Mayssa Attar

Pharmacokinetics and Pharmacodynamics of a Sustained-Release Dexamethasone Intravitreal Implant

PURPOSE To determine the pharmacokinetics and pharmacodynamics of a sustained-release dexamethasone (DEX) intravitreal implant (Ozurdex; Allergan, Inc.). METHODS Thirty-four male monkeys (Macaca fascicularis) received bilateral 0.7-mg DEX implants. Blood, vitreous humor, and retina samples were collected at predetermined intervals up to 270 days after administration. DEX was quantified by liquid chromatography-tandem mass spectrometry, and cytochrome P450 3A8 (CYP3A8) gene expression was analyzed by real-time reverse transcription-polymerase chain reaction. RESULTS DEX was detected in the retina and vitreous humor for 6 months, with peak concentrations during the first 2 months. After 6 months, DEX was below the limit of quantitation. The C(max) (T(max)) and AUC for the retina were 1110 ng/g (day 60) and 47,200 ng · d/g, and for the vitreous humor were 213 ng/mL (day 60) and 11,300 ng · d/mL, respectively. The C(max) (T(max)) of DEX in plasma was 1.11 ng/mL (day 60). Compared with the level in the control eyes (no DEX implant), CYP3A8 expression in the retina was upregulated threefold up to 6 months after injection of the implant (0.969 ± 0.0565 vs. 3.07 ± 0.438; P < 0.05 up to 2-month samples). CONCLUSIONS The in vivo release profile of the DEX implant in an animal eye was similar to the pharmacokinetics achieved with pulse administration of corticosteroids (high initial drug concentration, followed by a prolonged period of low concentration). These results are consistent with those in clinical studies supporting the use of the DEX implant for the extended management of posterior segment diseases.

Ophthalmic drug delivery considerations at the cellular level: drug-metabolising enzymes and transporters.

Ophthalmic drugs typically achieve < 10% ocular bioavailability. A drug applied to the surface of the eye may cross ocular–blood barriers where it may encounter metabolising enzymes and cellular transporters before it distributes to the site of action. Characterisation of ocular enzyme systems and cellular transporters and their respective substrate selectivity have provided new insight into the roles these proteins may play in ocular drug delivery and distribution. Altered metabolism and transport have been proposed to contribute to a number of ocular disease processes including inflammation, glaucoma, cataract, dry eye and neurodegeneration. As ocular enzyme and transport systems are better characterised, their properties become an integral consideration in drug design and development.

Intracameral Sustained-Release Bimatoprost Implant Delivers Bimatoprost to Target Tissues with Reduced Drug Exposure to Off-Target Tissues

Abstract Purpose: To explore the ocular distribution of bimatoprost after intracameral administration of a biodegradable sustained-release bimatoprost implant (Bimatoprost SR) versus repeated topical administration of bimatoprost 0.03% ophthalmic solution in dogs. Bimatoprost SR and topical bimatoprost 0.03% previously were shown to have similar intraocular pressure-lowering effects in humans in a phase 1/2 clinical trial. Methods: Twenty-four beagle dogs received either once-daily topical bimatoprost 0.03% for 7 days or a bilateral intracameral administration of Bimatoprost SR (15 μg). At predetermined time points, ocular tissues were collected and concentrations of bimatoprost and bimatoprost acid were quantified using liquid chromatography–tandem mass spectrometry. Results: Bimatoprost SR administration enhanced delivery of study drug to a site of action [iris–ciliary body (ICB)] compared with topical bimatoprost (Cmax [bimatoprost+bimatoprost acid] = 18,200 and 4.13 ng/g, respectively). However, distribution of drug to tissues associated with prostaglandin analog (PGA)-related side effects (i.e., bulbar conjunctiva, eyelid margins, and periorbital fat) was limited following Bimatoprost SR administration (Cmax [bimatoprost+bimatoprost acid] = BLQ [beneath the limit of quantitation] to 0.354 ng/g) compared with topical dosing (Cmax [bimatoprost+bimatoprost acid] = 36.6–2,110 ng/g). Conclusions: Bimatoprost SR administration in dogs selectively delivered drug to the ICB with low or undetectable drug levels in ocular surface and extraocular tissues. Use of Bimatoprost SR for glaucoma treatment may reduce the incidence of adverse events typically associated with topical PGAs by targeting bimatoprost delivery to the key site of action of the PGA class and reducing exposure to off-target tissues.

Ocular pharmacokinetics and tolerability of bimatoprost ophthalmic solutions administered once or twice daily in rabbits, and clinical dosing implications

Purpose Fixed-combination medications can benefit patients requiring multiple agents to lower their intraocular pressure (IOP), but combining agents with complementary mechanisms of action is challenging if their dosing frequency differs. This study compares in vivo pharmacokinetic and ocular tolerability of bimatoprost 0.01% ophthalmic solutions dosed once or twice daily. Reports of twice-daily dosing in glaucoma patients are also reviewed. Methods New Zealand White rabbits were administered bimatoprost 0.01% monotherapy or fixed-combination bimatoprost 0.01%/brimonidine 0.1%, once or twice daily in both eyes for 4 days. Ocular tissues were harvested and analyzed by liquid chromatography-tandem mass spectrometry. The pharmacokinetic parameters calculated included maximum observed concentration, time to maximum concentration, and area under the concentration-time curve. Results Due to extensive metabolism, bimatoprost concentration was below the quantitation limit by 1 hour post-dose in all samples. Bimatoprost acid exposure, however, could be measured up to 6–8 hours post-dose and was similar in the aqueous humor and iris-ciliary body (pharmacological site of action) of animals treated once or twice daily with either bimatoprost 0.01% or fixed-combination bimatoprost 0.01%/brimonidine 0.1%. Increasing dosage frequency in rabbits did not raise the incidence of drug-related conjunctival hyperemia (most common adverse event associated with bimatoprost use in humans), suggesting comparable ocular tolerability of the once- and twice-daily regimens for each formulation. Conclusion Bimatoprost 0.01% administered once or twice daily as monotherapy and in fixed-combination with brimonidine 0.1% in rabbits show similar pharmacokinetic profiles of bimatoprost acid, especially in the iris-ciliary body. Key findings from previous clinical studies suggest that by varying the concentration of benzalkonium chloride (a preservative with corneal penetration-enhancing properties), formulations of bimatoprost 0.01% can be administered once or twice daily. These findings support development of bimatoprost 0.01%-based fixed-dose combination therapies administered twice daily for patients who require multiple adjunctive medications to control their IOP.