Intraoral (IO) administration is a unique route that takes advantage of transmucosal absorption in the oral cavity to deliver a drug substance locally or systemically. IO delivery can also enhance or enable oral administration, providing a better therapeutic benefit/safety risk profile for patient compliance. However, there are relatively few systematic biopharmaceutics assessments for IO delivery to date. Therefore, the goals of this study were to i) identify the most relevant in vitro permeability models as alternatives to porcine oral tissues (gold standard) for predicting human IO absorption and ii) establish guidelines for biopharmaceutics assessment during early drug development for IO delivery. Porcine kidney LLC-PK1 cells provided the strongest correlation of transmucosal permeability with porcine oral tissues followed by human Caco-2 cells. Furthermore, cultured human buccal tissues predicted high/low permeability classification and correlated well with porcine oral tissues, which are used for predicting clinical IO absorption. In the meantime, we introduced maximum absorbable dose and dose number in the oral cavity for IO delivery assessment as well as a decision tree to provide guidance for biopharmaceutics assessment during early drug development for IO delivery.