l -Boronophenylalanine ( l- BPA) Dipeptide Prodrugs Enhance Boron Delivery to Tumors, Facilitate Tumor Eradication and Induce Tumor Vaccine Effect in Mice Following Neutron Irradiation

Boron neutron capture therapy (BNCT) is an emerging cancer treatment modality that has seen a resurgence due to its efficacy and the advent of compact neutron beam devices. BNCT is a binary cancer treatment that combines selective boron delivery with epithermal neutron irradiation. When ¹⁰B-enriched compounds accumulate in tumor cells and capture low-energy neutrons, the reaction generates high-linear energy transfer (LET) α-particles and ⁷Li nuclei that deposit their energy within a few microns, producing irreparable DNA double-strand breaks and tumor cell death while sparing normal tissues. The clinical success of BNCT critically depends on the selective delivery of sufficient boron to tumors. Currently, 4- l-boronophenylalanine (l-BPA) is the leading BNCT agent, as it exploits LAT-1 (SLC7A5), a transporter highly expressed in many solid tumors. However, l-BPA suffers from poor solubility, severely limiting the maximum intravenous dose that can be administered and thereby constraining therapeutic efficacy. Here we report the synthesis and evaluation of highly soluble ¹⁰B-enriched l-BPA-based dipeptides as next-generation boron delivery agents. These dipeptides, incorporating hydrophilic peptide linkages, enabled substantially higher intravenous bolus doses in mice compared to l-BPA while maintaining favorable biodistribution. In vivo, the dipeptides were rapidly cleaved by endogenous proteases, releasing l-BPA de novo within tumors. Neutron irradiation of syngeneic CT26 tumors in dipeptide-treated mice produced complete and durable tumor regressions, accompanied by immunological effects including a vaccine response and abscopal tumor suppression. By contrast, l-BPA administered at its solubility-limited maximal dose under the same irradiation conditions produced only a transient tumor growth delay, without durable regression or evidence of immune activation. These findings establish the dipeptides as prodrugs that overcome the solubility limitations of l-BPA, enabling higher-dose administration with improved tumor control and immune-mediated benefits. This strategy has the potential to expand patient management options and enhance the therapeutic impact of BNCT.