Lowmolecularweight Delivery Agents

In the 1950s and early 1960s clinical trials of BNCT were carried out using boric acid and some of its derivatives as delivery agents. These simple chemical compounds had poor tumor retention, attained low T:Br ratios and were nonselective.26,27 Among the hundreds of low-molecular-weight (LMW) boron-containing compounds that were synthesized, two appeared to be promising. One, based on arylboronic acids,28 was (l)-4-dihydroxy-borylphenylalanine, referred to as boronophe-nylalanine or BPA (Figure 6.1, 1). The second, a polyhedral borane anion, was sodium mercaptoundecahydro-closo-dodecaborate,29 more commonly known as sodium borocaptate or BSH (2). These two compounds persisted longer in animal tumors compared with related molecules, attained T:Br and T:Bl boron ratios greater than 1 and had low toxicity. 10B-enriched BPA, complexed with fructose to improve its water solubility, and BSH, have been used clinically for BNCT of brain, as well as extracranial tumors. Although their selective accumulation in tumors is not ideal, the safety of these two drugs following intravenous (i.v.) administration has been well established.30,31

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