Identifying a Cardiac PTD Figure 5. CTP specifically transduces cardiac tissue in vivo. Confocal analysis (206) was performed on tissues from heart, liver, kidney, lung, skeletal muscle and brain (a–f) from mice euthanized 30 minutes after intravenous injection of CTP-biotin-SA488 conjugate (10 mg/Kg) or PBS+SA488. Slides were counter-stained with DRAQ5, a nuclear stain. CTP-SA488 – green, Nuclei – blue. N = 3 in each group; scale bars represent 100 uM. doi:10.1371/journal.pone.0012252.g005 CTP was compared with 8K, a well characterized cationic protein transduction domain. Mice were injected with 10 mg/Kg dose of either CTP-SA488, 8K-SA488 or CON-SA488 conjugate and euthanized 30 minutes post-injection (Fig. 8). Mice treated with 8K-SA488 showed robust transduction of hepatocytes as well as kidney glomeruli with very little uptake in heart tissue. In contrast, CTP-SA488 conjugate showed only robust transduction of heart tissue with some uptake in the kidney glomerular capillaries and none by liver or spleen. The CON-SA488 complex did not show appreciable uptake in any organ. It is important to note that all of the analysis of CTP transduction in vivo was performed with the L-form, the naturally occurring form, of the peptide. Preliminary experiments using a non-degradable D-form have shown a far more efficient transduction that persists for extended periods of time (data not shown). Thus, it appears that there is degradation of the L-CTP complexes over time. Discussion The clinical application of potentially effective biological therapies for common acute cardiac conditions, like myocardial infarction, has been limited by efficiency and specificity of delivery of therapeutic agents. For example, for gene therapy approaches, such as plasmid DNA, delivery to the heart is very inefficient whereas there are significant time delays associated with cardiac Figure 6. Pattern of distribution of CTP-biotin-SA488 over time in vivo. Mice were injected intravenously with CTP-biotin-SA488 (10 mg/Kg) and euthanized 15, 30 or 120 minutes post-injection. Heart, kidneys and liver were cross-sectioned, counter-stained with DRAQ5 and confocal microscopy performed. N = 1 in each group; scale bars represent 100 uM. doi:10.1371/journal.pone.0012252.g006 PLoS ONE | www.plosone.org 8 August 2010 | Volume 5 | Issue 8 | e12252