M01 as a novel drug enhancer for specifically targeting the blood-brain barrier

Structural and biophysical insights into targeting of claudin-4 by a synthetic antibody fragment

Claudins are a 27-member family of ~25 kDa membrane proteins that integrate into tight junctions to form molecular barriers at the paracellular spaces between endothelial and epithelial cells. As the backbone of tight junction structure and function, claudins are attractive targets for modulating tissue permeability to deliver drugs or treat disease. However, structures of claudins are limited due to their small sizes and physicochemical properties-these traits also make therapy development a challenge. Here we report the development of a synthetic antibody fragment (sFab) that binds human claudin-4 and the determination of a high-resolution structure of it bound to claudin-4/enterotoxin complexes using cryogenic electron microscopy. Structural and biophysical results reveal this sFabs mechanism of select binding to human claudin-4 over other homologous claudins and establish the ability of sFabs to bind hard-to-target claudins to probe tight junction structure and function. The findings provide a framework for tight junction modulation by sFabs for tissue-selective therapies.

The Basic Requirement of Tight Junction Proteins in Blood-Brain Barrier Function and Their Role in Pathologies

This review addresses the role of tight junction proteins at the blood-brain barrier (BBB). Their expression is described, and their role in physiological and pathological processes at the BBB is discussed. Based on this, new approaches are depicted for paracellular drug delivery and diagnostics in the treatment of cerebral diseases. Recent data provide convincing evidence that, in addition to its impairment in the course of diseases, the BBB could be involved in the aetiology of CNS disorders. Further progress will be expected based on new insights in tight junction protein structure and in their involvement in signalling pathways.

Blood-Brain Barrier Disruption for the Treatment of Primary Brain Tumors: Advances in the Past Half-Decade

PURPOSE OF REVIEW

To review relevant advances in the past half-decade in the treatment of primary brain tumors via modification of blood-brain barrier (BBB) permeability.

RECENT FINDINGS

BBB disruption is becoming increasingly common in the treatment of primary brain tumors. Use of mannitol in BBB disruption for targeted delivery of chemotherapeutics via superselective intra-arterial cerebral infusion (SIACI) is the most utilized strategy to modify the BBB. Mannitol is used in conjunction with chemotherapeutics, oligonucleotides, and other active agents. Convection-enhanced delivery has become an attractive option for therapeutic delivery while bypassing the BBB. Other technologic innovations include laser interstitial thermal therapy (LITT) and focused ultrasound (FUS) which have emerged as prime modalities to directly target tumors and cause significant local BBB disruption. In the past 5 years, interest has significantly increased in studying modalities to disrupt the BBB in primary brain tumors to enhance treatment responses and improve clinical outcomes.

An inhibitor of claudin-5 interactions, M01, alleviates neuroinflammation and vasogenic edema after blood-spinal cord barrier dysfunction

Molecular docking modeling has confirmed that M01 (C30H28N4O5) acts as a potent inhibitor of claudin-5. Our prior data indicated that claudin-5 is essential to the structural integrity of the blood-spinal cord barrier (BSCB). The aim of this study was to investigate the effect of M01 on the integrity of the BSCB and its effect on neuroinflammation and vasogenic edema after blood-spinal cord barrier dysfunction in in-vitro and in-vivo models. Transwell chambers were used to construct an in-vitro model of the BSCB. Fluorescein isothiocyanate (FITC)-dextran permeability and leakage assays were performed to validate the reliability of the BSCB model. Semiquantitative analysis of inflammatory factor expression and nuclear factor-κB signaling pathway protein levels was performed using western blotting. The transendothelial electrical resistance of each group was measured, and the expression of a tight junction protein ZO-1 was determined via immunofluorescence confocal microscopy. Rat models of spinal cord injury were established by the modified Allen's weight-drop method. Histological analysis was carried out by hematoxylin and eosin staining. Locomotor activity was evaluated with Footprint analysis and the Basso-Beattie-Bresnahan scoring system. The M01 (10 μM) reduced the release of inflammatory factors and degradation of ZO-1 and improved the integrity of the BSCB by reversing vasogenic edema and leakage. M01 may represent a new strategy for the treatment of diseases related to BSCB destruction.

Claudin targeting as an effective tool for directed barrier modulation of the viable epidermis

Tight junction (TJ) formation is vital for epidermal barrier function. We aimed to specifically manipulate TJ barriers in the reconstructed human epidermis (RHE) by claudin-1 and -4 knockdown (KD) and by claudin-binding fusion proteins of glutathione S-transferase and modified C-terminal fragments of Clostridium perfringens enterotoxin (GST-cCPE). Impedance spectroscopy and tracer permeability imaging were employed for functional barrier assessment and investigation of claudin contribution. KD of claudin-1, but not claudin-4, impaired the paracellular barrier in vitro. Similarly, claudin-binding GST-cCPE variants weakened the paracellular but not the stratum corneum barrier. Combining both TJ targeting methods, we found that claudin-1 targeting by GST-cCPE after claudin-4 KD led to a marked decrease in paracellular barrier properties. Conversely, after claudin-1 KD, GST-cCPE did not further impair the barrier. Comparison of GST-cCPE variants with different claudin-1/claudin-4 affinities, NHS-fluorescein tracer detection, and immunostaining of RHE paraffin sections showed that GST-cCPE variants bind to extrajunctional claudin-1 and -4, which are differentially distributed along the stratum basale-stratum granulosum axis. GST-cCPE binding blocks these claudins, thereby specifically opening the paracellular barrier of RHE. The data indicate a critical role for claudin-1 in regulating paracellular permeability for ions and small molecules in the viable epidermis. Claudin targeting is presented as a proof-of-concept for precise barrier modulation.

Modifying the blood-brain barrier by targeting claudin-5: Safety and risks

The blood-brain barrier is a major obstacle to the delivery of drugs to the central nervous system. In the blood-brain barrier, the spaces between adjacent brain microvascular endothelial cells are sealed by multiprotein complexes known as tight junctions. Among the many components of the tight junction, claudin-5 has received the most attention as a target for loosening the tight-junction seal and allowing drugs to be delivered to the brain. In mice, transient knockdown of claudin-5 and the use of claudin-5 binders have been shown to enhance the permeation of small molecules from the blood into the brain without apparent adverse effects. However, sustained knockdown of claudin-5 in mice is lethal within 40 days, and administration of an anti-claudin-5 antibody induced convulsions in a nonhuman primate. Here, we review the safety concerns of claudin-5-targeted technologies with respect to their clinical application.

Modulating the Blood-Brain Barrier: A Comprehensive Review

The highly secure blood-brain barrier (BBB) restricts drug access to the brain, limiting the molecular toolkit for treating central nervous system (CNS) diseases to small, lipophilic drugs. Development of a safe and effective BBB modulator would revolutionise the treatment of CNS diseases and future drug development in the area. Naturally, the field has garnered a great deal of attention, leading to a vast and diverse range of BBB modulators. In this review, we summarise and compare the various classes of BBB modulators developed over the last five decades-their recent advancements, advantages and disadvantages, while providing some insight into their future as BBB modulators.