Blocking of efflux transporters in rats improves translational validation of brain radioligands

Abstract

Background Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs. Methods PET tracers targeting serotonin 5-HT2A receptors ([18F]MH.MZ, [18F]Altanserin, [11C]Cimbi-36, [11C]Pimavanserin), serotonin 5-HT7 receptors ([11C]Cimbi-701, [11C]Cimbi-717 and [11C]BA-10) and dopamine D2/3 receptors ([18F]Fallypride) were used in the study. The brain uptake and target-specific binding of these PET radiotracers were evaluated in rats with and without inhibition of P-gp. Rat data were subsequently compared to the results obtained in pigs. Results Without P-gp inhibition, the amount of target-specific binding in the rat brain was sufficient to justify further translation for three out of eight evaluated tracers. With P-gp inhibition, results for five out of eight tracers justified further translation. The performance in pigs could correctly be predicted for six out of eight tracers when rat data obtained under P-gp inhibition were used, compared to four out of eight tracers without P-gp inhibition. Conclusions P-gp strongly affects the uptake of PET tracers in rodents, but false prediction outcomes can be reduced by evaluating a tracer under P-gp inhibition.

Synaptic Density and Neuronal Metabolic Function Measured by Positron Emission Tomography in the Unilateral 6-OHDA Rat Model of Parkinson’s Disease

“…Anaesthesia was induced using 3% isoflurane in oxygen. All rats were placed in a 2 × 2 custom made rat holder (illustration in Figure 1 ), enabling simultaneous scanning of four rats ( Keller et al, 2017 ; Shalgunov et al, 2020 ; Casado-Sainz et al, 2021 ). While in the custom-made rat holder, the rats were kept under anaesthesia with a constant flow of isoflurane (∼2% isoflurane in oxygen).…”

“…The regions (depicted in Figure 3 and Supplementary Figure 5 ) included in this study were: anterior cingulate cortex, medial prefrontal cortex, motor cortex, nucleus accumbens, orbitofrontal cortex, striatum, thalamus, and ventral midbrain (a region covering both the ventral tegmental area and substantia nigra). The dorsomedial striatum and dorsolateral striatum were manually delineated and used in the study ( Shalgunov et al, 2020 ; Casado-Sainz et al, 2021 ). All images and co-registration were visually checked for accuracy following spatial transformation.…”

Synaptic density and neuronal metabolic function measured by PET in the unilateral 6-OHDA rat model of Parkinson’s disease

“…The regions (depicted in Figure 3 and Supplementary Figure 4) included in this study were: anterior cingulate cortex, medial prefrontal cortex, motor cortex, nucleus accumbens, orbitofrontal cortex, striatum, thalamus, and ventral midbrain (a region covering both the ventral tegmental area and substantia nigra). The dorsomedial striatum and dorsolateral striatum were manually delineated and used in the study (Shalgunov et al, 2020; Casado-Sainz et al, 2021). All images and co-registration were visually checked for accuracy following spatial transformation.…”

The role of neuroimaging in Parkinson’s disease

“…This heterogeneity of α-Syn makes it difficult to establish in vitro assays to optimize lead compounds (Kotzbauer et al, 2017). It should also be noted that initial in vivo evaluation of candidate tracers for brain imaging is usually performed in rodents, and there are well-described discrepancies between BBB permeability in rodents compared to larger animals including humans (Shalgunov et al, 2020;Syvänen et al, 2009).…”