Abstract/Summary

Poly(amidoamine) (PAMAM) dendrimers are nanosized, highly defined, hyperbranched polymeric vehicles designed for targeted delivery of drugs or bioactive molecules. Whilst heavily researched, their mechanism of interaction with biological systems is barely understood. Here, medium-generation PAMAMs (-COOH-terminus (G4.5); -NH2-terminus (G5)) were characterized by UV-Vis and fluorescence, small-angle X-ray scattering and computational simulations. Biophysical (surface pressure measurements, neutron reflectometry) and microbiological techniques were used to assess modulating factors (i.e. solvent pH, residual methanol) of dendrimer-membrane interactions and related toxicity. PAMAM surface groups are charged at pH 7, and in phosphate buffer solution the dendrimers assumed a compact, near-globular shape (radius ≈2.5 – 3 nm). PAMAM G5 penetrated anionic phosphatidylglycerol (DPPG, model bacterial lipid) monolayers rapidly but showed lesser membrane-activity on supported bilayers. Electrostatic effects could be potential drivers but also hinder PAMAM G4.5 penetration into DPPG monolayers. Across techniques, it was shown that solvent pH influenced dendrimer core (tertiary amines) and surface charge, with higher impact on structural characteristics of zwitterionic PAMAM G4.5 than of G5. At pH 4, the charge effect led to higher penetration levels into DPPG monolayers and bilayers for both dendrimers (G5 > G4.5) compared to pH 7. Methanol as co-solvent affected PAMAM radii and spectroscopic properties across the pH range tested. Additionally, it increased DPPG monolayer penetration compared to the methanol-free buffer environment, which could be explained by both, the impact on PAMAMs and on lipid layer organization. Anti-bacterial efficacy was studied on gram-negative and gram-positive bacteria. Amine-terminated PAMAMs led to growth-inhibition of most strains, but specifically gram-positive Staphylococcus spp. which are rich in anionic membrane lipids (i.e. PG lipids). Whilst overall less inhibitory than G5, PAMAM G4.5 was bactericidal against Staphylococcus saprophyticus. Taken together, our findings highlight the significance of the PAMAM characteristics and the solvent-PAMAM-lipid interplay and explain, at least partially, potential drivers of PAMAM membrane-toxicity.