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Introduction: Delivering drugs to the central nervous system (CNS) remains a major challenge due to the blood-brain barrier, restricting the entry of drugs into the brain. This limitation contributes to the ongoing lack of effective treatments for CNS diseases. To improve the process of drug discovery and development, it is crucial to streamline methods that measure clinically relevant parameters, allowing for good selection of drug candidates. Area Covered: In this paper, we discuss the essential prerequisites for successful CNS drug delivery and review relevant methods. We emphasize the need for closer collaboration between in vitro and in vivo scientists to improve the relevance of these methods and increase the success rate of developing effective CNS therapies. While our focus is on small molecule drugs, we also touch on some aspects of larger molecules. Expert Opinion: Significant progress has been made in recent years in method development and their application. However, there is still work to be done before the use of in silico models, in vitro cell systems, and AI can consistently offer meaningful correlations and relationships to clinical data. This gap is partly due to limited patient data, but a lot can be achieved through in vivo research in animal models.

Purpose: To investigate the pharmacokinetics (PK) of ceftazidime-avibactam (CAZ-AVI) in critically ill patients undergoing continuous venovenous hemodiafiltration (CVVHDF), and compare with a general phase III trial population.

Methods: A prospective PK study was conducted in critically ill patients who received CVVHDF for acute kidney injury, treated with CAZ-AVI (1000/250 mg or 2000/500 mg q8h). Plasma and CVVHDF-circuit samples were collected to determine CAZ-AVI concentrations. Individual PK parameters at steady-state were estimated using non-compartmental analysis. For visual comparison, plasma concentrations from CVVHDF patients were overlaid with simulated data from patients not receiving CVVHDF based on previously developed population PK models.

Results: A total of 35 plasma samples and 16 CVVHDF-circuit samples were obtained from four patients, with two patients sampled on two separate occasions. Median total clearance and volume of distribution were 4.54 L/h and 73.2 L for CAZ and 10.5 L/h and 102 L for AVI, respectively. Median contribution of CVVHDF to total clearance was 19.8% for CAZ and 5.3% for AVI. Observed CAZ-AVI PK profiles were generally within the 90% confidence interval of model predictions, but the observed concentrations were notably lower early (0-2 h) and higher later (4-8 h) in the dosing interval, suggesting a higher volume of distribution.

Conclusions: These results suggest that the CAZ-AVI dose regimens used in this study can be applicable in critically ill patients undergoing CVVHDF, despite the different shape of the PK profiles observed in this population. Further research with a larger patient cohort is warranted to validate and refine these findings.

Peripheral nervous system (PNS) toxicity assessment in non-clinical safety studies is challenging and relies mostly on histopathological assessment. The present work aims to identify blood-based biomarkers that could detect peripheral neuropathy in rats upon exposure to neurotoxic compounds. Three anticancer agents (oxaliplatin, cisplatin, paclitaxel) and a developmental compound (NVS-1) were assessed in male rats (Wistar Han). Clinical and/or functional endpoints (i.e., electronic Von Frey, Cold Plate, and Paw Pressure tests) and blood biomarkers (i.e., neurofilament light chain (NfL), neurofilament heavy chain (NF-H), microtubule-associated protein Tau (Tau), neuron specific enolase (NSE), vascular endothelial growth factor A (VEGFA), and glial fibrillary acidic protein (GFAP)) were assessed. Drug exposure and histopathological evaluations were conducted on selected nervous tissues. Oxaliplatin, cisplatin and paclitaxel treatment resulted in a significant decrease of nociceptive thresholds. Clinical signs suggestive of PNS toxicity were observed with NVS-1. NfL was consistently increased in the NVS-1 study and correlated with moderate microscopic findings in dorsal root ganglia (DRG). Only minimal microscopic findings were observed in oxaliplatin-treated animals, whereas no treatment-related microscopic findings were observed in animals treated with cisplatin and paclitaxel. For all compounds, exposure was confirmed in the PNS tissues. Clinical and functional changes were observed with all the compounds evaluated. NfL levels in plasma proved to be the most sensitive indicator of PNS toxicities, capturing moderate nervous degeneration in DRG. A combined approach that includes both functional assessments and biomarker measurements offers a more comprehensive evaluation than histopathological analysis alone when monitoring drug-induced neurotoxicity in rat models.

Background: The primary objective of this study was to advance our understanding of active drug uptake at brain barriers in higher species than rodents, by examining oxycodone brain concentrations in pigs.

Methods: This was investigated by a microdialysis study in healthy and endotoxemic conditions to increase the understanding of inter-species translation of putative proton-coupled organic cation (H+/OC) antiporter-mediated central nervous system (CNS) drug delivery in health and pathology, and facilitate the extrapolation to humans for improved CNS drug treatment in patients. Additionally, we sought to evaluate the efficacy of lumbar cerebrospinal fluid (CSF) exposure readout as a proxy for brain unbound interstitial fluid (ISF) concentrations. By simultaneously monitoring unbound concentrations in blood, the frontal cortical area, the lateral ventricle (LV), and the lumbar intrathecal space in healthy and lipopolysaccharide (LPS)-induced inflammation states within the same animal, we achieved exceptional spatiotemporal resolution in mapping oxycodone transport across CNS barriers.

Results: Our findings provide novel evidence of higher unbound oxycodone concentrations in brain ISF compared to blood, yielding an unbound brain-to-plasma concentration ratio (Kp,uu,brain) of 2.5. This supports the hypothesis of the presence of the H+/OC antiporter system at the blood-brain barrier (BBB) in pigs. Despite significant physiological changes, reflected in pSOFA scores, oxycodone blood concentrations and its active net uptake across the BBB remained nearly unchanged during three hours of i.v. infusion of 4 µg/kg/h LPS from Escherichia coli (O111:B4). Mean Kp,uu,LV values indicated active uptake also at the blood-CSF barrier in healthy and endotoxemic pigs. Lumbar CSF concentrations showed minimal inter-individual variability during the experiment, with a mean Kp,uu,lumbarCSF of 1.5. LPS challenge caused a slight decrease in Kp,uu,LV , while Kp,uu,LumbarCSF remained unaffected.

Conclusions: This study enhances our understanding of oxycodone pharmacokinetics and CNS drug delivery in both healthy and inflamed conditions, providing crucial insights for translating these findings to clinical settings.

Background Chemotherapy-induced peripheral neuropathy (CIPN) represents a major unmet medical need that currently has no preventive and/or curative treatment. This is, among others, driven by a poor understanding of the contributive role of drug transport across biological barriers to target-site exposure. Methods Here, we systematically investigated the transport of 11 small-molecule drugs, both, associated and not with CIPN development, at conventional (dorsal root ganglia, sciatic nerve) and non-conventional (brain, spinal cord, skeletal muscle) CIPN sites. We developed a Combinatory Mapping Approach for CIPN, CMA-CIPN, combining in vivo and in vitro elements. Results Using CMA-CIPN, we determined the unbound tissue-to-plasma concentration ratio (K-p,K-uu) and the unbound intracellular-to-extracellular concentration ratio (K-p,K-uu,K-cell), to quantitatively assess the extent of unbound drug transport across endothelial interfaces and parenchymal cellular barriers of investigated CIPN-sites, respectively, in a rat model. The analysis revealed that unique pharmacokinetic characteristics underly time-dependent accumulation of the CIPN-positive drugs paclitaxel and vincristine at conventional (dorsal root ganglia and sciatic nerve) and non-conventional (skeletal muscle) CIPN sites. Investigated CIPN-positive drugs displayed intracellular accumulation contrary to CIPN-negative drugs nilotinib and methotrexate, which lacked this feature in all investigated tissues. Conclusions Hence, high unbound drug intracellular and extracellular exposure at target sites, driven by an interplay of drug transport across the endothelial and parenchymal cellular barriers, is a predisposing factor to CIPN development for CIPN-positive drugs. Critical drug-specific features of unbound drug disposition at various CIPN- sites provide invaluable insights into understanding the pharmacological/toxicological effects at the target-sites which will inform new strategies for monitoring and treatment of CIPN.

Introduction: Chemotherapy-induced peripheral neuropathy (CIPN) is a dose-limiting adverse event observed in patients receiving paclitaxel, associated with initial pathological changes in the peripheral nervous system, i.e., distal nerves and dorsal root ganglia (DRG). The prevalence of CIPN in patients receiving paclitaxel formulated i) in polyethylated castor oil with ethanol (CreEL-PTX), ii) as albumin-bound (nab-PTX), and iii) in XR17 micelles (micellar-PTX), is unexpectedly varying. We hypothesize that the discrepancy in CIPN prevalence could be governed by differences in the extent of paclitaxel distribution across blood-to-tissue barriers at the CIPN-sites, caused by the specific formulation.

Methods: The recently developed Combinatory Mapping Approach for CIPN was used to determine the unbound tissue-to-plasma concentration ratio K_p,uu,tissue, after a 4-h infusion of 4 mg/kg CreEL-PTX, 4 mg/kg nab-PTX or 1 mg/kg micellar-PTX in male and female Sprague Dawley rats. K_p,uu,tissue was determined in conventional (DRG, sciatic nerve) and non-conventional (brain, spinal cord, skeletal muscle) CIPN-sites.

Results: Based on our data, the Cremophor-free paclitaxel formulations were associated with a higher distribution of paclitaxel to CIPN-sites than CreEL-PTX, e.g., K_p,uu,DRG of 0.70 and 0.60 for nab-PTX and micellar-PTX, respectively, in comparison to 0.27 for CreEL-PTX (p < 0.01). In addition, the fraction of unbound paclitaxel in plasma was on average 1.6-fold higher in nab- and micellar PTX arms and equal to 0.061 and 0.065, respectively, compared to 0.039 for the CreEL-PTX treatment arm (p < 0.0001).

Discussion: In the case of similar unbound paclitaxel concentration in the plasma of patients and assumed species-independent extent of paclitaxel transport across the barriers, nab- and micellar-PTX formulations can lead to higher paclitaxel exposure at CIPN-sites in comparison to CreEL-PTX.

Background

Oxycodone, a widely used opioid analgesic, has an unbound brain-to-plasma concentration ratio (K_p,uu) greater than unity, indicating active uptake across brain barriers associated with the putative proton-coupled organic cation (H⁺/OC) antiporter system. With this study, we aimed to elucidate oxycodone's CNS disposition during lipopolysaccharide (LPS)-induced systemic inflammation in Sprague–Dawley rats.

Methods

Using brain microdialysis, we dynamically and simultaneously monitored unbound oxycodone concentrations in blood, striatum, lateral ventricle, and cisterna magna following intravenous administration of oxycodone post-LPS challenge.

Results

Our results indicated a reduced, sex-independent brain net uptake of oxycodone across the blood–brain barrier (BBB) measured in the striatum. Notably, the LPS challenge has significantly altered the systemic pharmacokinetics (PK) of oxycodone, in a sex-specific manner, leading to lower clearance and higher blood concentrations in females compared to LPS-treated males and healthy rats of both sexes. Proteomic analysis using Olink Target 96 Mouse Exploratory assay confirmed the induction of systemic inflammation and neuroinflammation. The inflammation led to an increased paracellular transport, measured using 4 kDa dextran, while preserving net active uptake of oxycodone across both BBB and the blood-cerebrospinal fluid barrier (BCSFB), with K_p,uu values of 2.7 and 2.5, respectively. The extent of uptake was 1.6-fold lower (p < 0.0001) at the BBB and unchanged at the BCSFB after the LPS challenge compared to that in healthy rats. However, the mean exposure of unbound oxycodone in the brain following LPS was similar to that in healthy rats, primarily due to the LPS-induced changes in systemic exposure.

Conclusions

These findings highlight the dissimilar responses at blood–brain interfaces during LPS-induced inflammation. Advancing the knowledge of neuropharmacokinetic mechanisms, specifically those involving the H⁺/OC antiporter system, will enable the development of more effective therapeutic strategies during inflammation conditions.

Background

Triptans are potent 5-HT_1B/1D/1F receptor agonists used in migraine therapy, thought to act through peripheral mechanisms. It remains unclear whether triptans cross the blood-brain barrier (BBB) sufficiently to stimulate central 5-HT_1B/1D/1F receptors. This study investigates the disposition of eletriptan and sumatriptan in central nervous system (CNS) and peripheral nervous system (PNS) regions and predicts regional 5-HT_1B/1D/1F receptor occupancies at clinically relevant concentrations.

Methods

Using the Combinatory Mapping Approach (CMA) for regions of interest (ROI), we assessed the unbound tissue-to-plasma concentration ratio (K_{p, uu, ROI}) in rats at steady state across CNS (hypothalamus, brain stem, cerebellum, frontal cortex, parietal cortex, striatum, hippocampus, whole brain, and spinal cord) and PNS (trigeminal ganglion and sciatic nerve) regions. We used K_{p, uu, ROI} values to estimate unbound target-site concentrations and 5-HT_1B/1D/1F receptor occupancies in humans.

Results

We observed heterogenous triptan transport across CNS and PNS regions with the highest extent of unbound drug transport across the blood-nerve barrier in the trigeminal ganglion (K_{p, uu, TG}: eletriptan: 0.519, and sumatriptan: 0.923). Both drugs displayed restricted entry across the BBB (K_{p, uu, whole brain}: eletriptan: 0.058, and sumatriptan: 0.045) combined with high inter-regional variability. We estimated near-complete receptor occupancy in the trigeminal ganglion, while lower occupancies were observed in the whole brain, irrespective of the drug or receptor subtype. For instance, eletriptan was predicted to achieve 84% 5-HT_1B receptor occupancy in the trigeminal ganglion and 37% in the whole brain at clinically relevant concentrations.

Conclusions

This study suggests that despite low BBB transport, both eletriptan and sumatriptan achieve unbound concentrations sufficient to stimulate 5-HT_1B, 5-HT_1D, and 5-HT_1F receptors not only in the trigeminal ganglion, but also in the CNS. Further research is needed to determine whether central mechanisms contribute to triptan’s antimigraine effect and/or side effects.

The pyrilamine-sensitive proton-coupled organic cation (H⁺/OC) antiporter system facilitates the active net uptake of several marketed organic cationic drugs across the blood-brain barrier (BBB). This rare phenomenon has garnered interest in the H⁺/OC antiporter system as a potential target for CNS drug delivery. However, analysis of pharmacovigilance data has uncovered a significant association between substrates of the H⁺/OC antiporter and neurotoxicity, particularly drug-induced seizures (DIS) and mood- and cognitive-related adverse events (MCAEs). This preclinical study aimed to elucidate the CNS regional disposition of H⁺/OC antiporter substrates at therapeutically relevant plasma concentrations to uncover potential pharmacokinetic mechanisms underlying DIS and MCAEs. Here, we investigated the neuropharmacokinetics of pyrilamine, diphenhydramine, bupropion, tramadol, oxycodone, and memantine. Using the Combinatory Mapping Approach for Regions of Interest (CMA-ROI), we characterized the transport of unbound drugs across the BBB in specific CNS regions, as well as the blood-spinal cord barrier (BSCB) and the blood-cerebrospinal fluid barrier (BCSFB). Our findings demonstrated active net uptake across the BBB and BSCB, with unbound ROI-to-plasma concentration ratio, K_p,uu,ROI, values consistently exceeding unity in all assessed regions. Despite minor regional differences, no significant distinctions were found when comparing the whole brain to investigated regions of interest, indicating region-independent active transport. Furthermore, we observed intracellular accumulation via lysosomal trapping for all studied drugs. These results provide new insights into the CNS regional neuropharmacokinetics of these drugs, suggesting that while the brain uptake is region-independent, the active transport mechanism enables high extracellular and intracellular drug concentrations, potentially contributing to neurotoxicity. This finding emphasizes the necessity of thorough neuropharmacokinetic evaluation and neurotoxicity profiling in the development of drugs that utilize this transport pathway.

Despite extensive research, targeted delivery of substances to the brain still poses a great challenge due to the selectivity of the blood -brain barrier (BBB). Most molecules require either carrier- or receptor -mediated transport systems to reach the central nervous system (CNS). These transport systems form attractive routes for the delivery of therapeutics into the CNS, yet the number of known brain endothelium -enriched receptors allowing the transport of large molecules into the brain is scarce. Therefore, to identify novel BBB targets, we combined transcriptomic analysis of human and murine brain endothelium and performed a complex screening of BBBenriched genes according to established selection criteria. As a result, we propose the high -affinity cationic amino acid transporter 1 (SLC7A1) as a novel candidate for transport of large molecules across the BBB. Using RNA sequencing and in situ hybridization assays, we demonstrated elevated SLC7A1 gene expression in both human and mouse brain endothelium. Moreover, we confirmed SLC7A1 protein expression in brain vasculature of both young and aged mice. To assess the potential of SLC7A1 as a transporter for larger proteins, we performed internalization and transcytosis studies using a radiolabelled or fluorophore-labelled anti-SLC7A1 antibody. Our results showed that SLC7A1 internalised a SLC7A1-specific antibody in human colorectal carcinoma (HCT116) cells. Moreover, transcytosis studies in both immortalised human brain endothelial (hCMEC/D3) cells and primary mouse brain endothelial cells clearly demonstrated that SLC7A1 effectively transported the SLC7A1specific antibody from luminal to abluminal side. Therefore, here in this study, we present for the first time the SLC7A1 as a novel candidate for transport of larger molecules across the BBB.