P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are major efflux transporters that contribute to multidrug resistance and limit intestinal absorption. Gefitinib is an EGFR tyrosine kinase inhibitor and a substrate of P-gp and BCRP. In our previous study, CDK 4/6 inhibitor IV, BX795, and CGP60474 were identified as dual P-gp/BCRP inhibitor candidates. This study evaluated the effects of these novel inhibitors on gefitinib oral absorption. A modified LC-MS/MS method for gefitinib was validated. The assay showed excellent linearity over the concentration range of 2–200 ng/mL and met FDA acceptance criteria. Pharmacokinetic studies were conducted in rats after intravenous injection of gefitinib (2 mg/kg) or oral administration of gefitinib (5 mg/kg), either alone or in combination with elacridar (positive control) or each dual inhibitor candidate (10 mg/kg). Non-compartmental analysis indicated that CDK 4/6 inhibitor IV, BX795, and elacridar significantly increased AUC_last and oral bioavailability compared with gefitinib alone. Among the tested compounds, CDK 4/6 inhibitor IV produced the highest AUC, C_max, and oral bioavailability. These findings suggest that CDK 4/6 inhibitor IV and BX795 are promising dual P-gp/BCRP inhibitors for enhancing the oral absorption of gefitinib and support the potential of intestinal dual P-gp/BCRP inhibition as a strategy to improve systemic exposure of orally administered anticancer drugs.