Targeting peptides and positron emission tomography

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Abstract
Biologically active peptides have during the last decades made their way into conventional nuclear medicine diagnosis using single photon emission computed tomography (SPECT) and γ‐camera. Several clinical trails are also investigating the role of radiolabeled peptides for targeting radionuclide therapy. This has raised the question as to whether positron emission tomography (PET) can be used in order to obtain better quantitative information of the peptide distribution in tumor and healthy organs, i.e., to get a better dosimetry. Positron emitting analogs of the therapeutic radionuclides used have been produced and successfully applied in peptide pharmacokinetic measurements with PET. But the recent boom in 18FDG‐PET (18FDG = [18F]2‐deoxy‐2‐fluoro‐D‐glucose), and with this a worldwide increasing number of PET systems, has also inspired several research groups to hunt for alternative labels to be used for peptide diagnostics and PET. The rapid kinetic of short peptides agrees well with the short half‐lives of standard PET nuclides like 11C and 18F. Especially, 18F appears to be excellent for labeling bioactive peptides due to its favorable physical and nuclear characteristics. However, with present techniques labeling peptides with 18F is laborious and time‐consuming, and is not yet a clinical alternative. Other halogens like 75, 76Br and 124I are, from the chemical point of view, easier to apply. But an even better labeling alternative may be positron emitting metal ions like 55Co, 68Ga, and 110mIn since they tend to give better intracellular retention and thus a better signal‐to‐background ratio than the halogen labels. The main drawback with these radionuclides is that they are not readily available. Some of these radionuclides also emit γ in their decay that may affect the measuring properties of the PET equipment. This article reviews mainly the present situation of production and use of nonconventional positron emitters for peptide labeling. © 2003 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 66: 381–392, 2002