# Pharmacokinetics of Pullulan-Dexamethasone Conjugates in Retinal Drug Delivery
## 4 Discussion
A bigger size of the rabbit eye explains the longer half-life in rabbits, since it takes longer time for material transfer from the vitreous to the anterior chamber [58]. In humans, the volume of vitreous is about three times (\(\approx\)4.5 mL) bigger than in rabbits (\(\approx\)1.5 mL) [59], leading to even longer retention. Possible effects of inflammation on the vitreal kinetics of pullulan conjugates are not known. Based on the literature it is not likely that clearance of released dexamethasone would change due to inflammation [60]. But infiltrated macrophages due to ocular inflammation [61] might take up the particles [62], leading to faster drug release in the lysosomal conditions in the cells. _Interplay of polymer conjugate retention and drug release._ In this study, we used dexamethasone- some conjugate of pullulan, but did not measure concentrations of free pharmacologically active dexamethasone in the vitreous. We simulated free dexamethasone concentrations in the vitreous and aqueous humor before designing and performing labor intensive, expensive and analytically demanding in vivo studies to determine free and bound dexamethasone concentrations in the eye. Kinetic simulations were based on experimental kinetics of pullulan conjugates and in vitro release of dexamethasone from the conjugates [26]. For the first time in scientific literature, these simulations integrate the drug release rate and pharmacokinetics of particulate intravitreal drug delivery system. The results demonstrate that longer effective residence of dexamethasone in the rabbit vitreous (up to 16.5 days in rabbits and 25.5 days in humans) is clearly longer than the half-life of the delivery system (3.6 days) in the rabbit vitreous. This is possible because the residual nanoparticles still release dexamethasone that is active at small concentrations. At the threshold limit of dexamethasone activity (0.394 ng/mL) [54], the simulated amount of free drug in the eye is in the range of 1-4 ng in rabbit and human eyes. This is only less than 0.001% of the injected dexamethasone dose within the pullulan conjugate. Overall, this demonstrates the power of controlled release technology in intravitreal drug delivery and possibilities for further optimization. The minimum effective dexamethasone concentration may be different at different pathological conditions. Here we chose 1 nM as the minimum therapeutic concentration [54]. This concentration of dexamethasone should inhibit expression of vascular endothelial growth factor in the human vascular smooth muscle cultured cells. The concentration of endogenous cortisol is 14 nM in the human vitreous [63]. Since the anti-inflammatory potency of dexamethasone is about 25 times higher than that of cortisol [64], dexamethasone should be anti-inflammatory at 1 nM concentration. After administration of dexamethasone implant (dose 700 \(\mu\)g) to the vitreous of rhesus monkeys, the levels of dexamethasone remained above 1 nM for 3 months, but the efficacy may be extended even at lower levels of dexamethasone [65]. Thus, efficacy of pullulan conjugates may extend beyond 25 days as reported here, and this technology may be further developed for more extended ocular retention. Interestingly, our simulations on retention and release revealed that a major fraction of the conjugated dexamethasone dose will be eliminated from the eye as pullulan conjugate form (>99%), yet the drug action may be significantly prolonged. Ocular exposure to the released drug can be defined as intraocular bioavailability. Intraocular bioavailability of the released drug could be improved by increasing drug release rate (e.g., modification of chemical linker) and prolonging polymer-conjugate retention in the vitreous (e.g., increasing binding with vitreal components and molecular weight of polymer). The release and retention profiles of pullulan conjugated dexamethasone are very different from the intravitreal dexamethasone implants (e.g., Retisert).