In comparison with pristine sinomenine and carborane precursors, the calculations of molecular docking with matrix metalloproteinases (MMPs) and methylcarboranyl-n-butyl sinomenine showed improved interactions. Accordingly, methylcarboranyl-n-butyl sinomenine shows a high potential in the treatment of rheumatoid arthritis (RA) in the presence of slow neutrons. The reaction of potassium salt of sinomenie, which is generated from the deprotonation of sinomenine (1) using potassium carbonate in a solvent of N,N-dimethyl formamide, with 4-methylcarboranyl-n-butyl iodide, (2) forms methylcarboranyl-n-butyl sinomenine (3) in 54.3% yield as a new product. This new compound was characterized by (1)H, (13)C, and (11)B NMR spectroscopy, FT-IR spectroscopy, and elemental analyses to confirm its molecular composition. In addition to molecular docking interactions with MMPs, the in vitro killing effects of 3, along with its toxicity measurements, exhibited its potential to be the new drug delivery agent for boron neutron capture synovectomy (BNCS) and boron neutron capture therapy (BNCT) for the treatment of rheumatoid arthritis (RA) and cancers in the presence of slow neutrons, respectively.