Pharmacokinetics and microbiodistribution of 64Cu-labeled collagen-binding peptides in chronic myocardial infarction

Abstract

Objectives To evaluate the pharmacokinetics and microbiodistribution of 64Cu-labeled collagen binding peptides. Method The affinity constant (KD), association (ka) and dissociation rate constant (kd) for the peptide collagelin or its analogue (named CRPA) binding to collagen were measured by bio-layer interferometric analysis. Rats (n = 4–5) with myocardial infarction or normal were injected IV with the 64Cu-labeled peptides or 64Cu-DOTA as a control. Dynamic PET imaging was performed for 60 min at 7- to 8-week post-infarct. [18F]FDG PET imaging was performed to identify the viable myocardium. To validate the PET images, slices of heart samples from the base to the apex were analyzed using autoradiography and histology. Result The peptides bound to collagen with KD of ~ 0.9 μM. The 64Cu-peptides and 64Cu-DOTA accumulated in the infarct area (confirmed by autoradiography and histology images) within 1 minute of injection and were excreted rapidly via the renal system. The blood clearance curves were bi-phasic with the elimination half-lives, 21.9 ± 2.4, 26.2 ± 4.6 and 21.2 ± 2.1 min for 64Cu-CRPA, 64Cu-collagelin and the control 64Cu-DOTA, respectively. The clearance half-lives from the focal fibrotic tissue (24.1 ± 1.5, 25.6 ± 8.0 and 21.4 ± 1.3 min, respectively) and remote myocardium (20.8 ± 0.7, 21.0 ± 5.5 and 19.1 ± 2.4 min, respectively) were not significantly different. The uptake ratios of infarct-to-remote myocardium (1.93 ± 0.18, 2.15 ± 0.38 and 1.88 ± 0.08, respectively) for 64Cu-CRPA, 64Cu-collagelin and 64Cu-DOTA remained stable for the time period between 10 to 60 min. Conclusion The distribution of the 64Cu-collagelin probes corresponds to the heterogeneous distribution of expanded extracellular space in the setting of myocardial infarction. The overall washout rate from the fibrous tissue was determined by the slow washout rate (t1/2, ≥ 20 min) of the peptides from the extracellular space to the vasculature, not by the dissociation rate (t1/2, ≤ 2 min) of the 64Cu-peptides from collagen.

Radiotracers for Imaging of Fibrosis: Advances during the Last Two Decades and Future Directions

“…Surface Plasmon Resonance binding studies of the unconjugated and unlabeled LRELHLNNN peptide demonstrated sub-micromolar affinity (K d = 170 nM) to collagen type I. This is an improvement as compared to [ 68 Ga]Ga-CBP8 (2.1 µM) [9] [10,48,49] and collagelin (1.5-2 µM) [15][16][17]47,79], but direct comparison should be taken with caution, as different assays were used for affinity assessment for the different tracers. The binding of radiolabeled peptide correlated with the fibrosis grade in liver biopsies, as assessed by Sirius Red staining of collagen.…”

“…However, somewhat higher uptake and longer retention in the kidney and small intestine was observed for the 18 F-labeled counterpart. The faster clearance of the 68 Ga-labeled counterpart from the kidney presents a considerable advantage in terms of the possibility to image collagen type I in kidney, which might not be possible with the 18F-labeled counterpart [19] or CBP8 [10,48,49] and collagelin analogues [15][16][17]47,79]. However, the agent showed low in vivo stability in rodents, with only 10-15% of the intact fraction after 60 min.…”

Imaging the Cardiac Extracellular Matrix

“…In cardiac models, SPECT and PET probes based on a peptide known as collagelin accumulate in areas of myocardial infarction (Fig. 2.4) [33, 47]. In lung and liver models, other peptide-based probes accumulate in fibrotic tissue ([14, 78], p. 201).…”

“…This figure depicts positron-emission tomography images taken at several time points after probe injection. The top row shows a probe based on the collagelin analogue CRPA, and the bottom a probe based on collagelin [33]. (Figure Permission granted through RightsLink)…”