Background/Objectives: Antibody-based therapies often exhibit limited distribution within solid tumors due to the “binding-site barrier” (BSB). Our group has developed and validated the use of anti-idiotypic distribution enhancers (AIDEs), which transiently block antibody binding, improving intra-tumoral distribution and efficacy. This study evaluated 1HE and LG1, model anti-trastuzumab AIDEs, in combination with trastuzumab–PE24, a highly potent immunotoxin. Methods: The effects of 1HE on the whole-body disposition of radiolabeled trastuzumab were assessed in NCI-N87 tumor-bearing mice. Mechanistic pharmacokinetic/pharmacodynamic (PK/PD) modeling was employed to explore how AIDE binding kinetics influence antibody intra-tumoral distribution and immunotoxin potency. Trastuzumab–PE24 was developed by site-specific conjugation, enabled by self-splicing split intein, with cytotoxicity tested on various cell lines in vitro. The impact of 1HE and LG1 coadministration on trastuzumab–PE24 efficacy was evaluated in NCI-N87 xenograft-bearing mice. Results: 1HE coadministration decreased trastuzumab tumor maximum concentration, reducing tumor terminal slope by 8% and overall tumor exposure by 2.6%, without negatively affecting selectivity. Modeling predicted the optimal AIDE dissociation rate constant for trastuzumab–PE24 to be between 0.015 and 0.3 h⁻¹. The coadministration of trastuzumab–PE24 with 1HE and LG1 improved anti-tumor efficacy and extended median survival to 60 days (p = 0.0002). Conclusions: AIDE coadministration led to minimal negative impacts on overall tumor exposure, consistent with model simulations. AIDE coadministration improved the efficacy of trastuzumab–PE24 in NCI-N87 xenografts. Modeling further predicted that repeated AIDE administration with trastuzumab–PE24 could induce complete tumor regression. These findings highlight the advantages of the AIDE strategy, particularly when coadministered with highly potent immunotoxins.