MW4660 Hamilton

Journal of Antimicrobial Chemotherapy (1999) 44, 729–734 JAC Review

-Lactams: variations on a chemical theme, with some surprising biological results

J. M. T. Hamilton-Miller*

Department of Medical Microbiology, Royal Free and University College Medical School, London, UK

Introduction recently, to the extremely versatile penam and cephem structures that have sometimes produced surprising Until the middle of the 1970s virtually all the significant pharmacological results. developments in -lactam antibiotics had been achieved by adding different side chains to the classical penam and Modifications cephem nuclei.1 Notwithstanding the fact that, in earlier decades, Antifungal activity manipulation of the penicillin nucleus had failed to pro- duce any novel drugs of interest,2 the second half of the Modest activity against some pathogenic yeasts and fila- 1970s saw huge progress in useful alterations to the basic mentous fungi was reported for a cephalosporin whose penam and cephem ring structures. Penems (Ia) were side chain was an acid (N-benzyldithiocarbamate) having synthesized,3 naturally occurring carbapenems (Ib, e.g. intrinsic antifungal activity.20 Much more intriguing was the olivanic acids), oxapenams and monocyclic -lactams finding by the same authors that the aldehyde of penicillin (nocardicins and monobactams) were discovered,4–7 and V showed an antifungal action. Another interesting obser- penam sulphones (II) were found to be inhibitors of some vation that also does not seem to have been studied further, -lactamases.8 The cephalosporin nucleus was modified to is that unidentified degradation products from aqueous produce microbiologically active oxacephems9 (IIIa) and solutions of some first-generation cephalosporins inhibit carbacephems (IIIb).10 Not all changes were advantageous: the growth of certain dermatophytes.21 for example, oxocephems (IV),11 cephams (V)12 and cephems with the double bond in the 2,3 instead of the 3,4 Compounds with a modified -lactam ring position13,14 had little or no activity. Many of the active structures mentioned above, as such or The familiar antimicrobial agents in clinical use either have after suitable modification, have found their way into clinical an unmodified penam or cephem ring, or an alteration has practice, for example, imipenem, clavulanate, aztreonam, been made in the larger of the two rings (e.g. clavulanate, sulbactam, latamoxef and loracarbef. Many other related carbapenems, latamoxef, loracarbef). The four-membered compounds have been intensively studied but have either -lactam ring (azetidinone) common to both families been abandoned as clinical candidates or have not yet been remains unmodified. Indeed, it has become almost an fully developed. Iso-oxacephems (VI a ), also reported12 article of faith that the -lactam ring is sacrosanct. How- during the ‘fertile’ period mentioned above, have been ever, there have been other preconceptions about this reinvestigated recently as possible oral15 or parenteral16 group of antibiotics that have later proved on investigation antibiotics. Other similar compounds, synthesized later, also to be false, e.g. that a monocyclic -lactam could never show good antibacterial activity, e.g. isocephems (VI b ), 17 approach the activity of a bicyclic system,22 or that a free is o - a z a c e p h e m s 18 (VI c ) and isopenams (VI I ). 17 Penems are carboxyl group is essential.23 Thus this belief must be exam- still under development as potential therapeutic agents.19 ined carefully. It is the purpose of the present review to draw attention Changing the chemical constituents of the -lactam to further chemical modifications, most of them reported ring,24 or inserting a 5–6 double bond in the penam

*Correspondence address: Department of Medical Microbiology, Royal Free Campus, Rowland Hill Street, London NW3 2PF, UK. Fax: 44-171-435-9694; E-mail [email protected]

729 © 1999 The British Society for Antimicrobial Chemotherapy J. M. T. Hamilton-Miller

Figure. Structures of -lactam antibiotics. structure (creating a ‘dehydropenicillin’ (VIII),24,25 not to (ii) Replacing C5 in Xa with a N atom gives aza- -lactam be confused with an ‘anhydropenicillin’ (IX)26 in which the (pyrazolidinone) analogues (Xb). Certain of these in elements of water have been removed from the thiazo- the carbapenem series showed good activity against lidine ring) was not a successful strategy to improve Streptococcus pneumoniae, Klebsiella pneumoniae, antibacterial activity or to inhibit -lactamases. On the Escherichia coli, Enterobacter aerogenes and Serratia other hand, increasing the size of the ring from four- marcescens; however, the carbacephem analogues membered to five-membered had interesting results. were devoid of activity.27,28

(i) Expanding the ring by a ·CH2· moiety gives a -lactam (iii) It can be seen from structure X (in which the -lactam (Xa); while the compound derived from the penam ring is fused to its five-membered neighbour), that it is nucleus was not active, its penem and carbapenem not possible, for valency reasons, to substitute C5 with analogues did show antibacterial activity. Baldwin an oxygen atom. However, nature has provided a et al.24 have reported the synthesis but not the biologi- remarkably analogous unfused -lactam system in the cal activity of the -lactam analogue of ceftizoxime. form of lactivicin (XIa).29 Here a -lactam ring, an

730 -Lactams: variations on a chemical theme

isoxazolidinone closely related to cycloserine, is linked more than 25 years ago.39 They had less antibacterial activ- to a furan moiety. Surprisingly, lactivicin acts in many ity than their corresponding unbridged analogues. ways exactly like a ‘classical’ -lactam in that it binds Tricyclic carbacephems (XVIa), with a bridge between to penicillin binding proteins (PBPs), is a substrate C1 and N7 , were at first thought to have no useful activ- for and an inducer of -lactamases, shows increasing ity,40 but later derivatives both showed significant anti- activity against mutants that are hypersensitive to bacterial activity and inhibited class C -lactamases.41,42 -lactams, and is inactive against microorganisms that Bridged iso-oxa (XVIb) and isocephems (XVIc) were do not contain mucopeptide. found to be better inhibitors of class C -lactamases than These interesting properties suggested that analogues of the bridged monobactams (see above), and furthermore, lactivicin might be valuable antibiotics. As a result, the unlike the monobactams, some (especially the isocephem derivatives) had broad-spectrum antibacterial activity as 4-aminolactivicinic acid (4ALA) nucleus (XIb), was 43 synthesized and different side chains were added, following well. methods perfected for the syntheses of derivatives from the nuclei of the penicillins (6APA), cephalosporins (7ACA) Interactions with -aminobutyric acid (GABA) and monobactams (3AMA). The 4ALA analogues of cefotaxime and cephalothin showed high activity against, Examination of the three-dimensional structures of respectively, Enterobacteriaceae and staphylococci, and 3AMA, 6APA, 7ACA and the nocardicin nucleus (3- orally available prodrugs were also made.30 However, this aminonocardicinic acid) show them to be conformationally series was not proceeded with. rigid analogues of GABA, and as such they act as com- An even more radical departure from conventional petitive inhibitors of GABA aminotransferase.44 Inhibition thinking was made by Imming,31 who synthesized penam of this enzyme has an anti-convulsive effect. analogues in which the -ring had been enlarged to seven This relationship is fascinating because -lactam anti- or 13 members (the latter size—a -lactam—was con- biotics in current use are known to be capable of having sidered optimal). Chemical but not biological findings are precisely the opposite effect—i.e. to cause convulsions. It reported.31 is suggested that this epileptogenic activity may be due to inhibition of binding of GABA to its receptors.45 Compounds with extra rings ‘Bridged’ -lactams have an extra ring—created by cycliza- Inhibition of human and viral serine proteases tion of groups outside the main ring structure(s). These fall PBPs and many -lactamases have a serine motif at their into three categories. active centres, a property they share with a large class of enzymes known as serine proteases. Several of the latter (i ) Bridged monocyclic compounds. Bridged monobactams have been found to be inhibited by certain -lactams. (XIIa), sulfactams (XIIb) and other azetidinones, that contain two rings, have been reported to be good inhibitors 32–34 (i) Human leucocyte elastase is inhibited by cephalosporin of class C and, in some cases, class A -lactamases. 46 su l p h o n e s . The most active compounds had IC50 1 mg/L. (ii) Bridged penams and carbapenems. The tricyclic Inappropriate activity of this enzyme has been implicated 2,3-methylene penams exist in two stereoisomeric forms, in the tissue damage observed in certain chronic con- (XIIIa) and (XIIIb). The sulphones of these compounds ditions, such as cystic fibrosis, rheumatoid arthritis and show interesting differential properties, the former having emphysema. Further studies have identified inhibitors among cephems, penams, penems, monobactams and other good antibacterial activity and inhibiting class C -lacta- 47 mases, while the latter are poor antibacterial agents but are related structures. inhibitors of class A enzymes (penicillinase type).35,36 Thus it seems that the isomer is recognized by -lactamases as (ii) Chymotrypsin and to a lesser extent thrombin were a penem, and the isomer as a cephem. inhibited by some of the sulphone analogues synthesized 46 Bridged carbapenems (XIV), in which C1 and C2 are by Doherty et al. (IC50 1–10 mg/L). joined through a four-carbon linkage, make up the family of antibiotics given the trivial name ‘tribactams’,37 later (iii) Protease from cytomegalovirus (assemblin) is inhib- changed to trinems. Sanfetrinem showed broad-spectrum ited by monocyclic -lactams.48 Despite the fact that there activity38 and oral availability as a prodrug, but the future is little similarity between herpesvirus proteases and of this series is uncertain. other serine proteases (including -lactamases), covalent inactivation of the active site serine in assemblin has been (iii) Bridged cephems and analogues. The first tricyclic reported.49 This enzyme is important in capsid assembly, so cephems, bridged between C2 and C3 (XV), were reported inhibitors may point the way to antiviral agents.

731 J. M. T. Hamilton-Miller

Inhibition of human cytosolic phospholipase A2 Conclusions

This enzyme, thought to be involved with intracellular Penicillins have been in clinical use for 55 years, and generation of arachidonic acid, was inhibited by a penam 50 cephalosporins for about 20 years less. Their evolution as and almost as well by the corresponding penicilloate. This antibacterial agents has slowed from the breakneck pace of suggested that, contrary to the preceding examples, the the 1960s and 1970s, and it is now not easy to discern in inhibitory action in this case was due to a mechanism other which directions much further advance can be made. How- than the formation with the serine of a covalent acyl inter- ever, as outlined above, the ingenuity of medicinal chemists mediate at the active site of the enzyme. 51 combined with the extreme versatility of these molecules It is interesting that some 20 years previously phospho- means that their development can by no means yet be lipase A2 from Crotalus adamanteus had been reported to regarded as at the end of the line. Surely we will soon see a be inhibited by two penams (oxacillin and carbenicillin). useful inhibitor of class C -lactamases and, as for other pharmacological applications, perhaps we should be pre- pared for the unexpected. Inhibition of HIV protease Benzylpenicillin was the starting point for the synthesis of a series of compounds with great inhibitory activity against References HIV protease (an aspartate protease). The most active 1. Rolinson, G. N. (1988). The Garrod Lecture. The influence of member had an ED50 in a syncytium formation assay of 6-aminopenicillanic acid on antibiotic development. Journal of Anti- 50 nM. Unfortunately, none of the compounds had satis- microbial Chemotherapy 22, 5Ð14. factory pharmacokinetic properties, and this line of 2. Hamilton-Miller, J. M. T. (1967). Chemical manipulations of the 52 research has been terminated. penicillin nucleus: a review. Chemotherapia 12, 73Ð88. 3. Woodward, R. B. (1976). In Recent Advances in the Chemistry of Beta-Lactam Antibiotics. (Elks J., ed.), pp. 167Ð80. London: Royal Delivery of anticancer drugs Society of Chemistry, London, UK. 4. Advantage can be taken of the unique way in which many Brown, A. G., Butterworth, D., Cole, M., Hanscomb, G., Hood, J. D., Reading, C. et al. (1976). Naturally-occurring -lactamase cephalosporins fragment when their -lactam ring is inhibitors with antibacterial activity. Journal of Antibiotics 29, 668Ð9. broken, namely ejection of their substituent at C3. Atten- 5. Brown, A. G., Corbett, D. F., Eglington, A. J. & Howarth, T. T. tion has been drawn previously53 to the exploitation of this (1977). Structures of olivanic acid derivatives MM4550 and mechanism in ‘dual action’ antibiotics: here, a new anti- MM13902; two new, fused -lactams isolated from Streptomyces bacterial compound is produced if the original is attacked olivaceus. Journal of the Chemical Society Chemical Communica- by a -lactamase. Examples are cephalosporin MCO tions, 523Ð5. (which releases pyrothione, an antiseptic) and the cephalo- 6. Aoki, H., Sakai, H., Kohsaka, M., Konomi, T. & Hosoda, J. (1976). sporin/fluoroquinolone hybrids synthesized by Roche, Nocardicin A, a new monocyclic -lactam antibiotic. I. Discovery, which act as cephalosporins until hydrolysed, when a isolation and characterization. Journal of Antibiotics 29, 492Ð500. fluoroquinolone is released. 7. Asai, M., Haibara, K., Muroi, M., Kintaka, K. & Kishi, T. (1981). This process has now been taken a stage further in the Sulfazecin, a novel -lactam antibiotic of bacterial origin. Isolation design of targeted anticancer prodrugs.54 The strategy is as and chemical characterization. Journal of Antibiotics 34, 621Ð7. follows: a conjugate of -lactamase with a monoclonal anti- 8. English, A. R., Retsema, J. A., Girard, A. E., Lynch, J. E. & Barth, body specific for tumour-associated antigens binds to W. E. (1978). CP-45,899, a beta-lactamase inhibitor that extends malignant cells. Then a prodrug consisting of an adduct of the antibacterial spectrum of beta-lactams: initial bacteriological doxorubicin, a vinca alkaloid or a nitrogen mustard with a characterization. Antimicrobial Agents and Chemotherapy 14, cephalosporin—a covalent bond having been made at the 414Ð9. C3 position—is administered. The prodrug is activated 9. Narisada, M., Yoshida, T., Onoue, H., Ohtani, M., Okada, T., only at the surface of the tumour, where the -lactamase is Tsuji, T. et al. (1979). Synthetic studies on -lactam antibiotics. Pa r t bound: breaking the -lactam bond causes ejection of the 10. Synthesis of 7 -[ 2 - c a r b o x y - 2 - ( 4 - h y d r o x y p h e n y l ) a c e t a m i d o ] - 7 - me t h o x y - 3 - { [ ( 1 - m e t h y l - 1 H - t e t r a z o l - 5 - y l ) t h i o ] - m e t h y l } - 1 - o x a - 1 - d e t h i a - free cytotoxic drug. The advantage of this procedure is that 3-cephem-4-carboxylic acid disodium salt (6059-S) and its related the anticancer agent is much less toxic as a prodrug, so 1-oxacephems. Journal of Medicinal Chemistry 22, 757Ð9. systemic toxicity is reduced. 10. Guthikonda, R. N., Cama, L. D. & Christensen, B. G. (1974). Reduction in overall toxicity of an anticancer agent has Total synthesis of beta-lactam antibiotics. VIII. Stereospecific total also been reported by the reaction of a retinoid with an synthesis of ( ) 1-carbacephalothin. Journal of the American 17 isocephem, via an amide linkage at the C4 position. It is of Chemical Society 96, 7584Ð5. interest that the isocephem, which had the same side chain 11. Kim, C. U., Misco, P. F. & McGregor, D. N. (1979). Synthesis as benzylpenicillin, was highly microbiologically active, and antibacterial activity of 2-oxocephalosporins. Journal of Medici- although its retinoid conjugate was not. nal Chemistry 22, 743Ð5.

732 -Lactams: variations on a chemical theme

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