Systematic investigation of polyamidoamine dendrimers surface-modified with poly(ethylene glycol) for drug delivery applications: synthesis, characterization, and evaluation of cytotoxicity.

Surface modification of amine-terminated polyamidoamine (PAMAM) dendrimers by poly(ethylene glycol) (PEG) groups generally enhances water-solubility and biocompatibility for drug delivery applications. In order to provide guidelines for designing appropriate dendritic scaffolds, a series of G3 PAMAM-PEG dendrimer conjugates was synthesized by varying the number of PEG attachments and chain length (shorter PEG 550 and PEG 750 and longer PEG 2000). Each conjugate was purified by size exclusion chromatography (SEC) and the molecular weight (MW) was determined by (1)H NMR integration and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). NOESY experiments performed in D 2O on selected structures suggested no penetration of PEG chains to the central PAMAM domain, regardless of chain length and degree of substitution. CHO cell cultures exposed to PAMAM-PEG derivatives (< or =1 microM) showed a relatively high cell viability. Generally, increasing the degree of PEG substitution reduced cytotoxicity. Moreover, compared to G3 PAMAM dendrimers that were N-acetylated to varying degrees, a lower degree of surface substitution with PEG was needed for a similar cell viability. Interestingly, when longer PEG 2000 was fully incorporated on the surface, cell viability was reduced at higher concentrations (32 muM), suggesting increased toxicity potentially by forming intermolecular aggregates. A similar observation was made for anionic carboxylate G5.5 PAMAM dendrimer at the same dendrimer concentration. Our findings suggest that a lower degree of peripheral substitution with shorter PEG chains may suffice for these PAMAM-PEG conjugates to serve as efficient universal scaffolds for drug delivery, particularly valuable in relation to targeting or other ligand-receptor interactions.