Focused ultrasound in combination with microbubbles (FUS) provides an effective means to locally enhance the delivery of therapeutics to the brain. Translational and quantitative imaging techniques are needed to noninvasively monitor and optimize the impact of FUS on blood-brain barrier (BBB) permeability in vivo. Positron-emission tomography (PET) imaging using [¹⁸F]2-fluoro-2-deoxy-sorbitol ([¹⁸F]FDS) was evaluated as a small-molecule (paracellular) marker of blood-brain barrier (BBB) integrity. [¹⁸F]FDS was straightforwardly produced from chemical reduction of commercial [¹⁸F]2-deoxy-2-fluoro-D-glucose. [¹⁸F]FDS and the invasive BBB integrity marker Evan’s blue (EB) were i.v. injected in mice after an optimized FUS protocol designed to generate controlled hemispheric BBB disruption. Quantitative determination of the impact of FUS on the BBB permeability was determined using kinetic modeling. A 2.2 ± 0.5-fold higher PET signal (n = 5; p < 0.01) was obtained in the sonicated hemisphere and colocalized with EB staining observed post mortem. FUS significantly increased the blood-to-brain distribution of [¹⁸F]FDS by 2.4 ± 0.8-fold (V_T; p < 0.01). Low variability (=10.1%) of V_T values in the sonicated hemisphere suggests reproducibility of the estimation of BBB permeability and FUS method. [¹⁸F]FDS PET provides a readily available, sensitive and reproducible marker of BBB permeability to noninvasively monitor the extent of BBB disruption induced by FUS in vivo.

Domperidone and metoclopramide are widely prescribed antiemetic drugs with distinct neurological side effects. The impact of P-glycoprotein (P-gp)-mediated efflux at the blood–brain barrier (BBB) on brain exposure and BBB permeation was compared in vitro and in vivo using positron emission tomography (PET) imaging in rats with the radiolabeled analogs [¹¹C]domperidone and [¹¹C]metoclopramide. In P-gp-overexpressing cells, the IC₅₀ of tariquidar, a potent P-gp inhibitor, was drastically different using [¹¹C]domperidone (221 nM [198–248 nM]) or [¹¹C]metoclopramide (4 nM [2–8 nM]) as the substrate. Complete P-gp inhibition led to a 1.8-fold higher increase in the cellular uptake of [¹¹C]domperidone compared with [¹¹C]metoclopramide (p < 0.0001). Brain PET imaging revealed that the baseline brain exposure (AUC_brain) of [¹¹C]metoclopramide was 2.4-fold higher compared with [¹¹C]domperidone (p < 0.001), consistent with a 1.8-fold higher BBB penetration (AUC_brain/AUC_plasma). The maximal increase in the brain exposure (2.9-fold, p < 0.0001) and BBB penetration (2.9-fold, p < 0.0001) of [¹¹C]metoclopramide was achieved using 8 mg/kg of tariquidar. In comparison, neither 8 nor 15 mg/kg of tariquidar increased the brain exposure of [¹¹C]domperidone (p > 0.05). Domperidone is an avid P-gp substrate that was in vitro compared with metoclopramide. Domperidone benefits from a lower brain exposure and a limited risk for P-gp-mediated drug–drug interaction involving P-gp inhibition at the BBB.

Organic Anion-Transporting Polypeptides (OATPs) are known to control the liver uptake of many drugs. Non-hepatic expression of OATPs has been reported although functional importance for whole-body pharmacokinetics (WBPK) remains unknown. Glyburide is a well described substrate of several hepatic and non-hepatic OATPs. Dynamic whole-body positron emission tomography (DWB-PET) with [¹¹C]glyburide was performed in humans for determination of the importance of OATPs for liver uptake and WBPK. Seven healthy male subjects (24.7 ± 3.2 years) underwent [¹¹C]glyburide PET scan with concomitant blood sampling. All subjects underwent baseline [¹¹C]glyburide PET scan. Five subjects underwent a subsequent [¹¹C]glyburide PET scan after infusion of the potent OATP inhibitor rifampicin (9 mg/kg i.v.). The transfer constant (k_uptake) of [¹¹C]glyburide from blood to the liver was estimated using the integration plot method. The tissue exposure of [¹¹C]glyburide was described by the area under the time-activity curve (AUC) and corresponding tissue/blood ratio (AUCR). [¹¹C]glyburide was barely metabolized in both the baseline and rifampicin conditions. Parent (unmetabolized) [¹¹C]glyburide accounted for > 90 % of the plasma radioactivity. Excellent correlation was found between radioactive counting in arterial blood samples and in the image-derived input function, in both the baseline and rifampicin conditions (R² = 97.9 %, p < 0.01). [¹¹C]glyburide predominantly accumulated in the liver. Rifampicin decreased liver k_uptake by 77.3 ± 7.3 %, which increased exposure in blood, kidneys, spleen, myocardium and brain (p < 0.05). No significant change in AUCR was observed except in the liver (p < 0.01). [¹¹C]glyburide benefits from metabolic stability and high sensitivity to OATP inhibition which enables quantitative determination of OATP function. DWB-PET suggests negligible role for non-hepatic OATPs in controlling the tissue distribution of [¹¹C]glyburide.

[¹¹C]metoclopramide PET imaging provides a sensitive and translational tool to explore P-glycoprotein (P-gp) function at the blood-brain barrier (BBB). Patients with neurological diseases are often treated with cytochrome (CYP) modulators which may impact the plasma and brain kinetics of [¹¹C]metoclopramide. The impact of the CYP inducer carbamazepine or the CYP inhibitor ritonavir on the brain and plasma kinetics of [¹¹C]metoclopramide was investigated in rats. Data obtained in a control group were compared with groups that were either orally pretreated with carbamazepine (45 mg/kg twice a day for 7 days before PET) or ritonavir (20 mg/kg, 3 h before PET) (n = 4 per condition). Kinetic modelling was performed to estimate the brain penetration (V_T) of [¹¹C]metoclopramide. CYP induction or inhibition had negligible impact on the plasma kinetics and metabolism of [¹¹C]metoclopramide. Moreover, carbamazepine neither impacted the brain kinetics nor V_T of [¹¹C]metoclopramide (p > 0.05). However, ritonavir significantly increased V_T (p < 0.001), apparently behaving as an inhibitor of P-gp at the BBB. Our data suggest that treatment with potent CYP inducers such as carbamazepine does not bias the estimation of P-gp function at the BBB with [¹¹C]metoclopramide PET. This supports further use of [¹¹C]metoclopramide for studies in animals and patients treated with CYP inducers.

Organic anion-transporting polypeptide 2B1 (OATP2B1) is co-localized with OATP1B1 and OATP1B3 in the basolateral hepatocyte membrane, where it is thought to contribute to the hepatic uptake of drugs. We characterized a novel Slco2b1^(-/-) mouse model using positron emission tomography (PET) imaging with [¹¹C]erlotinib (a putative OATP2B1-selective substrate) and planar scintigraphic imaging with [^99mTc]mebrofenin (an OATP1B1/1B3 substrate, which is not transported by OATP2B1). Dynamic 40-min scans were performed after intravenous injection of either [¹¹C]erlotinib or [^99mTc]mebrofenin in wild-type and Slco2b1^(-/-) mice. A pharmacokinetic model was used to estimate the hepatic uptake clearance (CL₁) and the rate constants for transfer of radioactivity from the liver to the blood (k₂) and excreted bile (k₃). CL₁ was significantly reduced in Slco2b1^(-/-) mice for both radiotracers (p < 0.05), and k₂ was significantly lower (p < 0.01) in Slco2b1^(-/-) mice for [¹¹C]erlotinib, but not for [^99mTc]mebrofenin. Our data support previous evidence that OATP transporters may contribute to the hepatic uptake of [¹¹C]erlotinib. However, the decreased hepatic uptake of the OATP1B1/1B3 substrate [^99mTc]mebrofenin in Slco2b1^(-/-) mice questions the utility of this mouse model to assess the relative contribution of OATP2B1 to the liver uptake of drugs which are substrates of multiple OATPs.

Endotoxemia-induced inflammation may impact the activity of hepatocyte transporters, which control the hepatobiliary elimination of drugs and bile acids. ^99mTc-mebrofenin is a non-metabolized substrate of transporters expressed at the different poles of hepatocytes. ^99mTc-mebrofenin imaging was performed in rats after the injection of lipopolysaccharide (LPS). Changes in transporter expression were assessed using quantitative polymerase chain reaction of resected liver samples. Moreover, the particular impact of pharmacokinetic drug–drug interactions in the context of endotoxemia was investigated using rifampicin (40 mg/kg), a potent inhibitor of hepatocyte transporters. LPS increased ^99mTc-mebrofenin exposure in the liver (1.7 ± 0.4-fold). Kinetic modeling revealed that endotoxemia did not impact the blood-to-liver uptake of ^99mTc-mebrofenin, which is mediated by organic anion-transporting polypeptide (Oatp) transporters. However, liver-to-bile and liver-to-blood efflux rates were dramatically decreased, leading to liver accumulation. The transcriptomic profile of hepatocyte transporters consistently showed a downregulation of multidrug resistance-associated proteins 2 and 3 (Mrp2 and Mrp3), which mediate the canalicular and sinusoidal efflux of ^99mTc-mebrofenin in hepatocytes, respectively. Rifampicin effectively blocked both the Oatp-mediated influx and the Mrp2/3-related efflux of ^99mTc-mebrofenin. The additive impact of endotoxemia and rifampicin led to a 3.0 ± 1.3-fold increase in blood exposure compared with healthy non-treated animals. ^99mTc-mebrofenin imaging is useful to investigate disease-associated change in hepatocyte transporter function.