Development of Macrocyclic Peptide-Based Proteasome Inhibitors with Enhanced Blood-Brain Barrier Penetration for Treating Brain Neoplasms

Proteasome inhibitors are effective in treating hematologic cancers but have limited utility in brain tumors due to poor blood-brain barrier (BBB) penetration and metabolic instability. In this study, we developed novel macrocyclic peptide epoxyketone inhibitors with improved drug-like properties. Compounds were screened for cytotoxicity against brain cancer cell lines, permeability (PAMPA-BBB and Caco-2), and metabolic stability. Lead compound <b>10</b> demonstrated potent in vitro activity (IC₅₀ < 100 nM), low P-gp efflux, and favorable microsomal and plasma stability. In vivo pharmacokinetic studies in mice showed that compound <b>10</b> maintained therapeutic plasma levels and achieved measurable brain concentrations without toxicity. Co-administration of a P-gp inhibitor significantly enhanced brain exposure of compound <b>35</b>, confirming efflux as a key parameter. The incorporation of fluorinated phenyl and α,α-dimethylglycine moieties contributed to improved BBB permeability and metabolic stability. These findings support further development of macrocyclic epoxyketone inhibitors as promising candidates for brain cancer therapy.