Pseudo-irreversibility

The degree of depression of the maximal response to agonists with slow offset pseudo-irreversible... 

Ki of 6 nM against a guinea-pig uterine oxytocin preparation and at least 100-fold selectivity over rat vasopressin tissue preparations. These molecules show a pseudo-irreversible pharmacology, with an extended action against spontaneous contractibility of rat uterus 24 h post partum. 

Rivastigmine is a pseudo-irreversible inhibitor of both acetyl and butyryl cholinesterases. Thus although the drug initially blocks the enzymes, it is metabolized by them thereby giving the drug a relatively short half-life. The top dose is often necessary to achieve therapeutic efficacy, at which dose the central and peripheral cholinergic side effects become apparent. 

In unier tu cuklcuuy prevent proteolysis, the inhibitor should fulfill at least two requirements (1) inhibit the target enzyme in an irreversible or pseudo-irreversible manner and (2) suppress the enzyme activity before significant proteolysis has occurred. From a kinetic viewpoint, the classical example of vn irreversible protein inhibitor of NE is ai-PI. Proteolysis can also be prevented by reversible inhibitors if the in vivo inhibitor concentration is much greater than Kj, the equilibrium constant far inhibition (Ujvta >>Kj), resulting in a pseudo-irreversible behavior. 

Noncompetitive antagonism can occur if the antagonist binds to the same site as the agonist but does so irreversibly or pseudo irreversibly (i.e., very slow dissociation but no covalent binding). It also causes a shift in the dose-response curve to the right but does cause depression of the maximal response (not shown). 

As may be expected, criteria for the study of pseudo irreversible inhibitors are very similar to those for both affinity labels and mechanism-based inhibitors. However, because of the inherent reversibility of pseudoirrevers-ible inhibitors, it may be more difficult to obtain structural evidence for the covalent enzyme inhibitor adduct. Further, determination of the rate of reactivation and characterization of the products of the recovery process will also be of major importance in designating an inhibitor as pseudoirreversible. 

One of the most important mechanistic characteristics of COX-2 inhibition exhibited by many COX-2-selective inhibitors is the observed time-dependent inhibition of COX-2 but not of COX-1. A slow, noncovalent, pseudo-irreversable conformational change in the protein occurs after binding of the inhibitor, but this is only observed in binding with the COX-2 enzyme (353,398,399). As a result, a much lower concentration of inhibitor is required for effective inactivation of the COX-2 enzyme after a longer incubation, even for a... 

From an examination of Equation 6.1, and noted in Figure 6.4, if the rate of offset of the orthosteric antagonist is slow such that a correct re-equilibration cannot occur between the agonist, antagonist, and receptors during the period of response collection in the presence of antagonist, then essentially a pseudo-irreversible blockade of receptors will occur. 

Acetylcholinesterase (AChE) inhibitors are successfully used as drugs for the treatment of Alzheimer s disease. Physostigmine, also named eserine, has been isolated from the Calabar bean Physostigmina Venenosum). It is classified as a pseudo-irreversible inhibitor because it reacts with acetyl- as well as butyryl-cholinesterase (BChE) to form a carbamylated serine which is hydrolyzed again with a fi/2 of 40 min [73]. Physostigmine inhibits AChE with an IC50 of 27.9 nM and BChE with an IC50 of 16 nM. Its enantiomer is 350 times less active on AChE and 150 times less active on BchE [74]. 

Steady state kinetic measurements of initial rates are made under pseudo-irreversible Uni Uni kinetic conditions where all but one substrate are in substantial excess and no product is present. Furthermore, where a biocatalyst has more then one catalytic site, steady state analyses should be performed under conditions where only one of the sites is active at any one time, or else where the catalytic sites are at least able to operate completely independently of each other. In the latter case, measured values of kc t should then be divided through by the number of catalytic sites to provide a value per catalytic site, otherwise known as the turnover number per catalytic site. 

Rivastigmine, approved by the FDA in 2000, i,s a noncom-peliitve pseudo-irreversible carbamate AChEI that was... 

Rivastigmine is a centrally selective, arylcarbamate AChEI that was approved in 2000 for oral administration in the treatment of AD. It has an elimination half-life of 1.4 to 1.7 hours but is able to inhibit AChE for up to 10 hours. Because of the slow dissociation of the carbamylated enzyme, it has been referred to as a pseudo-irreversible AChEI (47). Like donepezil, rivastigmine exhibits a low level of hepatotoxicity. It is rapidly and extensively hydrolyzed in the CNS by cholinesterase with minimal involvement of CYP450. The phenolic metabolite is excreted primarily via the kidneys. 

The subject of non-equilibrium situations such as pseudo-irreversible inhibition in drug design has been reviewed by Dr David Swinney, who concluded that first, in the absence of mechanism-based toxicity, drug mechanisms that create transitions to a non-equilibrium state will be more efficient second, in the presence of mechanism-based toxicity, biochemical mechanisms that maintain equilibrium to achieve a tolerable balance between efficacy and toxicity are desired. Non-equilibrium states, of course, include slow-off rate and irreversible inhibition. 

Two non-competitive antagonists of 5-HT4-RS in guinea-pig ileum have also been described R 50595 [57, 58], a benzamide derivative (Table 2B) and FK 1052 a dihydropyridoindole derivative [59]. The mode of action of these drugs is not known. Do they pseudo-irreversibly bind to the agonist binding site, or to an allosteric site, or do they block the contraction by acting downstream to the 5-HT4-R ... 

Other unsaturated substrate analogs that have been tried as enzyme inhibitors include allyl amine and allyl alcohol. Allylamine is a pseudo-irreversible inhibitor of flavin-linked monoamine oxidase i.e., in the presence of allylamine, the enzyme shows a time-dependent inactivation that cannot be reversed by dialysis. When radiolabeled allylamine is used, radioactivity is incorporated at the same rate as the enzyme is inhibited. However, inhibition is relieved and radioactivity is removed from the enzyme upon incubation with the substrate, benzylamine. 

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