BACKGROUND Obesity is an emergent epidemic associated with morbidity, mortality, and psychosocial effects. One of the key gut hormones that controls appetite is peptide tyrosine-tyrosine 3-36 (PYY3-36) whose circulating half-life is only 8 min. A long-acting analogue of PYY3-36 would therefore have great potential as an antiobesity agent. The aims of this study were to investigate the effect of various aminoacid modifications of PYY3-36 on pharmacokinetics and their ability to suppress food intake. METHODS To investigate the pharmacokinetics of PYY3-36 and three modified analogues, serial sampling of plasma peptide levels via cannulation of the jugular vein was performed after subcutaneous injection of the peptide in rats (n=4 per peptide, 80 nmol/kg). To investigate the effect of these peptides on food intake, mice were injected subcutaneously (1000 nmol/kg) and food intake was assessed at timed intervals over 24 h (n=8 per peptide). RESULTS One-way ANOVA with post-hoc Dunnett's test was used in which each comparison was with the PYY3-36 or saline group. Plasma concentrations of the modified analogue, PYY-AP3H, were significantly higher than PYY3-36 up to 24 h post injection (p=0·0008 at 4 h, p=0·0028 at 24 h). The results confirm that modification of the native peptide, by addition of an α-helix stabilising sequence and histidine residues, lengthens the pharmacokinetic profile. Furthermore, PYY-AP3H significantly reduced food intake for up to 24 h compared with saline (p<0·0001) and native PYY3-36 (p<0·0001). CONCLUSIONS The rationally designed analogue, PYY-AP3H, has potential as a once-a-day subcutaneously administered preparation for the treatment of obesity. BACKGROUND UK Medical Research Council, Biotechnology and Biological Sciences Research Council, National Institute for Health Research (NIHR), an Integrative Mammalian Biology (IMB) Capacity Building award, an FP7-HEALTH-2009-241592 EuroCHIP grant, NIHR Imperial Biomedical Research Centre Funding Scheme.