Ophthalmic Drugs: Preclinical Pharmacokinetic (PK) Profiles and Research Strategies

There are a large number of patients suffering from ocular disease and visual impairments. According to the “World Report on Vision” published by the World Health Organization (WHO), there are at least 2.2 billion people worldwide with visual impairments, of which at least 1 billion have the potential for treatment. Common ocular diseases include refractive errors, cataracts, age-related macular degeneration (AMD), glaucoma, diabetic retinopathy (DR), corneal opacity, trachoma, dry eye syndrome, and conjunctivitis. Table 1 illustrates the causes, clinical manifestations, and treatments of common ophthalmic diseases. Uncorrected refractive errors (e.g., myopia or hyperopia), cataracts, and AMD are the main causes of moderate to severe visual impairments or blindness, which substantially affect patient quality of life. This article introduces the biological structure of the eye, ocular drug delivery, administration barriers, and preclinical study strategies, providing references for PK studies in ophthalmology.

Table 1. Common ocular disease

Biological structure of the eye

The eye consists of two major parts: the eyeball wall and the content of the eye. The eyeball wall is composed of three layers, including the outer, the middle, and the inner layers from the outside to the inside. The outer layer is a fibrous membrane composed of the cornea and sclera. The middle layer is the uvea composed of the iris, ciliary body, and choroid, and it is also known as the vascular layer or uveal layer. The inner layer is the retina.

Anatomically, the eye is divided into the anterior and posterior segments with the lens as the boundary (Figure 1). The structures of the lens and the structures in front of the lens are referred to as the anterior segment of the eye, which accounts for approximately one-third of the eye. The anterior segment includes the conjunctiva, cornea, iris, ciliary body, lens, and aqueous humor, playing an important role in studying ocular surface diseases and treatments. The structures behind the lens are referred to as the posterior segment of the eye, which accounts for approximately two-thirds of the eye. The posterior segment comprises the sclera, choroid, retina, vitreous body, and optic nerve. In the treatment of posterior segment diseases, ocular drug delivery methods, such as intravitreal injections, are commonly used to facilitate drug diffusion to the target areas and exert therapeutic effects. The clarification of the anterior and posterior segments of the eye during drug development plays a crucial role in studying the distribution and delivery of ophthalmic drugs.

Figure 1. Anatomy of the eye and routes of drug administration¹

*Solid black lines indicate different ocular tissues and green arrows indicate various routes of drug administration

Routes for ocular drug delivery

Routes for ocular drug delivery can be broadly classified into systemic and topical administrations. Systemic administration includes the traditional routes of administration, such as oral administration, intravenous injection, or intramuscular injection. Due to factors such as the blood–eye barrier, the amount of drug reaching the target tissues by systemic administration is limited, which leads to inadequate therapeutic effects. Therefore, for ophthalmic drugs, topical drug delivery is primarily used in clinical settings.

Topical ocular drug delivery is classified into three main categories: (1) topical administration, such as eye drops, eye ointments, and eye gels; (2) intraocular administration, such as intravitreal injections, intracameral injections, and intravitreal implants; and (3) periocular administration, such as subconjunctival injections, retrobulbar injections, peribulbar injections, and sub-tenon administrations. Ophthalmic drugs used to treat anterior segment diseases are often administered through ocular surface administration, while drugs used to treat posterior segment diseases are often administered through intraocular or periocular routes.

(1) For topical administration, tear drainage, blinking, and entering the systemic circulation through the nasolacrimal pathway result in only 5%–10% of the drug entering the corneal barrier.² After entering the aqueous humor through the cornea, the drug is quickly distributed into the iris and ciliary body, but it is difficult to enter the posterior segment of the eye. In addition, the eyelids have rich blood vessels, which may allow drugs to enter the bloodstream. Therefore, it is recommended that blood samples should be collected during the early stages of drug screening to examine the drug entry into the bloodstream and to provide support for later-stage studies on the effect of potential systemic exposure.

(2) For intravitreal injections, the drugs can diffuse to both the anterior segment tissues and the retina. It is worth noting that the drug distribution to the choroid occurs at a relatively slow rate due to the presence of tight junctions on the retinal pigment epithelium.

(3) For periocular injections, the drugs can penetrate the sclera to reach the retina and vitreous. This administration route is commonly used for drugs whose targets are in the posterior segment. The absorption, distribution, and metabolism of drugs through topical administration, intraocular or implant, and periocular administration are summarized in Table 2.

Table 2. Routes for ocular drug delivery and ocular pharmacokinetic (PK) profiles 

Barriers to ocular drug delivery

The biggest challenge in ocular drug delivery research is overcoming static and dynamic biological barriers to effectively deliver the drug to target ocular tissues. These barriers can be classified based on their anatomical location and functional characteristics, which are generally categorized as anterior segment barriers and posterior segment barriers.⁵ Anterior segment barriers include static barriers (such as the cornea, conjunctiva, blood-aqueous barrier, and efflux pumps) as well as dynamic barriers (such as tear drainage, conjunctival lymph, blood flow, and the aqueous humor).⁶ Posterior segment barriers include static barriers (such as the sclera, Bruch’s membrane, blood-retinal barrier, and efflux pumps) and dynamic barriers (such as the choroidal blood and lymph circulation) (Figure 2).⁷

Figure 2. Ocular biological barriers

Strategies for ophthalmic preclinical PK studies

(1) Selection of animal species in ophthalmic preclinical PK studies

Rabbits and monkeys are the most commonly used animal species for studying ophthalmic treatments due to their comparability to the human eyes^8,9. Considering the research costs and experimental operations, for small molecule drugs rabbits are often used for preclinical PK studies , whereas species relevant to drug efficacy are often chosen for preclinical studies of large molecule drugs.

The retinal pigment epithelium (RPE) plays a crucial role in drug distribution in the ocular tissues. It is also known as the uvea and consists of three continuous parts: the iris in the anterior segment, the ciliary body in the middle segment, and the choroid in the posterior segment. The pigment epithelial cells contain melanin, which can affect ophthalmic drug distribution and release, Drugs may also bind to melanin, leading to slow release or accumulation, thus affecting efficacy or toxicity evaluation.¹⁰ Albino experimental animals (e.g., CD-1 mice, Sprague–Dawley rats, New Zealand white rabbits) lack melanin in the choroid. Therefore, it is important to select appropriate animal models based on the drug properties for ophthalmic drug research in preclinical studies.

It is also necessary to consider the specificity of the ocular anatomical structures of different animal species. For example, only humans and non-human primates have well-developed macular structures, making monkeys the preferred experimental animals for pharmacodynamic studies of macular degeneration-like diseases.⁹ In general, a single animal model could not be used to fully simulate the drug efficacy in the human body. It is recommended to use a combination of multiple animal models for preclinical studies to comprehensively evaluate drug properties.

(2) PK models in ophthalmic preclinical PK studies

The distribution and elimination of drugs in different ocular tissues vary due to the influence of barriers, such as tears, the cornea, the blood-retinal barriers, and blood-aqueous barriers. The different ocular tissues can be recognized as separate compartments separated by barriers. The classic compartmental models could be applied to study the PK profiles for ophthalmic drugs.

Taking eyedrops as an example, the tears can be treated as a single-compartment model to study the PK of ophthalmic drugs in the tear. If a portion of the drug enters the body through the corneal barrier, the single-compartment model is not appropriate. In this case, it is recommended to use the anterior cornea, such as the tears, as one compartment and the cornea as the second compartment to perform data analysis using a two-compartment model. Moreover, in the study of aqueous humor and posterior segment tissues, based on the drug delivery process, a three-compartment model analysis can be performed with the anterior cornea as one compartment, the cornea as the second compartment, and the aqueous humor, vitreous, or retina as the third compartment (Figure 3).¹¹

Figure 3. Ophthalmic compartment models¹¹

Appropriate multi-compartment models are conducive to topical ocular PK studies. However, data modeling of this approach is based on different assumptions and is relatively complex and mathematically intricate. Improper use of this method may affect the accuracy of the data. In preclinical studies, non-compartment models are preferred to calculate ophthalmic PK parameters.

(3) Strategies for preclinical PK studies

Currently, there is no specific guidance from regulatory agencies for non-clinical PK research of ophthalmic drugs. The following suggestions for preclinical studies could be considered based on clinical guidance in China:

• For drugs using topical drug delivery (e.g., intravitreal injection), both topical and systemic exposure should be assessed.

• For drugs with systemic exposure or those administered through systemic administration, conventional systemic PK studies can be applied to assess systemic exposure and PK profiles such as drug distribution, metabolism, and elimination.

• For drugs distributed mainly in the ocular tissues and exerting drug efficacy within the eye, topical ocular PK evaluation should be performed, including the distribution, metabolism, and elimination processes of the drug in ocular tissues such as the aqueous and vitreous humor. Additionally, the distribution and target binding of drugs should be studied, with a focus on the systemic exposure of topical drug delivery, distribution, and accumulation of non-target organs, or drug distribution and target binding after systemic administration.

• The challenges of DMPK preclinical studies in ophthalmology include (1) the high demand for high-level animal experimental skills due to the delicate eye structures and the need for professional veterinary and anatomical teams to perform drug administration and ocular tissue collection; (2) the difficulty in ophthalmic sample bioanalysis is due to various ophthalmic collection matrices and the large number of samples with the form of small volumes and high sample handling requirements.

In early preclinical studies, comprehensive PK and pharmacodynamic studies should be conducted as much as possible. Regardless of systemic or ocular topical administration, PK studies need to be carried out according to the actual distribution of the drug.

(4) Case study: ophthalmic drug

WuXi AppTec DMPK has been conducting ophthalmic drug research since 2014, with more than 8 years of experience in ophthalmic drug PK studies. The research involves various animal species, including rodents, rabbits, dogs, and monkeys. We can offer multiple ocular drug deliveries (e.g., topical administration as eye drops, ointments or gels, anterior chamber injections, and intravitreal injections, allowing for precise anatomical dissection and collection of ocular tissues, followed by sample processing and bioanalysis.

Taking an eyedrop drug for the treatment of posterior segment diseases as an example, the drug’s concentration-time profile in the ocular tissues after a single instillation in Dutch-belted rabbits is shown (Figure 4).

Figure 4. Average tissue drug concentrations after a single topical administration in Dutch-belted rabbits, in anterior segment tissues (left) and posterior segment tissues (right)

The drug distribution shows a higher concentration in the anterior segment tissues and a lower concentration in the posterior segment tissues, which is consistent with the typical PK characteristics of topical eye formulations. Considering the efficacy test results, the lower drug concentration in the posterior segment tissues after topical administration has met the efficacy requirements. The results of this study provide data support for the drug to be administered through eyedrop formulation instead of eye injection, facilitating our clients’ drug development efforts.

Conclusion

Ocular diseases are usually chronic diseases, and their prevalence increases dramatically with age. It is expected that the population of ophthalmic patients will continue to grow due to factors such as population aging, excessive use of electronic devices, and air pollution. The rising prevalence of ophthalmic diseases and increased public health awareness are driving the growth of the ophthalmic drug market. We believe that ophthalmic drug research will become more comprehensive and patients with ophthalmic diseases will receive better treatment.

If you want to learn more details about the strategies for ophthalmology, please talk to a WuXi AppTec expert today to get the support you need to achieve your drug development goals.

Authors：Binbin Tian, Guang Yang, Jing Jin

Committed to accelerating drug discovery and development, we offer a full range of discovery screening, preclinical development, clinical drug metabolism, and pharmacokinetic (DMPK) platforms and services. With research facilities in the United States (New Jersey) and China (Shanghai, Suzhou, Nanjing, and Nantong), 1,000+ scientists, and over fifteen years of experience in Investigational New Drug (IND) application, our DMPK team at WuXi AppTec are serving 1,500+ global clients, and have successfully supported 1,200+ IND applications.