Ocular & Intrauterine Drug Delivery Systems
This unit explores two highly localized routes for controlled drug delivery. Ocular Drug Delivery Systems address the significant challenge of intraocular barriers (rapid tear drainage, corneal impermeability) and introduce advanced ophthalmic formulations like Ocuserts. Intrauterine Drug Delivery Systems (IUDs) examines the development, advantages, and long-term applications (primarily contraception) of localized delivery devices placed directly inside the uterus.
Syllabus & Topics
- 1Ocular Drug Delivery Systems (ODDS): Introduction: The eye is a highly protected organ. Conventional eye drops represent over 90% of ophthalmic formulations but are highly inefficient. When an eye drop (approx. 50 µL) is instilled, the normal tear volume is only 7 µL. Therefore, reflex blinking and precorneal fluid drainage (nasolacrimal drainage) immediately wash away the vast majority of the dose. Typically, less than 5% of the instilled drug penetrates the cornea to reach intraocular tissues. Rationale for ODDS: (1) Increase precarious residence time of the drug. (2) Decrease systemic absorption (which causes side effects, e.g., systemic beta-blocker effects from timolol eye drops). (3) Reduce frequency of administration. (4) Improve patient compliance.
- 2Intraocular Barriers & Overcoming Them: Barriers to absorption: (1) Precorneal tear film and rapid drainage (nasolacrimal duct). (2) Reflex blinking (washes away bulk of drop). (3) Corneal barrier (Cornea has a hydrophobic epithelium, hydrophilic stroma, hydrophobic endothelium – making it a ‘fat-water-fat’ layer. Drug must have balanced hydrophilic-lipophilic properties to cross). (4) Blood-aqueous block barrier. Methods to overcome barriers: (1) Viscosity enhancers: Adding polymers (HPMC, PVA, CMC) to eye drops to thicken the liquid, reducing drainage rate. (2) Prodrugs: Increasing lipophilicity (e.g., dipivefrin, a prodrug of epinephrine, penetrates cornea 17x better). (3) In situ gels: Liquid formulation that undergoes a phase transition to a gel upon reaching the eye (triggered by physiological temperature, pH, or ions), providing sustained residence. (4) Particulate systems: Micro/nanoparticles suspended in drops. (5) Ocular inserts (Ocuserts).
- 3Ocular Formulations & Ocuserts: Ocuserts (Ocular Inserts): Sterile, solid, or semisolid preparations placed in the cul-de-sac or conjunctival sac to provide prolonged drug release. Example: Ocusert® Pilo (Pilocarpine used for glaucoma). Structure: An elliptical, multi-layered device based on an elementary osmotic or diffusion pump design. It consists of a central reservoir of pilocarpine surrounded by two rate-controlling transparent polymer membranes (ethylene vinyl acetate – EVA), surrounded by a white retaining ring for visibility. Mechanism: Tear fluid diffuses through the EVA membrane into the core, dissolving the drug, which then diffuses outward at a continuous, steady zero-order rate. Advantages: Constant therapeutic effect 24 hours a day for 7 days (replacing daily 4x frequent eye drops), reduces total dose applied, virtually eliminates systemic side effects. Disadvantages: Foreign body sensation in the eye, device expulsion without patient realizing, difficulty in insertion.
- 4Intrauterine Drug Delivery Systems (IUDs): Introduction: IUDs are small, flexible (often T-shaped) medical devices inserted into the uterine cavity by a healthcare professional. Although initially used as pure physical barriers/irritants for contraception, modern IUDs serve as advanced, localized, controlled-release drug delivery platforms. Advantages: (1) True localized action: Provides extremely high local drug concentrations directly at the target tissue (endometrium) while minimizing systemic circulation levels. (2) Highly effective contraception (failure rate < 1%). (3) Reversible. (4) Long-term compliance: Once inserted, requires no daily adherence from the patient (set-and-forget). Disadvantages: Requires trained professional for insertion and removal, risk of pelvic inflammatory disease (PID) upon insertion, risk of uterine perforation or expulsion, menstrual irregularities (heavy bleeding with copper, unpredictable spotting with hormonal).
- 5Development of IUDs: Three generations of development: (1) First Generation (Inert/Non-medicated IUDs): E.g., Lippes Loop. Made of biologically inert plastic (polyethylene). Functioned solely by creating a sterile local inflammatory response in the uterus, making the environment hostile to sperm and implantation. High rates of bleeding/pain led to obsolescence. (2) Second Generation (Copper-bearing IUDs): E.g., Copper T 380A. The plastic T-frame is wrapped with copper wire. Copper ions continuously dissolve, exerting a direct spermicidal effect (toxic to sperm motility and viability) and amplifying the inflammatory response. Effective for 10-12 years. (3) Third Generation (Hormone-releasing IUDs – Medicated): Function as true active drug delivery systems. Instead of copper, they release a continuous, low, zero-order dose of a synthetic progestin (levonorgestrel) from a polymer reservoir on the stem of the T.
- 6Applications & Medicated IUD Examples: Progestasert: Was the first hormonal IUD, releasing progesterone (65 µg/day) from an EVA membrane reservoir. Lasted 1 year. Required frequent replacement. Mirena (Levonorgestrel-releasing intrauterine system – LNG-IUS): The gold standard hormonal IUD. The vertical stem contains a reservoir of levonorgestrel dispersed in a polydimethylsiloxane (silicone) matrix, covered by a rate-controlling membrane. Releases 20 µg of LNG per day. Mechanism: Thickens cervical mucus (blocking sperm entry), suppresses endometrial growth, and impairs sperm motility. Efficacy lasts 5-7 years. Applications of modern IUDs: (1) Primary: Long-acting reversible contraception (LARC). (2) Non-contraceptive benefits (especially LNG-IUS): Treatment of heavy menstrual bleeding (menorrhagia), dysmenorrhea, management of endometriosis, and providing localized endometrial protection during estrogen replacement therapy in menopausal women.
Learning Objectives
Exam Prep Questions
Q1. Why is the cornea described as a “fat-water-fat” barrier, and what does this mean for drug formulation?
The cornea consists of three main layers: Epithelium (highly lipophilic/fatty), Stroma (highly hydrophilic/watery, taking up 90% of corneal thickness), and Endothelium (highly lipophilic). Because of this alternating lipid–aqueous–lipid structure, a drug that is purely hydrophilic cannot cross the epithelium, and a drug that is purely lipophilic gets stuck in the stroma. To successfully penetrate the cornea, an ophthalmic drug must have balanced partition coefficient (both lipid and water solubility properties).
Q2. What is an “in situ” gel in ocular delivery?
An in situ (in place) gel is a liquid ophthalmic formulation that is easily instilled into the eye like regular drops. However, upon contact with the ocular environment, it undergoes an immediate phase transition into a semi-solid viscous gel. This transition is triggered by specific physiological factors in the eye: either a change in temperature (temperature-responsive polymers), a sudden change to ocular pH ~7.4 (pH-responsive), or the presence of ions in tears like Ca²⁺ or Na⁺ (ion-responsive). The resulting gel strongly resists drainage, dramatically increasing residence time and absorption.
Q3. What advantage does a hormonal IUD (like Mirena) have over oral contraceptive pills?
Constant delivery: Mirena releases levonorgestrel continuously (zero-order) directly into the uterus, avoiding peaks and troughs.
Localized action vs Systemic: Because it acts locally, systemic blood levels of levonorgestrel are much lower than with oral pills, minimizing hormonal side effects (mood changes, weight gain).
Compliance: Oral pills must be remembered daily; Mirena works continuously for 5–8 years without any patient effort.
Menorrhagia: It effectively reduces heavy menstrual bleeding, a benefit not reliably seen with all oral pills or copper IUDs.