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Published on: February 3, 2015
BLZ945 derivatives for PET imaging of colony stimulating factor-1 receptors in the brain
Berend van der Wildt1, Zheng Miao2, Samantha T Reyes2
1Molecular Imaging Program at Stanford (MIPS), Department of Radiology, Stanford University, School of Medicine, Stanford, CA, USA; Amsterdam UMC, Vrije Universiteit Amsterdam, Radiology & Nuclear Medicine, de Boelelaan 1117, Amsterdam, Netherlands.
Background:
The kinase colony stimulating factor-1 receptor (CSF-1R) has recently been identified as a novel therapeutic target for decreasing tumor associated macrophages and microglia load in cancer treatment. In glioblastoma multiforme (GBM), a high-grade cancer in the brain with extremely poor prognosis, macrophages and microglia can make up to 50% of the total tumor mass. Currently, no non-invasive methods are available for measuring CSF-1R expression in vivo. The aim of this work is to develop a PET tracer for imaging of CSF-1R receptor expression in the brain for future GBM patient selection and treatment monitoring.
Methods:
BLZ945 and a derivative that potentially allows for fluorine-18 labeling were synthesized and evaluated in vitro to determine their affinity towards CSF-1R. BLZ945 was radiolabeled with carbon-11 by N-methylation of des-methyl-BLZ945 using [11C]CH3I. Following administration to healthy mice, metabolic stability of [11C]BLZ945 in blood and brain and activity distribution were determined ex vivo. PET scanning was performed at baseline, efflux transporter blocking, and CSF-1R blocking conditions. Finally, [11C]BLZ945 binding was evaluated in vitro by autoradiography on mouse brain sections.
Results:
BLZ945 was the most potent compound in our series with an IC50 value of 6.9 ± 1.4 nM. BLZ945 was radiolabeled with carbon-11 in 20.7 ± 1.1% decay corrected radiochemical yield in a 60 min synthesis procedure with a radiochemical purity of >95% and a molar activity of 153 ± 34 GBq·μmol-1. Ex vivo biodistribution showed moderate brain uptake and slow wash-out, in addition to slow blood clearance. The stability of BLZ945 in blood plasma and brain was >99% at 60 min post injection. PET scanning demonstrated BLZ945 to be a substrate for efflux transporters. High brain uptake was observed, which was shown to be mostly non-specific. In accordance, in vitro autoradiography on brain sections revealed high non-specific binding.
Conclusions:
[11C]BLZ945, a CSF-1R PET tracer, was synthesized in high yield and purity. The tracer has high potency for the target, however, future studies are warranted to address non-specific binding and tracer efflux before BLZ945 or derivatives could be translated into humans for brain imaging.
Insights
This study developed a novel PET tracer, [11C]BLZ945, for imaging colony stimulating factor-1 receptor (CSF-1R) in the brain. While potent, further research is needed to address non-specific binding before clinical use in glioblastoma.
Area of Science:
- Neuroscience
- Radiochemistry
- Oncology
Background:
- Colony stimulating factor-1 receptor (CSF-1R) is a therapeutic target for reducing tumor-associated macrophages and microglia.
- Macrophages and microglia constitute up to 50% of glioblastoma multiforme (GBM) tumor mass.
- Non-invasive methods for in vivo CSF-1R expression measurement are lacking for GBM.
Purpose of the Study:
- To develop a positron emission tomography (PET) tracer for imaging CSF-1R expression in the brain.
- To enable future patient selection and treatment monitoring for GBM.
Main Methods:
- Synthesis and in vitro evaluation of BLZ945 and a fluorine-18 labeled derivative for CSF-1R affinity.
- Radiolabeling of BLZ945 with carbon-11 ([11C]BLZ945) via N-methylation.
- Ex vivo biodistribution and in vivo PET imaging in mice under baseline, efflux transporter blocking, and CSF-1R blocking conditions.
- In vitro autoradiography on mouse brain sections to assess binding.
Main Results:
- BLZ945 demonstrated high potency with an IC50 of 6.9 ± 1.4 nM.
- [11C]BLZ945 was synthesized with high radiochemical yield (>20%), purity (>95%), and molar activity (153 ± 34 GBq·μmol-1).
- PET imaging revealed high brain uptake of [11C]BLZ945, but it was largely non-specific and influenced by efflux transporters, with slow blood clearance and brain washout.
Conclusions:
- [11C]BLZ945, a CSF-1R PET tracer, was successfully synthesized with high yield and purity.
- The tracer exhibits high target potency but also significant non-specific binding and efflux transporter substrate properties.
- Further investigation into non-specific binding and tracer efflux is required before clinical translation for brain imaging in GBM patients.
