Purpose: Non-invasive PET imaging with radiolabeled RGD peptides for α(v)β(3) integrin targeting has become an important tool for tumor diagnosis and treatment monitoring in both pre-clinical and clinical studies. To better understand the molecular process and tracer pharmacokinetics, we introduced kinetic modeling in the investigation of (18)F-labeled RGD peptide monomer (18)F-FP-c(RGDyK) (denoted as (18)F-FPRGD) and dimer (18)F-FP-PEG3-E[c(RGDyK)](2) (denoted as (18)F-FPPRGD2).
Procedures: MDA-MB-435 tumor-bearing mice underwent 60 min dynamic PET scans following the injection of either (18)F-FPRGD or (18)F-FPPRGD2. Blocking studies with pre-injection of a blocking mass dose were performed for both monomeric and dimeric RGD groups. (18)F-FPRAD (RAD) was used as a negative control. Kinetic parameters (K(1), k(2), k(3), k(4)) of a three-compartment model were fitted to the dynamic data to allow quantitative comparisons between the monomeric and dimeric RGD peptides.
Results: Dimeric RGD peptide tracer showed significantly higher binding potential (Bp(ND) = k(3)/k(4), 5.87 ± 0.31) than that of the monomeric analog (2.75 ± 0.48, p = 0.0022, n = 4/group). The Bp(ND) values showed a significantly greater ratio (dimer/monomer ~2.1) than the difference in %ID/g uptake measured from static images (dimer/monomer ~1.5, p = 0.0045). Significant decrease in Bp(ND) was found in the blocked groups compared with the unblocked ones (dimer p = 0.00024, monomer p = 0.005, n = 4/group). Similarly, the RAD control group showed the lowest Bp(ND) value among all the test groups, as the RAD peptide does not bind to integrin α(v)β(3). Volume of distribution (V(T) = K(1)/k (2)(1 + k (3)/k (4))) could be separated into non-specific (V (ND) = K (1)/k (2)) and specific (V (S) = K (1) k (3)/(k (2) k (4))) components. Specific distribution volume (V(S)) was the dominant component of V(T) in the unblocked groups and decreased in the blocked groups. Unblocked RGD dimer also showed higher V(S) than that of the monomer (dimer V(S) = 2.38 ± 0.15, monomer V(S) = 0.90 ± 0.17, p = 0.0013, n = 4/group), well correlated with Bp(ND) calculations. Little difference in V(ND) was found among all groups. Moreover, parametric maps allowed quantitative analysis at voxel level and provided higher tumor-to-background contrast for Bp(ND) maps than the static images. Tumor heterogeneity in kinetic parameters was found in parametric images, which could not be clearly identified in static intensity images.
Conclusions: The pharmacokinetics of both monomeric and dimeric RGD peptide tracers was compared, and the RGD dimers showed significantly higher binding affinity than the monomeric analogs. Kinetic parameters were demonstrated to be valuable for separating specific and non-specific binding and may allow more sensitive and detailed quantification than simple standardized uptake value analysis.