Previously, we have reported the feasibility of the VAP-1-targeting peptide,
68Ga-DOTAVAP-P1, for PET imaging of inflammation in different rat models [
8‐
10]. However, as a limitation,
68Ga-DOTAVAP-P1 is cleared very rapidly from circulation and its
in vivo stability against degradation by enzymes is only moderate. In this study, we showed that the incorporation of a mini-PEG spacer in
68Ga-DOTAVAP-P1 enhanced its
in vivo stability and improved the PET imaging of inflammation.
The animal model used in our experiments involves turpentine oil injection-induced subcutaneous inflammation as described previously [
10]. In that study, we were able to show that the H & E staining of the inflamed site demonstrated infiltration of leucocytes and macrophages at the site of inflammation. The abscess centre with few cells, including residual injected oil, exudates and cell debris, was surrounded by an abscess wall. The dermis also appeared to be inflamed. In the present study, inflammation was evaluated in every animal by visually observing the pale colour of inflamed subcutaneous tissue. We performed
in vitro,
ex vivo and
in vivo experiments to evaluate the VAP-1 targeting, inflammation imaging efficacy and pharmacokinetics of
68Ga-DOTAVAP-PEG-P1 in comparison to the original
68Ga-DOTAVAP-P1. The incorporation of a mini-PEG spacer had no apparent effect on the
in vitro properties of the VAP-1 binding peptide; both peptides were stable in plasma incubations and their solubility was very similar. However, when i.v. administered,
68Ga-DOTAVAP-PEG-P1 showed significantly longer metabolic and elimination half-lives and slower total clearance compared to
68Ga-DOTAVAP-P1. Furthermore, our results revealed that while both peptides were able to visualise experimental inflammation by PET imaging,
68Ga-DOTAVAP-PEG-P1 showed a higher inflammation-to-muscle ratio than the original
68Ga-DOTAVAP-P1. As regards
68Ga-DOTAVAP-P1, the results of this study are in line with our previous publications [
8‐
10]. The renal excretion of
68Ga-DOTAVAP-PEG-P1 was slower, resulting in a significantly lower urinary bladder radioactivity in comparison to
68Ga-DOTAVAP-P1. The liver uptake was rather high for both peptides, which is, at least in part, due to the high number of VAP-1 receptors in the sinusoidal endothelial cells in the liver [
12]. Some degradation products of
68Ga-DOTA-peptides, such as free
68Ga, also tend to accumulate in the liver [
13]. Although modification with a mini-PEG spacer generally decreases liver uptake, the two peptides behaved quite similarly in our study, suggesting a VAP-1-specific binding in this tissue.
PEGylation has widely been used for improving the
in vivo kinetics of pharmaceuticals. However, the results of such modifications depend much on the nature of the lead compound and the choice of PEG linker [
14‐
20]. In most cases, PEGylation of radiopeptides has advantageous effects, such as increased metabolic half-life, decreased kidney uptake, and improved targeting and subsequent improved targeting for high-quality imaging. However, disadvantageous results have also been reported, e.g. the insertion of a long PEG chain may induce a higher liver uptake and reduce receptor binding [
16].
Modification with a mini-PEG spacer increased metabolic stability of VAP-1-targeting DOTA-peptide. In addition, it also improved in vivo imaging of inflammation suggesting that PEGylation had other highly pronounced in vivo effects beyond modification of pharmacokinetics. Although the modification with a mini-PEG spacer increased the target-to-background ratio, the SUV values in the inflamed area were still very low. Thus, further improvement of the tracer is warranted.