Please cite this article in press as: Romberg, B. et al., Poly(amino acid)s: Promising enzymatically degradable stealth coatings for liposomes, Int. J. Pharmaceut. (2006), doi:10.1016/j.ijpharm.2006.11.018 ARTICLE IN PRESS +Model IJP-9268; No. of Pages 4 International Journal of Pharmaceutics xxx (2006) xxx–xxx Poly(amino acid)s: Promising enzymatically degradable stealth coatings for liposomes Birgit Romberg a,∗ , Josbert M. Metselaar a , Lajos Baranyi b , Cor J. Snel a , Rolf B ¨ unger c , Wim E. Hennink a , Janos Szebeni d , Gert Storm a a Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences (UIPS), Utrecht University, P.O. Box 80082, 3508 TB Utrecht, The Netherlands b Vaccine and Immunology Research Institute, WA, District of Columbia, United States c Department of Anatomy, Physiology and Genetics, Uniformed Services University of the Health Sciences, Bethesda, MD, United States d Division of Retrovirology, Department of Vaccine Production and Delivery, US Military HIV Research Program, Walter Reed Army Institute of Research, Rockville, MD, United States Received 7 August 2006; received in revised form 3 November 2006; accepted 6 November 2006 Abstract Poly(amino acid)s (PAAs) were evaluated as coating polymers for long-circulating liposomes. The pharmacokinetics of PAA-coated liposomes were assessed in rats. Prolonged circulation times were obtained, comparable to those reported for poly(ethylene glycol) (PEG)-liposomes. Besides, the enzymatic degradability of PAAs was studied. PAAs – in free as well as liposome-associated form – are degradable by proteases, which is beneficial for reducing the risks of accumulation in vivo. Furthermore, complement activation by PAA-liposomes was evaluated in vitro and in vivo. Like other liposome types, they appear to activate the complement system. However, a role of endotoxin contamination of the PAA-liposome formulations used cannot be excluded in our complement activation studies. © 2006 Elsevier B.V. All rights reserved. Keywords: Long-circulating liposomes; Pharmacokinetics; Poly(amino acid)-coatings; Enzymatic degradability; Complement activation Long-circulating, sterically stabilized liposomes (often also referred to as ‘stealth’ liposomes) are frequently used as phar- maceutical carriers (Torchilin, 2005). Their delivery concept makes use of their passive targeting properties to tumors and sites of inflammation, where they can extravasate due to the enhanced permeability and retention (EPR) effect (Maeda et al., 2000). A steric stabilization layer of a hydrophilic polymer on the liposome surface such as the frequently used poly(ethylene glycol) (PEG), is responsible for a reduced uptake by cells of the reticulo-endothelial system which in turn results in prolonged circulation times as compared to non-coated lipo- somes. There are, however, some limitations associated with the use of polymers as liposome coatings. These polymers are generally not degradable by mammalian enzymes and upon uptake of the coated liposomes by cells, the polymer may accumulate and cause cell function impairment on the long ∗ Corresponding author. Tel.: +31 30 2536898; fax: +31 30 2517839. E-mail address: B.Romberg@pharm.uu.nl (B. Romberg). term (Moghimi and Szebeni, 2003). Polymer coatings may also hinder drug release and target cell interaction after liposome localization in the target region therefore leading to a reduced therapeutic efficacy (Boomer et al., 2003). Another point of concern, which does not only apply to coated liposomes but also uncoated, is that liposomes have been reported to activate the complement system in preclinical studies (Szebeni, 1998). In the clinical setting, adverse respiratory, hemodynamic and hematological changes may occur in patients upon comple- ment activation and potentially lead to hypersensitivity reactions (HSR). We recently designed alternative sterically stabilizing poly- mers based on poly(amino acid)s (PAAs), in particular poly(hydroxyethyl l-glutamine) (PHEG) and poly(hydroxy- ethyl l-asparagine) (PHEA), both coupled to a hydropho- bic anchor to graft them onto the liposome bilayer (Fig. 1) (Metselaar et al., 2003). This paper summarizes the current status of PAA-coated liposomes regarding circulation kinetics, enzy- matic degradability of the PAA-coating and activation of the complement system. 0378-5173/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ijpharm.2006.11.018