Physiologically Based Pharmacokinetic Modelling with Dynamic PET Data to Study the In Vivo Effects of Transporter Inhibition on Hepatobiliary Clearance in Mice

Performance and Sensitivity of [99mTc]Tc-sestamibi Compared with Positron Emission Tomography Radiotracers to Measure P-glycoprotein Function in the Kidneys and Liver

P-glycoprotein (P-gp, encoded in humans by the ABCB1 gene and in rodents by the Abcb1a/b genes) is a membrane transporter that can restrict the intestinal absorption and tissue distribution of many drugs and may also contribute to renal and hepatobiliary drug excretion. The aim of this study was to compare the performance and sensitivity of currently available radiolabeled P-gp substrates for positron emission tomography (PET) with the single-photon emission computed tomography (SPECT) radiotracer [99mTc]Tc-sestamibi for measuring the P-gp function in the kidneys and liver. Wild-type, heterozygous (Abcb1a/b(+/-)), and homozygous (Abcb1a/b(-/-)) Abcb1a/b knockout mice were used as models of different P-gp abundance in excretory organs. Animals underwent either dynamic PET scans after intravenous injection of [11C]N-desmethyl-loperamide, (R)-[11C]verapamil, or [11C]metoclopramide or consecutive static SPECT scans after intravenous injection of [99mTc]Tc-sestamibi. P-gp in the kidneys and liver of the mouse models was analyzed with immunofluorescence labeling and Western blotting. In the kidneys, Abcb1a/b() mice had intermediate P-gp abundance compared with wild-type and Abcb1a/b(-/-) mice. Among the four tested radiotracers, renal clearance of radioactivity (CLurine,kidney) was significantly reduced (-83%) in Abcb1a/b(-/-) mice only for [99mTc]Tc-sestamibi. Biliary clearance of radioactivity (CLbile,liver) was significantly reduced in Abcb1a/b(-/-) mice for [11C]N-desmethyl-loperamide (-47%), [11C]metoclopramide (-25%), and [99mTc]Tc-sestamibi (-79%). However, in Abcb1a/b(+/-) mice, CLbile,liver was significantly reduced (-47%) only for [99mTc]Tc-sestamibi. Among the tested radiotracers, [99mTc]Tc-sestamibi performed best in measuring the P-gp function in the kidneys and liver. Owing to its widespread clinical availability, [99mTc]Tc-sestamibi represents a promising probe substrate to assess systemic P-gp-mediated drug-drug interactions and to measure renal and hepatic P-gp function under different (patho-)physiological conditions.

Mixing Matrix-corrected Whole-body Pharmacokinetic Modeling Using Longitudinal Micro-computed Tomography and Fluorescence-mediated Tomography

PURPOSE

Pharmacokinetic modeling can be applied to quantify the kinetics of fluorescently labeled compounds using longitudinal micro-computed tomography and fluorescence-mediated tomography (μCT-FMT). However, fluorescence blurring from neighboring organs or tissues and the vasculature within tissues impede the accuracy in the estimation of kinetic parameters. Contributions of elimination and retention activities of fluorescent probes inside the kidneys and liver can be hard to distinguish by a kinetic model. This study proposes a deconvolution approach using a mixing matrix to model fluorescence contributions to improve whole-body pharmacokinetic modeling.

PROCEDURES

In the kinetic model, a mixing matrix was applied to unmix the fluorescence blurring from neighboring tissues and blood vessels and unmix the fluorescence contributions of elimination and retention in the kidney and liver compartments. Accordingly, the kinetic parameters of the hepatobiliary and renal elimination routes and five major retention sites (the kidneys, liver, bone, spleen, and lung) were investigated in simulations and in an in vivo study. In the latter, the pharmacokinetics of four fluorescently labeled compounds (indocyanine green (ICG), HITC-iodide-microbubbles (MB), Cy7-nanogels (NG), and OsteoSense 750 EX (OS)) were evaluated in BALB/c nude mice.

RESULTS

In the simulations, the corrected modeling resulted in lower relative errors and stronger linear relationships (slopes close to 1) between the estimated and simulated parameters, compared to the uncorrected modeling. For the in vivo study, MB and NG showed significantly higher hepatic retention rates (P<0.05 and P<0.05, respectively), while OS had smaller renal and hepatic retention rates (P<0.01 and P<0.01, respectively). Additionally, the bone retention rate of OS was significantly higher (P<0.01).

CONCLUSIONS

The mixing matrix correction improves pharmacokinetic modeling and thus enables a more accurate assessment of the biodistribution of fluorescently labeled pharmaceuticals by μCT-FMT.

In Vivo Imaging of Local Inflammation: Monitoring LPS-Induced CD80/CD86 Upregulation by PET

Purpose

The co-stimulatory molecules CD80 and CD86 are upregulated on activated antigen-presenting cells (APC). We investigated whether local APC activation, induced by subcutaneous (s.c.) inoculation of lipopolysaccharides (LPS), can be imaged by positron emission tomography (PET) with CD80/CD86-targeting ⁶⁴Cu-labelled abatacept.

Procedures

Mice were inoculated s.c. with extracellular-matrix gel containing either LPS or vehicle (PBS). Immune cell populations were analysed by flow cytometry and marker expression by RT-qPCR. ⁶⁴Cu-NODAGA-abatacept distribution was analysed using PET/CT and ex vivo biodistribution.

Results

The number of CD80⁺ and CD86⁺ immune cells at the LPS inoculation site significantly increased a few days after inoculation. CD68 and CD86 expression were higher at the LPS than the PBS inoculation site, and CD80 was only increased at the LPS inoculation site. CTLA-4 was highest 10 days after LPS inoculation, when CD80/CD86 decreased again. A few days after inoculation, ⁶⁴Cu-NODAGA-abatacept distribution to the inoculation site was significantly higher for LPS than PBS (4.2-fold). Co-administration of unlabelled abatacept or human immunoglobulin reduced tracer uptake. The latter reduced the number of CD86⁺ immune cells at the LPS inoculation site.

Conclusions

CD80 and CD86 are upregulated in an LPS-induced local inflammation, indicating invasion of activated APCs. ⁶⁴Cu-NODAGA-abatacept PET allowed following APC activation over time.

Electronic supplementary material

The online version of this article (10.1007/s11307-020-01543-3) contains supplementary material, which is available to authorized users.

Use of imaging to assess the activity of hepatic transporters

Introduction: Membrane transporters of the SLC and ABC families are abundantly expressed in the liver, where they control the transfer of drugs/drug metabolites across the sinusoidal and canalicular hepatocyte membranes and play a pivotal role in hepatic drug clearance. Noninvasive imaging methods, such as PET, SPECT or MRI, allow for measuring the activity of hepatic transporters in vivo, provided that suitable transporter imaging probes are available.Areas covered: We give an overview of the working principles of imaging-based assessment of hepatic transporter activity. We discuss different currently available PET/SPECT radiotracers and MRI contrast agents and their applications to measure hepatic transporter activity in health and disease. We cover mathematical modeling approaches to obtain quantitative parameters of transporter activity and provide a critical assessment of methodological limitations and challenges associated with this approach.Expert opinion: PET in combination with pharmacokinetic modeling can be potentially applied in drug development to study the distribution of new drug candidates to the liver and their clearance mechanisms. This approach bears potential to mechanistically assess transporter-mediated drug-drug interactions, to assess the influence of disease on hepatic drug disposition and to validate and refine currently available in vitro-in vivo extrapolation methods to predict hepatic clearance of drugs.

Synthesis and Structure-Affinity Relationship of Small Molecules for Imaging Human CD80 by Positron Emission Tomography

The costimulatory molecule CD80 is an early marker for immune activation. It is upregulated on activated antigen-presenting cells. We aimed at developing a tracer for imaging CD80 by positron emission tomography (PET). Novel CD80 ligands were synthesized and tested by SPR for affinity to human CD80 (hCD80) and displacement of endogenous ligands. Several compounds bound with one-digit nanomolar affinity to hCD80 and displaced CTLA-4 and CD28 at nanomolar concentrations. A structure-affinity relationship study revealed relevant moieties for strong affinity to hCD80 and positions for further modifications. Lead compound MT107 (7f) was radiolabeled with carbon-11. In vitro, [¹¹C]MT107 showed specific binding to hCD80-positive tissue and high plasma protein binding. In vivo, [¹¹C]MT107 accumulated in liver, gall bladder, and intestines but only scarcely in hCD80-positive xenografts. The unfavorable in vivo performance may result from high plasma protein binding and extensive biliary excretion.

Verification of image quality and quantification in whole-body positron emission tomography with continuous bed motion

OBJECTIVE

Whole-body dynamic imaging using positron emission tomography (PET) facilitates the quantification of tracer kinetics. It is potentially valuable for the differential diagnosis of tumors and for the evaluation of therapeutic efficacy. In whole-body dynamic PET with continuous bed motion (CBM) (WBDCBM-PET), the pass number and bed velocity are key considerations. In the present study, we aimed to investigate the effect of a combination of pass number and bed velocity on the quantitative accuracy and quality of WBDCBM-PET images.

METHODS

In this study, WBDCBM-PET imaging was performed at a body phantom using seven bed velocity settings in combination with pass numbers. The resulting image quality was evaluated. For comparing different acquisition settings, the dynamic index (DI) was obtained using the following formula: [P/S], where P represents the pass number, and S represents the bed velocity (mm/s). The following physical parameters were evaluated: noise equivalent count at phantom (NEC_phantom), percent background variability (N_10 mm), percent contrast of the 10 mm hot sphere (Q_{H, 10 mm}), the Q_{H, 10 mm}/N_10 mm ratio, and the maximum standardized uptake value (SUV_max). Furthermore, visual evaluation was performed.

RESULTS

The NEC_phantom was equivalent for the same DI settings regardless of the bed velocity. The N_10 mm exhibited an inverse correlation (r < - 0.89) with the DI. Q_H,10 mm was not affected by DI, and a correlation between Q_H,10 mm/N_10 mm ratio and DI was found at all the velocities (r > 0.93). The SUV_max of the spheres was not influenced by the DI. The coefficient of variations caused by bed velocity decreased in larger spheres. There was no significant difference between the bed velocities on visual evaluation.

CONCLUSION

The quantitative accuracy and image quality achieved with WBDCBM-PET was comparable to that achieved with non-dynamic CBM, regardless of the pass number and bed velocity used during imaging for a given acquisition time.

Bioimaging predictors of rilpivirine biodistribution and antiretroviral activities

Antiretroviral therapy (ART) has changed the outcome of human immunodeficiency virus type one (HIV-1) infection from certain death to a life free of disease co-morbidities. However, infected people must remain on life-long daily ART. ART reduces but fails to eliminate the viral reservoir. In order to improve upon current treatment regimens, our laboratory created long acting slow effective release (LASER) ART nanoformulated prodrugs from native medicines. LASER ART enables antiretroviral drugs (ARVs) to better reach target sites of HIV-1 infection while, at the same time, improve ART's half-life and potency. However, novel ARV design has been slowed by prolonged pharmacokinetic testing requirements. To such ends, tri-modal theranostic nanoparticles were created with single-photon emission computed tomography (SPECT/CT), magnetic resonance imaging (MRI) and fluorescence capabilities to predict LASER ART biodistribution. The created theranostic ARV probes were then employed to monitor drug tissue distribution and potency. Intrinsically 111Indium (111In) radiolabeled, europium doped cobalt-ferrite particles and rilpivirine were encased in a polycaprolactone core surrounded by a lipid shell (111InEuCF-RPV). Particle cell and tissue distribution, and antiretroviral activities were sustained in macrophage tissue depots. 111InEuCF-PCL/RPV particles injected into mice demonstrated co-registration of MRI and SPECT/CT tissue signals with RPV and cobalt. Cell and animal particle biodistribution paralleled ARV activities. We posit that particle selection can predict RPV distribution and potency facilitated by multifunctional theranostic nanoparticles.