Collecting antibodies and large molecule biomarkers in mouse interstitial brain fluid: a comparison of microdialysis and cerebral open flow microperfusion

Negligible In Vitro Recovery of Macromolecules from Microdialysis Using 100 kDa Probes and Dextran in Perfusion Fluid

Microdialysis is applied in neurointensive care to monitor cerebral glucose metabolism. If recoverable, macromolecules may also serve as biomarkers in brain disease and provide clues to their passage across the blood-brain barrier. Our study aimed to investigate the in vitro recovery of human micro- and macromolecules using microdialysis catheters and perfusion fluids approved for clinical use. In vitro microdialysis of a bulk solution containing physiological or supraphysiological concentrations of glucose, lactate, pyruvate, human IgG, serum albumin, and hemoglobin was performed using two different catheters and perfusion fluids. One had a membrane cut-off of 20 kDa and was used with a standard CNS perfusion fluid, and the other had a membrane cut-off of 100 kDa and was perfused with the same solution supplemented with dextran. The flow rate was 0.3 µl/min. We used both push and push-pull methods. Dialysate samples were collected at 2-h intervals for 6 h and analyzed for relative recovery of each substance. The mean relative recovery of glucose, pyruvate, and lactate was > 90% in all but two sets of experiments. In contrast, the relative recovery of human IgG, serum albumin, and hemoglobin from both bulk solutions was below the lower limit of quantification (LLOQ). Using a push-pull method, recovery of human IgG, serum albumin, and hemoglobin from a bulk solution with supraphysiological concentrations were above LLOQ but with low relative recovery (range 0.9%-1.6%). In summary, exchanging the microdialysis setup from a 20 kDa catheter with a standard perfusion fluid for a 100 kDa catheter with a perfusion solution containing dextran did not affect the relative recovery of glucose and its metabolites. However, it did not result in any useful recovery of the investigated macromolecules at physiological levels, either with or without a push-pull pump system.

Epitope(s) involving amino acids of the fusion loop of Japanese encephalitis virus envelope protein is(are) important to elicit protective immunity

Dengue vaccine candidates have been shown to improve vaccine safety and efficacy by altering the residues or accessibility of the fusion loop on the virus envelope protein domain II (DIIFL) in an ex vivo animal study. The current study aimed to comprehensively investigate the impact of DIIFL mutations on the antigenicity, immunogenicity, and protective efficacy of Japanese encephalitis virus (JEV) virus-like particles (VLPs) in mice. We found the DIIFL G106K/L107D (KD) and W101G/G106K/L107D (GKD) mutations altered the binding activity of JEV VLP to cross-reactive monoclonal antibodies but had no effect on their ability to elicit total IgG antibodies in mice. However, JEV VLPs with KD or GKD mutations induced significantly less neutralizing antibodies against JEV. Only 46% and 31% of the KD and GKD VLPs-immunized mice survived compared to 100% of the wild-type (WT) VLP-immunized mice after a lethal JEV challenge. In passive protection experiments, naïve mice that received sera from WT VLP-immunized mice exhibited a significantly higher survival rate of 46.7% compared to those receiving sera from KD VLP- and GKD VLP-immunized mice (6.7% and 0%, respectively). This study demonstrated that JEV DIIFL is crucial for eliciting potently neutralizing antibodies and protective immunity against JEV.

IMPORTANCE

Introduction of mutations into the fusion loop is one potential strategy for generating safe dengue and Zika vaccines by reducing the risk of severe dengue following subsequent infections, and for constructing live-attenuated vaccine candidates against newly emerging Japanese encephalitis virus (JEV) or Japanese encephalitis (JE) serocomplex virus. The monoclonal antibody studies indicated the fusion loop of JE serocomplex viruses primarily comprised non-neutralizing epitopes. However, the present study demonstrates that the JEV fusion loop plays a critical role in eliciting protective immunity in mice. Modifications to the fusion loop of JE serocomplex viruses might negatively affect vaccine efficacy compared to dengue and zika serocomplex viruses. Further studies are required to assess the impact of mutant fusion loop encoded by commonly used JEV vaccine strains on vaccine efficacy or safety after subsequent dengue virus infection.

Unravelling the brain metabolome: A review of liquid chromatography - mass spectrometry strategies for extracellular brain metabolomics

The analysis of the brain extracellular metabolome is of interest for numerous subdomains within neuroscience. Not only does it provide information about normal physiological functions, it is even more of interest for biomarker discovery and target discovery in disease. The extracellular analysis of the brain is particularly interesting as it provides information about the release of mediators in the brain extracellular fluid to look at cellular signaling and metabolic pathways through the release, diffusion and re-uptake of neurochemicals. In vivo samples are obtained through microdialysis, cerebral open-flow microperfusion or solid-phase microextraction. The analytes of potential interest are typically low in concentration and can have a wide range of physicochemical properties. Liquid chromatography coupled to mass spectrometry has proven its usefulness in brain metabolomics. It allows sensitive and specific analysis of low sample volumes, obtained through different approaches. Several strategies for the analysis of the extracellular fluid have been proposed. The most widely used approaches apply sample derivatization, specific stationary phases and/or hydrophilic interaction liquid chromatography. Miniaturization of these methods allows an even higher sensitivity. The development of chiral metabolomics is indispensable, as it allows to compare the enantiomeric ratio of compounds and provides even more challenges. Some limitations continue to exist for the previously developed methods and the development of new, more sensitive methods remains needed. This review provides an overview of the methods developed for sampling and liquid chromatography-mass spectrometry analysis of the extracellular metabolome.

Receptor-mediated drug delivery of bispecific therapeutic antibodies through the blood-brain barrier

Therapeutic antibody drug development is a rapidly growing sector of the pharmaceutical industry. However, antibody drug development for the brain is a technical challenge, and therapeutic antibodies for the central nervous system account for ~3% of all such agents. The principal obstacle to antibody drug development for brain or spinal cord is the lack of transport of large molecule biologics across the blood-brain barrier (BBB). Therapeutic antibodies can be made transportable through the blood-brain barrier by the re-engineering of the therapeutic antibody as a BBB-penetrating bispecific antibody (BSA). One arm of the BSA is the therapeutic antibody and the other arm of the BSA is a transporting antibody. The transporting antibody targets an exofacial epitope on a BBB receptor, and this enables receptor-mediated transcytosis (RMT) of the BSA across the BBB. Following BBB transport, the therapeutic antibody then engages the target receptor in brain. RMT systems at the BBB that are potential conduits to the brain include the insulin receptor (IR), the transferrin receptor (TfR), the insulin-like growth factor receptor (IGFR) and the leptin receptor. Therapeutic antibodies have been re-engineered as BSAs that target the insulin receptor, TfR, or IGFR RMT systems at the BBB for the treatment of Alzheimer's disease and Parkinson's disease.

Quantification of the Therapeutic Antibody Ocrelizumab in Mouse Brain Interstitial Fluid Using Cerebral Open Flow Microperfusion and Simultaneous Monitoring of the Blood-Brain Barrier Integrity

The increasing relevance of improved therapeutic monoclonal antibodies (mAbs) to treat neurodegenerative diseases has strengthened the need to reliably measure their brain pharmacokinetic (PK) profiles. The aim of this study was, therefore, to absolutely quantify the therapeutic antibody ocrelizumab (OCR) as a model antibody in mouse brain interstitial fluid (ISF), and to record its PK profile by using cerebral open flow microperfusion (cOFM). Further, to monitor the blood-brain barrier (BBB) integrity using an endogenous antibody with a similar molecular size as OCR. The study was conducted on 13 male mice. Direct and absolute OCR quantification was performed with cOFM in combination with zero flow rate, and subsequent bioanalysis of the obtained cerebral ISF samples. For PK profile recording, cerebral ISF samples were collected bi-hourly, and brain tissue and plasma were collected once at the end of the sampling period. The BBB integrity was monitored during the entire PK profile recording by using endogenous mouse immunoglobulin G1. We directly and absolutely quantified OCR and recorded its brain PK profile over 96 h. The BBB remained intact during the PK profile recording. The resulting data provide the basis for reliable PK assessment of therapeutic antibodies in the brain thus favoring the further development of therapeutic monoclonal antibodies.

In vivo monitoring of brain pharmacokinetics and pharmacodynamics with cerebral open flow microperfusion

In vivo investigation of brain pharmacokinetics and pharmacodynamics (PK/PD) is an integral part of neurological drug development. However, drugs intended to act in the brain may reach it at very low concentrations due to the protective effect of the blood-brain barrier (BBB). Consequently, very sensitive measurement methods are required to investigate PK/PD of drugs in the brain. Also, these methods must be capable of continuously assessing cerebral drug concentrations with verifiable intact BBB, as disrupted BBB may lead to compound efflux from blood into brain and to biased results. To date, only a few techniques are available that can sensitively measure drug concentrations in the brain over time; one of which is cerebral open flow microperfusion (cOFM). cOFM's key features are that it enables measurement of cerebral compound concentrations with intact BBB, induces only minor tissue reactions, and that no scar formation occurs around the probe. The membrane-free cOFM probes collect diluted cerebral interstitial fluid (ISF) samples that are containing the whole molecule spectrum of the ISF. Further, combining cOFM with an in vivo calibration protocol (e.g. Zero Flow Rate) enables absolute quantification of compounds in cerebral ISF. In general, three critical aspects have to be considered when measuring cerebral drug concentrations and recording PK/PD profiles with cOFM: (a) the BBB integrity during sampling, (b) the status of the brain tissue next to the cOFM probe during sampling, and (c) the strategy to absolutely quantify drugs in cerebral ISF. This work aims to review recent applications of cOFM for PK/PD assessment with a special focus on these critical aspects.

Brain pharmacokinetics of mono- and bispecific amyloid-β antibodies in wild-type and Alzheimer's disease mice measured by high cut-off microdialysis

BACKGROUND

Treatment with amyloid-β (Aβ) targeting antibodies is a promising approach to remove Aβ brain pathology in Alzheimer's disease (AD) and possibly even slow down or stop progression of the disease. One of the main challenges of brain immunotherapy is the restricted delivery of antibodies to the brain. However, bispecific antibodies that utilize the transferrin receptor (TfR) as a shuttle for transport across the blood-brain barrier (BBB) can access the brain better than traditional monospecific antibodies. Previous studies have shown that bispecific Aβ targeting antibodies have higher brain distribution, and can remove Aβ pathology more efficiently than monospecific antibodies. Yet, there is only limited information available on brain pharmacokinetics, especially regarding differences between mono- and bispecific antibodies.

METHODS

The aim of the study was to compare brain pharmacokinetics of Aβ-targeting monospecific mAb3D6 and its bispecific version mAb3D6-scFv8D3 that also targets TfR. High cut-off microdialysis was used to measure intravenously injected radiolabelled mAb3D6 and mAb3D6-scFv8D3 antibodies in the interstitial fluid (ISF) of hippocampus in wild-type mice and the App^NL-G-F mouse model of AD. Distribution of the antibodies in the brain and the peripheral tissue was examined by ex vivo autoradiography and biodistribution studies.

RESULTS

Brain concentrations of the bispecific antibody were elevated compared to the monospecific antibody in the hippocampal ISF measured by microdialysis and in the brain tissue at 4-6 h after an intravenous injection. The concentration of the bispecific antibody was approximately twofold higher in the ISF dialysate compared to the concentration of monospecific antibody and eightfold higher in brain tissue 6 h post-injection. The ISF dialysate concentrations for both antibodies were similar in both wild-type and App^NL-G-F mice 24 h post-injection, although the total brain tissue concentration of the bispecific antibody was higher than that of the monospecific antibody at this time point. Some accumulation of radioactivity around the probe area was observed especially for the monospecific antibody indicating that the probe compromised the BBB to some extent at the probe insertion site.

CONCLUSION

The BBB-penetrating bispecific antibody displayed higher ISF concentrations than the monospecific antibody. The concentration difference between the two antibodies was even larger in the whole brain than in the ISF. Further, the bispecific antibody, but not the monospecific antibody, displayed higher total brain concentrations than ISF concentrations, indicating association to brain tissue.

A call for better understanding of target engagement in Tau antibody development

Significant efforts have been channeled into developing antibodies for the treatment of CNS indications. Disappointment with the first generation of clinical Tau antibodies in Alzheimer's disease has highlighted the challenges in understanding whether an antibody can reach or affect the target in the compartment where it is involved in pathological processes. Here, we highlight different aspects essential for improving translatability of Tau-based immunotherapy.

Investigating brain uptake of a non-targeting monoclonal antibody after intravenous and intracerebroventricular administration

Monoclonal antibodies play an important role in the treatment of various diseases. However, the development of these drugs against neurological disorders where the drug target is located in the brain is challenging and requires a good understanding of the local drug concentration in the brain. In this original research, we investigated the systemic and local pharmacokinetics in the brain of healthy rats after either intravenous (IV) or intracerebroventricular (ICV) administration of EGFRvIII-T-Cell bispecific (TCB), a bispecific monoclonal antibody. We established an experimental protocol that allows serial sampling in serum, cerebrospinal fluid (CSF) and interstitial fluid (ISF) of the prefrontal cortex in freely moving rats. For detection of drug concentration in ISF, a push-pull microdialysis technique with large pore membranes was applied. Brain uptake into CSF and ISF was characterized and quantified with a reduced brain physiologically-based pharmacokinetic model. The model allowed us to interpret the pharmacokinetic processes of brain uptake after different routes of administration. The proposed model capturing the pharmacokinetics in serum, CSF and ISF of the prefrontal cortex suggests a barrier function between the CSF and ISF that impedes free antibody transfer. This finding suggests that ICV administration may not be better suited to reach higher local drug exposure as compared to IV administration. The model enabled us to quantify the relative contribution of the blood-brain barrier (BBB) and Blood-CSF-Barrier to the uptake into the interstitial fluid of the brain. In addition, we compared the brain uptake of three monoclonal antibodies after IV dosing. In summary, the presented approach can be applied to profile compounds based on their relative uptake in the brain and provides quantitative insights into which pathways are contributing to the net exposure in the brain.

Current Approaches to Monitor Macromolecules Directly from the Cerebral Interstitial Fluid

Gaining insights into the pharmacokinetic and pharmacodynamic properties of lead compounds is crucial during drug development processes. When it comes to the treatment of brain diseases, collecting information at the site of action is challenging. There are only a few techniques available that allow for the direct sampling from the cerebral interstitial space. This review concerns the applicability of microdialysis and other approaches, such as cerebral open flow microperfusion and electrochemical biosensors, to monitor macromolecules (neuropeptides, proteins, …) in the brain. Microdialysis and cerebral open flow microperfusion can also be used to locally apply molecules at the same time at the site of sampling. Innovations in the field are discussed, together with the pitfalls. Moreover, the ‘nuts and bolts’ of the techniques and the current research gaps are addressed. The implementation of these techniques could help to improve drug development of brain-targeted drugs.