Irreversible mechanism-based inhibitors

Propargylic, cyclopropyl, and fluoroallyl amines are powerful and irreversible mechanism-based inhibitors of MAOs. Oxidation by the enzyme affords a very active entity onto which a nucleophile of the enzyme, or of the cofactor, can be added (Figure 7.53). 

Irreversible inhibitors may be classified for convenience as active site directed inhibitors and suicide or irreversible mechanism based inhibitors (IMBIs). They bind to the enzyme by either strong non-covalent or strong covalent bonds. Inhibitors bound by strong non-covalent bonds will slowly dissociate, releasing the enzyme to carry out its normal function. However, whatever the type of binding, the enzyme will resume its normal function once the organism has synthesized a sufficient number of additional enzyme molecules to overcome the effect of the inhibitor. 

Suicide inhibitors, alternatively known as Kcat or irreversible mechanism based inhibitors (IMBIs), are irreversible inhibitors that are often analogues of the normal substrate of the enzyme. The inhibitor binds to the active site, where it is modified by the enzyme to produce a reactive group, which reacts irreversibly to form a stable inhibitor-enzyme complex. This subsequent reaction may or may not involve functional groups at the active site. This means that suicide inhibitors are likely to be specific in their action, since they can only be activated by a particular enzyme. This specificity means that drugs designed as suicide inhibitors could exhibit a lower degree of toxicity. 

Fig. 10.54 Clavulanic acid as an irreversible mechanism based inhibitor.

Previous editions of Selective Toxicity have been well received, and translated into German, Italian, Japanese, and Russian. This new edition, which reviews the literature up to September 1984, has been thoroughly revised to reflect current awareness of the subject. Many additions have been made, some of them quite substantial, but all blended into the original framework. There are new sections on drugs that influence the immune process, on inhibitors based on the transition state of the molecule, and on IMBIs, which are the irreversible mechanism-based inhibitors. There are new taxonomic tables of bacteria and protozoa, and an index of the 650 structural formulae used in the text. A new section (17.4) provides help in searching for the physical and biological properties of drugs, whether in compilations (books) or in computerized databanks. 

Affinity Labels. Active site-directed, irreversible inhibitors or affinity labels are usually substrate analogues that contain a reactive electrophilic functional group. In the first step, they bind to the active site of the target enzyme in a reversible fashion. Subsequentiy, an active site nucleophile in close proximity reacts with the electrophilic group on the substrate to form a covalent bond between the enzyme and the inhibitor, typically via S 2 alkylation or acylation. Affinity labels do not require activation by the catalysis of the enzyme, as in the case of a mechanism-based inhibitor. 

Finally, the whole process of reactive immunization opens up the opportunity of using mechanism-based inhibitors as haptens, capable of actively promoting a desired mechanism by contrast to their conventional use as irreversible enzyme inhibitors. 

In addition to the assessment of reversible inhibition, the role played by mechanism-based inhibitors (irreversible inhibitors) provides a focus during lead development, as it can result in a more profound and prolonged effect than that suggested by the therapeutic dose or exposure. Mechanism-based inhibition (MBI) occurs as a result of the CYP generating reactive intermediates that bind to the enzyme causing irreversible loss of activity. Oxidative metabolism via that CYP is only restored upon re-synthesis of that enzyme. Three mechanisms have been reported showing how intermediate species act as mechanism-based inhibitors ... 

Irreversible Inhibition with Mechanism-Based inhibitors (Suicide Substrates)... 

Interactions with metabolic enzymes fluorinated amino acids are peptidomi-metic units or reactive entities used to design either reversible enzyme inhibitors (analogues of substrates) or irreversible enzyme inhibitors (mechanism-based inhibitors). 

Fluorinated substrates that are transformed into activated entities capable of reacting in an irreversible manner with the enzyme (mechanism-based inhibitors or suicide substrates). In the case of a mechanism-based inhibitor, the enzyme does not add onto the substrate itself, but onto an intermediate... 

Because irreversible inhibitors tend to be reactive electrophiles, they can be difficult to use as drugs. Their reactivity can translate into instability and/or toxicity. A subclass of irreversible inhibitors is the mechanism-based inhibitor. Mechanism-based inhibitors... 

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