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Basic Science Investigation |
1 Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at UCLA, University of California, Los Angeles, California; 2 Department of Nuclear Medicine, Medical University of Innsbruck, Innsbruck, Austria; 3 Division of Radiopharmacy, University of Tuebingen, Tuebingen, Germany; and 4 Department of Nuclear Medicine, University of Freiburg, Freiburg, Germany
Correspondence: For correspondence or reprints contact: Gregory Z. Ferl, Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at UCLA, B2-085E CHS, 10833 Le Conte Ave., Los Angeles, CA 90095-6948. E-mail: gzferl{at}ucla.edu
Radiolabeled arginine-glycine-aspartate (RGD) peptides are increasingly used in preclinical and clinical studies to assess the expression and function of the 
vβ3 integrin, a cellular adhesion molecule involved in angiogenesis and tumor metastasis formation. To better understand the PET signal obtained with radiolabeled RGD peptides, we have constructed a compartmental model that can describe the time–activity curves in tumors after an intravenous injection. Methods: We analyzed 60-min dynamic PET scans obtained with 64Cu-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-RGD in 20 tumor-bearing severe combined immunodeficient (SCID) mice after a bolus dose (18,500 kBq [500 µCi]), using variations of the standard 2-compartment (4k) tissue model augmented with a compartment for irreversible tracer internalization. vβ3 binding sites were blocked in 5 studies with a coinjection of cold peptide. In addition, 20 h after injection, static PET was performed on 9 of 20 mice. We fitted 2k (k3 = k4 = 0), 3k (k4 = 0), 4k, and 4kc (k4 = constant) models to the PET data and used several criteria to determine the best model structure for describing 64Cu-DOTA-RGD kinetics in mice. Akaike information criteria (AIC), calculated from model fits and the ability of each model to predict tumor concentration 20 h after tracer injection, were considered. Results: The 4kc model has the best profile in terms of AIC values and predictive ability, and a constant k4 is further supported by Logan–Patlak analysis and results from iterative Bayesian parameter estimation. The internalization compartment allows quantification of the putative tracer internalization rate for each study, which is estimated here to be approximately an order of magnitude less than k3 and thus does not confound the apparent specific binding of the tracer to the tumor integrin during the first 60 min of the scan. Analysis of specific (S) and nonspecific or nondisplaceable (ND) binding using fitted parameter values showed that the 4kc model provided expected results when comparing vβ3 blocked and nonblocked studies. That is, specific volume of distribution, [VS = (K1k3)/(k2k4)], is much higher than is nondisplaceable volume of distribution, [VND = (K1/k2)], in nonblocking studies (2.2 ± 0.6 vs. 0.85 ± 0.14); VS and VND are about the same in the blocking studies (0.46 ± 1.6 vs. 0.56 ± 0.09). Also, the ratio of static tumor and plasma measurements at 60 and 10 min [CT(60)/CP(10)] is highly correlated (RS = 0.92) to tumor VS. Conclusion: We have developed and tested a compartmental model for use with the 64Cu-DOTA-RGD PET tracer and demonstrated its potential as a tool for analysis and design of preclinical and clinical imaging studies.
Key Words: compartmental model • pharmacokinetics • small-animal PET • RGD peptide • vβ3 integrin
Guest Editor: Adriaan A. Lammertsma, VU University Medical Center.
COPYRIGHT © 2009 by the Society of Nuclear Medicine, Inc.
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