Boronic enzyme inhibitors

The closest organic specie to the inorganic boric acid are the boronic acids generally described as R-B(OH)2. Boronic acids have been shown to act as inhibitors of the subtilisins. X-ray crystallographic studies of phenylboronic acid and phenyl-ethyl-boronic acid adducts with Subtilisin Novo have shown that they contain a covalent bond between the oxygen atom of the catalytic serine of the enzyme and the inhibitor boron atom (Matthews et al, 1975 and Lindquist Terry, 1974). The boron atom is co-ordinated tetrahedrally in the enzyme inhibitor complex. It is likely that boric acid itself interacts with the active site of the subtilisins in the same manner. 

Wash performance trials were performed to verify that the enzyme was liberated from the enzyme inhibitor complex in the wash and the expected washing performance could indeed be achieved. A range of toxicology studies were also conducted and they did not suggest that toxicological issues would be a show stopper especially when it was taken into account that the boronic acid was to be used in very low concentrations. 

Some boronic acid-based enzyme inhibitors undergo strong yet reversible covalent attachment to a nucleophile at the enzyme s active site, while others simply act as competitive inhibitors in their borate conjugate base form. Boronic acid-based inhibition of thrombin has been achieved <93MI109>, and that of P-lactamases has been particularly effective <95TL8399, 96M1688>. When compared to other covalent transition-state analog inhibitors of P-lactamases like phos-... 

Transition-state inhibitors stably mimic the transition state of the enzymatic reaction, and thereby interact with the substrate-bin-ding and catalytic machinery of the enzyme in a low-energy conformation. Transition-state analogs are competitive, reversible inhibitors, although some have extremely low Kj s and very slow off-rates. All proteases activate a nucleophile to attack a carbonyl, which leads to the formation of a tetrahedral intermediate that then collapses to form the enzyme products—two peptides. Thus, synthetic small molecules that mimic the tetrahedral intermediate of the protease reaction are attractive transition-state analogs. A classic class of protease transition-state inhibitors uses a boronic acid scaffold (4, 10). Boronic acid adopts a stable tetrahedral conformation in the protease active site that is resistant to nucleophilic attack. Boronic acid inhibitors, which are derivatized with different specificity elements, have been developed against every class of protease... 

Abbreviations used in tables include Ada = 1-adamantyl boro-AA-OH = boronic acid analogue of specified amino acid (aa) Cbz = benzyloxycarbonyl CMK = chloromethylketone EIM = enzyme inhibitor of monocytes -NA = p-nitroanilide ND = not determined NR = not reactive MeO-Suc = methoxy succinoyl Met(O) = methionine sulphoxide Pic = picolinyl PPE = porcine pancreatic elastase SLPI = secretory leukocyte proteinase inhibitor TFMK = trifluoromethylketone Z = benzyloxycarbonyl. 

The type-1 complex is a transition-state mimic and such boronic acid inhibitors are potent inhibitors. The greater affinity of these boronic acids as compared to the corresponding peptide-TFMKs (for example, compare (10-7) with TFMK (7-4) which had a Aj = 14 nM for HLE) might be explainable by the fact that in the boronic acid-enzyme complex there is an... 

In addition to being developed as enzyme inhibitors and boronolectins, boron-based compounds (not limited to boronic acid compounds although that is the focus of this chapter) are also being studied for their utility as boron neutron capture therapy (BNCT) agents [27, 28]. Such applications are based on the unique property of boron-10, which emit a particles upon irradiation with neutron. Since a particles travel only a few mm they are ideal for localized radiation therapy. Therefore, targeted delivery of high concentrations of boron agents can be used for BNCT of certain tumors. 

This chapter discusses tile application of boronic adds as potential enzyme inhibitors, artificial lectins (boronolectins), feed-back controlled drug delivery materials, boron neutron capture therapy agents, and other biologically active agents. The discussion focuses on the underlying chemical principles important for the various applications using selected examples. It does not strive to be comprehensive in terms of covering all the literature reports in this area, for which one can consult various reviews already [9, 38-40]. Boronic acids have also been used for the transport of various compounds such as ribonudeosides [41], amino acids [42], catecholamine [43], and saccharides across membranes [44-47]. However, such applications are more aimed for purification and separation and, therefore, will not be addressed in detail in this chapter. 

Various boronic acid compounds have been widely studied for their inhibition of different enzymes. The following discussion is divided based on the target enzymes and/or kind of approach used for the design of boronic acid-based enzyme inhibitors. At the end of the section, many examples are summarized in a table format for ref-... 

Figure 13.9 Boronic acid-based enzyme inhibitors listed in Table 13.1.

In addition to being used as enzyme inhibitors, boron compounds can also be used in boron neutron capture therapy (BNCT), which was first proposed in 1936 [27]. BNCT is based on the imique ability of boron-10 to transmute into hthium and emit a-particles upon irradiation with soft neutrons. Because a-partides are very damaging and only travel a very short distance, they are ideal candidates for localized cancer radiation therapy [28]. Successful application of BNCT requires the development of boron compounds that spedfically deliver substantial quantities of B to the target cells at very high concentrations [133-136]. 

The reaction is proposed to proceed from the anion (9) of A/-aminocatbonylaspattic acid [923-37-5] to dehydrooranate (11) via the tetrahedral activated complex (10), which is a highly charged, unstable sp carbon species. In order to design a stable transition-state analogue, the carboxylic acid in dihydrooronate (hexahydro-2,6-dioxo-4-pyrimidinecarboxylic acid) [6202-10-4] was substituted with boronic acid the result is a competitive inhibitor of dibydroorotase witb a iC value of 5 ]lM. Its inhibitory function is supposedly due to tbe formation of tbe charged, but stable, tetrabedral transition-state intermediate (8) at tbe active site of tbe enzyme. 

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