# Pharmacokinetics of Pullulan-Dexamethasone Conjugates in Retinal Drug Delivery
## 3 Results
### Pharmacokine Simulations
Calculation based on both Vv (932 and 1150 uL) reveal anterior route as the main elimination pathway for pullulan conjugate. In Figure 7, three volumes of distribution (932, 1150 and 1700 uL) were used to derive the lines, indicating anterior chamber as the only elimination route. Concentrations of free dexamethasone were simulated based on the pullulan conjugate retention in the eye and the published [26] release rate of dexamethasone from dexamethasone-pullulan conjugates. The simulated levels of free dexamethasone after intravitreal injection of BDP-pullulan-DEX in rats and rabbits are presented in Figure 8A,B. The simulated dexamethasone concentrations in the vitreous remain above the minimal active concentration 1 nM (or 0.394 ng/mL) [54] for 3.6, 16.5 and 25.5 days in the rats, rabbits and humans, respectively (Figure 8A-C). The simulations also reveal that only a small fraction of injected dexamethasone in the conjugate is released during the residence time of the polymeric conjugate in the eye (Table 3). Interestingly, the concentrations in the vitreous and aqueous humor differ more in the rabbits and humans than in the rats. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Parameter** & **Unit** & **Rat** & **Rabbit** & **Human** \\ \hline C\({}_{\text{max}}\) Vitreous & ng\(\cdot\)mL\({}^{-1}\) & 14 & 58 & 24 \\ T\({}_{\text{max}}\) Vitreous & h & 5 & 15 & 24 \\ C\({}_{\text{max}}\) Aqueous Humor & ng\(\cdot\)mL\({}^{-1}\) & 5 & 5 & 6 \\ T\({}_{\text{max}}\) Aqueous Humor & h & 5 & 15 & 24 \\ Aqueous humor/vitreous concentration ratio a & & 0.35 & 0.086 & 0.23 \\ Duration above minimal effective concentration & day & 3.6 & 16.5 & 25.5 \\ Dose of BDP-pullulan-DEX per eye & ug & 30 & 500 & 500 \\ Percent of the released DEX in the vitreous & \% & 0.8 & 2.5 & 4.7 \\ Percent of the released DEX in the aqueous humor & \% & 0.028 & 0.05 & 0.04 \\ \hline \hline \end{tabular}  
* Averaged at pseudo-steady state phase.  
\end{table} 
Table 3: Pharmacokine parameters of dexamethasone derived from simulations of intravitreally injected BDP-pullulan-DEX to rat, rabbit and human eyes. The dose of dexamethasone was 3, 50 and 50 ug per eye for rats, rabbits and humans, respectively. Figure 7: Maurice plot of intravitreally administered compounds in the rabbit eyes. The plot shows anteriorly eliminating compounds based on literature data: sucrose (0.342 kDa) [51], FITC-dextran (FD-10.5, FD-67 and FD-157 kDa) [52; 53]. The green, red and blue line are derived from Maurice equation by assuming 932, 1150 and 1700 uL as virtual volume of distribution. Location of BDP-pullulan-DEX NP (\(\sim\)75 kDa) data at close vicinity of the straight lines indicates anterior route of elimination in the rabbit eyes as the main elimination pathway. Figure 8: Simulation of released dexamethasone concentration in the vitreous and in the aqueous humor after intravitreal injection of BDP-pullulan-DEX in rat (**A**), rabbit (**B**) and human (**C**). The simulated doses of BDP-pullulan-DEX were 30, 500 and 500 \(\upmu\)g per eye for rats, rabbits and humans, corresponding to DEX doses of 3, 50 and 50 \(\upmu\)g per eye for rats, rabbits and humans, respectively. The dotted line (- - -) shows the minimum effective intravitreal concentration of dexamethasone for inhibiting the expression of VEGF, which is 1 nM (or 0.394 ng/mL) [54].