In vivo biodegradability and biocompatibility evaluation of novel alanine ester based polyphosphazenes in a rat model Swaminathan Sethuraman, 1,2 Lakshmi S. Nair, 3 Saadiq El-Amin, 3 Robert Farrar, 4 My-Tien N. Nguyen, 5 Anurima Singh, 6 Harry R. Allcock, 6 Yaser E. Greish, 7 Paul W. Brown, 7 Cato T. Laurencin 1,3,8 1 Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 2 Department of Chemical Engineering, Drexel University, Philadelphia, Pennsylvania 3 Department of Orthopaedic Surgery, University of Virginia, Charlottesville, Virginia 4 Department of Pathology, University of Virginia, Charlottesville, Virginia 5 Department of Biology, University of Virginia, Charlottesville, Virginia 6 Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 7 Intercollege Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania 8 Department of Biomedical Engineering, University of Virginia, Charlottesville, Virginia Received 2 August 2005; accepted 7 September 2005 Published online 2 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.30620 Abstract: Amino acid ester substituted polyphosphazenes are attractive candidates for various biomedical applications because of their biocompatibility, controllable hydrolytic degradation rates, and nontoxic degradation products. In this study, the bio- compatibility of three l-alanine ethyl ester functionalized polyphosphazenes was evaluated in a subcutaneous rat model. The polymers used in the study were poly[bis(ethylalanato)phos- phazene] (PNEA), poly[(50% ethylalanato) (50% methylphenoxy) phosphazene] (PNEA 50 mPh 50 ), and poly[(50% ethylalanato)(50% phenyl phenoxy) phosphazene] (PNEA 50 PhPh 50 ). Polymer disks of diameter 7.5 mm were prepared by a solvent evaporation technique and were implanted subcutaneously in rats. After 2, 4, and 12 weeks, the polymer along with the surrounding tissues were excised, prepared, and viewed by light microscopy to eval- uate the tissue responses of the implanted polymers. The tissue responses were classified as minimal, mild, or moderate, based on a biocompatibility scheme developed in our laboratory. Minimal inflammation was characterized by the presence of few neutro- phils, erythrocytes, and lymphocytes; mild response was charac- terized by the predominant presence of macrophages, fibroblasts, or giant cells; and moderate inflammation was characterized by the abundance of macrophages, giant cells, and by the presence of tissue exudates. The in vivo degradation profiles of the polymers at various time points were evaluated by gel permeation chroma- tography (GPC). PNEA and PNEA 50 mPh 50 matrices elicited vary- ing levels of tissue responses during the 12-week implantation period. At 2 weeks both polymers evoked a moderate response, and by 12 weeks the response was found to be mild. However, PNEA 50 PhPh 50 elicited a mild response at the end of 2 weeks and demonstrated a further decreased inflammatory response after12 weeks. The in vivo degradation of the polymers was followed by determining the molecular weights of the explanted polymer disks. PNEA and PNEA 50 mPh 50 disks showed significant de- crease in molecular weight after 2 weeks of implantation. The molecular weights of PNEA and PNEA 50 mPh 50 residues could not be determined by GPC after 12 weeks of implantation because of almost complete degradation. On the other hand the in vivo degradation of PNEA 50 PhPh 50 was found to be slow, with a 63% loss in molecular weight in 12 weeks. Furthermore, this polymer maintained its shape and structure during the entire study. Thus, these polymers demonstrated excellent tissue compatibility and in vivo biodegradability and can be potential candidates for various biomedical applications. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 77A: 679 – 687, 2006 Key words: polyphosphazenes; biocompatibility; polymers; tissue engineering INTRODUCTION Biodegradable polymers have been used as im- plants in vascular and orthopaedic surgery, matrices for the controlled release of bioactive molecules, as scaffolds for tissue engineering, and as absorbable sutures. 1 Along with the appropriate physical and mechanical properties, the biocompatibility of degrad- able polymers and their degradation products play an important role in determining their potential applica- tions. 2 Studies have shown that cell in-growth, tissue regeneration, and host response are dependent on the biocompatibility of these materials. Biodegradable al- iphatic ester polymers such as poly(glycolic acid) and poly(lactide-co-glycolide) (PLAGA) were among the first Food and Drug Administration (FDA) approved Correspondence to: C. T. Laurencin, M.D., Ph.D., University Professor Lillian T. Pratt Distinguished Professor, Chair of Orthopaedic Surgery, Department of Biomedical Engineer- ing, Department of Chemical Engineering, 400 Ray C. Hunt Drive, Suite 330, University of Virginia, Charlottesville, Vir- ginia 22903, USA; e-mail: laurencin@virginia.edu © 2006 Wiley Periodicals, Inc.