Advances in the understanding of retinal drug disposition and the role of blood–ocular barrier transporters

Progress in Ocular Drug Delivery: Challenges and Constraints

The eye has several dynamic and static barriers in place to limit the entry of foreign substances including therapeutics. As such, efficient drug delivery, especially to posterior segment tissues, has been challenging. This chapter describes the anatomical and physiological challenges associated with ocular drug delivery before discussing constraints with regard to formulation parameters. Finally, it gives an overview of advanced drug delivery technologies with a specific focus on recently marketed and late-stage clinical trial products.

ABCB1 and ABCG2 Together Limit the Distribution of ABCB1/ABCG2 Substrates to the Human Retina and the ABCG2 Single Nucleotide Polymorphism Q141K (c.421C> A) May Lead to Increased Drug Exposure

The widely expressed and poly-specific ABC transporters breast cancer resistance protein (ABCG2) and P-glycoprotein (ABCB1) are co-localized at the blood-brain barrier (BBB) and have shown to limit the brain distribution of several clinically used ABCB1/ABCG2 substrate drugs. It is currently not known to which extent these transporters, which are also expressed at the blood-retinal barrier (BRB), may limit drug distribution to the human eye and whether the ABCG2 reduced-function single-nucleotide polymorphism (SNP) Q141K (c.421C > A) has an impact on retinal drug distribution. Ten healthy male volunteers (five subjects with the c.421CC and c.421CA genotype, respectively) underwent two consecutive positron emission tomography (PET) scans after intravenous injection of the model ABCB1/ABCG2 substrate [¹¹C]tariquidar. The second PET scan was performed with concurrent intravenous infusion of unlabelled tariquidar to inhibit ABCB1 in order to specifically reveal ABCG2 function.In response to ABCB1 inhibition with unlabelled tariquidar, ABCG2 c.421C > A genotype carriers showed significant increases (as compared to the baseline scan) in retinal radiotracer influx K₁ (+62 ± 57%, p = 0.043) and volume of distribution V_T (+86 ± 131%, p = 0.043), but no significant changes were observed in subjects with the c.421C > C genotype. Our results provide the first evidence that ABCB1 and ABCG2 may together limit the distribution of systemically administered ABCB1/ABCG2 substrate drugs to the human retina. Functional redundancy between ABCB1 and ABCG2 appears to be compromised in carriers of the c.421C > A SNP who may therefore be more susceptible to transporter-mediated drug-drug interactions at the BRB than non-carriers.

Effects of Oral Vitamin D Supplement Therapy on Clinical Outcomes of Intravitreal Bevacizumab in Diabetic Macular Edema

Purpose

To assess the effects of oral vitamin D supplement therapy on clinical outcomes of intravitreal bevacizumab (IVB) injections in patients with diabetic macular edema (DME).

Methods

Seventy-one patients with center-involving DME received IVB injections three times monthly. Cases with serum 25-hydroxyvitamin D (25(OH)D) levels <30 ng/ml were divided into treatment and control groups. The treatment group received 50000 IU of oral vitamin D once a week for eight weeks. One month after the third IVB injection, changes in the best-corrected visual acuity (BCVA) and central macular thickness (CMT) were analyzed for each group.

Results

Thirty-seven patients had sufficient levels of 25 (OH) D, while 34 patients had insufficient levels. Nineteen cases with deficient levels of 25(OH)D were treated with oral vitamin D, while 15 patients were assigned to the control group. The mean of serum 25(OH)D in patients was 27.9 ng/ml [mean 20.3 ± 5.4 and 17.3 ± 5.4 ng/ml in control and treatment groups, respectively (P = 0.231)]. After three IVB injections, BCVA improved significantly in each group, but the difference between the study groups was not statistically significant. CMT decreased significantly in all the groups. The mean CMT reduction was more prominent in the vitamin D-treated group, but the difference between groups did not reach statistical significance (P = 0.29).

Conclusion

In DME patients with vitamin D deficiency, vitamin D supplement therapy had some beneficial effects on CMT reduction following three injections of IVB; nevertheless, these effects were not statistically significant. Definite conclusion needs further prospective studies with a larger sample size.

Quantitative Protein Expression in the Human Retinal Pigment Epithelium: Comparison Between Apical and Basolateral Plasma Membranes With Emphasis on Transporters

Purpose

Retinal pigment epithelium (RPE) limits the xenobiotic entry from the systemic blood stream to the eye. RPE surface transporters can be important in ocular drug distribution, but it has been unclear whether they are expressed on the apical, basal, or both cellular surfaces. In this paper, we provide quantitative comparison of apical and basolateral RPE surface proteomes.

Methods

We separated the apical and basolateral membranes of differentiated human fetal RPE (hfRPE) cells by combining apical membrane peeling and sucrose density gradient centrifugation. The membrane fractions were analyzed with quantitative targeted absolute proteomics (QTAP) and sequential window acquisition of all theoretical fragment ion spectra mass spectrometry (SWATH-MS) to reveal the membrane protein localization on the RPE cell surfaces. We quantitated 15 transporters in unfractionated RPE cells and scaled their expression to tissue level.

Results

Several proteins involved in visual cycle, cell adhesion, and ion and nutrient transport were expressed on the hfRPE plasma membranes. Most drug transporters showed similar abundance on both RPE surfaces, whereas large neutral amino acids transporter 1 (LAT1), p-glycoprotein (P-gp), and monocarboxylate transporter 1 (MCT1) showed modest apical enrichment. Many solute carriers (SLC) that are potential prodrug targets were present on both cellular surfaces, whereas putative sodium-coupled neutral amino acid transporter 7 (SNAT7) and riboflavin transporter (RFT3) were enriched on the basolateral and sodium- and chloride-dependent neutral and basic amino acid transporter (ATB0+) on the apical membrane.

Conclusions

Comprehensive quantitative information of the RPE surface proteomes was reported for the first time. The scientific community can use the data to further increase understanding of the RPE functions. In addition, we provide insights for transporter protein localization in the human RPE and the significance for ocular pharmacokinetics.

Serum and aqueous humor vitamin D levels in patients with diabetic macular edema

PURPOSE

To compare serum and aqueous humor (AH) vitamin D levels between the patients with diabetic macular edema (DME) and controls.

METHODS

A total of 65 subjects (30 DME, 35 control) were included. One-third of the control group had hypertension, dyslipidemia, or diabetes mellitus without diabetic retinopathy as underlying diseases. Serum and AH levels of 25-hydroxyvitamin D were measured in each subject. Multiple linear regression analysis was performed to investigate factors associated with serum and AH vitamin D levels.

RESULTS

There were no significant differences in serum vitamin D levels between the DME (14.3 ± 9.1 ng/mL) and control (16.2 ± 8.0 ng/mL) groups (P = 0.374). However, eyes with DME (41.6 ± 8.0 ng/mL) had a higher AH level of vitamin D than control eyes (25.5 ± 4.1 ng/mL, P < 0.001). AH vitamin D level was significantly associated with the presence of DME (β = 0.775, P < 0.001). Serum and AH levels of vitamin D were not significantly correlated (r = - 0.157, P = 0.211).

CONCLUSION

Serum vitamin D levels did not significantly differ between the DME and control groups. Localized vitamin D level in the eye was independent from systemic vitamin D level and it might be another indicator of DME severity.

New concepts in the diagnosis and management of choroidal metastases

The most frequent site of ocular metastasis is the choroid. The occurrence of choroidal metastases has increased steadily due to the longer survival of metastatic patients and the improvement of diagnostic tools. Fundoscopy, ultrasonography, and fluorescein angiography are now complemented by indocyanine green angiography and optical coherence tomography. Choroidal tumor biopsy may also confirm the metastatic nature of the tumor and help to determine the site of the primary malignancy. There is currently no consensus on the treatment strategy. Most patients have a limited life expectancy and for these complex treatments are generally not recommended. However, recent advances in systemic therapy have significantly improved survival of certain patients who may benefit from an aggressive ocular approach that could preserve vision. Although external beam radiation therapy is the most widely used treatment, more advanced forms of radiotherapy that are associated with fewer side effects can be proposed in select cases. In patients with a shorter life expectancy, systemic therapies such as those targeting oncogenic drivers, or immunotherapy can induce a regression of the choroidal metastases, and may be sufficient to temporarily decrease visual symptoms. However, they often acquire resistance to systemic treatment and ocular relapse usually requires radiotherapy for durable control. Less invasive office-based treatments, such as photodynamic therapy and intravitreal injection of anti-VEGF, may also help to preserve vision while reducing time spent in medical settings for patients in palliative care. The aim of this review is to summarize the current knowledge on choroidal metastases, with emphasis on the most recent findings in epidemiology, pathogenesis, diagnosis and treatment.

Differential expression of breast cancer-resistance protein, lung resistance protein, and multidrug resistance protein 1 in retinas of streptozotocin-induced diabetic mice

AIM

To investigate the altering expression profiles of efflux transporters such as breast cancer-resistance protein (BCRP), lung resistance protein (LRP), and multidrug resistance protein 1 (MDR1) at the inner blood-retinal barrier (BRB) during the development of early diabetic retinopathy (DR) and/or aging in mice.

METHODS

Relative mRNA and protein expression profiles of these three efflux transporters in the retina during the development of early DR and/or aging in mice were examined. The differing expression profiles of Zonula occludens 1 (ZO-1) and vascular endothelial growth factor-A (VEGFA) in the retina as well as the perfusion characterization of fluorescein isothiocyanate (FITC)-dextran and Evans blue were examined to evaluate the integrity of the inner BRB.

RESULTS

There were significant alterations in these three efflux transporters' expression profiles in the mRNA and protein levels of the retina during the development of diabetes mellitus and/or aging. The development of early DR was confirmed by the expression profiles of ZO-1 and VEGFA in the retina as well as the compromised integrity of the inner BRB.

CONCLUSION

The expression profiles of some efflux transporters such as BCRP, LRP, and MDR1 in mice retina during diabetic and/or aging conditions are tested, and the attenuated expression of BCRP, LRP, and MDR1 along with the breakdown of the inner BRB is found, which may be linked to the pathogenesis of early DR.

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

Vitreous humor analysis for the detection of xenobiotics in forensic toxicology: a review

Vitreous humor (VH) is a gelatinous substance contained in the posterior chamber of the eye, playing a mechanical role in the eyeball. It has been the subject of numerous studies in various forensic applications, primarily for the assessment of postmortem interval and for postmortem chemical analysis. Since most of the xenobiotics present in the bloodstream are detected in VH after crossing the selective blood-retinal barrier, VH is an alternative matrix useful for forensic toxicology. VH analysis offers particular advantages over other biological matrices: it is less prone to postmortem redistribution, is easy to collect, has relatively few interfering compounds for the analytical process, and shows sample stability over time after death. The present study is an overview of VH physiology, drug transport and elimination. Collection, storage, analytical techniques and interpretation of results from qualitative and quantitative points of view are dealt with. The distribution of xenobiotics in VH samples is thus discussed and illustrated by a table reporting the concentrations of 106 drugs from more than 300 case reports. For this purpose, a survey was conducted of publications found in the MEDLINE database from 1969 through April 30, 2015.

Delivery strategies for treatment of age-related ocular diseases: From a biological understanding to biomaterial solutions

Age-related ocular diseases, such as age-related macular degeneration (AMD), diabetic retinopathy, and glaucoma, result in life-long functional deficits and enormous global health care costs. As the worldwide population ages, vision loss has become a major concern for both economic and human health reasons. Due to recent research into biomaterials and nanotechnology major advances have been gained in the field of ocular delivery. This review provides a summary and discussion of the most recent strategies employed for the delivery of both drugs and cells to the eye to treat a variety of age-related diseases. It emphasizes the current challenges and limitations to ocular delivery and how the use of innovative materials can overcome these issues and ultimately provide treatment for age-related degeneration and regeneration of lost tissues. This review also provides critical considerations and an outlook for future studies in the field of ophthalmic delivery.

Activity of crizotinib over choroidal metastases in non-small-cell lung cancer (NSCLC)-ALK rearranged: a case report

BACKGROUND

Adenocarcinoma of the lung with EML4-ALK translocation is a rare subtype of Non Small-Cell Lung Cancer (NSCLC) that has recently shown to benefit from treatment with crizotinib. Despite the concerns about the efficacy of crizotinib over cerebral metastases, some reports have described its activity, although always after local treatment with radiotherapy. Recently it has been reported activity of crizotinib over choroidal metastases, again after radiotherapy.

CASE PRESENTATION

Herein we report a case of activity of crizotinib over choroidal metastases not previously treated with radiotherapy.

CONCLUSION

We suggest crizotinib may be active over choroidal metastases in a patient harboring ALK translocation with no need of radiotherapy.

Development of eye drops containing antihypertensive drugs: formulation of aqueous irbesartan/γCD eye drops

Aqueous nanoparticulated eye drop formulations based on γ-cyclodextrin (γCD) complexes were developed and tested in vitro. Three antihypertensive drugs, i.e. enalapril maleate, irbesartan and verapamil HCl, that have been shown to possess IOP-lowering activity were selected for this study. All three drugs displayed Bs-type phase-solubility diagrams in aqueous γCD solutions and had relatively low affinity for γCD. Irbesartan was selected for further formulation development. The drug was relatively stable at pH 4.5 but somewhat less stable at physiologic pH. However, presence of γCD in the aqueous media enhanced the chemical stability of irbesartan. Aqueous γCD-based eye drop formulations containing 1% and 2% (w/v) irbesartan were prepared and the effect of pH on the particles size distribution and drug release investigated. Only ∼2% of the drug was in solution in the pH 4.5 formulations but up to 45% in the pH 7 formulations. The pH 7 formulations, where larger fraction of the drug was in solution, displayed somewhat greater drug permeation flux but much lower drug permeation coefficients than the pH 4.5 formulations. Dynamic light scattering studies indicated the faster permeation was due to formation of smaller particles in presence tyloxapol.