The impact of ATP-binding cassette transporters in the diseased brain: Context matters

Summary

ATP-binding cassette (ABC) transporters facilitate the movement of diverse molecules across cellular membranes, including those within the CNS. While most extensively studied in microvascular endothelial cells forming the blood-brain barrier (BBB), other CNS cell types also express these transporters. Importantly, disruptions in the CNS microenvironment during disease can alter transporter expression and function. Through this comprehensive review, we explore the modulation of ABC transporters in various brain pathologies and the context-dependent consequences of these changes. For instance, downregulation of ABCB1 may exacerbate amyloid beta plaque deposition in Alzheimer’s disease and facilitate neurotoxic compound entry in Parkinson’s disease. Upregulation may worsen neuroinflammation by aiding chemokine-mediated CD8 T cell influx into multiple sclerosis lesions. Overall, ABC transporters at the BBB hinder drug entry, presenting challenges for effective pharmacotherapy. Understanding the context-dependent changes in ABC transporter expression and function is crucial for elucidating the etiology and developing treatments for brain diseases.

Graphical abstract

ABC transporters are expressed in various barriers within the CNS. Baltira et al. comprehensively outline the current knowledge on ABC transporter functionality in the diseased brain and conclude that changes in expression can impact disease etiology and treatment, depending on the context of disease, cell type, and transporter affected.

Introduction

Brain diseases are an important cause of disability and death worldwide, yet despite extensive research, treatment options remain limited. The blood-brain barrier (BBB) serves as a critical interface in preserving cerebral homeostasis. Several ATP-binding cassette (ABC) transporters are highly expressed at the BBB. ABC transporter functionality is highly relevant for brain diseases, as their dysregulation may not only contribute to disease etiology but may also limit treatment efficacy and/or enhance neurotoxicity. Gaining a comprehensive understanding of the functionality and cell type-specific regulation of ABC transporters under pathological conditions is, therefore, crucial for developing effective treatment strategies. Here, we provide an overview of the current evidence on the differential functionality of ABC transporters in brain diseases.

ABC transporters at barriers in the CNS

ABC transporters at the BBB

The brain is a vital organ that requires a continuous supply of oxygen, peptides, amino acids, and other molecules via the bloodstream for proper functioning. However, blood may also contain toxins or pathogens that are harmful to the brain. To safeguard against these threats, the BBB serves as a protective interface, composed of tightly connected brain microvascular endothelial cells (BMECs) surrounded by pericytes, astrocytic endfeet, microglia, and neurons, forming the neurovascular unit.¹ Crosstalk between these different types of cells is crucial for proper brain functionality and communication with the rest of the body.

BBB endothelial cells are intimately interconnected by junctions, preventing paracellular solute movement and establishing the polarity of the BBB.¹ While water and gaseous molecules can move paracellularly, most nutrients and drugs must traverse the luminal and abluminal endothelial membranes. The ability of molecules to passively diffuse depends on properties like molecular weight, charge, and lipophilicity. Molecules essential for brain functionality and unable to diffuse passively may enter the brain via carrier-mediated transport or receptor-mediated transcytosis. Additionally, the BBB features efflux transporters that can extrude many molecules, including pharmaceutical agents and (xeno)toxins that might otherwise enter via passive diffusion.²

ABC transporters, a superfamily of transmembrane proteins, use ATP hydrolysis to translocate molecules across cellular membranes.² This diverse group comprises various transporters expressed in different cell types, binding distinct substrates and serving diverse functions.³ A defining feature of ABC family members is the presence of at least one cytoplasmic domain known as the ABC or nucleotide-binding domain and one transmembrane domain. The canonical structure of ABC transporters includes four domains—two nucleotide-binding domains and two transmembrane domains. However, some members of the family are half-transporters, forming homodimers or heterodimers, or feature three transmembrane domains.⁴

ABCB1 (MDR1; P-glycoprotein) is the archetypical ABC transporter, discovered more than four decades ago in multidrug-resistant hamster ovary cells.⁵ The resolution of the crystal structure of murine Abcb1 in 2009 unveiled a large internal cavity for drug binding, accessible to the cytoplasm and the inner leaflet of the cell membrane.⁶ Upon ATP binding, the inward-facing cavity closes and opens outward, allowing exit of the drug into the extracellular space. ABCB1 recognizes a diverse range of structurally unrelated pharmaceutical molecules. Initially investigated in tumor cells for multidrug resistance, ABCB1 expression in various normal tissues, particularly those with a barrier function, indicated a protective role against toxic substance accumulation.⁷ Subsequent studies, including the development of Abcb1 knock-out mouse models, highlighted its significant impact on drug disposition, notably in limiting brain penetration.⁸ ABCB1’s predominant localization at the luminal membrane of brain endothelium underscores its role as a key guardian at the BBB.⁷

In the 1990s, ABCC1 was discovered as the first member of the ABCC or multidrug-resistance associated protein (MRP) family.⁹ ABCC1 transports some unconjugated hydrophobic anionic drugs like etoposide or vincristine, but is mainly involved in the transport of conjugates (e.g., glutathione-conjugates, glucuronides).¹⁰ ABCC1 is also present in endothelial cells of the BBB, but the localization of ABCC1 is most likely basolateral.^11,12 Wijnholds et al.¹³ showed that the brain penetration of etoposide is similar in Abcb1 and Abcb1;Abcc1 knock-out mice, suggesting ABCC1 does not protect the brain. Additionally, pharmacological inhibition of ABCC1 markedly decreased brain uptake of a typical ABCC1 substrate (17β-estradiol-17β-D-glucuronide), suggesting that ABCC1 facilitates substrate uptake.¹¹

Among 11 ABCC family members, 9 are classified as MRPs.¹⁴ Only ABCC4 demonstrated functional protection of the brain from several camptothecin analogs.¹⁵ Similarly, ABCC2 may protect the brain, but this is less well-established. Luminal expression of ABCC2 was detected in C57Bl/6 mice, but not in FVB mice.¹² ABCC2 reduces phenytoin brain penetration in TR-rats (spontaneous Abcc2-null mutant) compared with wild type (WT).¹⁶ ABCC2 is hard to detect in the BBB of rats, but seems to be upregulated following pilocarpine-induced convulsive status epilepticus (SE).¹⁷ Although other ABCC transporter subtypes may be present at the BBB, none of them have been shown to change drug brain distribution.

In the late 1990s, Doyle et al.¹⁸ identified ABCG2, also known as breast cancer resistance protein (BCRP or ABCG2). ABCG2 comprises only one nuclear binding domain and one transmembrane domain, requiring at least dimerization for functionality.⁴ ABCG2 shares substrate specificity and tissue distribution with ABCB1.¹⁹ The important role of ABCG2 in the BBB is often concealed by the action of ABCB1 for solutes that are dual ABCB1/ABCG2 substrates. However, when ABCB1 is absent, further deletion of ABCG2 can cause a considerably increased brain accumulation of many agents. Thus, ABCB1 and ABCG2 are the most dominant drug efflux transporters at the BBB.

ABC transporter expression research has recently shifted toward using targeted proteomics via liquid chromatography-tandem mass spectrometry (LC-MS/MS). Unlike traditional qualitative techniques, LC-MS/MS has provided quantitative data on ABC transporter expression in healthy BMECs, facilitating inter-species and intra-species expression level comparisons (Table 1).^{20,21,22,23,24,25} Studies consistently indicate that ABCB1 is the predominant ABC transporter in mice and rats, while ABCG2 and ABCC4 are typically expressed at lower levels. Interestingly, humans and monkeys exhibit lower ABCB1 expression but higher ABCG2 levels compared with mice. ABCC4 expression is lower in primates compared with rodents, often falling below detectable levels. Other transporters like ABCC2 and ABCC5 are generally undetectable across species. Notably, proteomics findings do not always align with transcriptomic data, emphasizing the importance of assessing transporter expression at the protein level.²⁶ ABCB1 and ABCG2’s dominance in the BBB is validated by proteomic data. Higher ABCG2 expression and lower expression of ABCB1 in humans indicate a potentially greater role of ABCG2 in the human BBB than inferred from studies in mice. Therefore, this review mostly addresses ABCB1, ABCG2, and ABCC1 in the diseased brain.

Table 1.

Reported proteomic quantifications of ABC transporter expression at the BBB in isolated brain microvessels from various species and strains

Mouse									Rat
Strain	ddY		FVB		C57Bl/6		Balb/c		Wistar		S/D		Fischer 344
	Mean	SE	Mean	SE	Mean	SE	Median	Range	Mean	SE	Mean	SE	Median	Range
ABCB1	16.4	1.30	14.2	1.60	17.8	1.20	8.57	BLQ-12.8	19.2	1.10	19.0	2.00	22.0	19.6–25.9
ABCC4	1.33	0.14	1.27	0.21	1.51	0.27	BLQ	–	1.46	8.00 × 10−2	1.60	0.29	BLQ	–
ABCG2	3.74	0.32	3.21	0.49	5.48	0.37	3.84	0.29–9.16	5.74	0.500	4.15	0.29	4.59	2.50–4.91
Source	Uchida et al.22						Bao et al.20		Hoshi et al.25				Bao et al.20

Human							Monkey
	Mean	SE	Median	Range	Mean	SE	Mean	SE
ABCB1	6.06	1.69	3.38	1.00–7.42	2.58	0.930	5.46	0.520
ABCC4	0.195	6.90 × 10−2	BLQ	–	ND	ND	0.303	4.00 × 10−3
ABCG2	8.14	2.26	6.21	2.23–16.5	2.22	0.610	14.3	0.470
Source	Uchida et al.21		Bao et al.20		Al-Majdoub et al.23		Ito et al.24

Concentrations are shown in fmol/μg protein.

BLQ, below limit of quantification; n/d, not detected.

ABC transporters at the blood-cerebrospinal fluid barrier and blood-spinal cord barrier

The blood-cerebrospinal fluid (CSF) barrier comprises the choroid plexus (CP) and arachnoid barrier cells. CSF is produced and expelled by the CP, flowing through the ventricles and subarachnoid space before reabsorption into the bloodstream via arachnoid villi (Figure 1).²⁷ CSF cushions the brain, providing mechanical support and protection from sudden impact. It supports brain homeostasis by delivering nutrients, regulating ions, and removing waste. However, the CSF’s surface area in contact with the brain is only a fraction of the BBB’s, and it replenishes every 4–5 h, making drug delivery via CSF relatively inefficient.

Figure 1.

CNS barriers

The blood-CSF barrier (BCSFB) comprises the CP and arachnoid barrier cells. CSF is produced and expelled by the CP, flowing through the ventricles and subarachnoid space before reabsorption into the bloodstream via arachnoid villi. At the CP and the meninges endothelial cells are fenestrated. In these tissues, the barrier is formed by epithelial cells and arachnoid cells. At the BBB and the BSCB, endothelial cells form the barrier being connected by tight junctions. They are supported by astrocytes and pericytes, which generally are less abundant in the BSCB compared with the BBB, leading to lower tight junction expression and a potentially more permissive barrier. Created with BioRender.com together with Servier Medical Art under a Creative Commons Attribution 3.0 Unported License.

ABC transporters have been detected in the CP epithelial cells and in the arachnoid barrier cells that separate the dura with fenestrated capillaries and the arachnoid from the subarachnoid space.^28,29,30 In rats, the location of ABCB1 in CP epithelial cells is in the apical membrane, but the expression is much less compared with the BBB as assessed by quantitative western blot (WB).²⁸ Conversely, ABCC1 expression is much lower in BBB microvessels. ABCC1 is localized in the basolateral membranes of the CP epithelial cells and reduces the entry of substrates from the blood into the CSF.¹³ Similarly, ABCC4 is expressed basolaterally in CP epithelial cells, but its functionality has not yet been demonstrated.

The blood-spinal cord barrier (BSCB) is less well studied, but the anatomy seems to be very similar to the BBB. The principal barrier is formed by tightly connected endothelial cells that are supported by astrocytes and pericytes. However, the BSCB seems to be more permeable, likely due to lower tight junction protein expression.³¹

ABC transporters in CNS disorders

As outlined above, ABC transporters are crucial gatekeepers of the CNS. Changes in their expression and function may impact the brain penetration of potentially harmful agents and pharmaceuticals, especially in the context of brain disorders where pathological factors, such as inflammatory signals, excessive glutamate, and oxidative stress, may affect transporter functionality.^32,33 Below, we will discuss the existing data on alterations in ABC transporter functionality in various brain diseases (Figure 2). Understanding the reliability of this evidence requires insight into the experimental methods and disease models used in each study. We, therefore, compiled a comprehensive list of studies (Tables S1–S12) providing these details. Figure 3 visually represents the distribution of techniques utilized across different diseases and the extent of investigation into specific ABC transporters. Overall, while techniques vary widely, most studies focus on ABCB1/ABCB1, followed by ABCG2/ABCG2, ABCC1/ABCC1, and, sporadically, ABCC2-5/ABCC2-5.

Figure 2.

Graphical overview of cell type-specific changes in ABC transporter functionality in various brain diseases

Changes for which robust experimental evidence exists are highlighted in green (up) or pink (down). Changes for which the literature has yet to produce conclusive results are depicted as gray. Created with BioRender.com.

Figure 3.

Overview of the frequency of techniques employed and the extent of investigation into the various ABC transporters per studied disease

(A and B) Diseases are ranked from least (CJD) to most-studied (brain tumors). Data are based on the list of studies provided in the supplemental tables. (A) Of all ABC transporters, the vast majority of studies investigate ABCB1, followed by ABCG2 and ABCC1 and, sporadically, ABCC2-5. (B) In general, while the applied techniques vary per disease, antibody-based methods predominate, except for PD and psychiatric disorders.

Alzheimer’s disease

Alzheimer’s disease (AD) is a prevalent neurodegenerative disorder marked by progressive cognitive decline, making it the leading cause of dementia.³⁴ Its multifaceted pathogenesis, including Aβ accumulation, tau aggregation, and microglial dysfunction, remains incompletely understood despite extensive research. Several etiologies have been proposed, such as amyloid-β (Aβ) accumulation, tau aggregation, and microglial dysregulation. Notably, Aβ pathology has garnered significant attention and ABC transporters seem to be implicated in Aβ clearance in AD patients, as well as AD animal models (Table S1).

ABCB1 is the primary ABC efflux transporter studied in Aβ clearance as first observed in non-endothelial cell lines and inside out vesicles.³⁵ Studies in postmortem brain samples of non-demented elderly found an inverse relationship between Aβ deposition and ABCB1 expression.³⁶ Animal studies using AD mouse models revealed that ABCB1 depletion or inhibition hampers Aβ clearance, elevating Aβ brain levels.³⁷ This work was supported by later studies.^37,38,39 Yet, Ito et al.⁴⁰ showed that verapamil failed to diminish Aβ brain clearance in healthy rats, implying negligible ABCB1-mediated efflux. However, verapamil may not completely abolish ABCB1 function, as seen in genetic knockouts.

Interestingly, various AD animal models have shown a decline in ABCB1 levels and transport activity via WB, fluorescence cell imaging, and radiolabeled drugs.^39,41 Thus, Aβ accumulation seems to cause downregulation of ABCB1 expression, albeit not to a level that it increases the brain accumulation of a substrate like digoxin.^42,43 Consistent findings were observed in AD patient tissue using immunohistochemistry (IHC).⁴⁴ Furthermore, higher [¹¹C]verapamil uptake by positron emission tomography (PET) indicated reduced ABCB1 functionality at the BBB in AD patients and animal models.^45,46,47 So, intriguingly, the transport protein involved in the clearance of Aβ is downregulated by the accumulation of Aβ, potentially implying a self-reinforcing effect on Aβ accumulation. The molecular mechanisms responsible for ABCB1 downregulation by Aβ remain unclear. Hartz et al.⁴⁸ proposed that Aβ might decrease ABCB1 expression via ubiquitination, as observed in vitro and in vivo using human tissue and AD animal models. Nonetheless, factors besides ABCB1 likely contribute to the clearance of Aβ.

As outlined earlier, ABCG2 and ABCB1 have overlapping substrate specificities and cooperate in expelling drugs from the brain. In vitro experiments using human brain endothelial cells suggest that ABCG2 restricts the apical to basolateral flux of Aβ across endothelial cell layers, but has no effect on basolateral to apical efflux.⁴⁹ Notably, the absence of basolateral-to-apical efflux of Aβ may be due to the absence of a suitable basolateral influx transporter (e.g., LRP1) in these model systems. Zhang et al.³⁸ reported ABCG2-mediated Aβ transport in human cell lines overexpressing ABCG2. Additionally, they found upregulation of ABCG2 in the vasculature of 3xTg AD mice and AD patients with cerebral amyloid angiopathy (CAA). Upregulation was also observed in AD patients without CAA, although not statistically significant. Aβ brain accumulation after intravenous administration significantly increased in Abcg2^−/− mice relative to WT mice.³⁸ However, these effects of ABCG2 in restricting Aβ influx over the BBB may not be relevant for AD. When AD-relevant mouse models were crossed with ABC transporter knock-out mice, greater Aβ accumulation was found in Abcb1^−/− and Abcc1^−/−, but not in Abcg2^−/−, mice.⁵⁰ In line with this, Abcg2^−/−;Tg-SwDI mice do not harbor significantly different Aβ40 levels compared with Tg-SwDI mice.⁵¹ Similarly, no evidence was found for ABCG2-mediated Aβ efflux using mouse brain capillaries.³⁹ These studies do not preclude a role for ABCG2 in Aβ efflux, since ABCB1 is the more dominant BBB efflux transporter in mice. To address this issue, the comparison between Abcb1^−/− and Abcb1;Abcg2^−/− mice would be required. If ABCG2 does efflux Aβ, its upregulation may be a compensatory and protective mechanism against amyloid plaques and inflammation,⁵² although evidence in AD patients compared with controls in expression, transcription, and immunohistochemical signal is lacking.^38,53

ABCC1, alongside ABCB1 and potentially ABCG2, plays a role in Aβ clearance and AD pathology. Most important, Krohn et al.⁵⁰ showed in amyloid precursor protein (APP)/presenilin 1 (PS1) transgenic mice, carrying APP and PS1 mutations, that ABCC1 deficiency exacerbates Aβ plaque deposition, attributed to ABCC1 deficiency rather than altered Aβ production. ABCC1 is unlikely to mediate Aβ clearance via BMECs due to its likely basolateral location in BMECs, directing substrates into the brain.^11,12 Even if ABCC1 were at the luminal membrane, its absence may not significantly impact Aβ clearance across the BBB, since ABCB1 is still present at the luminal membrane of BMECs and would likely fulfill this role.

Besides clearance via the BBB into the blood, Aβ may also be cleared into the CSF via bulk flow of interstitial fluid (ISF).⁵⁴ Although it was generally assumed that ISF primarily originated from water diffusion across the BMECs,⁵⁵ more recently the concept of the glymphatics has been proposed.⁵⁶ The source of glymphatics would be CSF flowing inward along perivascular brain-penetrating arteries. The systolic pressure would drive the CSF into the peri-arterial vascular space and further into the brain parenchyma. This latter process would be facilitated by the presence of water channels (aquaporin-4) in the astrocytic endfeet surrounding these blood vessels. The resulting net flow would move toward the perivascular space of the draining venules and further into the meningeal lymphatic vessels, allowing the clearance of macromolecules produced in the brain parenchyma, such as Aβ. However, there is an ongoing debate about the glymphatics concept,⁵⁷ but this falls outside the scope of this review. Importantly, studies linking ABC transporters to the bulk flow of ISF have not been described. However, considering the profound effect of ABCC1 loss on Aβ clearance, such investigations are warranted.

Αβ has been found to cause ABCC1 upregulation in primary mouse astrocytes, which could propose a putative mechanism against Aβ-mediated oxidative stress.⁵⁸ Another member of the family, the ABCC4 transporter, has also been described to have elevated transcriptional and expression levels in AD patients by applying WB and quantitative qRT-PCR.⁵³

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease affecting both the upper and lower motor neurons, leading to diverse symptoms, including dysarthria, difficulties in speech, and respiratory incapacity, the primary cause of death.⁵⁹ Approximately 10% of all ALS diagnoses are familial, with the remainder sporadic.⁶⁰ Mutations in genes such as SOD1, TDP43, and FUS explain only part of the familial cases, leaving much of the pathogenesis unexplained, similar to many other neurodegenerative diseases.

The ABC transporters' functionality in ALS has been investigated for approximately a decade (Table S2), focusing on transgenic models carrying ALS-linked mutations of the Sod1 gene. Studies in Sod1-mutated mice and rats have consistently shown enhanced ABCB1 expression and functionality.^61,62,63,64 Qosa et al.⁶⁵ demonstrated elevated ABCB1 immunoreactivity in spinal cord capillaries isolated from Sod1^G86R-mutated mice, as well as elevated levels in astrocytes derived from ALS patients with SOD1 or FUS mutations. Intriguingly, similar results were found when brain and spinal cord capillaries were incubated with conditioned medium from these astrocytes. The authors proposed a mechanism of ABCB1 upregulation mediated by astrocytes through nuclear factor κB (NF-κB) signaling.⁶⁵ NF-κB is a key mediator of inflammatory diseases and potentially a regulator of ABCB1 in other neurological diseases.⁶⁶ Similarly, Mohamed et al.⁶⁷ observed that astrocytes from familial SOD1^A4V and sporadic ALS patients can enhance the expression and functionality of ABCB1 in endothelial cells in vitro. These data imply that elevated ABCB1 transporter activity in the BBB mediates pharmacoresistance observed in certain ALS patients.⁶⁸ Indeed, Jablonski et al.⁶⁹ highlighted that ABCB1/ABCG2 inhibition with elacridar can significantly improve the treatment response in Sod1^G86R mice.

However, due to the lack of models representing sporadic ALS, which constitutes 90%–95% of the cases, a major challenge in modeling the disease remains.⁶⁰ Animal models predominantly feature mutations affecting SOD1. However, SOD1-driven ALS only represents 10%–20% of familial cases and 1%–2% of sporadic cases, raising questions about the clinical relevance of these models.⁷⁰ Jablonski et al.⁶³ first studied ABCB1 and ABCG2 in postmortem tissue from two sporadic and one familial ALS cases, observing elevated levels in isolated spinal cord tissue homogenates. Further studies in isolated cerebral tissue capillaries of Sod1^G93A rats revealed elevated transport activity and immunoreactivity of ABCB1 and ABCG2 with disease progression. A recent study expanded on this, examining the immunoreactivity of ABCB1 and ABCG2 in a larger patient cohort.⁷¹ IHC in 17 sporadic and 8 familial ALS cases highlighted enhanced ABCB1 immunoreactivity in reactive astroglial cells in the motor cortex of all ALS patients, with modest ABCG2 immunoreactivity in glial cells and blood vessels of the motor cortex.

In summary, several studies suggest an overexpression of primarily ABCB1. However, further research is necessary to confirm these findings and explore the sporadic form of the disease, which, despite its prevalence, remains relatively understudied.

Parkinson’s disease

Parkinson’s disease (PD) is a prevalent neurodegenerative disorder characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta. Patients experience various motor symptoms like rigidity and bradykinesia, along with non-motor issues such as anosmia and complex gastrointestinal disturbances.⁷² Besides neuronal loss, the formation of alpha-synuclein protein aggregates called Lewy bodies is a hallmark pathology.⁷³ While the exact cause remains elusive, one hypothesis implicates impaired BBB integrity, potentially allowing neurotoxic compound accumulation.⁷⁴ This concept stems from observations linking certain pesticides to increased PD risk via neurotoxicity. In fact, some of these pesticides are substrates of ABCB1.⁷⁵ A less functional BBB could also be a result of decreased efflux transport (see also Table S3).

Indeed, a general decrease in ABCB1 efflux transport activity has been described in PD. Reduced vascular ABCB1 mRNA levels were observed in PD brain tissue via in situ hybridization.⁷⁶ This aligns with previous findings showing diminished ABCB1 functionality in advanced-stage PD patients using [¹¹C]verapamil PET imaging,^77,78 although this was not evident in early-stage patients.⁷⁹ Loss of ABCB1 in PD patients might exacerbate the disease, akin to AD. Disturbances in ABCB1 functionality could facilitate the entrance of neurotoxic compounds, hastening dopaminergic neuron death.⁸⁰ In contrast, while 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration in mice mimics the symptoms of PD by causing dopaminergic degeneration, it causes moderate ABCB1 upregulation, implying that this model may not fully replicate ABC transporter changes in PD.⁸¹ There is no direct link between ABCG2 and PD; however, the antioxidative properties of urate, a substrate of ABCG2, provide an indirect association. Individuals with a specific ABCG2 single nucleotide polymorphism leading to hyperuricemia tend to develop PD later in life.⁸² More extensive studies of ABCB1 and other ABC transporters are crucial for understanding their involvement in this disease.

Multiple sclerosis

Multiple sclerosis (MS) is a chronic immune-mediated CNS disease, characterized by abnormal infiltration of peripheral immune cells into the CNS, damaging the myelin coating surrounding neuronal axons. This leads to brain inflammation and diverse neurological symptoms.⁸³ Radiological and pathological data show BBB disturbances, enabling peripheral immune cells migration into the brain. However, the mechanism underlying BBB integrity loss is not yet elucidated.⁸⁴ As ABC transporters are a major part of the BBB, their dysregulation may play a critical role in MS pathology (Table S4).

Initial studies on ABC transporters in MS patients revealed decreased ABCB1 immunoreactivity in endothelial cells within MS lesions, particularly in vessels surrounded by perivascular immune cell infiltrates.⁶⁶ This coincided with ABCB1 overexpression in hypertrophic astrocytes. Likewise, decreased ABCB1 expression was observed in endothelial vessels in an acute experimental autoimmune encephalomyelitis (EAE) rat model, concomitant with increased CD4⁺ T cell infiltration, a common feature in MS lesions.⁶⁶ The researchers proposed that CD4⁺ T cells play a key role in ABCB1 downregulation, as unstimulated CD4⁺ T cells from MS patients significantly impaired ABCB1 functionality in human endothelial cells in vitro, unlike those from healthy controls. Activated CD4⁺ T cells from both MS patients and healthy controls also attenuated ABCB1 function. These findings suggest that MS-educated CD4⁺ T cells cause ABCB1 downregulation in the BBB. Subsequent studies confirmed ABCB1 loss in the BBB and ABCB1 gain in astrocytes in lesions of MS patients and EAE mice.⁸⁵ This focal reduction of ABCB1 at the BBB in affected regions was leveraged to enable targeted delivery of therapeutic agents while averting neurotoxicity in unaffected brain regions. Expanding on their work on ABCB1, Kooij et al.⁸⁶ also observed enhanced ABCC1 and ABCC2 expression in reactive astrocytes. Additionally, ABCC1, ABCC2, and ABCG2 were over-expressed in foamy macrophages found in demyelinated lesions.⁸⁷ ABCB1 and ABCC1 were shown to facilitate the excretion of chemokine (C-C motif) ligand 2 (CCL2), which promotes the migration of monocytes over transwells.⁸⁶ Further research showed that ABCB1-mediated CCL2 secretion by brain endothelial cells facilitated the CD8⁺ T cells influx during neuroinflammation.⁸⁸ Inducing EAE in Abcb1 knock-out mice revealed decreased lesional CD3 T cell infiltration, reduced cytokine production, and milder symptoms.⁸⁹ Immune cells from Abcb1 knock-out mice were less functional in vitro, supporting ABCB1’s proposed role in proinflammatory cytokine secretion and suggesting ABCB1 as an immunomodulatory factor in MS-mediated inflammation. The earlier finding that CD4⁺ T cells mediate the downregulation of ABCB1⁶⁶ may seem paradoxical as it would reduce chemotactic signaling by the microenvironment when the disease progresses. However, it may contribute to the self-limiting nature of the lesions, along with the immune-suppressive phenotype of foamy macrophages.⁸⁷ Unfortunately, irreversible damage has already occurred in these brain areas. More recent studies have shown that brain-homing Th17.1 cells, a specific subset of CD4 T cells, play a dominant role in MS. Importantly, these cells express high levels of ABCB1, which contributes to poor glucocorticoid responsiveness.⁹⁰

These findings sketch an intriguing role for ABC transporters, in particular for ABCB1 as a pro-inflammatory mediator. Consequently, the inhibition of ABCB1 may not only help to improve the delivery of anti-inflammatory agents, but it may also directly interfere with the chemotactic attraction of T cells.

Epilepsy

Epilepsy is a prevalent neurological disorder, manifesting through spontaneous recurrent seizures from abnormal neuronal activity.⁹¹ This activity can be focal to one or several specific brain regions or generalized.⁹² Despite various antiseizure medications being available, approximately one-third of patients remain resistant.⁹³ ABC transporters seem to be implicated in this resistance (Table S5).

Epilepsy was the first neurological disease associated with ABC efflux transporter upregulation. In 1995, Tishler et al.⁹⁴ demonstrated increased ABCB1 levels in resected brain tissue from refractory epilepsy patients. Subsequent studies employing cDNA arrays, IHC, and WB on tissue from patients with refractory epilepsy showed ABCB1 overexpression predominantly in the endothelial cells.⁹⁵ Furthermore, ABCB1 was detected in reactive astrocytes and neurons within epileptic lesions of patients.^96,97 Similar findings were observed in electrically or pharmacologically induced rat models of epilepsy, highlighting enhanced ABCB1 expression in glial, neuronal, and endothelial tissues.^98,99,100 Studies also revealed that the pharmacological inhibition of ABCB1 by tariquidar reversed the resistance of spontaneous epileptic seizures to phenobarbital in a rat model of temporal lobe epilepsy (TLE).¹⁰⁰ Additional PET studies visualized drug efflux by ABCB1 in epileptic patients using the substrate [¹¹C]verapamil. These studies suggested that enhanced ABCB1 functionality in specific brain regions of patients with TLE was positively correlated with seizure frequency.^101,102

Like ABCB1, members of the ABCC family also appear upregulated in epilepsy. Dombrowski et al.⁹⁵ first showed increased mRNA levels of ABCC2 and ABCC5 in epileptic tissue from patients using cDNA arrays. Further studies demonstrated ABCC1 protein in astrocytic endfoot processes in glial cells and neurons, with ABCC2 also found in the endothelium and reactive astrocytes of epilepsy patients.^97,103,104 Similarly, ABCC1 and ABCC2 were found in endothelial cells, reactive astrocytes, and neuronal cells of rats subjected to seizures.^17,99,105 Upregulation of ABCC5 and ABCC6 mRNA was shown in isolated human brain-derived microvascular endothelial cells by Kubota et al.¹⁰⁶

There are limited reports on ABCG2 overexpression in epilepsy. In rats with electrically induced SE, overexpression was observed in blood vessels and astrocytes using IHC and WB.¹⁰⁵ In patients, Aronica et al.¹⁰⁷ found strong immunoreactivity and high protein levels in the microvasculature of epileptogenic brain tumors, but not in tissue samples from two other epileptogenic pathologies like hippocampal sclerosis and focal cortical dysplasia (FCD).¹⁰⁷ It is possible that ABCG2's presence was due to the tumor since ABCG2 expression in brain tumor vasculature is common (as discussed later). A recent study using qRT-PCR found increased ABCG2 expression in tissue from patients with mesial TLE, with only marginal elevation in FCD.¹⁰⁸ ABCG2 may be important since it is constitutively expressed at the BBB, but epilepsy-induced upregulation is less likely.

Several important anti-seizure drugs are substrates of ABCB1, ABCC1, ABCC2, and ABCG2.^109,110 Their presence at the BBB and significant upregulation in epileptic tissues possibly contribute to drug resistance, a response triggered by the disease. In rats, overexpression of ABCB1 seems to be linked to seizures, likely related to cellular stress and/or neuronal damage.⁹⁹ ABCB1 overexpression may be initiated through extensive glutamate signaling during seizures,¹¹¹ leading to the activation of cyclo-oxygenase-2.^99,112 However, the mechanism behind the upregulation of other ABC transporters remains elusive.

Tuberous sclerosis

Tuberous sclerosis complex (TSC) is an autosomal dominant multisystem genetic disorder caused by mutations in the TSC1 or TSC2 genes, encoding hamartin and tuberin proteins, respectively, that regulate mammalian target of rapamycin (mTOR).¹¹³ The disease manifests with benign tumors in multiple organs, including the brain, leading to diverse symptoms, such as epilepsy and neuropsychiatric disorders. TSC-specific brain lesions consist of abnormal cells with mixed glioneural phenotypes named cortical tubers, subependymal nodules, and subependymal giant cell astrocytomas.¹¹³ Epileptogenic tubers are frequent and the seizures are often drug resistant.¹¹³ The mechanism of resistance is not fully understood, but might also be associated with ABC transporter expression. Limited reports using IHC on patient tissue have explored ABC transporter family alterations in TSC (Table S6).

The first indication of ABC transporter alterations in TSC was described in a case report, demonstrating elevated ABCB1 immunoreactivity in epileptogenic cortical tubers from a TSC patient with refractory epilepsy.¹¹⁴ This finding was later confirmed in three more TSC patients with refractory epilepsy with additional staining for ABCC1 in glial and neuronal cells.¹¹⁵ Strong immunoreactivity of ABCB1 and ABCC1 was observed in various cell types, including abnormal balloon cells, dysplastic neurons, astrocytes, microglial cells, and some brain vessels. Subsequent studies revealed the same results for ABCB1 in a TSC patient with a germline mutation in TSC2.¹¹⁶ A few years later, Lazarowski et al.¹¹⁷ investigated ABCG2 in the same three patients and found ABCG2 upregulation in brain vessel endothelial cells. However, unlike ABCB1 and ABCC1, no significant ABCG2 upregulation was found in other cells of the cortical tubers.

The few existing reports indicate upregulation of drug resistance-associated ABC transporters in TSC, potentially contributing to poor antiseizure medication response, akin to epilepsy of other etiologies. However, interventions to reduce the consequences of the underlying genetic defect include mTOR inhibitors, such as everolimus, which are also substrates of ABCB1.¹¹⁸ Hence, ABCB1-mediated drug resistance is likely. Further research is necessary to draw robust conclusions on ABC transporter functionality in TSC, as now only IHC has been applied and the number of subjects used in the studies is very small.

Brain tumors

Brain cancer encompasses a diverse range of intracranial malignancies. CNS tumors are classified according to their location, cell of origin, proliferation rate, and molecular background.¹¹⁹ Primary tumors originate from brain cells and include diffuse gliomas, meningiomas, and medulloblastomas. Secondary tumors are brain metastases derived from tumors originating outside the brain, with lung, breast cancer, and melanoma being the most common sources. Symptomatic brain metastases are increasing, partially due to more efficacious treatment of peripheral lesions, underscoring the BBB’s role as a barrier to effective pharmacotherapy. The BBB in brain tumors, herein referred to as blood-brain tumor barrier (BBTB) is generally leakier than the BBB, but systemically administered drugs still have impeded access to the tumors.¹²⁰ In high-grade gliomas, BBTB integrity varies within lesions with a leaky core, but much less leaky adjacent tumor-invasive zones. Similarly, brain metastases may enhance vascular permeability, but smaller lesions may remain shielded.

ABC transporters at the BBB/BBTB and within brain tumor cells may impact therapy. The first studies detected ABCB1 expression in human glioma samples via measuring mRNA levels and immunocytochemistry.^121,122 Later studies confirmed ABCB1 expression in tumor vessels and cells, with increased mRNA or protein levels across different tumor grades (Table S7).^{123,124,125,126,127} Weaker staining was observed in proliferating microvessels.¹²⁶

Initial evidence of ABCG2 expression in glioblastoma (GBM) tumor vessels was obtained through laser capture microscopy and qPCR, revealing upregulated mRNA levels in tumor vessels and perivascular parenchyma.¹²⁸ Strong ABCG2 immunoreactivity was found in tumor vessels of GBM¹⁰⁷ and pediatric glioma.¹²⁹ ABC transporters, particularly ABCG2, are also expressed in cancer stem-like cells, a therapy-resistant subpopulation within GBM.¹¹⁹ This resistance is multifactorial, but includes drug efflux by ABC transporters.¹³⁰ ABCG2 presence is found in subsets of human and mouse brain tumor cells.^131,132,133 Together, ABCB1 and ABCG2 restrict the BBB penetration of many drugs and can even restrict drug efficacy when the BBTB is leaky.¹³⁴

ABCC transporters have also been found in human gliomas.^135,136 ABCC1 was detected in tumor cells and vessels linking it to drug resistance.^{123,136,137,138} However, its substrate range is narrower compared to ABCB1 and ABCG2, and its localization is likely basolateral rather than apical.^10,14 Consequently, while ABCC1 may contribute to GBM resistance, it likely does not reduce the BBTB drug penetration. Other ABCCs have been reported in brain tumors, but their functionality remains unclear (see Table S7).

As outlined earlier, conventional protein quantification methods like ELISA, IHC, and WB have limitations. Bao et al.²⁰ implemented LC-MS/MS proteomics to quantify protein expression in microvessel homogenates of GBM and normal brain tissue. They found lower expression of ABCB1 and ABCG2 in GBM. Notably, the recovery of microvessels was higher, likely due to microvascular proliferation in GBM. It likely also implies that newly formed microvessels from the tumor core are over-represented in the homogenates relative to pre-existing brain microvessels from invasive regions of GBM.

This may also apply to the study of Dusart et al.,¹³⁹ who used sophisticated algorithms to deconvolute bulk RNA sequencing (RNA-seq) data from cortex and GBM samples into constituent cell type-enriched profiles. They observed ABCB1 enrichment in normal brain microvessels, but not in the GBM vasculature. ABCG2 was not examined. Notably, in adult low-grade gliomas (LGGs), lacking microvascular proliferation, ABCB1 expression mirrored that of the control vasculature. Comparable results were reported by Schaffenrath et al.,¹⁴⁰ using RNA-seq on isolated microvessels. Overall, ABCB1 and ABCG2 seem to be present in the brain tumor vasculature. Their expression in LGGs resembles that of normal brain tissues, but likely diminishes in the proliferative endothelium of GBM core regions. PET imaging with the substrate [¹¹C]tariquidar in LLG patients demonstrated transporter functionality.¹⁴¹ Consequently, invasive GBM regions are probably well protected by ABC transporters.

In other CNS tumors like medulloblastomas and ependymomas, ABC transporters have been studied less extensively. Hashimoto et al.¹⁴² analyzed 36 medulloblastoma samples using a range of techniques, finding overexpression of ABCC1 but not of other ABC transporters. This result aligns with ABCC1 upregulation found in medulloblastomas by IHC and qRT-PCR.¹⁴³ ABCB1 expression in medulloblastomas is conjecture, with Sawada et al. failing to detect it by IHC in 4 out of 4 medulloblastomas,¹²⁶ while others found low RNA expression levels¹⁴³ of ABCB1 by IHC in 99 out of 233 samples.¹⁴⁴ ABCG2 expression was demonstrated in the presumed cancer stem cell fraction of human medulloblastoma cells.^145,146 Data regarding ABCB1 and/or ABCG2 expression in the medulloblastoma vasculature are lacking. However, ABCG2 was detected in the endothelium of ependymomas.¹²⁷

Limited data exist on ABC transporter expression in metastatic brain tumors. Demeule et al.¹⁴⁷ observed reduced ABCB1 levels in human melanoma metastasis using WB. By applying IHC, Yonemori et al.¹⁴⁸ detected ABCG2 in blood vessels of HER2-positive breast cancer brain metastasis. Quantitative protein expression showed decreased ABCB1 and ABCG2 in the vasculature of lung and breast cancer brain metastases.¹⁴⁹ However, RNA-seq data from microvessels isolated from lung adenocarcinoma brain metastases suggested similar expression levels of ABCB1 and ABCG2 as in normal brain vasculature.¹⁴⁰ Since brain metastases originate from various primary tumor types, BBB leakiness, and ABC transporter expression may vary.¹⁵⁰

Stroke

Stroke, an acute medical event, occurs when the blood supply to a brain region is obstructed (ischemic) or when a blood vessel ruptures (hemorrhagic), leading to severe neurological damage and death.¹⁵¹ Notably, stroke ranks second globally in causes of death, necessitating better neuroprotective strategies. Ischemic strokes predominate and appear to be associated with ABC transporter overexpression¹⁵² (Table S8).

Endothelial cells subjected to ischemic conditions often serve as in vitro models of ischemic stroke. Increased ABCB1 protein levels have been observed in rat endothelial cells submitted to oxidative stress from reactive oxygen species exposure, hypoxia/reoxygenation,^153,154 or oxygen-glucose deprivation (OGD) in rat and mouse endothelial cells.^155,156 However, inconsistent results were observed with in vivo mouse and rat models of ischemic stroke via transient intraluminal middle cerebral artery occlusion (MCAO). While some studies found no change in Abcb1a mRNA in rats,¹⁵⁷ others observed increased ABCB1 at various time points after MCAO.^155,158 Additionally a time-dependent ABCB1 protein increase was reported in rats following 6 h lasting MCAO.¹⁵⁹ The greatest effect was seen at 6 h, but at that time the infarction volume already involved 25% of the brain. They also found that the concentration of two typical ABCB1 substrates was reduced at 6 h relative to the non-substrate sodium fluorescein, but the potential effect of tissue damage on the circulation and drug delivery was not addressed.

Inconclusive findings extend to other ABC transporters beyond ABCB1. In rats subjected to MCAO, positive ABCG2 IHC staining and increased ABCG2 mRNA levels in the peri-infarcted region were reported,¹⁵⁷ but the IHC specimens lacked typical ABCG2 staining in brain vessels. Kilic et al.¹¹ reported a temporary 25%–50% decrease in ABCC1 levels 3 h after MCAO in mice, although the effect was similar in the ischemic and contralateral hemispheres. ABCC1 would facilitate brain entry of 17β-estradiol-17β-D-glucuronide, mitigating stroke-induced cell death. Park et al.,¹⁶⁰ however, suggested that ABCC1 protects against stroke-induced cell death via oxidized glutathione clearance and that ABCC1 expression was increased in stroke-affected brain tissue. ABCC4 was upregulated in vitro in mouse endothelial cells under OGD,¹⁵⁶ while ABCC5 was purportedly upregulated in peri-infarcted neurons.¹⁵⁷ In summary, the influence of stroke on ABC transporters remains poorly elucidated. The possibility that alterations in the expression or functionality of ABCC1, affect the efficacy of certain neuroprotective agents, calls for further extensive investigation.

Psychiatric disorders

Psychiatric disorders encompass various conditions marked by mood and behavioral shifts, impacting patients’ well being. Their origins are multifaceted, involving genetic and environmental factors.¹⁶¹ Notably, many patients show resistance to antipsychotic or antidepressant therapy.^162,163 The mechanisms of resistance are currently unknown. ABC transporter overexpression may contribute to this resistance,¹⁶⁴ but the supporting literature is rather limited (Table S9 and Extended section 3.9).

Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease (CJD) is a rare transmissible neurological disorder caused by prion accumulation.¹⁶⁵ The pathologic form of normal neuronal host prion protein drives the pathogenesis of CJD by causing deposits in the brain that lead to neurodegeneration. Patients experience rapid and progressive cognitive decline and various neurological symptoms. Only one study has investigated ABC transporters' expression in CJD (Table S10).

Vogelgesang et al.¹⁶⁶ quantified ABCB1 by IHC in 10 CJD patients and aged-matched control patients with other diseases. They demonstrated that ABCB1 is significantly decreased in endothelial cells from the cortex, leptomeninges, and white matter of CJD patients. Co-occurrence of Aβ, which they observed in 5 out of 10 CJD patients, did not further reduce ABCB1 expression. Although two aged-matched control patients suffered from GBM, a condition known to alter ABC transporter functionality (see section brain tumors), the effect size was substantial. Unfortunately, IHC of ABCG2 or ABCC1 was not conducted.

Polyglutamine diseases

Polyglutamine (polyQ) diseases encompass nine inherited disorders driven by expanded polyQ-tract containing proteins.¹⁶⁷ Notable examples include Huntington’s disease (HD) and Machado-Joseph disease. Pathological protein aggregation in neuronal nuclei as seen with mutant huntingtin in HD results in progressive neurodegeneration. Symptoms typically start around age 40, featuring cognitive and motor function decline.¹⁶⁸ Although the integrity of the BBB is clearly compromised in HD patients and mouse models,¹⁶⁹ research on ABC transporters in polyQ diseases is scarce, limited to studies using the R6/2 mouse model of HD or Drosophila (Table S11 and extended section 3.11).

Traumatic brain injury

Traumatic brain injury (TBI) results from physical head trauma, subclassified by the cause (e.g., brain penetrating objects or blunt force) and severity (mild, moderate, or severe).¹⁷⁰ Logically, the severity and duration of TBI symptoms largely depend on the type and severity of the injury. There are a few reports in the literature on ABC transporter functionality in TBI (Table S12). Unfortunately, they all focus on different TBI sub-classifications, complicating drawing overarching conclusions on ABC transporter functionality in TBI.

Pop et al.¹⁷¹ studied a juvenile rat model of severe penetrating TBI and revealed reduced ABCB1 expression 60 days after injury via WB and IHC. They suggested a connection between Aβ accumulation and cognitive changes later in life.¹⁷¹ However, the reduction in ABCB1, while statistically significant, is minor (∼20%), leaving the potential contribution to Aβ accumulation uncertain, especially given minimal effects in Abcb1-deficient mice lacking the protein entirely (see section ABC transporters at the BBB).

Willyerd et al.¹⁷² reported WB and IHC data from a small cohort of severe blunt force TBI patients undergoing decompressive craniotomy. Compared with postmortem frontal cortex control tissue, they observed potential ABCC1 overexpression while ABCB1 seemed unaffected. Although enhanced ABCC1 expression fits with the upregulation seen in other diseases inducing local stress conditions, there are limitations. First, while representing an admirable effort, the group sizes of the WB (5 vs. 7) and IHC (2 vs. 3) cohorts were rather small. Second, although the IHC showed clear differences, the WBs suggested that part of the immuno-reactivity might be due to 90- and 50-kDa-sized fragments instead of full-length ABCC1, which may have an unknown functionality.

Vita et al.¹⁷³ examined a mouse model of repeated mild blunt force TBI, noting increased Abcb1a and Abcb1b mRNA levels in the hippocampus over 5 days after injury. In contrast, Abcg2 and Abcc1 mRNA levels demonstrated a non-significant trend toward an increase and decrease, respectively. Correspondingly, ABCB1 protein levels increased in the dorsal cortex proximal to the injury and the hippocampus 5 days after injury, although the effect size was modest (∼40% increase).

In summary, there are only a few papers concerning ABC transporter regulation and functionality after TBI, reporting incongruent results in terms of regulation of ABCB1: upregulation, downregulation, or no change. Of course, this could be attributed to each study focusing on different sub-classifications of TBI. However, limited effect sizes and small sample sizes suggest a potential for random effects. Further studies are required to draw definitive conclusions, but current evidence suggests any impact of TBI on ABCB1 expression is marginal at best.

Therapeutic implications and future perspectives

This review highlights alterations in ABC transporter functionality in various brain diseases. The changes are diverse, influenced by disease type and affected cell type. Given the protective role of ABC transporters, even unchanged expression may impact disease outcomes. Understanding these dynamics is crucial for therapeutic intervention, emphasizing the need for continued research in this area.

Traditionally, WB and IHC have been the predominant techniques employed to investigate ABC transporters. However, more recently RNA-seq and targeted proteomics have been implemented, offering more quantitative data and enabling cross-species comparisons in bulk material. In years to come, single-cell and spatial transcriptomics and proteomics will become more dominant. In fact, there are already several repositories available for mining. These will likely contain a wealth of information that may further boost our understanding of ABC transporter expression and function in the diseased brain.

In certain neurodegenerative diseases like AD and PD, a decrease in ABCB1 expression at the BBB has been observed, prompting questions regarding causality. While the exact relationship remains unclear, ABC transporters are implicated in the pathogenesis of these disorders. First, they play a role in clearing Aβ, and their decline may render the CNS more susceptible to Aβ neurotoxicity. Second, in PD, they inhibit the uptake and accumulation of neurotoxic substances. However, whether these changes are causal or consequential remains speculative. Boosting ABC transporter expression emerges as a potential therapeutic strategy, with preliminary evidence suggesting that pharmacological activation of pregnane X receptor can enhance ABC transporter expression and function at the BBB.³⁹ Similarly, stimulating ABCC1 has shown promise in enhancing Aβ clearance in AD models.⁵⁰ However, these approaches must consider potential drawbacks, such as ABCB1’s role in limiting the brain penetration of pharmaceutical compounds, and may only be effective in early stage disease before irreversible damage occurs, necessitating accurate diagnostic tools for early detection.

In neuro-inflammatory and excitatory brain disorders like ALS, MS, and epilepsy, ABC transporter functionality increases, potentially affecting therapy outcomes. Elevated transporter activity may lead to drug resistance, observed in refractory epilepsy and TSC due to upregulation of ABCB1, ABCG2, ABCC1, and/or ABCC2. In gliomas, GSCs express ABC transporters (mainly ABCG2, ABCB1, and ABCC1), shielding them from therapies. ABC transporters at the BBB restrict drug access to GBM cells invading adjacent normal brain tissue. Inhibiting ABC transporters presents a viable strategy to enhance drug delivery in such cases, targeting primarily ABCB1 and ABCG2 and possibly also ABCC2 and ABCC4.

ABCB1 and ABCC1 also play roles beyond drug transport, including chemokine secretion driving T cell invasion into the inflamed brain lesion in MS. Conversely, downregulation of ABCB1 in BMECs occurring in advanced lesions reduces chemokine excretion and immune cell infiltration. This dampening effect, along with other immune-suppressive factors (e.g., infiltration of lipid-laden macrophages) may contribute to the self-limiting nature of the lesions. However, once lesions stabilize, irreversible damage has already occurred. Prophylactic inhibition of ABCB1 and ABCC1 could potentially reduce chemokine release early on, mitigating inflammation and damage.

Most ABCB1 and later ABCG2 inhibitors were developed in the late 1980s and 1990s to treat multidrug-resistant cancers. However, clinical trials were disappointing, prompting the pharmaceutical industry to abandon the field. This is unfortunate; selective and potent inhibitors would be required to improve drug delivery to sanctuary sites like the brain. Elacridar, for instance, may be used to enhance CNS access of many ABCB1 and selected ABCG2 substrates.¹⁷⁴ Unfortunately, potent inhibitors for ABCC1 or ABCC2 are lacking. Existing compounds are often model compounds with micromolar potency and may also be substrates for ABCB1 and/or ABCG2, hindering CNS accumulation.

In conclusion, there is ample evidence that ABC transporter expression is altered in brain diseases and that such changes are important for disease etiology and treatment. Future research using omics approaches will aid in elucidating the interplay between the transporters and the disease. Existing data, however, already support the concept of pharmacological modulation to achieve more effective treatments for several brain diseases.