Ocular Pharmacokinetic Research Challenges
2012 - MPI Research
There are multiple challenges that can be encountered when conducting ocular pharmacokinetic (PK) research. These challenges often involve three areas:
Dissection of Ocular Tissue
Consider the situation where a drug is applied to the eye by topical administration. The drug will be distributed to tissues down the concentration gradient from front to back, and from the outside to inside of the anterior chamber as well. Any drug applied in this way reaches the posterior tissues, such as choroid and retina, by moving into and through the conjunctiva, fornix, sclera, and choriocapillaris. Drug flux from compartment-to-compartment is constrained by the unique anatomy of the eye and is facilitated by blood flow in the choriocapillaris. In addition to the concentration gradient and blood flow, physico-chemical properties of the drug such as water solubility and LogP are also clearly important for drug movement, and need to be considered.
Proper dissection of ocular tissues requires experience, precision, and extensive knowledge of ocular anatomy. Significant differences in drug concentrations in adjoining tissues and fluids are often observed in ocular tissues for a myriad of reasons. A lack of attention to these details can result in cross-contamination between compartmentalized tissues that are physically close. For example:
- Formulations for topical ocular administration contain relatively high drug concentrations. This leads to very high drug concentrations in the tear and surface tissues, such as cornea and conjunctiva, at early time points.
- Large drug concentration gradients between tissues can also occur due to the compartmentalized and diverse tissues and fluids encountered in the eye. The sclera is a highly fibrous matrix with water and few cells, the choroid is highly vascular, and the retina consists of neurons and supporting cells. It is not surprising that drug concentration differences between these tissues can be considerable.
- Without proper dissection technique, tissues and fluids of relatively low drug concentration, such as the trabecular meshwork, aqueous humor or vitreous, can become contaminated from the high drug concentration tissues. If the target tissue is the retina, the drug concentration in the adjacent choroid may be considerably higher than in the retina. Consequently, the dissection of the retina must be performed while maintaining scrupulous separation from the high drug concentrations in the choroid. This is a delicate and essential component of the ocular PK process.
Ocular Tissue and Fluid Processing
Tissue preparation can be problematic with respect to homogenization and extraction efficiency of drug from different matrices. The eye contains significant connective tissue and the homogenization of these tissues, without inadvertent heating of the sample, can be challenging. Important considerations include:
- A tissue from one species (conjunctiva, for example) may differ from another species in drug extraction efficiency. Specific dedicated equipment is recommended and difficult tissues can be homogenized using glass beads, cooling, and special containers.
- Specific matrices used to generate standard samples must be carefully weighed. If a drug is to be dosed in rabbits, then collecting control tissues from this species can be costly because of the small amounts available from single animals. An economical option is to run standards using tissues from larger species that have comparable extraction efficiency to the test species.
Lower Limit Of Quantitation (LLOQ) requirements can be problematic in ocular PK studies due to the small amounts of certain tissues (or fluids) that can be collected from any single animal. For example, the wet weight of rabbit trabecular meshwork (TM) is about 0.25 mg (a volume of 0.25 mL), which is over 100-fold less sample volume than what is usually available for standard quantitative assays of drug in plasma. In real numbers, to analyze drug in TM, a quantifiable signal must be obtained on 0.050 pg (50 femtograms) for a LLOQ requirement of 100 pg/mL (injection of the entire sample). A highly skilled analyst and stringent attention to detail are requirements for obtaining this level of sensitivity.
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