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The 26S proteasome is an essential regulator of protein homeostasis and a clinically validated therapeutic target in multiple myeloma (MM). Rapaprotin, a novel macrocycle identified from a rapamycin-inspired rapafucin library, disrupts 26S proteasome function by inducing disassembly of the 19S regulatory particle in the 26S proteasome, leading to apoptosis in MM cells. Its bioactivation requires prolyl endopeptidase (PREP)-mediated cleavage to generate Rapaprotin-L, a negatively charged, linear metabolite with potent proteasome-disassembly activity. Using the PRISM cancer cell line profiling platform, we identified high P-glycoprotein (P-gp/ABCB1) expression as a major determinant of Rapaprotin resistance in solid tumor cell lines. Efflux assays confirmed Rapaprotin-L, but not its parent Rapaprotin, as a high-efficiency P-gp substrate. Co-treatment with the third-generation P-gp inhibitor tariquidar restored the intracellular accumulation of Rapaprotin-L, reinstating proteasome inhibition and consequent apoptosis in Rapaprotin-resistant colorectal cancer cell lines. Strong synergy between Rapaprotin and tariquidar was observed in a 3D spheroid model. These results establish P-gp as a key mediator of resistance to Rapaprotin and identify a rare example of a negatively charged Rapaprotin-L as a P-gp substrate. Together, these findings expand the potential therapeutic scope of Rapaprotin beyond hematologic malignancies to a broader range of solid tumors.