Aggregation behavior of polyisoprene-pluronic graft copolymers in selective solvents.

Novel amphiphilic graft copolymers composed of a polyisoprene (PIP) backbone with Pluronic side chains, polyisoprene-g-Pluronic, have been synthesized using a "graft onto" technique. Small-angle neutron scattering (SANS) has been used to characterize the conformation of the P123 and P103 Pluronic graft copolymers in selective solvents such as ethanol and hexane and in a nonselective solvent, tetrahydrofuran (THF). The results indicated that, in a selective solvent for the side chain Pluronics (e.g., ethanol), "crew-cut" micelles were formed with a large core of radius ∼ 120 Å; data were fitted with a core-shell model. In a good solvent for the backbone (e.g., hexane), "flowerlike" micelles were formed with a small inner radius of ∼64 Å. In the nonselective solvent, a swollen polymer coil was found, which was described using the Guinier-Debye model. As THF/ethanol and THF/hexane can be prepared in any ratio, it was possible to vary the solvent composition gradually in order to study the transition from swollen coil to micelle. When going from 100% THF to 100% ethanol, the transition to micellar behavior was observed at a ratio of 20:80 (v/v %) THF/ethanol for both grafted copolymers and 40:60 (v/v %) THF/hexane for grafted P123 copolymers.

Effect of temperature, cosolvent, and added drug on Pluronic-flurbiprofen micellization.

Structural changes in the micellization of Pluronics P103 and P123, as a function of temperature, cosolvent (ethanol, 10 v/v %), and the addition of the hydrophobic drug flurbiprofen, were investigated by SANS and tensiometry. Addition of ethanol increases the critical micellization concentration of the Pluronics (making the polymer more soluble), while increasing the repulsive interactions between the flurbiprofen-Pluronic spherical complexes. However, increasing temperature and addition of drug increases both the aggregation number and core radius and leads to a more dehydrated core. The addition of flurbiprofen to Pluronic P103 was also found to reduce the critical micellization temperature from between 15 and 20 °C to below 10 °C and at higher drug concentrations leads to an attractive interaction between micelles and eventually phase separation.

Growth and shrinkage of pluronic micelles by uptake and release of flurbiprofen: variation of pH.

The micellization of Pluronic triblock copolymers (P103, P123, and L43) in the presence of flurbiprofen at different pH was studied by small-angle neutron scattering (SANS), pulsed-field gradient stimulated-echo nuclear magnetic resonance (PFGSE-NMR), and surface tension measurements. Addition of flurbiprofen to the Pluronic at low pH leads to an increase in the fraction of micellization, aggregation number, and the core radius of the micelles. However, changing the pH to above the pKa of flurbiprofen in an ethanol/water mixture (∼6.5) reduces the fraction of micellization and results in a weaker interaction between the drug and micelles due to the increased drug solubility in aqueous solution.

Flurbiprofen encapsulation using pluronic triblock copolymers.

Pulsed-field gradient stimulated-echo nuclear magnetic resonance (NMR) and surface tension measurements have been used to study the effect of drug addition on the micellization behavior of pluronic triblock copolymers (P103, P123, and L43). The addition of 0.6 wt% flurbiprofen to Pluronic P123 and P103 solutions reduced their cmc and promoted micellization. Also, a substantial increase in the hydrodynamic radius of Pluronic P103 from 5 to 10 nm was observed, along with an increased fraction of polymer micellized, demonstrating that the polymers solubilize this nonsteroidal anti-inflammatory drug.

Investigation into 64Cu-labeled Bis(selenosemicarbazone) and Bis(thiosemicarbazone) complexes as hypoxia imaging agents.

BACKGROUND: Cu-diacetyl-bis(N4-methylthiosemicarbazone) [Cu-ATSM], although excellent for oncology applications, may not be suitable for delineating cardiovascular or neurological hypoxia. For this reason, new Cu hypoxia positron emission tomography (PET) imaging agents are being examined to search for a higher selectivity for hypoxic or ischemic tissue at higher oxygen concentrations found in these tissues. Two approaches are to increase alkylation or to replace the sulfur atoms with selenium, resulting in the formation of selenosemicarbazones. METHODS: Three 64Cu-labeled selenosemicarbazone complexes were synthesized and one was screened for hypoxia selectivity in vitro using EMT-6 mouse mammary carcinoma cells. Rodent biodistribution and small animal PET images were obtained from BALB/c mice implanted with EMT-6 tumors. One alkylated thiosemicarbazone was synthesized and examined. RESULTS: Of the three bis(selenosemicarbazone) ligands synthesized and examined, only 64Cu-diacetyl-bis(selenosemicarbazone) [64Cu-ASSM] was isolated in high-enough radiochemical purity to undertake cell uptake experiments where uptake was shown to be independent of oxygen concentration. The bis(thiosemicarbazone) complex synthesized, 64Cu-diacetyl-bis(N4-ethylthiosemicarbazone) [64Cu-ATSE], showed hypoxia selectivity similar to 64Cu-ATSM although at a higher oxygen concentration. Biodistribution studies for 64Cu-ASSM and 64Cu-ATSE showed high tumor uptake at 20 min (64Cu-ASSM, 10.33+/-0.78% ID/g; 64Cu-ATSE, 7.71+/-0.46% ID/g). PET images of EMT-6 tumor-bearing mice visualized the tumor with 64Cu-ATSE and revealed hypoxia selectivity consistent with the in vitro data. CONCLUSION: Of the compounds synthesized, only 64Cu-ASSM and 64Cu-ATSE could be examined in vitro and in vivo. Although the stability of bis(selenosemicarbazone) complexes increased upon addition of methyl groups to the diimine backbone, the fully alkylated species, 64Cu-ASSM, demonstrated no hypoxia selectivity. However, the additional alkylation present in Cu-ATSE modifies the hypoxia selectivity and in vivo properties when compared with Cu-ATSM.

Hypoxia-targeting copper bis(selenosemicarbazone) complexes: comparison with their sulfur analogues.

The first copper bis(selenosemicarbazone) complexes have been synthesized, using the ligands glyoxal bis(selenosemicarbazone), pyruvaldehyde bis(selenosemicarbazone), and 2,3-butanedione bis(selenosemicarbazone). Their spectroscopic properties indicate that they are structurally analogous to their well-known square-planar sulfur-containing counterparts, the copper bis(thiosemicarbazone) complexes. Spectroscopic comparison of the sulfur- and selenium-containing complexes provides insight into their electronic structure. The effects on spectroscopic and redox properties of replacing sulfur with selenium, and of successive addition of methyl groups to the ligand backbone, are rationalized in terms of their electronic structure using spin-unrestricted density functional calculations. These suggest that, like the sulfur analogues, the complexes have a very low-lying empty ligand-based pi-orbital immediately above the LUMO, while the LUMO itself has d(x2)-(y2) character (i.e., is the spin partner of the HOMO). Replacement of S by Se shifts the oxidation potentials much more than the reduction potentials, whereas alkylation of the ligand backbone shifts the reduction potentials more than the oxidation potentials. This suggests that oxidation and reduction involve spatially different orbitals, with the additional electron in the reduced species occupying the ligand-based pi-orbital rather than d(x2)-(y2). Density functional calculations on the putative singlet Cu(I)-reduced species suggest that this ligand pi-character could be brought about by distortion away from planarity during reduction, allowing the low-lying ligand pi-LUMO to mix into the d(x2)-(y2)-based HOMO. The analogy in the structure and reduction behavior between the sulfur- and selenium-containing complexes suggests that labeled with positron emitting isotopes of copper (Cu-60, Cu-62, Cu-64), the complexes warrant biological evaluation as radiopharmaceuticals for imaging of tissue perfusion and hypoxia.