N-[C]Methyl-AMD3465 PET as a Tool for In Vivo Measurement of Chemokine Receptor 4 (CXCR4) Occupancy by Therapeutic Drugs

Purpose: Chemokine receptor 4 (CXCR4) is overexpressed in many cancers and a potential drug target. We have recently developed the tracer N-[C]methyl-AMD3465 for imaging of CXCR4 expression by positron emission tomography (PET). We investigated the pharmacokinetics of N[C]methyl-AMD3465 in rats bearing a C6 tumor and assessed whether the CXCR4 occupancy by the drug Plerixafor® can be measured with this PET tracer. Procedure: A subcutaneous C6 tumor was grown in Wistar rats. Dynamic N-[C]methyl-AMD3465 PET scanswith arterial blood samplingwas performed in control rats and rats pretreatedwith Plerixafor® (30mg/kg, s.c). The distribution volume (VT) of the tracer was estimated by compartmentmodelingwith a two-tissue reversible compartment model (2TRCM) and by Logan graphical analysis. The nondisplaceable binding potential (BPND)was estimatedwith the 2TRCM.Next, CXCR4 receptor occupancy of different doses of the drug Plerixafor® (0.5–60 mg/kg) was investigated. Results: The tumor could be clearly visualized by PET in control animals. Pretreatment with 30 mg/ kg Plerixafor® significantly reduced tumor uptake (SUV 0.65 ± 0.08 vs. 0.20 ± 0.01, p G 0.05). N[C]Methyl-AMD3465was slowlymetabolized in vivo, with 70 ± 7%of the tracer in plasma still being intact after 60 min. The tracer showed reversible in vivo binding to its receptor. Both 2TRCM modeling and Logan graphical analysis could be used to estimate VT. Pre-treatment with 30 mg/kg Plerixafor® resulted in a significant reduction inVT (2TCRM0.87 ± 0.10 vs. 0.23 ± 0.12, p G 0.05) and BPND (1.85 ± 0.14 vs. 0.87 ± 0.12, p G 0.01). Receptor occupancy by Plerixafor® was dosedependent with an in vivo ED50 of 12.7 ± 4.0 mg/kg. Logan analysis gave comparable results. Conclusion: N-[C]Methyl-AMD3465 PET can be used to visualize CXCR4 expression and to calculate receptor occupancy. VT determined by Logan graphical analysis is a suitable parameter to assess CXCR4 receptor occupancy. This approach can easily be translated to humans and used for early drug development and optimization of drug dosing schedules.