There are tumors that are formed in various tissues and organs whose development and rate of proliferation are strongly hormone-dependent. For example, the steroid hormones have receptors that are localized in the cell's nucleus,
Figure 2.36 Some nucleoside derivatives the key target for high LET radiation. It was this observation that prompted the exploration of hormones as BNCT agents. Sweet synthesized the first carboranyl derivative of estradiol (Figure 2.37).81
Subsequently, Wellmann and Gabel have synthesized boron analogues of hormonal antagonists and other estrogenic and androgenic compounds.82 One of the advantages of these boronated steroid hormones is their relatively low
Figure 2.38 Boronated hormonal antagonists
Figure 2.38 Boronated hormonal antagonists molecular weights, offering the potential for the rapid targeting of tumor cells in contrast to other growth factors. Key requirements for such compounds are that they be chemically stable under in vivo conditions, be non-toxic and retain their hormonal activities. Several of these esterogen-related compounds have demonstrated the anticipated specificity for malignant cells; however, the maximum number of 10B atoms that could be achieved with these carboranyl compounds was in the order of 105-106 atoms of 10B atoms per cell.
There has been renewed interest in the synthesis of boron-containing steroids.83 Also, a recent work in this area has exemplified the coupling of a boron-containing moiety to a ligand possessing binding specificity for intracellular hormone receptors (Figure 2.38).84
Though these compounds have intrinsic advantages, especially with regard to the rapidity of tumor targeting, the fact that adequate receptor site density is a sine qua non in the development of useful BNCT agents has not been adequately addressed.
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