Molecular Imaging of ABCB1 and ABCG2 Inhibition at the Human Blood-Brain Barrier Using Elacridar and 11C-Erlotinib PET

Development and validation of an LC-MS/MS method for simultaneous quantification of eight drugs in plasma and brain: Application in a pharmacokinetic study in mice

A selective and sensitive liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneous quantitation of a cassette of 8 drugs, including docetaxel, erlotinib, loperamide, riluzole, vemurafenib, verapamil, elacridar and tariquidar. Stable isotopically labeled compounds were available for use as internal standards for all compounds, except for tariquidar for which we used elacridar-d4. Sample pre-treatment involved liquid-liquid extraction using tert-butyl-methyl ether as this resulted in good recovery and low ion suppression. Chromatographic separation was achieved using a Zorbax Extend C18 analytical column and a linear gradient from 20 % to 95 % methanol in 0.1 % (v/v) formic acid in water. MS/MS detection using multiple reaction monitoring was done in positive ionization mode. We validated this assay for human and mouse plasma and mouse brain homogenates. The calibration curves were linear over a range 1-200 nM for each drug in the mix, except for tariquidar probably due to the lack of a stable isotope labeled analog. The intra-day and inter-day accuracies were within the 85-115 % range for all compounds at low, medium and high concentrations in the three different matrices. Similarly, the precision for all compounds at three different concentration levels ranged below 15 %, with the exception of tariquidar in mouse plasma and brain homogenate and riluzole in brain homogenate. Pilot studies have confirmed that the method is suitable for the analysis of mouse plasma samples and brain homogenates following cassette dosing of this mixture in mice.

ATP-binding cassette transporter inhibitor potency and substrate drug affinity are critical determinants of successful drug delivery enhancement to the brain

BACKGROUND

Pharmacotherapy for brain diseases is severely compromised by the blood-brain barrier (BBB). ABCB1 and ABCG2 are drug transporters that restrict drug entry into the brain and their inhibition can be used as a strategy to boost drug delivery and pharmacotherapy for brain diseases.

METHODS

We employed elacridar and tariquidar in mice to explore the conditions for effective inhibition at the BBB. Abcg2;Abcb1a/b knockout (KO), Abcb1a/b KO, Abcg2 KO and wild-type (WT) mice received a 3 h i.p. infusion of a cocktail of 8 typical substrate drugs in combination with elacridar or tariquidar at a range of doses. Abcg2;Abcb1a/b KO mice were used as the reference for complete inhibition, while single KO mice were used to assess the potency to inhibit the remaining transporter. Brain and plasma drug levels were measured by LC-MS/MS.

RESULTS

Complete inhibition of ABCB1 at the BBB is achieved when the elacridar plasma level reaches 1200 nM, whereas tariquidar requires at least 4000 nM. Inhibition of ABCG2 is more difficult. Elacridar inhibits ABCG2-mediated efflux of weak but not strong ABCG2 substrates. Strikingly, tariquidar does not enhance the brain uptake of any ABCG2-subtrate drug. Similarly, elacridar, but not tariquidar, was able to inhibit its own brain efflux in ABCG2-proficient mice. The plasma protein binding of elacridar and tariquidar was very high but similar in mouse and human plasma, facilitating the translation of mouse data to humans.

CONCLUSIONS

This work shows that elacridar is an effective pharmacokinetic-enhancer for the brain delivery of ABCB1 and weaker ABCG2 substrate drugs when a plasma concentration of 1200 nM is exceeded.

Overcoming Barriers in Glioblastoma-Advances in Drug Delivery Strategies

The world of cancer treatment is evolving rapidly and has improved the prospects of many cancer patients. Yet, there are still many cancers where treatment prospects have not (or hardly) improved. Glioblastoma is the most common malignant primary brain tumor, and even though it is sensitive to many chemotherapeutics when tested under laboratory conditions, its clinical prospects are still very poor. The blood-brain barrier (BBB) is considered at least partly responsible for the high failure rate of many promising treatment strategies. We describe the workings of the BBB during healthy conditions and within the glioblastoma environment. How the BBB acts as a barrier for therapeutic options is described as well as various approaches developed and tested for passing or opening the BBB, with the ultimate aim to allow access to brain tumors and improve patient perspectives.

BCRP drives intrinsic chemoresistance in chemotherapy-naïve breast cancer brain metastasis

Although initially successful, treatments with chemotherapy often fail because of the recurrence of chemoresistant metastases. Since these tumors develop after treatment, resistance is generally thought to occur in response to chemotherapy. However, alternative mechanisms of intrinsic chemoresistance in the chemotherapy-naïve setting may exist but remain poorly understood. Here, we study drug-naïve murine breast cancer brain metastases (BCBMs) to identify how cancer cells growing in a secondary site can acquire intrinsic chemoresistance without cytotoxic agent exposure. We demonstrate that drug-naïve murine breast cancer cells that form cancer lesions in the brain undergo vascular mimicry and concomitantly express the adenosine 5'-triphosphate-binding cassette transporter breast cancer resistance protein (BCRP), a common marker of brain endothelial cells. We reveal that expression of BCRP by the BCBM tumor cells protects them against doxorubicin and topotecan. We conclude that BCRP overexpression can cause intrinsic chemoresistance in cancer cells growing in metastatic sites without prior chemotherapy exposure.

Peering into the Brain's Estrogen Receptors: PET Tracers for Visualization of Nuclear and Extranuclear Estrogen Receptors in Brain Disorders

Estrogen receptors (ERs) play a multitude of roles in brain function and are implicated in various brain disorders. The use of positron emission tomography (PET) tracers for the visualization of ERs' intricate landscape has shown promise in oncology but remains limited in the context of brain disorders. Despite recent progress in the identification and development of more selective ligands for various ERs subtypes, further optimization is necessary to enable the reliable and efficient imaging of these receptors. In this perspective, we briefly touch upon the significance of estrogen signaling in the brain and raise the setbacks associated with the development of PET tracers for identification of specific ERs subtypes in the brain. We then propose avenues for developing efficient PET tracers to non-invasively study the dynamics of ERs in the brain, as well as neuropsychiatric diseases associated with their malfunction in a longitudinal manner. This perspective puts several potential candidates on the table and highlights the unmet needs and areas requiring further research to unlock the full potential of PET tracers for ERs imaging, ultimately aiding in deepening our understanding of ERs and forging new avenues for potential therapeutic strategies.

Brain exposure of osimertinib in patients with epidermal growth factor receptor mutation non-small cell lung cancer and brain metastases: A positron emission tomography and magnetic resonance imaging study

Brain metastases (BMs) are associated with poor prognosis in epidermal growth factor receptor mutation-positive (EGFRm) non-small cell lung cancer (NSCLC). Osimertinib is a third-generation, irreversible, EGFR-tyrosine kinase inhibitor that potently and selectively inhibits EGFR-sensitizing and T790M resistance mutations with efficacy in EGFRm NSCLC including central nervous system (CNS) metastases. The open-label phase I positron emission tomography (PET) and magnetic resonance imaging (MRI) study (ODIN-BM) assessed [¹¹ C]osimertinib brain exposure and distribution in patients with EGFRm NSCLC and BMs. Three dynamic 90-min [¹¹ C]osimertinib PET examinations were acquired together with metabolite-corrected arterial plasma input functions at: baseline, after first oral osimertinib 80 mg dose, and after greater than or equal to 21 days of osimertinib 80 mg q.d. treatment. Contrast-enhanced MRI was performed at screening and after 25-35 days of osimertinib 80 mg q.d.; treatment effect was assessed per CNS Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 and per volumetric changes in total BM using a novel analysis approach. Four patients (aged 51-77 years) completed the study. At baseline, ~1.5% injected radioactivity reached the brain (ID_max[brain] ) 22 min (median, T_max[brain] ) after injection. Total volume of distribution (V_T ) in whole brain was numerically higher compared with the BM regions. After a single oral osimertinib 80 mg dose, there was no consistent decrease in V_T in whole brain or BMs. After greater than or equal to 21 days' daily treatment, V_T in whole brain and BMs were numerically higher versus baseline. MRI revealed 56%-95% reduction in total BMs volume after 25-35 days of osimertinib 80 mg q.d. treatment. The [¹¹ C]osimertinib crossed the blood-brain and brain-tumor barriers and had a high, homogeneous brain distribution in patients with EGFRm NSCLC and BMs.

Radiolabeled EGFR TKI as predictive imaging biomarkers in NSCLC patients – an overview

Non-small cell lung cancer (NSCLC) has one of the highest cancer-related mortality rates worldwide. In a subgroup of NSCLC, tumor growth is driven by epidermal growth factor receptors (EGFR) that harbor an activating mutation. These patients are best treated with EGFR tyrosine kinase inhibitors (EGFR TKI). Identifying the EGFR mutational status on a tumor biopsy or a liquid biopsy using tumor DNA sequencing techniques is the current approach to predict tumor response on EGFR TKI therapy. However, due to difficulty in reaching tumor sites, and varying inter- and intralesional tumor heterogeneity, biopsies are not always possible or representative of all tumor lesions, highlighting the need for alternative biomarkers that predict tumor response. Positron emission tomography (PET) studies using EGFR TKI-based tracers have shown that EGFR mutational status could be identified, and that tracer uptake could potentially be used as a biomarker for tumor response. However, despite their likely predictive and monitoring value, the EGFR TKI-PET biomarkers are not yet qualified to be used in the routine clinical practice. In this review, we will discuss the currently investigated EGFR-directed PET biomarkers, elaborate on the typical biomarker development process, and describe how the advances, challenges, and opportunities of EGFR PET biomarkers relate to this process on their way to qualification for routine clinical practice.

The Development of Positron Emission Tomography Tracers for In Vivo Targeting the Kinase Domain of the Epidermal Growth Factor Receptor

Multiple small molecule PET tracers have been developed for the imaging of the epidermal growth factor receptor (EGFR). These tracers target the tyrosine kinase (TK) domain of the receptor and have been used for both quantifying EGFR expression and to differentiate between EGFR mutational statuses. However, the approaches for in vivo evaluation of these tracers are diverse and have resulted in data that are hard to compare. In this review, we analyze the historical development of the in vivo evaluation approaches, starting from the first EGFR TK PET tracer [11C]PD153035 to tracers developed based on TK inhibitors used for the clinical treatment of mutated EGFR expressing non-small cell lung cancer like [11C]erlotinib and [18F]afatinib. The evaluation of each tracer has been compiled to allow for a comparison between studies and ultimately between tracers. The main challenges for each group of tracers are thereafter discussed. Finally, this review addresses the challenges that need to be overcome to be able to efficiently drive EGFR PET imaging forward.

Complete inhibition of ABCB1 and ABCG2 at the blood-brain barrier by co-infusion of erlotinib and tariquidar to improve brain delivery of the model ABCB1/ABCG2 substrate [11C]erlotinib

P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) restrict at the blood-brain barrier (BBB) the brain distribution of the majority of currently known molecularly targeted anticancer drugs. To improve brain delivery of dual ABCB1/ABCG2 substrates, both ABCB1 and ABCG2 need to be inhibited simultaneously at the BBB. We examined the feasibility of simultaneous ABCB1/ABCG2 inhibition with i.v. co-infusion of erlotinib and tariquidar by studying brain distribution of the model ABCB1/ABCG2 substrate [¹¹C]erlotinib in mice and rhesus macaques with PET. Tolerability of the erlotinib/tariquidar combination was assessed in human embryonic stem cell-derived cerebral organoids. In mice and macaques, baseline brain distribution of [¹¹C]erlotinib was low (brain distribution volume, V_T,brain < 0.3 mL/cm³). Co-infusion of erlotinib and tariquidar increased V_T,brain in mice by 3.0-fold and in macaques by 3.4- to 5.0-fold, while infusion of erlotinib alone or tariquidar alone led to less pronounced V_T,brain increases in both species. Treatment of cerebral organoids with erlotinib/tariquidar led to an induction of Caspase-3-dependent apoptosis. Co-infusion of erlotinib/tariquidar may potentially allow for complete ABCB1/ABCG2 inhibition at the BBB, while simultaneously achieving brain-targeted EGFR inhibition. Our protocol may be applicable to enhance brain delivery of molecularly targeted anticancer drugs for a more effective treatment of brain tumors.

Pleiotropic Roles of ABC Transporters in Breast Cancer

Chemotherapeutics are the mainstay treatment for metastatic breast cancers. However, the chemotherapeutic failure caused by multidrug resistance (MDR) remains a pivotal obstacle to effective chemotherapies of breast cancer. Although in vitro evidence suggests that the overexpression of ATP-Binding Cassette (ABC) transporters confers resistance to cytotoxic and molecularly targeted chemotherapies by reducing the intracellular accumulation of active moieties, the clinical trials that target ABCB1 to reverse drug resistance have been disappointing. Nevertheless, studies indicate that ABC transporters may contribute to breast cancer development and metastasis independent of their efflux function. A broader and more clarified understanding of the functions and roles of ABC transporters in breast cancer biology will potentially contribute to stratifying patients for precision regimens and promote the development of novel therapies. Herein, we summarise the current knowledge relating to the mechanisms, functions and regulations of ABC transporters, with a focus on the roles of ABC transporters in breast cancer chemoresistance, progression and metastasis.

A combo-strategy to improve brain delivery of antiepileptic drugs: Focus on BCRP and intranasal administration

The breast cancer resistance protein (BCRP) is an efflux transporter expressed at the apical surface of human brain endothelial cells of the blood-brain barrier (BBB). It was proposed as one of the transporters responsible for the development of drug resistance to several central nervous system (CNS) drugs, including antiepileptic drugs (AEDs). In this context, the present work aimed to characterize the interaction between new-generation AEDs, lacosamide, levetiracetam and zonisamide, and BCRP, in order to investigate whether intranasal administration can successfully avoid the impact of BCRP on brain drug distribution, preventing the development of refractory epilepsy. Firstly, BCRP substrates and/or inhibitors were identified resorting to intracellular accumulation and bidirectional transport assays on Madin-Darby canine kidney (MDCK) cells and the transfected cell line with human ABCG2 (MDCK-BCRP). Furthermore, in vivo pharmacokinetic studies were carried out for BCRP substrates with and without elacridar, a well-known P-gp and BCRP modulator, to assess the impact of efflux inhibition on brain drug distribution. The extent of drug equilibration between plasma and brain was compared after intravenous (IV) and intranasal administration to mice. Among the three tested AEDs, zonisamide was the only AED identified as BCRP substrate in vitro, as demonstrated by the net flux ratio of 2.73, which decreased 53.85 % in the presence of a BCRP inhibitor, Ko143. Lacosamide revealed to inhibit BCRP in all tested concentrations (2.5-75 µM), exhibiting a significant increase (p < 0.001) of the intracellular accumulation of a BCRP substrate (Hoechst 33342) in MDCK-BCRP cells. Levetiracetam did not behave as a BCRP substrate nor inhibitor. After IV administration, the plasma concentrations of zonisamide were unaffected by elacridar, but its extent of brain exposure increased three-fold (as assessed by AUC_t, 674.12 vs 284.47 µg.min/mL). These results corroborate the previous in vitro findings, suggesting that BCRP is involved in the transport of zonisamide through the BBB. In opposition, no significant changes were found in plasma or brain concentrations after the administration of zonisamide by intranasal route, indicating that the influence of BCRP is less relevant than for IV route. In addition, direct nose-to-brain delivery of zonisamide, given by the direct transport percentage, was approximately 49 %. Altogether, these assays demonstrated that the impact of BCRP on the delivery of zonisamide to the brain is lower after intranasal administration, probably due to direct nose-to-brain transport. Therefore, the intranasal administration of AEDs may be a relevant strategy to avoid the impact of efflux transporters at the BBB and the development of drug resistance.

Enhancing Brain Retention of a KIF11 Inhibitor Significantly Improves its Efficacy in a Mouse Model of Glioblastoma

Glioblastoma, the most lethal primary brain cancer, is extremely proliferative and invasive. Tumor cells at tumor/brain-interface often exist behind a functionally intact blood-brain barrier (BBB), and so are shielded from exposure to therapeutic drug concentrations. An ideal glioblastoma treatment needs to engage targets that drive proliferation as well as invasion, with brain penetrant therapies. One such target is the mitotic kinesin KIF11, which can be inhibited with ispinesib, a potent molecularly-targeted drug. Although, achieving durable brain exposures of ispinesib is critical for adequate tumor cell engagement during mitosis, when tumor cells are vulnerable, for efficacy. Our results demonstrate that the delivery of ispinesib is restricted by P-gp and Bcrp efflux at BBB. Thereby, ispinesib distribution is heterogeneous with concentrations substantially lower in invasive tumor rim (intact BBB) compared to glioblastoma core (disrupted BBB). We further find that elacridar—a P-gp and Bcrp inhibitor—improves brain accumulation of ispinesib, resulting in remarkably reduced tumor growth and extended survival in a rodent model of glioblastoma. Such observations show the benefits and feasibility of pairing a potentially ideal treatment with a compound that improves its brain accumulation, and supports use of this strategy in clinical exploration of cell cycle-targeting therapies in brain cancers.

Botryllamide G is an ABCG2 inhibitor that improves lapatinib delivery in mouse brain

Introduction: Transporters comprising the blood-brain barrier complicate delivery of many therapeutics to the central nervous system. The present study ascertained whether the natural product botryllamide G is viable for in vivo inhibition of ABCG2 using lapatinib as a probe for ABCB1 and ABCG2-mediated efflux from the brain. Methods: Wild-type and Mdr1a/Mdr1b (-/-) mice were treated with botryllamide G and lapatinib ("doublet therapy"), and while a separate cohort of wild-type mice was treated with botryllamide, tariquidar and lapatinib ("triplet therapy"). Results: Botryllamide G demonstrates biphasic elimination with a rapid distribution, decreasing below the in vitro IC50 of 6.9 µM within minutes, yet with a relatively slower terminal half-life (4.6 h). In Mdr1a/Mdr1b (-/-) mice, doublet therapy resulted in a significant increase in brain lapatinib AUC at 8 h (2058 h*ng/mL vs 4007 h*ng/mL; P = .031), but not plasma exposure (P = .15). No significant differences were observed after 24 h. Lapatinib brain exposure was greater through 1 h when wild-type mice were administered triplet therapy (298 h*pg/mg vs 120 h*pg/mg; P < .001), but the triplet decreased brain AUC through 24 h vs. mice administered lapatinib alone (2878 h*pg/mg vs 4461hr*ng/mL; P < .001) and did not alter the brain:plasma ratio. Conclusions: In summary, the ABCG2 inhibitor, botryllamide G, increases brain exposure to lapatinib in mice lacking Abcb1, although the combination of botryllamide G and tariquidar increases brain exposure in wild-type mice only briefly (1 h). Additional research is needed to find analogs of this compound that have better pharmacokinetics and pharmacodynamic effects on ABCG2 inhibition.

Proof-of-Concept Study of Drug Brain Permeability Between in Vivo Human Brain and an in Vitro iPSCs-Human Blood-Brain Barrier Model

The development of effective central nervous system (CNS) drugs has been hampered by the lack of robust strategies to mimic the blood-brain barrier (BBB) and cerebrovascular impairments in vitro. Recent technological advancements in BBB modeling using induced pluripotent stem cells (iPSCs) allowed to overcome some of these obstacles, nonetheless the pertinence for their use in drug permeation study remains to be established. This mandatory information requires a cross comparison of in vitro and in vivo pharmacokinetic data in the same species to avoid failure in late clinical drug development. Here, we measured the BBB permeabilities of 8 clinical positron emission tomography (PET) radioligands with known pharmacokinetic parameters in human brain in vivo with a newly developed in vitro iPSC-based human BBB (iPSC-hBBB) model. Our findings showed a good correlation between in vitro and in vivo drug brain permeability (R² = 0.83; P = 0.008) which contrasted with the limited correlation between in vitro apparent permeability for a set of 18 CNS/non-CNS compounds using the in vitro iPSCs-hBBB model and drug physicochemical properties. Our data suggest that the iPSC-hBBB model can be integrated in a flow scheme of CNS drug screening and potentially used to study species differences in BBB permeation.

The influence of the coadministration of the p-glycoprotein modulator elacridar on the pharmacokinetics of lapatinib and its distribution in the brain and cerebrospinal fluid

Background Lapatinib is a small-molecule tyrosine kinase inhibitor of human epidermal receptor 2 (HER2) and EGFR that has currently been approved for the treatment of HER2-positive advanced and metastatic breast cancer (BC). The ATP-binding cassette (ABC) family of transporters includes P-glycoprotein (P-gp; ABCB1) and breast cancer resistance protein (BCRP; ABCG2), which substantially restrict the penetration of drugs, including chemotherapeutics, through the blood-brain barrier and blood-cerebrospinal fluid barrier. The aim of this study was to investigate the effects of elacridar, an ABCB1 and ABCG2 inhibitor, on the brain and cerebrospinal fluid uptake of lapatinib. Methods Rats were divided into two groups: one group received 5 mg/kg elacridar and 100 mg/kg lapatinib (an experimental group), and the other group received 100 mg/kg lapatinib (a control group). Lapatinib concentrations in the blood plasma (BP), cerebrospinal fluid (CSF) and brain tissue (BT) were measured by liquid chromatography coupled with tandem mass spectrometry. Results Elacridar significantly increased lapatinib penetration into the CSF and BT (C_max increase of 136.4% and 54.7% and AUC_0-∞ increase of 53.7% and 86.5%, respectively). The C_max of lapatinib in BP was similar in both experimental groups (3057.5 vs. 3257.5 ng/mL, respectively). Conclusion This study showed that elacridar influenced the pharmacokinetics of lapatinib. The inhibition of ABCB1 and ABCG2 transporters by elacridar substantially enhanced the penetration of lapatinib into the CSF and BT. The blocking of protein transporters could become indispensable in the treatment of patients with breast cancer and brain metastases.

Pharmacokinetics of the P-gp Inhibitor Tariquidar in Rats After Intravenous, Oral, and Intraperitoneal Administration

Background and objective

P-glycoprotein (P-gp), a transmembrane transporter expressed at the blood–brain barrier, restricts the distribution of diverse central nervous system-targeted drugs from blood into brain, reducing their therapeutic efficacy. The third-generation P-gp inhibitor tariquidar (XR9576) was shown to enhance brain distribution of P-gp substrate drugs in humans. Oral bioavailability of tariquidar was found to be low in humans requiring the compound to be administered intravenously, which hinders a broader clinical use. The objective of the present study was to investigate the plasma pharmacokinetics of tariquidar in rats after single intravenous, oral, and intraperitoneal administration.

Methods

Two different tariquidar formulations (A and B) were used, both at a dosage of 15 mg/kg, respectively. Formulation A was a solution and formulation B was a microemulsion which was previously shown to improve the oral bioavailability of the structurally related P-gp inhibitor elacridar in mice.

Results

In contrast to human data, the present study found a high bioavailability of tariquidar in rats after oral dosing. Oral bioavailability was significantly higher (p = 0.032) for formulation B (86.3%) than for formulation A (71.6%). After intraperitoneal dosing bioavailability was 91.4% for formulation A and 99.6% for formulation B.

Conclusion

The present findings extend the available information on tariquidar and provide a basis for future studies involving oral administration of this compound.

Physical blood-brain barrier disruption induced by focused ultrasound does not overcome the transporter-mediated efflux of erlotinib

Overcoming the efflux mediated by ATP-binding cassette (ABC) transporters at the blood-brain barrier (BBB) remains a challenge for the delivery of small molecule tyrosine kinase inhibitors (TKIs) such as erlotinib to the brain. Inhibition of ABCB1 and ABCG2 at the mouse BBB improved the BBB permeation of erlotinib but could not be achieved in humans. BBB disruption induced by focused ultrasound (FUS) was investigated as a strategy to overcome the efflux transport of erlotinib in vivo. In rats, FUS combined with microbubbles allowed for a large and spatially controlled disruption of the BBB in the left hemisphere. ABCB1/ABCG2 inhibition was performed using elacridar (10 mg/kg i.v). The brain kinetics of erlotinib was studied using ¹¹C-erlotinib Positron Emission Tomography (PET) imaging in 5 groups (n = 4-5 rats per group) including a baseline group, immediately after sonication (FUS), 48 h after FUS (FUS + 48 h), elacridar (ELA) and their combination (FUS + ELA). BBB integrity was assessed using the Evan's Blue (EB) extravasation test. Brain exposure to ¹¹C-erlotinib was measured as the area under the curve (AUC) of the brain kinetics (% injected dose (%ID) versus time (min)) in volumes corresponding to the disrupted (left) and the intact (right) hemispheres, respectively. EB extravasation highlighted BBB disruption in the left hemisphere of animals of the FUS and FUS + ELA groups but not in the control and ELA groups. EB extravasation was not observed 48 h after FUS suggesting recovery of BBB integrity. Compared with the control group (AUC_Baseline = 1.4 ± 0.5%ID.min), physical BBB disruption did not impact the brain kinetics of ¹¹C-erlotinib in the left hemisphere (p > .05) either immediately (AUC_FUS = 1.2 ± 0.1%ID.min) or 48 h after FUS (AUC_FUS+48h = 1.1 ± 0.3%ID.min). Elacridar similarly increased ¹¹C-erlotinib brain exposure to the left hemisphere in the absence (AUC_ELA = 2.2 ± 0.5%ID.min, p < .001) and in the presence of BBB disruption (AUC_FUS+ELA = 2.1 ± 0.5%ID.min, p < .001). AUC_left was never significantly different from AUC_right (p > .05), in any of the tested conditions. BBB integrity is not the rate limiting step for erlotinib delivery to the brain which is mainly governed by ABC-mediated efflux. Efflux transport of erlotinib persisted despite BBB disruption.

A Proof-of-Concept Study to Inhibit ABCG2- and ABCB1-Mediated Efflux Transport at the Human Blood-Brain Barrier

The adenosine triphosphate-binding cassette transporters P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) are 2 efflux transporters at the blood-brain barrier (BBB) that effectively restrict brain distribution of dual ABCB1/ABCG2 substrate drugs, such as tyrosine kinase inhibitors. Pharmacologic inhibition of ABCB1/ABCG2 may improve the efficacy of dual-substrate drugs for treatment of brain tumors, but no marketed ABCB1/ABCG2 inhibitors are currently available. In the present study, we examined the potential of supratherapeutic-dose oral erlotinib to inhibit ABCB1/ABCG2 activity at the human BBB. Methods: Healthy men underwent 2 consecutive PET scans with ¹¹C-erlotinib: a baseline scan and a second scan either with concurrent intravenous infusion of the ABCB1 inhibitor tariquidar (3.75 mg/min, n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, n = 7; 650 mg, n = 8; or 1,000 mg, n = 2). Results: Although tariquidar administration had no effect on ¹¹C-erlotinib brain distribution, oral erlotinib led, at the 650-mg dose, to significant increases in volume of distribution (23% ± 13%, P = 0.008), influx rate constant of radioactivity from plasma into brain (58% ± 26%, P = 0.008), and area under the brain time-activity curve (78% ± 17%, P = 0.008), presumably because of combined partial saturation of ABCG2 and ABCB1 activity. Inclusion of further subjects into the 1,000-mg dose group was precluded by adverse skin events (rash). Conclusion: Supratherapeutic-dose erlotinib may be used to enhance brain delivery of ABCB1/ABCG2 substrate anticancer drugs, but its clinical applicability for continuous ABCB1/ABCG2 inhibition at the BBB may be limited by safety concerns.