A thermoresponsive polymer based on poly N-isopropylacrylamide (PNIPAM) was synthesized and radioiodinated to explore its potential use in localized radiotherapy. The synthesized PNIPAM polymer was functionalized with L-tyrosinamide to facilitate radioiodination. The content of tyrosinamide groups in the polymer was assayed spectrophotometrically (lambda = 275nm). The functionalized polymer showed a cloud point temperature of 29-31 degrees C and phase separation at 35 degrees C, as revealed by Differential Scanning Calorimetry (DSC) and Dynamic Light Scattering (DLS). The phase transition temperature is conducive for preferential localization of the polymer at the site of injection due to changes in the polymer conformation at body temperature. For in vivo demonstration, the biodistribution studies of radioiodinated polymer were carried out in Swiss mice bearing fibrosarcoma tumor. Biodistribution studies showed a retention of 30% of the injected labeled polymer, PNIPAM-125 I-tyrosinamide, in the solid tumor tissues after 2 h of intratumoral injection. Although the activity decreased with time, 3-4% of the injected dose (i. d) was found to be retained in the tumor on 5 d post injection. The results suggest the potential use of thermosensitive polymer based on poly N-isopropylacrylamide for locoregional radionuclide therapy. (C) 2013 Wiley Periodicals, Inc.