Discovery of a High-Affinity Fluoromethyl Analog of [¹¹C]5-Cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([¹¹C]CPPC) and Their Comparison in Mouse and Monkey as Colony-Stimulating Factor 1 Receptor Positron Emission Tomography Radioligands

Abstract

11 C]CPPC has been advocated as a radioligand for colony-stimulating factor 1 receptor (CSF1R) with the potential for imaging neuroinflammation in human subjects with positron emission tomography (PET). This study sought to prepare fluoro analogs of CPPC with higher affinity to provide the potential for labeling with longer-lived fluorine-18 (t 1/2 = 109.8 min) and for delivery of higher CSF1R-specific PET signal in vivo. Seven fluorine-containing analogs of CPPC were prepared and four were found to have high inhibitory potency (IC 50 in low to sub-nM range) and selectivity at CSF1R comparable with CPPC itself. One of these, a 4-fluoromethyl analog (Psa374), was investigated more deeply by labeling with carbon-11 (t 1/2 = 20.4 min) for PET studies in mouse and monkey. [ 11 C]Psa374 showed high peak uptake in monkey brain but not in mouse brain. Pharmacological challenges revealed no CSF1R-specific binding in either species at baseline. [ 11 C]CPPC also failed to show specific binding at baseline. Moreover, both [ 11 C]Psa374 and [ 11 C]CPPC showed brain efflux transporter substrate behavior in both species in vivo, although Psa374 did not show liability toward human efflux transporters in vitro. Further development of [ 11 C]Psa374 in non-human primate models of neuroinflammation with demonstration of CSF1Rspecific binding would be required to warrant the fluorine-18 labeling of Psa374 with a view to possible application in human subjects.

PET Imaging of Innate Immune Activation Using 11C Radiotracers Targeting GPR84

“…Most recently, Horti et al reported a 11 C–CPPC radiotracer targeting CSF1R (a candidate biomarker of innate immune cell-driven inflammation) to have 55% higher binding in the mouse brain when injected with LPS (10 mg/kg) compared to a control animal . This radiotracer displayed significant nonspecific binding since the CSF1R knockout mouse still showed an extensive signal. , Another example is the 11 C-GSK1482160 radiotracer, which binds purinergic receptor subtype 7 (P2 × 7)expressed on activated microglia induced by LPS (5 mg/kg) at a 3.2-fold higher amount than binding in saline-treated mice brains . Although this is a promising fold difference in binding, P2 × 7 is a suboptimal biomarker for innate immune activation since it is not solely expressed on myeloid cells .…”

Diagnostic and Therapeutic Radiopharmaceuticals: A “Hot” Topic

“…Moreover, their results showed that [ 11 C]13 interacts with mouse and monkey P-gp efflux pumps in vivo, precluding any utility for robust PET imaging of CSF1R in these species. 14 In this context, to overcome poor brain permeability, the pro-radiotracer approach is discussed in ref 15. This approach might be used to improve brain uptake of molecules bearing free carboxylic acid which is masked by a brain-penetrant chemical functionality that is released after BBB penetration.…”

“…[ 11 C] 13 produced a CSF1R-specific signal in mouse and monkey. Moreover, their results showed that [ 11 C] 13 interacts with mouse and monkey P-gp efflux pumps in vivo , precluding any utility for robust PET imaging of CSF1R in these species …”

Candidate Tracers for Imaging Colony-Stimulating Factor 1 Receptor in Neuroinflammation with Positron Emission Tomography: Issues and Progress

“…Left images are coronal, middle are sagittal, and right images are horizontal. The figure is taken from Altomonte et al (ref ) with permission.…”

“…10 min before tracer) increased [ 11 C]CPPC brain radioactivity uptake 2-fold (2.9 SUV at 4 min), denoting a possible interaction between CPPC blocker and brain efflux transporters. 53 Further evidence for such an interaction was found by imaging [ 11 C]CPPC in dual P-gp and BCRP knockout mice. In these mice, [ 11 C]CPPC showed substantially higher peak brain radioactivity uptake (2.7 SUV at 4.5 min) than in WT mice at baseline, comparable with brain radioactivity uptake in WT mice after CPPC blockade.…”