No abstract

We describe the synthesis of linear-hyperbranched lipids for liposome preparation based on linear poly(ethylene glycol) (PEG) and hyperbranched polyglycerol (PG). Molecular weights were adjusted to values around 3000 g/mol with varying degrees of polymerization of the linear and the branched segments in analogy to PEG-based stealth lipids; polydispersities were generally low and below 1.3. The hydrophobic anchors were introduced into the lipid structures as initiators for the anionic polymerization of ethylene oxide and are either based on cholesterol or on different aliphatic glyceryl ethers. Complete incorporation of the apolar initiators was evidenced by MALDI-ToF analysis at all stages of the reaction. The linear-hyperbranched polyether lipid is incorporated as the polyfunctional shell in liposome formulations together with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The resulting liposomes were subsequently characterized via dynamic light scattering (DLS) and small angle neutron scattering (SANS) as well as transmission electron microscopy (TEM), demonstrating the formation of unilamellar liposomes in the size range of 40 to 50 nm.

On the basis of a new acetal-protected glycerol monomethacrylate monomer (cis-1,3-benzylidene glycerol methacrylate/BGMA) a series of potentially biocompatible and partially biodegradable homo- and block copolymers were synthesized. ATRP polymerization of BGMA yielded well-defined polyacrylates with pendant benzylidene acetal groups and high glass transition temperatures (115−130 °C). This hydrophobic poly(cis-1,3-benzylidene glycerol methacrylate) could be readily transformed into the hydrophilic and water-soluble poly(1,3-dihydroxypropyl methacrylate), referred to as poly(isoglycerol methacrylate) (PIGMA). It exclusively contains primary hydroxyl groups and therefore differs significantly from the commonly known poly(glycerol methacrylate) (PGMA). Block copolymer systems based on poly(lactide) and BGMA were realized via two orthogonal living polymerization techniques starting from a bifunctional initiator, employing first atom transfer radical polymerization (ATRP) of BGMA and in the second step organo-base catalyzed polymerization of l- or d-lactide. This route provides well-defined block copolymers of low polydispersity (PDI 1.12−1.17) and molecular weights in the range of 7000 to 30 000 g/mol (NMR). Rapid and highly selective acetal hydrolysis of the PBGMA block resulted in the release of the hydrophilic and water-soluble poly(1,3-dihydroxypropyl methacrylate) (poly(isoglycerol methacrylate), PIGMA). Acidic hydrolysis of the acetal protecting groups of poly(BGMA)-b-poly(lactide) copolymers proceeded smoothly to amphiphilic structures, notably without affecting the potentially labile polyester block. The novel PIGMA-b-PLLA copolymers are capable of supramolecular self-assembly to spherical aggregate structures in aqueous environment. The polymers generally exhibited low aggregation constants (CAC: 8−20 mg/L). Because of the unique feature of stereocomplex formation of poly(lactide), the corresponding aggregate morphology could be adjusted by mixing two nearly identical PIGMA-b-PLA copolymers with enantiomeric poly(lactide blocks) in a 1:1 ratio. In this case the uniformly shaped micelles (20 nm) changed to large vesicles with diameters ranging from 600 to 1400 nm. These features render this new type of amphiphilic block copolymers promising for drug delivery applications.

Langmuir films of multifunctional, hydrophilic polyethers containing a hydrophobic cholesterol group (Ch) were studied by surface pressure-mean molecular area (π-mmA) measurements and Brewster angle microscopy (BAM). The polyethers were either homopolymers or diblock copolymers of linear poly(glycerol) (lPG), linear poly(glyceryl glycidyl ether) (lPGG), linear poly(ethylene glycol) (lPEG), or hyperbranched poly(glycerol) (hbPG). Surface pressure measurements revealed that the homopolymers lPG and hbPG did not stay at the water surface after spreading and solvent evaporation, in contrast to lPEG. Because of the incorporation of the Ch group in the polymer structure, stable Langmuir films were formed by Ch-lPG(n), Ch-lPGG(n), and Ch-hbPG(n). The Ch-hbPG(n), Ch-lPEG(n), Ch-lPEG(n)-b-lPG(m), Ch-lPEG(n)-b-lPGG(m), and Ch-lPEG(n)-b-hbPG(m) systems showed an extended plateau region assigned to a phase transition involving the Ch groups. Typical hierarchically ordered morphologies of the LB films on hydrophilic substrates were observed for all Ch-initiated polymers. All LB films showed that Ch of the Ch-initiated homopolymers is able to crystallize. This strong tendency of self-aggregation then triggers further dewetting effects of the respective polyether entities. Fingerlike morphologies are observed for Ch-lPEG(69), since the lPEG(69) entity is able to undergo crystallization after transfer onto the silicon substrate.

Aiming at controlled modification of liposomal surface structures, we describe a postpreparational approach for surface derivatization of a new type of multifunctional, sterically stabilized liposomes. Application of dual centrifugation (DC) resulted in high encapsulation efficiencies above 50% at very small batch sizes with a total volume of 150 μL, which were conductive to fast and efficient optimization of variegated surface modification reactions. Cholesterol-polymer amphiphiles, including complex hyperbranched polyether structures bearing 1-4 terminal alkynes, were used in DC formulations to provide steric stabilization. The alkyne moieties were explored as anchors for the conjugation of small molecules to the liposomal surface via click chemistry, binding 350-450 fluorophores per liposome as examples for surface active molecules. Using Förster resonance energy transfer (FRET) spectroscopy, the conjugation reaction as well as the uptake of FRET-labeled liposomes by RBE4 cells was monitored, and the distribution of the fluorescent lipids among cellular structures and membranes could be studied. Thus, the combination of clickable hyperbranched amphiphiles and dual centrifugation provides access to well-defined liposomal formulations with a variety of surface moieties.