Proportion of the dose required by the physiologically-based pharmacokinetic (PBPK) model to be sublingually absorbed to exactly recover the (a) area under the curve (AUC; i.e., AUC0– ∞ and AUC0–τ for single and multiple dose studies, respectively) and (b) peak concentration (Cmax) observed in the clinical trial (i.e., ideal proportion) across dose. Blue and orange circles (●), triangles (▲), and diamonds (◆) represent sublingual tablet and solution data obtained from Harris et al. [26], Schuh and Johanson [23], and Dong et al. [28] respectively. Blue and orange dotted lines represent linear regression models with logarithmically transformed dose for buprenorphine tablets and solution, respectively. Respective shaded areas represent the 95% confidence interval (CI) of the regression models. Associated linear-log equations are shown in the upper right corners (where dose is in milligrams and logarithm base is 10), with coefficients of determination (R2) shown in the lower-left corners. The final buprenorphine PBPK model uses the average of the AUC- and Cmax‑optimized equations, i.e., proportion sublingually absorbed equals 38.1 – 19.7 × log(Dose) and 53.3 – 25.6 × log(Dose) for sublingual tablets and solution, respectively.