[¹⁸F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging

Abstract

Selective inhibition of glycine transporter 1 (GlyT1) has emerged as a potential approach to alleviate N-methyl-d-aspartate receptor (NMDAR) hypofunction in patients with schizophrenia and cognitive decline. ALX5407 is a potent and selective inhibitor of GlyT1 derived from the metabolic intermediate sarcosine (N-methylglycine) that showed antipsychotic potential in a number of animal models. Whereas clinical application of ALX5407 is limited by adverse effects on motor performance and respiratory function, a suitably radiolabeled drug could represent a promising PET tracer for the visualization of GlyT1 in the brain. Herein, [18F]­ALX5407 and the corresponding methyl ester, [18F]­ALX5406, were prepared by alcohol-enhanced copper mediated radiofluorination and studied in vitro in rat brain slices and in vivo in normal rats. [18F]­ALX5407 demonstrated accumulation consistent with the distribution of GlyT1 in in vitro autoradiographic studies but no brain uptake in μPET experiments in naı̈ve rats. In contrast, the methyl ester [18F]­ALX5406 rapidly entered the brain and was enzymatically transformed into [18F]­ALX5407, resulting in a regional accumulation pattern consistent with GlyT1 specific binding. We conclude that [18F]­ALX5406 is a promising and easily accessible PET probe for preclinical in vivo imaging of GlyT1 in the brain.

Next Generation Copper Mediators for the Efficient Production of 18F‐Labeled Aromatics

“…To demonstrate the practical suitability of the novel Cu mediators, we next applied them for the preparation of several PET‐tracers: [ 18 F]R91150, [16] [ 18 F]ALX5407, [17] [ 18 F]MNI1126, [18] 3‐( S )‐ and ( R )‐[ 18 F]FPhes, [19] and ( S )‐αMe‐3‐[ 18 F]FPhe. [20] …”

“…To demonstrate the practical suitability of the novel Cu mediators, we next applied them for the preparation of several PET-tracers: [ 18 F]R91150, [16] [ 18 F]ALX5407, [17] [ 18 F]MNI1126, [18] 3-(S)-and (R)-[ 18 F]FPhes, [19] and (S)-αMe-3-[ 18 F]FPhe. [20] Radiosynthesis of [ 18 F]R91150, a promising 5-HT 2A -selective PET-tracer, [16b] from the corresponding Bpin precursor 7 was performed according to the developed protocol using Cu(4-PhPy) 4 (ClO 4 ) 2 as the mediator.…”

“…After subsequent deprotection and HPLC purification, the tracer was obtained in activity yields (AYs) [11] of 23 � 5 % (Scheme 3), as compared to AYs of 10-15 % obtained under standard conditions [Cu(Py) 4 (OTf) 2 in DMA/ nBuOH]. Application of the method for preparation of [ 18 F]ALX5407, a glycine transporter type 1 (GlyT 1 ) radioligand, [17] from the appropriate Bpin precursor 8 using Cu(4-PhPy) 4 (ClO 4 ) 2 improved AYs (30 � 5 % vs. 14 � 6 %), allowed for a reduction of precursor and mediator amounts from 30 to 10 μmol, and thus simplified preparative HPLC purification of the radiolabeled product (Scheme 3).…”

Mutated Isocitrate Dehydrogenase (mIDH) as Target for PET Imaging in Gliomas

“…Alternatively, the entire 4-trifluoromethoxyphenyl substituent or the 3,3,5-trimethyl-cyclohexyl group (which is also located in a hydrophobic sub-pocket, Figure 8 B) could be replaced by a suitable 18 F-labeled cycloalkyl, aryl or heteroaryl scaffold, although the effects of these modifications on the inhibitory potency are more difficult to predict. Finally, it is worth noting that the presence of a carboxyl group often reduces the rate (or extent) of brain penetration [ 101 , 102 ], which could impede PET imaging with short-lived radionuclides. BAY1436032 analogs lacking a (free) carboxyl group have in several cases been shown to retain high inhibitory potency ( Table 3 ).…”

Visualizing GABA transporters in vivo: an overview of reported radioligands and future directions

“…Alternatively, the radioligand [ 18 F]39 could behave like a prodrug and the ester could hydrolyse after crossing the BBB to give the carboxylic acid [ 18 F]44 that binds to GAT1. While such prodrugs have been used for preclinical PET imaging [ 83 , 84 ], they complicate quantitative analysis and kinetic modelling of imaging data. Hence, the radioligand [ 18 F]39 is not suitable for the in vivo imaging of neuronal GATs.…”