i> analysis of the <sup>99m</sup>Tc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (<b>10a′</b>) compared to the peptide with the unmodified DPA chelate (<b>9a′</b>). In vivo biodistribution analysis of <b>9a′</b> and <b>10a′</b> showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for <b>10a′</b> compared to predominantly hepatic accumulation for <b>9a′</b>. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted <i>fac</i>-[M<sup>I</sup>(CO)<sub>3</sub>]<sup>+</sup> complexes in future studies
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