Recent progress in the syntheses and biological evaluation of boronated porphyrins for boron neutron-capture therapy

Boronated porphyrins are an important class of tumor-localizing agents in two bimodal therapies for cancer currently under study experimentally and clinically; boron neutron-capture therapy (BNCT) and photodynamic therapy (PDT). The desirable properties for the boronated porphyrins are that they are easily synthesized, pure and well-characterized drugs, and that in vivo, they are stable, tumor-specific, with high tumor:blood and tumor:normal tissue boron concentration ratios, and cause minimal toxicity. A large number of new porphyrins and their syntheses are presented herein. The focus is primarily on porphyrins published within the past 5 years, but the implications and trends from porphyrins studied in vivo over the past 15 years are also reviewed. Many possess quite unusual, novel structures and others have appended cell-targeting moieties for greater tumor specificity. Besides the commonly used closo- and nido-o-carboranes other boron cages and modes of attachment are presented. These boron cages can selectively alter the lipophilic, hydrophilic and amphiphilic properties of the porphyrins as well as their boron content. New delivery modalities have also greatly improved the targeting potential of compounds previously deemed unsuitable for applications in BNCT.