In vitro, ex vivo, and in vivo evaluation of ophthalmic ointments containing dexamethasone and tobramycin

This investigation compares <i>in vitro</i> release, <i>ex vivo</i> release and permeation, and <i>in vivo</i> ocular pharmacokinetics to render biologically informed evaluations of ophthalmic semi-solid drug products containing dexamethasone (hydrophobic) and tobramycin (hydrophilic). Both drugs were formulated with three petrolatum matrices (IGI® 320 A, IGI® 386, or Spectrum®) with distinct rheological character and benchmarked against the reference listed drug, Tobradex®. Temperature-sweep rheology revealed that IGI® 386 most closely reproduced the viscoelastic profile of the reference product. USP Apparatus I release testing with surfactant-free medium provided maximal discrimination for dexamethasone (Tobradex® > IGI® 320 A > IGI® 386 > Spectrum®), and rank-order release rates correlated strongly with <i>ex vivo</i> corneal permeation and <i>in vivo</i> corneal exposure. In contrast, tobramycin required a polysorbate-containing medium to resolve formulation differences <i>in vitro</i>, yet those differences did not persist <i>ex vivo</i> or <i>in vivo</i>, consistent with its rapid dissolution and diffusion, which attenuate matrix effects. The data demonstrate that drug solubility dictates the choice of biorelevant release conditions in petrolatum-based ophthalmic ointments: surfactant-free media capture formulation-dependent release for hydrophobic actives, whereas hydrophilic actives may yield artifactual discrimination when surfactant is present. However, formulations indistinguishable <i>in vitro</i> were typically similar in their <i>in vivo</i> ocular pharmacokinetics. By integrating tiered models, the framework enhances understanding of critical quality attributes, supports regulatory decision-making, and may help reduce reliance on animal studies, thereby expediting the development of therapeutically equivalent generic ophthalmic ointments.