Inhibitors of 3-lactamases

The benzocarbacephems 22 (Fig. 11.12), with (X = Cl or F) or without (X = H) a halogen leaving group, were synthesized by a copper-mediated intramolecular aromatic substitution as the main step.73 74 These tricyclic (3-lactams are also competitive inhibitors of (3-lactamases and not substrates. 

The first clinical application of the second strategy (to overcome bacterial resistance by neutralizing the /3-lactamases) was the combination of clav-ulanic acid (5.12) and amoxicillin. The efficacy of clavulanic acid has stimulated research on other inhibitors of /3-lactamases, leading to the discovery of a number of other inactivators such as sulbactam (5.13), 6/3-bromopenic-illanic acid (5.14), and olivanic acid (5.15) [44] [45],... 

Some cephalosporins can be both substrates and inhibitors of /3-lactamases. The acyl-enzyme intermediate can undergo either rapid deacylation (Fig. 5.4, Pathway a) or elimination of the leaving group at the 3 -position to yield a second acyl-enzyme derivative (Fig. 5.4, Pathway b), which hydrolyzes very slowly [35][53], Thus, cephalosporins inactivate /3-lactamases by a mechanism similar to that described above for class-II inhibitors. It has been hypothesized that differences in the rate of deacylation of the acyl-enzyme intermediates derive from their different abilities to form H-bonds. A H-bond to NH in Fig. 5.4, Pathway a, may be necessary to assure a catalytically essential conformation of the enzyme, whereas the presence of a H-bond acceptor in Fig. 5.4, Pathway b, may drive the enzyme to an unproductive conformation. The ratio between hydrolysis and elimination, and, consequently, the relative importance of substrate and inhibitor behaviors of cephalosporins, is determined by the nature of the leaving group at C(3 ). An appropriate substitution at C(3 ) of cephalosporins may, therefore, increase the /3-lactamase inhibitory properties and yield potentially better antibiotics [53]. 

Recently, a structure-inhibition-activity relationship study revealed that carbapenem derivatives could lead to a new class of specific inhibitors of bacterial /3-lactamases [54], Indeed, phosphonyl derivatives, 5.16, and their cyclic analogues, 5.17 - i.e., structures not based on the /1-lactam nucleus -can also afford effective inhibitors of /3-lactamases [55][56],... 

Figure 22 Penam sulfone 39, a mechanism-based inhibitor of /3-lactamase, used as a hapten to generate scFv antibodies, FT6 and FT12, with a /3-lactamase activity.

Several classes of (3-lactamases, often encoded in transmissible plasmids, have spread worldwide rapidly among bacteria, seriously decreasing the effectivenss of penicillins and other (3-lactam anti-biotics.t y Most (3-lactamases (classes A and C) contain an active site serine and are thought to have evolved from the dd transpeptidases, but the B typey has a catalytic Zn2+. The latter, as well as a recently discovered type A enzyme,2 hydrolyze imipenem, currently one of the antibiotics of last resort used to treat infections by penicillin-resistant bacteria. Some (3-lactam antibiotics are also powerful inhibitors of (3-lactamases.U/aa/bb These antibiotics may also have uses in inhibition of serine proteasesCC/dd such as elastase. Some antibiotic-resistant staphylococci produce an extra penicillin-binding protein that protects them from beta lactams.ee Because of antibiotic resistance the isolation of antibiotics from mixed populations of microbes from soil, swamps, and lakes continues. Renewed efforts are being... 

Current commercial inhibitors of /3-lactamases include clavulanic acid (an oxapenam see Table 1), sulbactam, and tazobactam (two penam sulfones see Table 1). They are effective only against the class A serine /3-lactamases and they are administrated in the form of antibiotic/inhibitor combinations <2006BP930> Augmentin (amoxicillin/clavulanic acid), Timentin (ticarcillin/clavulanic acid), Unasyn (ampicillin/Sulbactam), Zosyn (piperacillin/tazobactam). 

All of these effects, fluorescence artifacts, edge effects, insolubility, etc., have been understood for some time. But even taking them all into account still wasn t enough to explain a mysterious problem that had probably been observed in many labs. Dr Brian Shoichet of UCSF elaborates We were looking for inhibitors of 3-lactamase. We found all these hits and they were all noncompetitive, which was weird, and they were all time-dependent, which was weird. None of those things were impossible, but to have them all We had twenty of these molecules and so we said, Well, let s just make sure they re not inhibiting chymotrypsin. And they did. Then we said, Let s make sure they don t inhibit dihydrofolate reductase. And they did. We said, Let s make sure they don t inhibit P-galactosidase. And they did. We knew we were in trouble. Usually, when that happens in pharma, people drop the project. ... 

A new development in prescribing penicillins is to give clavulanic acid 13.14) at the same time. This is a product, isolated from Streptomyces clavuligerus, which has a /3-lactam structure, but only a little anti-bacterial action. On the other hand, it is a strong inhibitor of /3-lactamases, and hence is synergistic with ampicillin and amoxycillin, with which it is most often administered. 

The disclosure of the potent activity of clavulanic acid has also led to the preparation of several structurally related compounds. " Thio-analogues of clavulanic acid have received some attention and the sulphone (162) is a powerful inhibitor of /3-lactamases produced by many pathogenic bacteria. 3-Carboxyisopenam sulphone (163) has been prepared in 4 steps from 4-iodomethylazetidinone. The sodium salt of the product was obtained by... 

The sulfated compounds MM 13902 (3, n = (5) and MM 17880 (4) are also broad-spectmm agents, but not as potent as thienamycia and all lack any significant activity against Pseudomonas (73). Many carbapenems are excellent inhibitors of isolated P-lactamases, particularly the olivanic acid sulfoxide MM 4550 (3, n = 1) (3). The possible mechanism of action of the carbapenems as inhibitors of P-lactamases has been discussed in some detail (74). Other carbapenems such as PS-5 (5) (75), the carpetimycins (76), asparenomycins (77), and pluracidomycins (8) are all highly active as antibiotics or P-lactamase inhibitors. The parent nucleus itself (1, X = CH2) is intrinsically active, but chemically unstable (9). 

The olivanic acids (general structure, Fig. 5.5D) are naturally-occurring /3-lactam antibiotics which have, with some difficulty, been isolated from culture fluids of Strep, olivaceus. They are broad-spectrum antibiotics and are potent inhibitors of various types of/3-lactamases. 

Maiti SN, Kamalesh Babu RP, Shan R (2006) Overcoming Bacterial Resistance Role of /3-Lactamase Inhibitors. 2 207-246 Motohashi N, see Michalak K (2007) 8 223-302... 

Joyeau, R. Molines, H. Labia, R. Wakselman, M. /V-aryl 3-halogenated azetidin-2-ones and benzocarbacephems, inhibitors of P-lactamases. J. Med. Chem. 1988, 31, 370-374. 

Abstract Resistance to modern antibiotics is currently a major health concern in treating infectious diseases. Abuse, overuse, and misuse of antibiotics in treating human illness have caused the pathogens to develop resistance through a process known as natural selection. The most common mechanism of resistance to -lactam antibiotics is the production of /3-lactamases, which destroy -lactam antibiotics before they reach the bacterial target. Over the last two decades, combination therapy involving treatment with a -lactam antibiotic and a /3-lactamase inhibitor has become very successful in controlling -lactamase-mediated bacterial resistance. Currently available inhibitors like... 

The interaction of class-I inhibitors with /3-lactamases is niorc comp ex. Subsequent to acylation, the five-membered ring is cleaved at the etero... 

Combinations of /3-lactamase inhibitors with /3-lactam antibiotics are very useful in the treatment of infections, since they are relatively immune to the emergence of new resistance. However, a /3-lactamase resistant to inactivation by clavulanic acid has been identified [52],... 

Fig. 5.4. Inactivation of /3-lactamases by cephalosporins (Fig. 5.1, Pathway b). The mechanism of this inactivation is similar to that of class-II inhibitors (Fig. 5.3, Pathway b) and is based on the slow hydrolysis of the acyl-enzyme complex (Pathway b). The normal deacylation of the acyl-enzyme complex represented by Pathway a results in the lost of antibacterial activity of the drug. The ratio between Pathways a and b is determined by the nature of the...

Monobactams have been investigated as p-lactamase inhibitors <98CHE1308, 98CHE1319>. The ketene-imine route to P-lactams was used to obtain 1,3,4-trisubstituted derivatives with high trans selectivity. The enolate from 4-hydroxy-y-lactone reacted with the imine (Ar CH NAr ) to give 59, vdiich cyclized in the presence of lithium chloride at low temperature to yield 60. The compounds were assayed for cholesterol absorption inhibition and 61 (R = = OH, R = F) was found to be a potent inhibitor of 3-hydroxy-3-... 

Inhibition of enzymatic inactivation Enzymatic inactivation of 3-lactam antibiotics is a major mechanism of antibiotic resistance. Inhibition of 3-lactamase by 3-lactamase inhibitor drugs (eg, sulbactam) results in synergism. 

Penam p-Lactamase Inhibitors. Penam is the trivial name of 4-thia-l-azabicyclo[3.2.0]heptane. The report that 6-/3-bromopenicillanic acid, [2(5)-(2or. 5 . 6/8)]-6-bromo-3,3-dimethyl-7-oxo-4-thia-l-azabicyclo [3.2.0]lieptane-2-carboxylic acid, (R = Br, R1 = H. R = R1 = CHi) is a potent inhibitor led to intense study both of this compound and analogues. The microbiology profile of 6-/3-bromopenicillanic acid has been reported and the compound has progressed to clinical trials. Mechanistic studies have demonstrated that the dihydrothiazine derivative is responsible for inactivation of /3-lactamases. 

Inhibition of a staphylococcal /3-lactamase by certain dipeptides has been reported 102), but it is not clear whether the active site was directly involved. Other remotely related and unrelated compounds as well as metals have been implicated, but the results are largely inconclusive 2,9,20,44) Of the nonspecific inhibitors, the thiol reagents have been most extensively tested and usually found ineffective. This is not surprising in view of the available data on the amino acid composition of /3-lactamases (Table III) which show total absence of cysteine. However, interesting exceptions have been reported. Thiol reagents inhibit the Zn2+-dependent /3-lactamase II of B. cereus 87, 66) and the /3-lac-... 

Bycroft et al. [83] have reported a series of semisynthetic penicillin derivatives such as, 6-spiro-epoxypenicillins F, G (Fig. 8) possessing both (3-lactamase inhibitory and antibacterial activity (Fig. 8). It has been found that novel chlorinated 6-spiro-epoxypenicillins F are potent in vitro inhibitors of a range of chemically important (3-lactamases [84], whereas, 6-spirocyclopropylpenems, G, show a reduced level of (3-lactamase inhibitory activity. The significance of the five fold difference between the turnover numbers for F(a) and G(b) (differ only in their stereochemistry at one center) was found to be in close comparison with the turnover number of 20,000, reported for the established (3-lactamase inhibitor, sulbactam [140]. Thus, the notable (3-lactamase inhibitory and antibacterial properties of these spiro-(3-1 actams depend upon the substituents and the stereochemistry of the epoxide. 

The action mechanism of a novel class of monobactams, inhibitors for the class A (3-lactamases has been reported in 1999 and is showed in Scheme 103 [310-313]. As exemplified by structure I, the inhibitor acylated rapidly the active site serine of (3-lactamase and the tosylate was released from species II. The acyl-enzyme underwent fragmentation, resulting in enzyme inhibition by formation of three distinct products, depending on the type of functionality linked to the inhibitor (III, IV, or V, Scheme 115). 

The progress made at the molecular biology level in the comprehension of the mechanisms of action of /3-lactam drugs, and the mechanisms of bacterial resistance. This now allows the rational design of synthetic drugs, particularly in the field of /3-lactamase inhibitors acting as suicide substrates (see Section 2.03.12). 

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