Clavulanic acid, structure

Fig. 5.5 A, clavulanic acid B, latamoxef C, 1-carbapenems D, olivanic acid (general structure) E, thienamycin F, meropenem G, 1-carbacephems H, loracarbef.

After clavulanic acid, the penicillanic acid derivatives (particularly the corresponding sulfone analogs) have been the subject of intense research in the -lactamase inhibitor area. From this extensive investigation, two compounds (sulbactam and tazobactam) from this class have been successfully introduced into clinical use. The penicillanic acid sulfones are /3-laclamasc inhibitors that are quite homologous to clavulanate in both their mechanism of action and in the spectrum of -lactamases susceptible to their action. The first notable success in this field was the discovery of sulbactam 7 (Fig. 7), which was reported by Pfizer chemists in 1978 and shown to possess potent inhibitory activity, principally for class A //-lactamases. It had greater affinity for class C types than clavulanate. From careful comparison of its structure to clavulanate, a rational basis for the similarities between the two is apparent. Both lack a C-6 substituent. Since the absence (or presence) of this substituent is an important, but not exclusive, factor in //-lactamase recogni-... 

Mechanistic investigations have shown that these compounds behave as suicide inhibitors (preferably called mechanism-based inactivators) in the sense that they are recognized by /3-lactamases as substrates, but the great stability of the acyl-enzyme intermediate blocks turnover of the enzyme [46] [47]. /3-Lactamase inhibitors can be divided into two classes, class I and class II class-I inhibitors (e.g., clavulanic acid (5.12)), in contrast to those of class II (e.g., olivanic acid (5.15)), have a heteroatom at position 1 that can lead to ring opening at C(5). The mechanistic consequences of this difference in structure are illustrated by the general scheme in Fig. 5.3. 

In the search for structural diversity, and novel therapeutic agents, unique ring structures like the 1,2,5-thiadiazole have always captured the imagination of chemists. Often, as in the case of timolol (4), the interest is rewarded. In the early 1990s, a simple thiadiazole was appended to a penem in the development of the structure-activity relationships for a series of ) -lactamase inhibitors. The result was enhanced penetration of the bacterial membrane and a broader spectrum of activity versus clavulanic acid <9lJAN33l>. 

Beta-lactamase inhibitors include clavulanic acid, sulbactam and tazobactam. They are structurally related to the beta-lactam antibiotics however the antibacterial activity of these compounds is very weak or negligible. They are strong inhibitors of bacterial beta-lactamases and can protect beta-lactam antibiotics from hydrolysis by these enzymes. 

The four oxapenams [(67), (68 R = CH2OH, CH2OCHO, C02H)] structurally related to clavulanic acid (58) also have been isolated from fermentation broths of Streptomyces species. 

Clavulanic acid is produced by Streptomyces clavuligerus and is structurally related to penicillins. This naturally occurring compound is a specific and irreversible inhibitor of a wide range of bacterial -lactamases and, therefore, can enhance the activity of penicillins and cephalosporins against many resistant organisms. Potassium clavulanate can be administered both orally and perenter-ally in combination with antibiotics sensitive to the action of -lactamases. 

The diverse origin of two structurally similar compounds is exemplified by the (3 -lactam antibiotics isopenicillin N (1) and clavulanic acid (2). While these molecules are structurally and functionally similar, they are derived by quite different routes. Isopenicillin N is formed by the direct cyclization of the tripeptide (3) (B-80MI10400) while clavulanic acid appears to be elaborated directly from a three-carbon intermediate of the glycolytic pathway (possibly phosphoenolpyruvate, 4) and glutamic acid (5) (B-80M110401). 

Further, the discovery of 7-a-methoxy cephalosporins [5] from Streptomyces in 1971, carbapenems [6], thienamycin [7], clavulanic acid [8], sulbactum [9] as well as the totally synthetic oxapenems [10], oxacephams [11], and other bicyclic (3-lactams stimulated the search for novel antibiotics. More recent dedicated efforts to find new active molecules and modify the penicillin and cephalosporin structure have resulted in the discovery of simple monocyclic (3-lactams such as norcardicins and monobactams [12, 13]. Yet another dimension has been added to the (3-lactam research with the recent discovery of tricyclic (3-lactam antibiotics called trinems [14]. Thus, (3-lactam antibiotics in general can be classified into several groups based on their structures (Fig. 1). 

P Swaren, D Golemi, S Cabantous, A Bulychev, L Maveyraud, S Mobashery, JP Samana. X-ray structure of the Asn276Asp variant of the Escherichia coli TEM-1 (3-lactamase direct observation of electrostatic modulation in resistance to inactivation by clavulanic acid. Biochemistry 38 9570-9576, 1999. 

The successive discoveries of cephalosporin C (1945), cephamycin (1971), thienamycin (1976), clavulanic acid (1975), nocardicin (1976), sulfazecin (1981), etc. The structural diversity found in the natural compounds inspired the medicinal chemists for side-chain modifications of the penam and penem cores (see Section 2.03.11). 

Structural studies of clavulanic acid dehydrogenase, which catalyzes the biosynthesis of clavulanic acid 54 (R1 = Rz = H) from clavulanate-9-aldehyde, have provided a deeper understanding of the mechanism of the reduction process <2007B1523>. 

For an example with a four-membered ring, we go back to (3-lactams. A serious problem with [3-lactam antibiotics is that bacteria develop resistance by evolving enzymes called [3-lactamases, which break open the four-membered ring. In 1984, a team from Beechams reported the exciting discovery of some very simple inhibitors of these enzymes all based on the core structure named clavulanic acid. This too was a [3-lactam but a much simpler one than the penicillins we saw earlier. 

The natural penicillins, primarily G and V, have a relatively narrow spectrum. They act mostly on gram-positive organisms. The fact that proper selection of precursors could lead to new variations in the penicillin side chain offered the first source of synthetic penicillins. Penicillin V, derived from a phenoxy-acetic acid precursor, attracted clinical use because of its greater acid tolerance, which made it more useful in oral administration. Also, the widespread use of penicillin eventually led to a clinical problem of penicillin-resistant staphylococci and streptococci. Resistance for the most part involved the penicillin-destroying enzyme, penicillinase, which attacked the beta-lactam structure of the 6-aminopenicillanic acid nucleus (6-APA). Semisynthetic penicillins such as ampicillin and carbenicillin have a broader spectrum. Some, such as methicillin, orafi-cillin, and oxacillin, are resistant to penicillinase. In 1984, Beecham introduced Augmentin, which was the first combination formulation of a penicillin (amoxicillin) and a penicillinase inhibitor (clavulanic acid). Worldwide production of semisynthetic penicillins is currently around 10,000 tons/year, the major producers are Smith Kline Beecham, DSM, Pfizer, and Toyo Jozo. 

Enzymatic inactivation or modification of antibiotics has been discussed by many authors [179-182, 186-188], As described earlier, /(-lactams may be susceptible to /(-lactamases. During the past 30-odd years, several -lactams have been synthesized that are less susceptible to these enzymes. Such drugs include (i) newer types of /(-lactam structures, e.g. carbapenems (37), cephamycins (40) and carbacephems (53), and (ii) modifications of the side-chains of existing penams (38) or cephems (39) [317]. Nevertheless, the wide diversity of /(-lactamases [180,181,318] means that organisms producing enzymes with broad-spectrum activity may be able to resist some members of the /(-lactam group, /(-lactamase inhibitors such as clavulanic acid (36), and the penicillanic acid sulphone (54) derivatives, tazobactam (55) and sulbactam (56) have been combined with, and protect, appropriate /(-lactamase-susceptible penicillins, with useful clinical results. Most extended-spectrum /(-lactamases are susceptible to these inhibitors, but newer -lactamase inhibitors may still be needed. 

The azetidinone ring system 43 is an important structural feature of the powerful b-lactam famiUes of antibiotics and appears in many other natural products such as clavulanic acid. Free radical-based routes to this ring system are remarkable for their variety and range. Four distinct radical-based disconnections have been investigated for azetidinone preparation. Disconnection a impUes closure of a carbamoyl-type radical onto an imsaturated acceptor group. Disconnection b imphes a closure of an amidoalkyl (a-carbamoyl) radical onto an enamide acceptor. Disconnection c points to an amidyl radical ring closure onto an alkene acceptor. Finally, disconnection d connotes ring closure of an acyl radical onto an imine acceptor (Scheme 11). 

As resistance to 3-lactam antibiotics increased because of the expression of a variety of 3-lactamases, many groups focused their efforts on discovering compounds that could be more reactive to 3-lactamases without having significant intrinsic antibiotic activities of their own and pharmacokinetic properties that would be similar to 3-lactam antibiotics. This focus led to the discoveries of clavulanic acid (18) and monobactam sulfazecins (19). Nature effectively stabilized the latter monobactam structure by the addition of a A-sulfamic acid. The 3-lactamase inhibitor clavulanic acid was combined with amoxicillin, which... 

Clavulanic acid (80), isolated from Strepto-myces clavuligeruSy is similar in structure to the penicillins, except oxygen replaces sulfur in the five-membered ring (123). Clavulanic acid has weak antibacterial activity, but is a potent inhibitor of p-lactamases (124). A mixture of clavulanic acid and the j8-lactamase-sensitive amoxycillin was introduced in 1981 as Augmentin and has proved to be an effective combination to combat jS-lactamase-pro-ducing bacteria (125). In 2001,20 years after its launch, Augmentin is the best-selling antibacterial worldwide. 

Clavulanic acid is also a mechanism-based inhibitor and its mode of action is believed to involve ring opening of the initially formed acyl-enzyme complex (18) to the keto-derivative (19), which may then tautomerise to the hydrolytically more stable -amino-acrylate (20) Scheme 6.4). This transiently inhibited form may hydrolyse to re-release active enzyme or react further with the enzyme to produce irreversibly inhibited forms. It has been shown that approximately 115 molecules of clavulanic acid are destroyed per molecule of enzyme before the j8-lactamase is irreversibly inactivated. Whilst irreversibly inactivated forms are known to exist, the nature of these products is not yet known. Possible structures are (21) and... 

Edwards RG, Dewdney JM, Dobrzanski RJ, Lee D. Immunogenicity and allergenicity studies on two beta-lactam structures, a clavam, clavulanic acid, and a carbape-nem structure-activity relationships. Int Arch Allergy Appl Immunol 1988 85(2) 184-9. 

Imipenem caused positive dipstick tests for leukocytes in patients with agranulocytosis and normal urinary sediments. This phenomenon was reproducible in vitro with imipenem, meropenem, and clavulanic acid. Sulbactam, tazobactam, three penicillins, three cephalosporins, and the basic structures of penicillins, cephalosporins, and monobactams tested negative (50). 

Tazobactam, USP. Tazobactam is a penicillanic acid sulfone that is similar in structure to sulbactam. It is a nioK potent /3-lactamase inhibitor than sulbactam and ha.- 3 slightly broader spectrum of activity than clavulanic acid. Ii has very weak antibacterial activity. Tazobactam is available in fixed-dose, injectable combinations with piperacillin, a broad-spectrum penicillin consisting of an 8 I ratio of pipci- acillin sodium to tazobactam sodium by weight and ma-keted under the trade name Zosyn. The pharmacokineticsprotein bound, experience ven little metabolism, and are excreted in active forms in the urine in high concentrations. 

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