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10 CH AP ÇCTEMR + Lancer HO) Antibacterial Antibiotics JOHN M. BEALE, JR HISTORICAL BACKGROUND Sir Alexander Fleming”s accidental discovery of the antibac- temil properties of penicillin in 1929 is largely credited with Wituting the modem antibiotic era. Notuntil 1938, however, when Florey and Chain introduced penicillin into therapy, "2 dd practical medical exploitarion of this important discovery 2002, available begin to be realized. Centuries earlier, humans had learned do me crude preparations empirically for the topical treal- ment of infections, which we now assume to be effective because of the antibiotic substances present. As early as 500 + bn, molded curd of soybean was used in Chinese folk medicine to treat boils and carbuncles. Moldy cheese had aho been used for centuries by Chinese and Ukrainian peas- mais to treat infected wounds. The discovery by Pasteur and Jouber in 1877 that anthrax bacilli were killed when grown da culture in the presence of certain bacteria, along with Wmlar observations by other microbiologists, led Vuil- dem” 10 define antibiosis (literally “against life”) as the hiological concept of survival of the fittest, in which one maaism destroys another to preserve itself, The word anti- húrio was derived from this root. The use of the term by “e lay public, as well as the medical and scientific communi- es has become so widespread that its original meaning has ecome obscured. bs 1942, Waksman” proposed the widely cited definition q an amibiotic or amibrotic substance is a substance pro- dxed by microorganisms, which has the capacity of inhibit- mm lhe growih and even of destroving other microorga- ms Later proposals” have sought both to expand and Mo pestrict the definition to include any substance produced Ru livimp organism that is capable of inhibiting the growih Reumival of one or more species of microorganisms in low mcentrations, The advances made by medicinal chemists emedify naturally occurring antibiotics and to prepare syn- Mie anilogues necessitated the inclusion of semisynihetic aynithetic derivatives in the definition. Therefore, a sub- axe is classified as an antibiotic if the following conditions E mel; 609345, 2005 + 298:74, 2000 « Science 295; fed, Chem, 44 Oh, C:), Am mm. Chem: Seo 385: 1573, 1900 = Bivorg, Me Êo w Malaria, the genoma asc cof ocre ds 2, available E tis a product of metabolism (although it may be duplicated or “sen have beem anticipated by chemical synthesis). E tis à synthetic product produced as a structural analogue of a nemrilly occurring amtibiotic À antagonizes the growth or survival of one or more species of inlemorganiams. A his effective in low concentrations The solution of the antibacterial antibiotie tyrocidin from de soil bacterium Bacillus brevis by Dubois suggested the pobuble existence 0f many antibiotic substances in nature und provided the impetas for the search for them. An organ- ized search of the order Actinomyeetales led Waksman and ussociates to isolate sireptomycin from Streprompyces griseus. The discovery that this antibiotic possessed in vivo activity against Mycobacterium tuberculosis in addition to numerous species of Gram-negative bacilli was electrifying. KH was now evident that soil microorganisms would provide a rich source of antibiotics. Broad screening programs were instituted to find amtibiotics that might be effective in the treatment of infections hitherto resistant to existing chemo- therapeutic agents, as well as to provide safer and more ef- fective chemotherapy, The discoveries of broad-spectrum antibacterial antibiotics such as chloramphenicol and the tet- racychines, antifungal amtibiotics such às nystatin and grisco- fulvin (sec Chapter 8), and the ever-inereasing number of antibioties that may be used to treat infectious agents that have developed resistance to some of the older antibiotics attest to the spectacular success of this approach as it has been applied in research programs throughout the world, CURRENT STATUS Commercial and scientific interest in the amtibrotic field has led to the isolation and identification of antibiotic substances that may be numbered in the thousands. Numerous semisyn- thetic and synthetic derivatives have been added to the total Very few such compounds have found application in general medical practice, however, because in addition to the ability to combat infections or neoplastic disease, an antibiotic must possess other attributes. First, it must exhibit sufficient selec- tive toxicity to be decisively effective against pathogenic microorganisms or neoplastic tissue, on the one hand. with- out causing significant toxic effects, on the other. Second, an antibiotic should be chemically stable enough to be isolated, processed, and stored for a reasonable length of time without deterioration of potency. The amenability of am antibiotic for oral or parenteral administration to be converted into suitable dosage forms to provide active drug in vivo is also important. Third, the rates of biotransformation and elimina- tion of the antibiotic should be slow enough to allow a con- vement dosing schedule, yet rapid and complete enough to facilitate removal of the drug and its metabolites from the body soon after administration has been discontinued, Some groups of antibiotics, because of certaim unique properbes, have been designated for specialized uses, such as the treut- ment of tuberculosis or fungal infections. Others are used for cancer chemotherapy. These antibiotics are described along with other drugs of the same therapeutic class: antifungal and antitubercular antibiotics are discussed in Chapter 8, and antineoplastic antibiotics are discussed in Chapter |2., The spectacular success of antibiotics in the treatment of 299 300 Wilson and Gisvold's Texthook of Organic Medicinal and Pharmaceutical Chemistry human diseases has prompted the expansion of their use into a number of related fields. Extensive use of their antimicro- bial power is made in veterinary medicine. The discovery that low-level administration 0f antibioties to meat-produc- ing animals resulted in faster growth, lower mortality, and beer quality has led to the use of these products as feed supplements. Several antibiotics aré used to control bacterial and fungal discases of plants, Their use in food preservation is being studied carefully. Indeed, such uses of antibiotics have necessitated careful studies of their long-term effects on humans and their effects on various commercial pro- cesses. For example, foods that contain low-level amounts of antibiotics may be able to produce allergic reactions in hypersensitive persons, or the presence of antibiotics in milk may interfere with the manufacture of cheese. The success of antibiotics in therapy and related fields has made them one of the most important products of the drug industry today. The quantity of antibiotics produced in the United States cach year may now be measured in several millions of pounds and valued at billions of dollars. With research activity stimulated to find new substances to treat viral infections that now are combated with only limited success and wilh the promising discovery that some antibiot- les are active against cancers that may be viral in origin, the future development of more antibiotics and the increase in the amounts produced seem to be assured COMMERCIAL PRODUCTION The commercial production of antibiotics for medicinal use follows a general pattem, differing in detail for each antibi- otic. The general scheme may be divided into six steps: (a) preparation of a pure culture of the desired organism for use in inoculation of the fermentation medium; (b) fermentation, during which the antibiotic is formed; (c) isolation of the antibiotic from the culture medium; (d) purification; (e) as- says for potency, sterility, absence of pyrogens, and other necessary data; and (1) formulation into acceptable and stable dosage forms. SPECTRUM OF ACTIVITY The ability of some antibiotics, such as chloramphenicol and the tetracyclines, to antagonize the growth ol numerous pathogens has resulted im their designation as broad-spec- trum antibiotics. Designations of spectrum of activity are of somewhat limited use to the physician, unless they are based on clinical effectiveness of the antibiotic against specific microorganisms. Many of the broad-spectrum antibiotics are active only in relatively high concentrations against some of the species of microorganisms often included in the “spectrum.” MECHANISMS OF ACTION The manner in which antibiotics exert their actions agains susceptible organisms vartes, The mechanisms of action of some of the more common antibioties are summarized il Table 10-1. In many instances, the mechanism of action notfully known: for a few (e.g. penicillins), the site of action is known, but precise details of the mechanism are still under: investigation. The biochemical processes of microorganisme ure a lively subject for research, for an understanding of those mechanisms that are peculiar to the metabolic systems of infectious organisms is lhe basis for the future develop ment of modem chemotherapeutic agents, Antibioties ta) TABLE 10-1 Mechanisms of Antibiotic Action Site of Action Antibiotic Process Interrupted Type of Activity Celt wall Bacitracin Mucopeptide synthesis Bactericidal Cephalosporins Cell wall crons-linking Bactencidal Cycloserine Synthesis of cell wall peptides Bactenicidal | Pemicilhn Cell wall cross-Linking Bactericidal l Vancomyein Mucopeptide synthesis Hactericical Í CeH membrane Amphotericin B Membrane function Fungicidal | Nystatin Membtans function Fumgicidal | , Polympxins Membrane integnty Bactenicidal | Ribasormes Chiloramphentcol Protein synthesis Bacteriostntic SOS subunit Erythrompcia Protein synthesis Bacterivstatio Linçompeins Protein synthesis Bacteriostatic | 308 subunit Aminoglycosides Protein synthesis and fidelity Bactencidal Tetracyclines Protein synthesis Bactenastatic Nucleir acids Actinomycin DNA and mRNA synthesis Pancicdal Griseofulvia Cell division, microtubulo assembly Fungistatic DINA andor RNA Mitomyein O DNA aynthento Pancidal Rifampin mRNA synthesis Buctericidal 302 Wilson and Gisvold's Texthook of Organic Medicinal and Pharmaceutical Chemistry Binding studies with intiated benzyl penicillin have shown that the mechanisms of action 0f various S-lactam antibioties are much more complex than previously assumed. Studies in E coli have revealed as many as seven different functional proteins, cach with an important role in cell wall biosynthesis.* These pemicillin-binding proteins (PBPs) have the following functional properites: e PBIPS | and 4 are transpeptidases involved in peptidoglycan symlhesis associated with cell elongation. knhibition results in spheroplast formation and rapid cell Iysis,* º caused by autolysins (bacterial enzymes that create nicks in the cell wall for attachment of new peptidoglycan units or for separation of daughter cells during cell division""). e PBP 2 is a transpeptidase involved in maimtaiming the rod shape of bacil. Inhibition results in ovoid or round forms thai undergo delayed Iysis, * PBP3is a transpeptidase required for septum formation during cell division. Inhibition results in the formation of filamen- tous forms containing rod-shaped units that cannot separate ris not yet clear whether inhibition of PBP 3 is Jethal to the bacterium e PBPs 4 ihrough 6 are carboxypeptidases responsible for the hydrolysis of D-alamne==alanine terminal peptide bonds of the cross-linking peptides. Inhibition of these enzymes 15 ap- parently not Tethal to the bacterium, even though cleavage of the terminal bealanine bond is required before peptide cross- timkage The various Bactam amtilbrotics difler in their affinities for PBPs. Pemicillin G binds preferentially to PBP 3, whereas the first-generation cephalosporins bind with higher affinity to PBP 1,. In contrast to other penicillins and to cephalospo- nos, which can bind to PBPs 1,2, and 3, amdinocillin binds onty to PBP 2. THE PENICILLINS Commercial Production and Unitage Until 1944, it was assumed that the active principle in peni- cilhin was a single substance and that variation im activity of different products was due to lhe amount of incrt materials in the samples. Now we know that during the biological elaboration of the antibiotic, several closely related com- pounds may be produced, These compounds differ chemi- cally in the acid moiety of the amide side chain. Variations in this moiety produce differences in antibiotic effect and in physicochemical properties, including stability. Thus, one can speak of penicillins as a group of compounds and iden- Ufy cach penieillin specifically. As each of the different pemi- cillins was first isolated, letter designations were used in the United States; the British used Roman numerals. Over 30 penicillins have been isolated from fermentation mixtures. Some of these occur naturally; others have been biosynlhesized by altering the culture medium to provide certain precursors that may be incorporated as acy] groups. Commercial production of biosynthetic penicillins today de- pends chiefly on various stiains of Penicillium notatum and P. chrysogenum. In recent years, many more penicillins have been prepared semisynthenicallv, and undoubtedly, many more will be added to the list im attempts to find superior products, Becuuse the penicillin first used in chemotherapy was not a pure compound and exhibited varying activity among sam- ples, il was necessary to evaluate it by microbiological assay. The procedure for assay was developed at Oxford, England, and the value became known as the Orford unit: | Oxtond unit is defined as the smallest amount of penteillin that will TABLE 10-2 Structure of Penicillins o ] ] al CON CHCOOH R-C—NH HH” CIC Hado Generic Name Chemical Name R Group Penicilin G Benzylpentcillin Pemteilin V Phenoxyimnethy|penicillin Methicilin 2,6-Dimelhoxyphenyi- pendelilin Nafcilln 2-Ethoxy=|-naplthyi- peniciltim Oxncillin 5-Meihyl-3-phenyl-4- isoxarolyIpenicillin Cloxacilha S-Muethy]-3(2. chlorophenyl 1-3- nsoxagolyIpenicillin Dictoxacilin 5-Menhyl-5-12,6- dichloropheny] + isoxazolyipenicilhin Dee-Aminobenzy!- peniciltin Ampicillia Amexicilio pe-Amino-p- IydrgxyheneyI penieilhim Cyclncilin |-Aminoçylohexyl- peniciltin Carbenicillin mCarbox yhenayl- penicilha a Carboxy-d-Uhbeny- pemicillin Ticarciltin am- say. and, ford will CHs TABLE 10-2—Continued Generic Name Chemical Name R Group Piperaciltin ur-(4-EthyI-2,3-dioxo-]- piperazinylearbonyi- I ii amino jbenzyipenicillin NH mes) k parti Mezlocillio c(1-Methanesilfony|-2- oxoimidazolidino- carbony amino jbenay] penicillin PM nhibit, in vitro, the growth of a strain of Staphylococcus in SU ml. of culture medium under specified conditions. Now fal pure crystalline penicillin is available, the United States Parmacopoeia (USP) defines unit as the antibiotic activity WL6 pg of USP penicillin G sodium reference standard. The weight=unit relationship of the penicillins varies with he acy| substituent and with the salt formed of the free acid: Emg of penicillin G sodium is equivalent to 1,667 units, | ng of penicillin G procaine is equivalent to 1,009 units, and Umg of penicillin G potassium is equivalent to 1,530 units. The commercial production of penicillin has increased mukedly since its introduction. As production increased, he cost dropped correspondingly. When penicillin was first duilable, 100,000 units sold for 820, Currently, the same quintity costs less than a penny. Fluctuations in the produc- dom of penicillins through the years have reflected changes alhe relative popularity of broad-spectrum antibiotics and fencillins, the development of penicillin-resistant strains of devcral pathogens, the more recent introduction of semisyn- delic penicillins, the use of penicillins in animal feeds and He veterinary purposes, and the increase in marketing prob- dems im q highly competitive sales area Table 10-2 shows the general structure of the penicillins Mú relates the structure of the more familiar ones to their nous designations. Womenciature e nomenciature of penicillins is somewhat complex and Rj cumbersome. Two numbering systems for the fused Meyclic heterocyclic system exist. The Chemical Abstracts em imitiates the numbering with the sulfur atom and as- dois lhe ring nitrogen the 4 position. Thus, penicillins are Bim as 4-thia-l-azabicyclo[3.2.0]heptanes, according to iptem. The numbering system adopted by the USP is mverse of the Chemical Abstracts procedure, assigning Chapter 10 = Antibacierial Antibiotics 303 number | to the nitrogen atom and number 4 to the sulfur atom. Three simplified forms of penicillin nomenclature have been adopted for general use. One uses the name “penam”” for the unsubstituted bicyclic system, including the amide carbonyl group, with one of the foregoing number ing systems às just described. Thus, penicillins generally are designated according to the Chemical Abstracts system as S-acylamino-2,2-dimethyIpenam-3-carboxylic acids. The second, seen more frequently in the medical literature, uses the name “penicillanic acid" to describe the ring system with substituents that are generally present (Le, 2,2-di methyl and 3-carboxyl). A third form, followed in this chap- ter, uses trivial nomenciature to name the entire 6-carbon- Ylaminopenicillanic acid portion of the molecule penicillin and then distinguishes compounds on the basis of the R group of the acyl portion of the molecule. Thus. penicillin G is named henzylpenicillin, penicillin V is phenoxymeth- ylpenicillin, methicillin is 2.6-dimethoxyphenyIpenicillin, and so on. For the most part. the larer two systems serve well for naming and comparing closely similar pentcillin structures, but they are too restrictive to be applied to com- pounds with unusual substituents or to ring-modified deriva- tives, Stereochemistry The penicillin molecule contains three chiral carbon atoms (€-3, 0-5, and €-6). AJ naturally occurring and microbio- logically active symthetic and semisynthetic penicillins have the same absolute configuration about these three centers The carbon atom bearing the acylamino group (C-6) has the L configuration, whereas the carbon to which the carboxyl group is attached has the p configuration. Thus, the acylam ino and carboxyl groups are trans to each other. with the former in the a and the latter in the B orientation relative to the penam ring system, The atoms composing the 6- aminopenicillanic acid portion of the structure are derived biosynthetically from two amino acids, L-cysteine (8-1. C- 5, €.6, €-7, and 6-amino) and t-valine (2,2-dimethy], C-2, €-3, N-4, and 3-carboxyl). The absolute stereochemistry of the penicillins is designated 38:5R:6R, as shown below [0] o mei 4 - e 4 o - . N 5 CH, 8 5.8 CM, NH 5 A NH—— bo Ka 7 | 5 , a , = ú ó 4% CH; d 1 (2 y A = ==) HO HO Chemical Abstracts usp pe 5 pe ] | hs ; Na E p= q E, Penam + ta) [5] HO à H Penicillanio Acid NH 1] &” Chemical Abstracts Chapter 1) = Antibacterial Antibiotics 305 and penaldic acid, The second path involves a com- gement of penicillanic acid to a penillic acid a series of intramolecular processes that remain to ate completely. Pemilho acid (an imidazoline-2- Múviio noi) readily decarboxylates and sulfers hy- Ho fing opening under acidic conditions to form a sec- ingor end product of acid-catalyzed penicillin degrada- c acid, Pemeilloic acid, the mujor product under weaklv acidic to alkaline (as well as enzy- dividi lyric conditions, cannot be detected as an inter- e under strongly acídic conditions, It exists in equilib- penamaldic acid, however, and undergoes atom in acid to form penilloic acid. The third product of the degradation is penicilloaldehyde, hy decarboxylation of penaldic acid (a derivative of yde). Wing the pH of aqueous solutions within a range Sand refngerating the solutions, aqueous prepara- Soluble penicillins may be stored for up to several felationship of these properties to the pharma- penicillins has been reviewed by Schwartz and * Some buffer systems, particularly phos phates exert a favorable effect on pemicillim stability, of the pH effect, Finholt et al.2 showed that fflees may catalyze penicillin degradation, however, e adjusted to obtain the requisite ions. Hydroalco- of pemcillin G potassium are about as un- us solutions” Because penicillins are inacti- E metal roms such as zine and copper, it has been at the phosphates and the citrates combine with úls to prevent their existence as free tons in solu- tityo co are o y | NCHCHO (CH,))C—CHCO,H the IDA | | | pt gran “E(CHaa are Ss” co SH NH,HCI : | | CO,C(CH) có pes dz E) do NH—CHCO,H I p- +Butyl a-phthalimidomalon- p-Penicillamine HCI da aldhydate OCH lily 1. HANNH, 2. sq. HCI os s CIMOCH CONHCH—GH “etc OC e HAN CH—H CICHO, Co) N | j e NH—CHCO,H (atada do DEL saai ; : OC(CH,); OC(CH,, 1. Hei 2. Pyridine camocconncH—cH” otcHys —1-KOM (one equiv) CUHOCH;CONHCH—GH >c(cH) I [Oo CL CANSCESNCA, di ia I | Npc po NH—CHCO,H CO—N—CHCO,K OH Figure 10-2 = Synthesis of phenoxymethylpenicillin Oxidizing agents also inactivate penicillins, but reducing agents have little effect on them, Temperature affects the rate of deterioration; although the dry salts are stable at room temperature and do not require refrigeration, prolonged heat- ing inactivates the penicillins. Acid-catalyzed degradation im the stomach contributes strongly to the poor oral absorption of penicillin. Thus, ef- forts to obtain penicillins with improved pharmacokinetic and microbiological properties have focused on acyl func- tionalities that would minimize sensitivity of the S-lactam cmg to acid hydrolysis while maintaiming antibacterial ac- tivity. Substitution of an electron-withdrawing group in the a position of benzylpenicillin markedly stabilizes the pemcil- lin to acid-catalyzed hydrolysis, Thus, phenoxymethyIpent- cillin, «= aminobenzylpenicilhin, and a-halobenzyIpenicillin are significantly more stable than benzylpenicillin in acid solutions. The increased stability imparted by such electron- wiihdrawing groups has been attributed to decreased reactiv- ity (nucleophilicity) of the side chain amide carbony! oxygen atom toward participation in S-laciam ring opening to form penicillenic acid. Obviouslv, o-aminobenzyIpenicillin (am- picillin) exists as the protonated form im acidie (as well as neutral) solutions, and the ammonium group is known to be powerfully electron-withdrawing. Bacterial Resistance Some bacteria, in particular most species of Gram-negative bacilli, are naturally resistant to the action of penicillins, Other normally sensitive species can develop pemicillin re- sistance (either through natural selection of resistant individ- 306 Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry 5 R-CONH-CH—CH C(CHola | se CO-N—CH—cO,H ue A Penicillin e a Fá COM o ;CH=NH=CH=CO,H CHcyoS DE N= Í EHTA a NÇ HS—C—cH, = No CH, > Rea OM no sa CH, ç 0H / 4,0 coM A Penicillonic Acid Penililc Acid Re “DO, HO di P— CCHys 8- CICHs R-CONH—G=CH—NH—CH—COsH R—CONH—CH—CH om R—CONH—CHy—CH pe COM H5-C—-CH, coM Nº tom NT “com CH, H H Penamaldic Acid Penicillaic Acid Penillole Acid o CH—-C—CH-COH + R—CONH-CH—CHO Es A—CONH=CH,;—CHO HS NH; COM : Penicilamina Penaídic Acid Penicilloaldehydeo Figure 10-3 =» Degradation of penicillins. uals or through mutation). The best understood and, proba- bly, the most important biochemical mechanism of penicillin resistance is the bacterial elaboration of enzymes that inacti- vate penicillins. Such enzymes, which have been given the nonspecific name penicillinases, are of two general types: B-lactamases and acylases. By Far the more important of these are the S-lactamases, engzymes that catalyze the hy- drolytic opening of the 8-lactam ring of penicillins to pro- duce inactive penicilloic acids, Synthesis of bacterial 5-lac- tamases may be under chromosomal or plasmid R factor control and may be either constitutive or inducible (stimu- lated by the presence of the substrate), depending on the bacterial species. The well-known resistance among sirains of Staphylococcus aureus is apparently entirely due to the production of an inducible S-lactamase, Resistance among Gram-negative bacilh, however, may result from other, poorly characterized “resistance factors” or constitutive 5 lactamase elaboration. 8-Lactamases produced by Gram- negative bacilli appear to be cytoplasmic enzymes that re- main in the bacterial cell, whereas those elaborated by 5. aureus are synthesized in the cell wall and released extracel- lulariy. B-Lactamases from different bacterial species may be classified?º by their structure, their substrate and inhib- itor specificities, their physical properties (pH optimum, isa electric point, molecular weight, etc.) and their immunologs cal properties. Specific acvlases, engymes that can hydroly ze the acybum ino side chain of penicillins, have been obtained from sevent species of Grum-negative bacteria, but their possible role bacterial resistance has not been well defined. These qm zymes find some commercial use in the preparation 0f6 aminopenicillanic acid (6-APA) for the preparation of sem synlhetic penicillins. 6-APA is less active and hydrolyzal more rapidly (enzymatically and nonenzymatically) fas penicillin. Another important resistance mechanism, especially E Gram-negative bacteria, is decreased permeability to penidk lins. The cell envelope in most Gram-negative bactena more complex than in Gram-positive bacteria. It contains outer membrane (linked by lipoprotein bridges to the pepih doglycan cell wall) not present in Gram-positive bacigi which creates a physical barrier to the penetration of am ics. especially those that are hydrophobic.” Small hyda philic molecules, however. can traverse the outer mem through pores formed by proteins called porins.* Altera of the number or nature of porins in the cell envelope? dl 308 Wilvon and Gisvoled's Texthook of Organic Medicinal and Pharmaceutical Chemistry fore, may be an important component of carbenicillin's ac- tivity against some ampicillin-resistant organisms. f-Lacta- mases produced by Pseudomonas spp., however, readily hydrolyze carbenicillin. Although carbenicillin às also some- what resistant to staphylococcal S-lactamase, it is consider- ably less so than methicillin or the isoxazoyl penicillins, and its inherent antistáphylococeal activity is less impressive than that of the penicillinase-resistant penicillins. The peni- cillinase-resistant penicillins, despite their resistance to most B-lactamases, however, share the lack of activity of penícil- lin G against Gram-negative bacilli, primarily because of an inability to penetrate the bacterial cell envelope. Compared with the aminoglycoside antibiotics, the po- tency of carbenicillin against such Gram-negative bacilli as Pseudomonas acruginasa, Proteus vulgaris, and Klebsiella pnemmoniae is much less impressive. Large parenteral doses are required to achieve baciericidal concentrations in plasma and tissues, The low toxicity of carbenicillin (and the pemicil- lins in general), however, usually permits (in the absence of allergy) the use of such high doses without untoward effects. Furthermore, carbemcillin (and other penicillins), when combined with aminoglycosides, exerts a synergistic bacteri- cidal action against bacterial species sensitive to both agents, frequently allowing the use of a lower dose of the more toxic aminoglvcoside than is normally required for treatment of a life-threatening infection. The chemical incompatibility of penicillins and aminoglycosides requires that the two antibi- otics be administered separately; otherwise, both are inacti- vated. Iyengar et al. showed that acylation of amino groups in the aminoglycoside by the B-lactam of the penicillin oc- curs. Unlike the situation with ampicillin, the introduction of asymmetry at the «-benzy] carbon in carbenicillin imparts little or no stercoselectivity of antibacterial action; the indi- vidual enantiomers are nearly equally active and readily epi- merized to the racemate in aqueous solution. Because dt is a derivative of phenyImalonic acid, carbenicillin readily de- carboxylates to benzylpemeillin in the presence of acid; therefore, it is not active (as carbemicillin) orally and must be administered parenterally, Estenfication of the a-carboxy! group (e.g.cas lhe 5-indanyl ester) partially protects the com- pound from acid-catalyzed destrucuon and provides an or- ally active derivative that is hydrolyzed to carbenicillin in the plasma, The plasma levels of free carbenicillin achieved with oral administration of such esters, however, may not suffice for effective treatment of serious infections caused by some species of Gram-negative bacilli, such as P, acrugi- Hosa. A series of a-acylureido-substituted penicillins, exempli- fied by azlocillin, mezlocillin, and piperacillin, exhibit greater activity against certain Gram-negative bacilli than carbenicillin. Although the acylureidopenicillins are acy- lated derivatives of ampicillin, the antibacterial spectrum of activity of the group is more like that of carbenicillin, The aevlureidopenicillins are, however, superior to carbenicillin against Alebsieila spp., Enterobacter spp. and P. aerugi- nosa. This enhanced activity às apparently not due to &- lactamase resistance, in that both inducible and plasmid- mediated S-lactamases hydrolyze these penicillins. More facile penetration through the cell envelope of these particu- lar bacterial species is the most likely explanation for the greater potency. The acylureidopenicillins, unlike ampicil lin, are unstable under acidic conditions; therefore, they am not available for oral administration. Protein Binding The nature of the acylamino side chain also determines the extent to which penicillins are plasma protein bound, Quant tative structure-activity relationship (QSAR) studies of the binding of penicillins to human serum” * indicate that he drophobic groups (positive 7 dependence) im the side chair appear to be largely responsible for increased binding lo serum proteins. Pemicillins with polar or ionized substituents, in the side chain exhibit low-to-intermediate fractions dd protein binding. Accordingly, ampicillin, amoxicillin, and cyclacillin experience 25 to 30% protein binding, and cas benicillin and ticarcillin show 45 to 55% protein binding Those with nonpolar, lipophilic substituents (nafeillin ash isoxazoyl penicillins) are more than 90% protein bound. The) penicillins with less complex acyl groups (benzylpemeillin, phenoxymethylpenicillin, and methicillin) fall in the range) of 35 10 80%, Protein binding is thought to restrict the tissue! availability of drugs if the fraction bound is sufficiently higiol thus, the tissue distribution of the penicillins in the highly bound group may be inferior to that of other pemicillins The similanity of biological half-lives for various penicillins, however, indicates that plasma protein binding has little eb fecton duration of action, All of the commercially available penicillins are secreted actively by the renal active transpoll system for anions. The reversible nature of protein bindil does not compete effectively with the active tubular secs tion process. EEFES O FEREZELRREES E FEREESETES: Allergy to Penicillins Allergic reactions to various pemicillins, ranging in seven from a vanety of skin and mucous membrane rashes to ds fever and anaphylaxis, constitute the major problem assoc ated with the use of this class of antibiotics. Estimates pla the prevalence of hypersensitivity to penicillin G Uroughod the world between | and 10% of the population. In the Unial States and other industrialized countries, it is nearer de higher figure, ranking penicillin the most common cause df drug-induced allergy. The penicillins most frequently impls cated in allergic reactions are penicillin G and ampla Virtually all commercially available penicillins, howem have been reported to cause such reactions; in fact, cross sensitivity among most chemical classes of 6-acylaminoper icillanic acid derivatives has been demonstrated The chemical mechanisms by which penicillin prepare tions become antigenic have been studied extensively EM dence suggests that pemcillins or their rearrangement po ucts formed in vivo (e.g. penicillenic acids)*! react lysine e-amino groups of proteins to form pemicilloy pa teins, which are major antigenic determinants. * elinical observations with the biosynthetic penicillins G am V indicated a higher incidence of allergic reactions w unpurificd, amorphous preparations than with highly p fied, crystalline forms, suggesting that small amounts dl highly antigenic penicilloyl proteins present in umpunhal samples were a cause. Polymeric impuríties in amprlho ty ue Ce eu vd he dosage forms have been implicated us possible antigenic deerminamis and a possible explanation for the high fre quency of allergic reactions with this particular semisyn lhetic penicíllin. Ampicillin is known to undergo pH-depen- dent polymerization reactions (especially in concentrated adutons) that involve nucleophilic attack of the side chain amino group of one molecule on the S-lactam carbonyl car- don atom of a second molecule, and so on. The high fre quency of antigemicity shown by ampicillin polymers to- geiher with their isolation and characterization in some umpioiltin preparations supports the theory that they can con- inbute to ampicillin-induced allergy* Classification Vanous designations have been used to classify penicillins, Igsed on their sources, chemistry, pharmacokinetic proper lies, resistance to enzymatic spectrum of activity, and clini- dal uses (Table 10-33. Thus. penicillins may be biosynthetic, emisynthetic, or (potentially) synthetic; acid-resistant or poe orully or conly) parenterally active; and resistant to B Itumases (penicillinases) or not. They may have a narrow, mermediate, broad, or extended spectrum of antibacterial aeuvity and may be intended for multipurpose or limited glinicul use. Designations of the activity spectrum as narrow, memediate, broad, or extended are relative and do not nec gsarily imply the breadth of therapeutic application. Indecd, Me classification of pemeillin G as a “narmow-spectrum"” utlbrotic has meaning only relative to other penicillins. AÍ mugh the S-lactamase-resistant penicillins have a spectrum DE activity similar to that of penicillin G, they generally are jserved for the treatment of infections caused by penicillin E-resistant, B-lactamase-producing 5. aureus because their uvity against most penicillin G-sensitive bacteria is sig- Bificanty inferior. Similarly, carbemeillin and tearcillin unlly are reserved for the treatment of infections caused dy ampicillin-resistant, Gram-negative bacill because they alter no advantage (and have some disadvantages) over am- peillin or penicillin G in infections sensitive to them through the kidneys by active tubular to maintain an effecuve concentration in blood have led to the development of “repository” forms of this drug. Suspen- Chapter 10 = Antibacterial Amtibicria 309 Products Penidillin 6. For years, the most popular penicillin has been penicillin 6. or benzylpenicillin. In fact, with the ex ception of patients allergic to it, penicillin G remains the agent of choice for the treatment of more different kinds of bacterial infection than any other antibiotic. ltwas first made available as the water-soluble salts of potassium, sodium, and calcium. These salts 01 penicillin are inactivated by the gastric juice and are not effecuve when agdministered orulhy unless antacids, such as calcium carbonate, aluminum hy- droxide, and magnesium trisilicate, or a strong buffer, such as sodium citrate, is added, Also, because penicillin is abs sorbed poorly from the intestnal tract, oral doses must be very large, about 5 times the amount necessary with paren- teral administration. Only after the production of pemicillin had increased enough to make low-priced penteillin aval able did the oral dosage forms become popular. The water- soluble potasstum and sodium salis are used orally and par- enterally to achieve high plasma concentrations of penicillin G rapidly. The more water-soluble potassium salt usually is preferred when large doses are required. Situations in which hyperkalemia is a danger, however, as in renal failure, re- quire use of the sodium salt; the potusstum salt is preferred for patients on salt-free diets or with congestive heart condi- LLONS H =] H NA, , Penieillin G ) me N EN, Benzyipenicillin " F E — CHs . [od Je o HO The rapid elimination of penteillin from the bloodstream secretion and the need TABLE 10-3 Classification and Properties of Penicillins Oral Acid Absorption Penicillim Source Resistance (%) Meiirvipenicillin Bic yithenio Poor Por (20) Prenoymethy Ipeni Eiosyinthetio Good Good (60) Meihicilir Semisynthetic Pixar Nil Natcilhin Semisynthetic Fair Vaniable Araeillin Semisynthetic Checa Fair (30% Choaci lin Semieynthretic Good Good (50) Qahermacil im Semi yrthotic Good Good (5) dmpiihim Sermisymiletio Chond Fair (40) Mies hcilin Semisynthetlc Good Good (75) Cateniciltin Semisymihetic Poor Nil Ticarcilliny Semisymthotic Poor Nil EMeslocilhi h Sermesynthetho Poor Na peracillim Semi syntlgetie Poor Nil Plasma fi-Lactamase Protein Resistance Spectrum Clinical Binding (%) (5. aureus) of Activity Use Sbt) Nai Intermediate Miultipurpose 55-80 No Intermediate Meualthpasmpersas HO -s) Ves Niro Limited use sm) Yes Narrow Limited une HS Yes Narrow Limite use ER Yes Narrora Limited use 5-DR Yes Narrow Limited is 20-35 Na Hiro Multipurpone B0-25 No Broad Multipurpese SH Nes Extended Limitod que 45 Nm Extended Limiteul une St) No Extended Limite) usa so No Extended Limited use med Win) aver, pean É us ty to UT vater - For This from Drms 1, ef- alt of jeth- es à nsol- *pen- in so- lin), bihat icinal Reacting 2,6-dimethoxybenzoyl chloride with 6-APA forms 6-(2,6-dimethoxybenzamido)penicillanic acid. The sodium salt is a white, crystalline solid that is extremely soluble in water, forming clear, neutral solutions, As with ulher penicillins, itis very sensitive to moisture, losing about half of its activity in 5 days at room temperature. Refrigera- ton at 5ºC reduces the loss in activity to about 20% in the ame period. Solutions prepared for parenteral use may be hept as long as 24 hours if refrigerated. lt is extremely sensi- five to acid (a pH of 2 causes 50% loss of activity in 20 minutes); thus, it cannot be used orally. Methicillin sodium is particularly resistant to inactivation by ihe penicillinase found in staphylococei and somewlhat more resistant than penicillin G to penicillinase from Bacil- huscereus. Methicillin and many other penicillinase-resistant penicillins induce penicillinase formation, an observation dhat has implications concerning use of these agents in the itaiment of penicillin G=sensitive infections. Clearly, the me of a penicillinase-resistant penicillin should not be fol- lowed by pemicillin G. The absence of the benzyImethylene group of penicillin Gand the steric protection afforded by the 2- and 6-methoxy groups make this compound particularly resistant to enzy- matic hydrolysis. Methicillin sodium has been introduced for use in the Teument of staphylococeal infections caused by strains re- aistant to other penicillins. It is recommended that it not be med in general therapy, to avoid the possible widespread development of organisms resistant to il. The incidence of interstitial nephritis, a probable hyper- ensitivity reaction, is reportedly higher with methicillin than wi other penicillins. Qxacillin Sodium, USP. Oxacillin sodium, (5-methyl- iphenyl-4-isoxazolyDpenicillin sodium monohydrate (Pro- Saphlin), is the salt of a semisynthetic penicillin that is hahly resistant to inactivation by penicillinase. Apparently, lhe stenic effects of the 3-pheny! and 5-methyl groups of the mazolyl ring prevent the binding of this penicillin to the Elactamase active site and, hereby, protect the lactam ring om degradation in much the same way as has been sug- gested for methicillin. H is also relatively resistant to acid Indrolysis and, therefore, may be administered orally with good effect AA HH À E = E ti dia 5 ce a bica Pi NR. PÁ E N so CH TN CH, 4 = Na õ : o Oxaeillin Sodium Oxacillin sodium, which is available in capsule form, is pesonably well absorbed from the gastrointestinal tract, par- heularty im fasting patients. Effective plasma levels of oxa- lin are obtained in about | hour, but despite extensive pasmo protein binding, it is excreted rapidly through the dineys, Oxacillin experiences some first-pass metabolism ie liver to the 5-hydroxymethyl derivative. This metabo- de has antibacterial activity comparable to that of oxacillin Chapter 10 = Antibacterial Antibioties 31 but is less avidly protein bound and more rapidly excreted. The halogenated analogues cloxacillin, dicloxacillin, and floxacillin experience less 5-methyl hydroxylation The use of oxacillin and other isoxazolyIpenicillins should be restricted to the treatment of infections caused by staphy- lococci resistant to penicillin G. Although their spectrum of activity is similar to that of penicillin G, the isoxazolyIpeni- cillins are, in general, inferior to it and the phenoxymeth- ylpenicillins for the treaiment of infections caused by peni- cillin G-sensitive bacteria. Because they cause allergic reactions similar to those produced by other penicillins, the isoxazolylpenicillins should be used with great caution in patients who are penicillin sensitive. Cloxacillin Sodium, USP. The chlorine atom ortho to the position of attachment of the phenyl ring to the isoxazole ring enhances the activity of cloxacillin sodium, [3-(o-chlo- rophenyl>+5-methyl-4-isoxazolyl |penicillin sodium mono- hydrate (Tegopen), over that of oxacillin, not by increasing its intrinsic antibacterial activity but by enhancing its oral absorption, leading to higher plasma levels. In almost all other respects, it resembles oxacillin. Cloxaeillin Sodium Dicloxacillin Sodium, USP. The substitution of chlo- nine atoms on hoth carbons ortho to the position of attach- ment of the phenyl ring to the isoxazole ring is presumed to enhance further the stability of the oxacillin congener dicloxacillin sodium, [3-(2,6- dichlorophenyl)-5-methyl-d- isoxazolyl|penicillin sodium monohydrate (Dynapen, Pa- thocil, Veracillin) and to produce high plasma concentrations of it. lts medicinal properties and use are similar to those of cloxacillin sodium. Progressive halogen substitution, how- ever, also increases the fraction bound to protein in the plasma, potentially reducing the concentration of free amtibi- otic in plasma and tissues, Its medicinal properties and use are the same as those of cloxacillin sodium F— À MR ' E [1 b. Ah em 2 SM] CHs po] [1] a = Dicloxacillin Sodium Nafcillin Sodium, USP, Nafeillin sodium, 6-(2-ethoxy- E-naphthylpenicillin sodium (Unipen), is another semisyn- 312 thetic peniciltin that resulted from the search for penicil- linase-resistant compounds, Like methicilln, nafeillin has substiuems in positions ortho to the point of attachment of the aromatic ring to the carboxamide group of penicillin, No doubt, the ethoxy group and the second ring of the naphiha- lene group play steric roles im stabilizing nafcillin against penicillinase. Very similar structures have been reported to produce similar results in some substituted 2-biphenyipeni- cillins. HE o f , al “a Na” Po o Nateillin Sodium Unlike methicillin, nafeilhn is stable enough in acid to permitits use by oral administration. When il is given orally, its absorption is somewhat slow and incomplete, but satisfac- tory plasma levels may be achieved in about | hour. Rela- úvely small amounts are excreted through the kidnevs; most is excreted in the bile, Even though some eyclic reabsorption from the gut may occur, nafeillin given orally should be readministered every 4 to 6 hours. This salt is readily soluble in water and may be administered intramuscularly or intrave- nously to obtain high plasma concentrations quickly for the treatment of serious infections. Nafeillin sodium may be used in infections caused solely by penieillin G-resistant staphylococei or when streptococel are present also. Although it is recommended that it be used exclusively for such resistant infecuons, nafeillin is also ef- fective against pneumococei and group A B-hemolytic strep- tococei. Because. like other pemeillins, il may cause allergic side effects, it should be administered with care. Ampiciíllin, USP. Ampicillin, 6-[D-a-aminopheny lacet- amido |penicillanie acid, p-o-aminobenzylpenicillin (Penbri- ten, Polycillin, Omnipen, Ameill, Principen), mects another goal of the research on semisynthetic penicillins—an anti- bacterial spectrum broader than that of penicillin G. This product is active against the same Gram-positive organisms that are susceptible to other penicillins, and it is more active against some Gram-negative bacteria and enterococei than are other pemicillins. Obviously, the acamino group plays am important role in the broader activity, but the mechanism for its action is unknown. ht has been suggested that the amino group confers qn ability to cross cell wall barriers that are impenetrable to other penicillins. p(—=-Ampicillin, prepared from p=(=)-a-aminophenviacetic acid, às signifi- cantly more active than 2-( + -ampicillin. es rs Í 4 H is a E Nr NH; y Me Y “CH á - =) ni Ampicilin, USP Wilson and Gisvold's Texthook af Organic Medicinal and Pharmaceutical Chemistry Ampicillin is not resistant to penicillinase, and 1 produces the allergic reactions and other untoward effects found in pemicillin-sensitive patients. Because such reactions are rely tively rare, however, it may be used to treat infections caused by Gram-negative bacilli for which a broad-spectrum antihe otic, such as a tetracycline or chloramphenicol, may be indi cated bur not preferred because of undesirable reactions ar lack of bactericidal effect. Ampicillin is not'so widely aclive however, that it should be used as à broad-spectrum antibi otic in the same manner as the tetracyclines. It is particuluily useful for the treatment of acute urinary tract infections caused by E. coli or Proteus mirabilis and is the agent of choice against Haemophilus influenza infections. Ampiaik lin together with probenecid, to inhibit its active tubular ex cretion, has become a treatment of choice for gonorhes recent years. 9-Lactamase-producing strains of Gram-negr tive bacteria that are highly resistant to ampicillin, however appear to be increasing in the world population, The threa from such resistant strains is particularly great with Hd fluenzae and N. gonorrhocar because there are few alterar tive therapies for infections caused by these orgamsms Incomplete absorption and excretion of effective concentr tons in the bile may contribute to the effectiveness of amp cillin in the treatment of salmonellosis and shigellosis, Ampicillin is water soluble and stable in acid. The proto ated e-amino group of ampicillin has a pK, of 7.3, and it is protonated extensively in acidic media, which explain ampicillin's stability to acid hydrolysis and instability to ah kaline hydrolysis. [tis administered orally and is absorbal from the intestinal tract to produce peak plasma concentra tons in about 2 hours. Oral doses must be repeated aboul every 6 hours because it is excreted rapidly and unchangal through the kidnevs. It is available as 4 white, crystaline anhydrous powder that is sparingly soluble in water ora the colorless or slightly buff-colored crystalline triliydmme that is soluble in water. Either form may be used for om administration, in capsules or as a suspension. Earlier claim of higher plasma levels for the anhydrous form than for tt trihydrate following oral administration have been de puted.-** The white, crystalline sodium sal is very solubl in water, and solutions for infections should be administra! within | hour afier being made. ps pair ESeszspaet Es . SETE Bacampicillin Hydrochioride, usp. Bacampicillm he drochloride (Spectrobid) is the hydrochloride salt of lhe q ethoxycarbonyloxyethyl ester of ampicillin. It is a produ of ampicillin with no antibacterial activity. After oral absomo tion, bacampicillin is hydrolyzed rapidly by esterases inlh plasma to form ampicillin. A Sa o] | | da HH da Ne" EHs , NH ; ns c A DD + Ô Bacampicilin Hydrochioride Oral absorption of bacampicillin is more rapid and cu plete than that of ampicillin and less affected by food. 314 occasional vomiting, and diarrhea). lt seems doubtful that the high doses required for the treatment of serious systemic infectons could be tolerated by most patients, Indanyl car- benicillin occurs as the sodium salt, an off-white, bitter pow- der that is freely soluble in water. lt is stable in acid. h should be protected from moisture to prevent hydrolysis of the ester. Ticarcillin Disodium, Sterile, USP. Ticarcillin diso- dium, «-carboxy-3-thienylpenicillin (Ticar), is an isostere of carbemicillin in which the phenyl group is replaced by a thienyl group. This semisynthetic penicillin derivative, like carbenicillin, is unstable in acid and, therefore, must be ad- ministered parenterally. It is similar to carbenicillin in anti- bacterial spectrum and pharmacokinetic properties. Two ad- vantages for ticarcillin are claimed: (a) slightly better pharmacokinetic properties, including higher serum levels and a longer duration of action; and (b) greater in vitro po- tency against several species of Gram-negative bacilh, most notably 2. aeruginosa and Bacteroides fragilis. These ad- vantages can be crucial in the treatment of serious infections requiring high-dose therapy. x Re | ' nao Ha e" CHy ) Dea Pra Na q Ticarcillin Disodium Mezlocillin Sodium, Sterile, USP. Mezlocillin (Mez- lin) is an acylureidopenicillin with an antibacterial spectrum similar to that of carbenieillin and ticarcillin, however, there are some major differences, It is much more active against most Klebsielta spp.. P. aeruginosa, anaerobic bacteria (e.g. Streptococcus faecalis and B. fragilis). and H. influenzae. is recommended for the treatment of serious infections caused by these organisms. q fi HM NH ES neo é - NH To a AOH5 sa No EM É ) e + nd == O ne Meziocillin Sodium Mezlocillin is not generally effective against B-lactamase- producing bacteria, nor is it active orally. It is available as a white, crystalline, water-soluble sodium salt for injection. Solutions should be prepared freshly and, if not used within 24 hours, refrigerated, Mezlocillin and other acylureidopeni- elimos , nliles cratera, esonto qsnaliueas qlrasroçar oie netics. Peak plasma levels, half-life, and area under the time curve increase with increased dosage. Mezlocillin has less Wilson and Gisvold's Texihook of Organic Medicinal and Pharmaceutical Chemistry effect on bleeding time than carbemicillin, and itis less likely to cause hypokalemia. Piperacillin Sodium, Sterile, USP. Piperacillin (Pi pracil) is the most generally useful of the extended-spectrum acylureidopenicillins. It is more active than mezlocilim) against susceptible strains of Gram-negative acrobic bacill) such as Serraria marcescens, Proteus, Enterobacter, and Ob trobacter spp., and P. aeruginosa. Mezlocillin, however, ap pears to be more active against Providencia spp. and k pneumoniae. Piperacillin is also active against anacrobi bacteria, especially B. fragitis and 5. faecalis (enterococeus) B-lactamase-producing strains of these organisms are, how ever, resistant to piperacillin, which is hydrolyzed by 5, mp reus B-lactamase. The B-lactamase susceptibility of pipene cillin is not absolute because S-lactamase-producina ampicillin-resistant strains of N. gonorrhocae and H. im fluenzae are susceptible to piperacillin. PoBEsOZrrroOMe ma - Piperacillin Sodium Piperacillin is destroyed rapidly by stomach acid; them fore, it is active only by intramuscular or intravenous súmi) istration, The injectable form is provided as the while, ente talline, waler-soluble sodium sal. lts pharmacokinaill properties are very similar to those of the other acylureido penicillins. P-LACTAMASE INHIBITORS The strategy of using a B-lactamase inhibitor in combinahos with a S-lactamase-sensitive penicillin in the therapy fa infections caused by B-lactamase-producing bacterial st has, until relatively recently, failed to live up to its obvio promise, Early attempts to obtain synergy against such res tant strains, by using combinations consisting of a Had mase-resistant penicillin (e.g., methicillin or oxacillin) E competitive inhibitor and a B-lactamase-sensitive peniciil (e.g.. ampicillin or carbenicillin) to kill the organisms, pm with limited success. Factors that may contribute to (heh ure of such combinations to achieve synergy include (al failure of most lipophilc penicillinase-resistant penteihim to penetrate the cell envelope of Gram-negative bacia effecane, comcenhuinas, (1) fas seversice 'Cining ol po cillinase-resistant penicillins to S-lactamase, requiring hi concentrations to prevent substrate binding and hydro likely n (PE gotrum tocilin | bacilh. and Ci- ver, ap- and K nerobie oceus). e. how= y 5, aus pipera- eucing, — EH. mM mbinatiom verapy for) nal strain ts opvisaa siring h ' aydrol and (c) the induction of S-lactamases by some penicillinase- resistant penicillins. The discovery of the naturally occurring, mechanism- based inhibitor clavulanic acid, which causes potent and pro- gessive inactivation of B-lactamases (Fig. 10-4), has created enewed interest in B-lactam combination therapy. This in- teresthas led to the design and synthesis of additional mecha- asm-based B-lactamase inhibitors, such as sulbactam and robactam, and the isolation of naturally occurring, B-Inc- tums, such as the thienamyeins, which both inhibit S-lacta- maes and interact with PBPs. The chemical events leading to the inactivation of B-lacta- tuses by mechanism-hased inhibitors are very complex. In 4 mview of the chemistry of B-lactamase inhibition, Knowles” has described two classes of B-lactamase inhibi- tors: class | inhibitors that have a heteroatom leaving group & position | (e.g.. clavulanic acid and sulbactam) and class Winhibitors that do not (e.g., the carbapenems). Unlike com- pelitive inhibitors, which bind reversibly to the enzyme they ihibit, mechanism-based inhibitors react with the enzyme di much the same way that the substrate does. With lhe Ixumases, an acylenzyme intermediate is formed by reac- bmofthe S-lactam with an active-site serine hydroxyl group Wiihe enzyme. For normal substrates, the acylenzyme inter- mediute readily undergoes hydrolysis, destroying the sub- Hint and frecing the enzyme to attack more substrate. The viensyme intermediate formed when a mechanism-based Ibibitor is attacked by the enzyme is diverted by tauto- persm to à more stable imine form that hydrolyzes more oa cao Md | Ea Figure 10-4 = Mechanism-based inhibition of -lactamases Chapter 10 = Antibacterial Amtibiotics 315 slowly to eventually free the enzyme (transiemt inhibition), or for a class | inhibitor, a second group on the enzyme may be attacked to inactivate it. Because these inhibitors are also substrates for the enzymes that they inactivate, they are sometimes referred to as ““suicide substrates."” Because they cause prolonged inactivation of certain B- lactamases, class [ inhibitors are particularly useful in combi- nation with extended-spectrum, S-lactamase-sensitive peni- eillins to treat infections caused by B-lactamase-producing bacteria, Three such inhibitors, clavulanic acid, sulbactam, and tazobactam, are currently marketed in the United States for this purpose. À class 1 inhibitor, the carbapenem deriva- tive imipenem, has potent antibacterial activity in addition to its ability to cause transient inhibition of some 5-lactamases. Certain antibacterial cephalosporins with a leaving group at the C-3 position can cause transient inhibition of B-lacta- mases by forming stabilized acylenzyme intermediates. These are discussed more fully below in this chapter. The relative susceptibilities of various B-lactamases to inactivation by class | inhibitors appear to be related to the molecular properties of the enzymes* ** * gLactamases belonging to group A, a large and somewhat heterogenous group of serine enzymes, some with narrow (e.g., penicil- linases or cephalosporinases) and some with broad (j,e., gen- eral B-lactamases) specificities, are generally inactivated by class 1 inhibitors. A large group of chromosomally encoded serine S-lactamases belonging to group € with specificity for cephalosporins are, however, resistant to inactivation by class [ inhibitors. A small group of Zn” * -requiring metallo- q nr aaa Inactivation 7x ad EnOH + Hydrolysis Products í e o sd o A a ! rt En E ÇA o fre Eogep PR Tá ao 6 oo En ad- as in ins lis, cid ore sa 1H able n. a per nar- ss Of ives very | the ctum tpar used! skin mitos -pro= mtibi- EO smucture creates considerable ring strain and increases the reactivity of the B-lactam to ning-opening reactions. The side Chain is unique in two respects: it is a simple |-hydroxyethyl group instead of the familiar acylamino side chain, and it is mented to the bicyclic ring system rather than having the usual 8 orientation of the penicillins and cephalosporins. fe remaining feature is a 2-aminoethylhioether function ai C.2 The absolute stereochemistry of thienamycin has been determined to be 58:65:88. Several additional structurally telated antibiotics have been isolated from various Strepro- mes spp. including the four epithienamycins, which are Iemenic to thienamycin at €-5, C-6, or C-8, and derivatives m which the 2-aminoethylthio side chain is modified. 5 * Sa Thienamycin O Thienamycin displays outstanding broad-spectrum amti- facterial properties in vitro," ltis highly active against most erobic and anacrobic Gram-positive and Gram-negative xcteria, including S. aureus, P, aeruginosa, and B. fragilis, hmbermore, it is resistant to inactivation by most G-lacta- mues elaborated by Gram-negative and Gram-positive bac- tena and, therefore, is effective againstmany strains resistant b penicillins and cephalosporins, Resistance to lactamases pears to be a function of the a-|-hydroxyethyl side chain hecuse this property às lost in the 6-nor derivative and epi- enamycins with S stereochemistry show variable resis- ace to the differem B-lactamases. dn unfortunate property of thienamycin is its chemical ms ability im solution. It is more susceptible to hydrolysis both acidic and alkaline solutions than most B-lactam mibiotics, because of lhe strained nature of its fused ring diem containing an endocyclic double bond. Furthermore, E its optimally stable pH between 6 and 7, thienamycin lerpoes concentration-dependent inactivation. This inac- jon is believed to result from intermolecular aminolysis ittho B-lactam by the cysteamine side chain of a second olecule. Another shortcoming is its susceptibility to hy- Eolytic inactivation by renal dehydropeptidase-l (DHP-I),º Chapter 10 = Antibacierial Antiblotics 317 which causes it to have an unacceptably short half-life in vivo, Imipenem-Cilastatin, USP. Imipenem is N-formimi- doylthienamycin, the most successful of a series of chemi- cally stable derivatives of thienamycin in which the primary amino group is converted to a nonnucleophilic basic func- ton Cilastatin is an inhibitor of DHP-L The combination (Primaxin) provides a chemically and enzymatically stable form of thienamycin that has clinically useful pharmacoki- netic properties. The half-life of the drug is nonetheless short (ty, — 1 hour) because of renal tubular secretion of imipenem. Imipenem retains the extraordinary broad-spectrum antibac- térial properties 0f thienamyein, lts bactercidal activity re- sults from the inhibition 0f cell wall synthesis associated with bonding to PBPs |, and 2. Imipenem is very stable to most fHactamases. [tis an inhibitor of B-lactamases from certain Gram-negative bacteria resistant to other S-lactam antibiotics, e.g., P. acruginosa, S. marcescens, and Entero- bacter spp. Imipenem is indicated for the treatment of a wide vanety of bacterial infections of the skin and tissues, lower respira- tory tract, bones and joints, and genitourinary tract, as well as of septicemia and endocarditis caused by f-lactamase- producing strains of susceptible bacteria, These include uero- bic Gram-positive organisms such as $. aureus, $. epider- múdis, enterococei, and viridans streptococci; acrobic Gram- negative bacteria such as E, coli, Klebsiella, Serratia, Pro- videncia, Haemophilus, Citrobacter, and indole-positive Proteus spp., Morganella morganii, Acinetobacter and En- terobacter spp. and P, aeruginosa and anaerobes such as B. fragitis and Clostridium, Peptococeus, Peptidostrepto- coceus, Enhacterium, and Fusobacterium spp. Some Pseu- domonas spp. are resistant, such as P. maltophília and P. cepacia, as aré some methicillin-resistant staphylococei, Im- ipenem is effective against non=-lactamase-producing strains of these und additional bacterial species, but other less expensive and equally effective antibiotics are preferred for the treatment of infections caused by these organisms. The imipenem-cilastatin combination is marketed as a sterile powder intended for the preparation of solutions for intravenous infusion, Such solutions are stable for 4 hours at 25ºC and up to 24 hours when refrigerated. The concomitant Imipenem-Cilastatin 318 Wilsor and Gisvoldls Texthook of Organio Medicinal and Pharmaceutical Chemistry admmistratos ol imipenem and an aminoglycoside antibi- the preparation. Meropenem appears to be less epileptogeme eil otic results in synergistic antibacterial activity in vivo. The than imipenem when the two agents are used in the treatment tur two types of antibiotics are. however, chemically incompati- of bacterial meningitis, ble and should never be combined in the same intruvenous bottle. INVESTIGATIONAL CARBAPENEMS The extended spectrum of antibacterial activity associated with the carbapenems together with their resistance to inacti- vation by most S-lactamases make this class of B-lactams an attractive target for drug development. In the design of new carbapenems., structural variations are being investi- gated with the objective of developing analogues with ad- vantages over imipenem. Improvements that are particularly desired include stability to hydrolysis catalyzed by DHP-L* stability 10 bacterial metallo- B-lactamases (“carbapene- mases”Pº that hydrolyze imipenem, activity against MRSA; and increased potency against P. aeruginosa, espe- cualhy imipenem-resistant straims. Enhanced pharmacoki- netic properties, such as oral bioavailability and a longer duration of action, have heretofore received line emphasis in carbapenem analogue design Early struciure= activity studies established the critical im- portance of the 4º position of the double bond, the 3-car- boxyl group, and the 6-a-hydroxyethyl side chain for both broad-spectrum antibacterial activity and B-lactamase stabil- ty in carbapenems. Modifications, therefore, have concen- trated on variations at positions | and 2 of the carbapenem nucleus. The incorporation of a S-methyl group at the 1 position gives the carbapenem stability to hydrolysis by renal DHP-[.º* ** Substituents at the 2 position, however, appear to affect primanly the spectrum of antibacterial activity of the carbapenem by mfluencing penetration into bacteria. The capability of carbapenems to existas gwitterionic structures (as exemplified by imipenem and biapenem), resulting from the combined festures ola baste amine function attached to the 2 position and the 3-carboxyl group, may enable ihese molecules to enter bacteria via their charged porn channels. Meropenem. Meropenem is à second-generation car- bapenem that, to date, has undergone the most extensivo clinical evaluation.” It has recently been approved as Mer- rem for lhe treatment of infections caused by multiply-resis- tant bacteria and for empirical therapy for serious infections, such as bacrerial menimgitis, sepúcemia, pneumonia, and peritonitis. Meropenem exhibits greater potency against Gram-negative and anacrobic bacteria than does imipenem, butit is shghtly less active against most Gram-positive spe- cles. Tt is not effective against MRSA. Meropeném às not hydrolyzed by DHP-I and is resistant to most B-lactamases, mcluding a few carbapenemases that hydrolyze carbapenem. Like imipenem, meropenem is not active orally, lt is pro- vided as a sterile Iyophilized powder to be made up in normal saline or 5% dextrose solution for parenteral administration. Approximately 70 to 80% of unchanged meropenem is ex- creted in the urine following intravenous or intramuscular administration. The remainder is the inactive metabolite formed by hydrolytic cleavage of the B-lactam ring. The lower incidence of nephrotoxicity of meropenem (compared wih imipenem) has been corelated ih us greater stability to DHP- and the absence of the DHP-L inhibitor cilastatin in Biapenem. Biapenem is a newer second-peneratim carbapenem with chemical and microbiological propertis similar to those of meropenem.º” Thus, it has broad-spes Cc trum antibacterial activity that includes most acrobic Gram E negative and Gram-positivo bacteria and anacrabes. Hi ep penem is stable to DHP-I*” and resistant to most fact thai | mases. It is claimed to be less susceptible to metalh-d ig luctamases than either imipenem or meropenem, Ju is nao tea setive orally. OH PR HC” ii dO CHs CEPHALOSPORINS Historical Background The cephalosporins are S-lactam antibiotics isolated fe Cephalosporium spp. or prepared semisynthetically. Ma of the antibiotics introduced since 1965 have been serio thetic cephalosporins. Interest in Cephalosporwm É began in 1945 with Giuseppe Brotzu"s discovery tal al tures of €. acremonium inhibited the growth of a wide ety of Gram-positive and Gram-negative bacteria. Aba and Newton in Oxford, having been supplied cul the fungus in 1948, isolated three principal antibiotc com nents: cephalosporin Pl, a steroid with minimal antibao activity, cephalosporin N, later discovered to be idem with synnematin N (a penicillin derivalive now calhal 
