With slower decrease of the in vitro binding activity dehalogenation or degradation, hypothesis which however has to be further investigated. A prerequisite for the use of a ligand as tracer for imaging purposes is a higher in vivo accumulation in tumor tissue, compared to the healthy organs. Although organ distribution studies in nude mice bearing SKRC 52 tumors revealed a higher uptake in the tumor than in most of the healthy organs the blood values were higher, resulting in an enhanced background, which is a drawback for the use of the native peptide as imaging agent. With progression of time a reduction of the absolute uptake values in healthy organs and the tumor is noticed. This reduction is SCH772984 stronger in the healthy tissues than in the tumor for a circulation period of up to 60 min, resulting in an increase of the tumor-to-organ ratios. Thereafter however, a further uptake reduction in tumor and organs leads to a decrease of the tumor-to-organ ratios, which is disadvantageous for in vivo applications. The fast washout from the tumor is in concert with the results of the in vitro kinetic and internalization experiments and might also be explained by an intracellular dehalogenation or degradation process. Similar results were also revealed for other healthy organs, indicating an in vivo specificity of CaIX-P1. The enhanced blood values are explained through an interaction of the peptide with serum proteins, such as albumin or through in vivo deiodination of the radioligand. HPLC analysis of blood at 1 h after injection of 131I-labeled CaIX-P1 revealed that the majority of the radioactivity was associated to serum proteins. In addition, free iodide and small peptide fragments were detected. In vivo deiodination of directly radiolabeled peptides has been described in the literature. This problem can be adressed through chemical modifications of the peptide. A possible way to enhance resistance to deiodination of peptides is the protection of the radioiodinated N-terminal tyrosine with a t-butyloxycarbonyl group. Alternatively, further radiolabeling approaches, such as metal labeling through a chelator might be applied in order to improve labeling stability and reduce radioactive background. In case of CaIX-P1 the high blood value is additionally explained by the metabolic properties of the peptide. Stability experiments in human serum demonstrated a degradation of CaIX-P1 through serum proteases. Mass spectrometry revealed a degradation of a tyrosine molecule. Since direct iodination is performed on the side group of tyrosine, the degradation might lead to free 125I-labeled tyrosine residues that circulate in the bloodstream. In this way the organ distribution of CaIX-P1 is negatively influenced by both radiolabeling and metabolic instability. Therefore, a major issue of further investigation is the serum stabilization of the CaIX-P1 peptide.