Address for correspondence and reprints: Professor A. Bryskier, Hoescht Marion Roussel, Direction des Recherches Anti-infectieuses, Clinical Pharmacology, 102, route de Noisy, 93230 Romainville, France Tel: +33 (0)1 49 91 51 21 Fax: +33 (0)1 49 91 50 20
The C-3′ quaternary ammonium cephems belong to group IV of the microbiological classification of cephalosporins. This group is divided into two subgroups according to the position of the quaternary ammonium moiety (C-3 or C-7). These compounds are structurally related to the third-generation cephalosporins (or group III of the microbiological classification), but in addition are featured with two or more of the following properties: broad-spectrum activity including Pseudomonas aeruginosa; activity against Enterobacteriaceae producing type 1 β-lactamase; the presence of a quaternary moiety. The zwitterionic properties of the C-3′ quaternary cephalosporins allow rapid penetration through the outer membrane of Gram-negative bacteria, stability to and low affinity for type 1 β-lactamases in the periplasmic space and high affinity for the penicillin binding proteins (PBPs). Among the currently available analogues cefpirome and cefozopran exhibit a well-balanced antibacterial spectrum against Gram-negative bacilli and Gram-positive cocci. Cefepime is less active against Staphylococcus aureus.
Since cephalothin and cephaloridine were introduced in 1964, cephalosporins have become a widely used and rapidly expanding class of antibacterial agents. A large number of derivatives have been synthesized by modifying the 7-amino-cephalosporinic-acid (7 ACA).
In recent years a number of highly active cephem derivatives have been reported, and some of them developed and introduced into clinical practice.
Many of the currently marketed injectable cephalosporins, such as cefotaxime, ceftriaxone or ceftazidime have the common feature of a 2-amino-5-thiazolyl-acetamido group at the 7-β position of the cephem nucleus. They show excellent activity against Gram-negative bacilli, except Pseudomonas aeruginosa, and moderate activity against Staphylococcus aureus isolates susceptible to oxacillin and methicillin (MSSA).
The wide-spread use of these agents was followed by the emergence of resistant mutant strains of microorganisms such as Enterobacter cloacae, Serratia marcescens and P. aeruginosa which constitutively produced high level of type 1 β-lactamases. One of the most apparent defects of these compounds is the lack of activity against Enterobacteriaceae producing derepressed AmpC enzymes 1.
Despite the number of cephalosporins and other antibacterial agents available, the treatment of patients with severe infections remains a common problem in daily clinical practice. Any progress should therefore be welcomed.
Among significant advances, the recent introduction of a positively-charged substituent at the C-3′ position of the cephem nucleus has overcome certain short-comings of earlier 2-amino-5-thiazolyl cephalosporins including activity against P. aeruginosa and Gram-negative bacilli producing type 1 β-lactamases.
NOVELTIES IN THE FIELD OF CHEMICAL STRUCTURES
The substituent at the C-3 position and the side chain at the 7-β position of the cephem nucleus are key functions to alter the antibacterial activity. Extensive modifications of the 7-β acylamido side chain in combination with the C-3 substitution has led to many useful cephalosporin drugs.
A significant step in the advance of the chemistry of cephalosporins was the introduction of the 2-amino-5-thiazolyl nucleus. The combination of this moiety with the syn (Z)-methoxyimino chain found in cefuroxime gave rise to the new so-called third generation cephalosporins of which cefotaxime was the lead compound 2. The introduction of the 2-amino-5-thiazolyl moiety greatly enhances the antibacterial activity against Gram-negative bacilli. The addition of the syn (Z)-methoxyimino residue enhanced the stability to broad spectrum β-lactamase hydrolysis.
After the discovery of cefotaxime, one of the aims of research in the field of cephalosporins was to modify the C-3 heterocyclic moiety to increase the plasma halflife. This target was reached with the synthesis of ceftriaxone 3. Many of the earlier compounds have a N -methyl-tetrazol thio at the C-3 position of the cephem nucleus which was found to give rise to problems of disulfiram-like reactions and coagulation abnormalities in man.
The goal of the research in the field of cephalosporins was to improve the antibacterial activity against P. aeruginosa, S. aureus and other Gram-positive cocci and Gram-negative bacilli producing type 1 β-lactamases. Cefsulodin and ceftazidime were the first cephalosporins with improved activity against P. aeruginosa to be used in this clinical setting. They bear a 1-pyridinium group at the C-3 position. This observation prompted the researchers to prepare a new series of cephalosporins with a 2-amino-5-thiazolyloxiimino chain at the 7-β-position and a positively charged group at C-3 (azolium group). Most of the C-3′ quaternary ammonium cephems are characterized by a condensed azolium moiety except for cefluprenam (Figure 1).
All these derivatives are zwitterionic compounds and have no net charge, since the negative charge of the carboxylic acid group is (internally) neutralized (compensated for) by the positive charge of the C-3′ quaternary ammonium group.
Ceftazidime is the earlier member of the oxi imino-2-amino-5-thiazolyl quaternary ammonium cephalosporins. Other cephalosporins contain a C-3′ pyridinium such as cefsulodin, cefpimizole or cephaloridine, but none of them possess a 2-amino-5-thiazolyl moiety. Ceftazidime is a dianionic cephalosporin, in contrast to other compounds (e.g. cefpirome and cefepime) which are zwitterionic derivatives.
Ceftazidime contains a carboxypropyloxiimino substituent attached to the acetyl chain. This additional carboxylic group provides acidic (negative charge) properties to this compound. The carboxylate anion of the cephem ring is neutralized by the positively charged quaternary ammonium group. The extra negative charge strongly influences the biological properties of ceftazidime (Figure 2).
Modifications of the 2-amino-5-thiazolyl ring have been performed. It was found that substitution of the thiazolyl ring with a chlorine atom increased the antibacterial activity against P. aeruginosa, whereas the activity against other Gram-negative bacilli decreased. Introduction of a 5-amino-2-thiadiazolyl nucleus instead of a 2-amino-5-thiazolyl ring enhanced the antipseudomonal activity but slightly decreased the overall activity against Enterobacteriaceae 4.
Studies have demonstrated that analogues bearing an hydroxyimino residue exhibit higher activity against S. aureus, but they are less active against Gram-negative bacteria than their methoxyimino counterparts. Introduction of a monofluoromethoxyimino group at the 7-position led to a two-fold increase in activity against most bacteria as compared with the methoxyimino counterpart (cefluprenam).
Among all the C-3′ quaternary ammonium cephalosporins, only a few of them, such as cefpirome, cefepime and cefozopran were introduced into clinical practice. A number of compounds are in clinical development in Japan, such as cefclidin, cefluprenam and cefoselis. In the field of C-3′ quaternary ammonium cephems, research continues in order to overcome the production of extended spectrum β-lactamases by Enterobacteriaceae, to increase the antipseudomonal activity and to try to obtain compounds active against methicillin-resistant isolates of S. aureus.
Extended-spectrum β-lactamases are recognized for their abilities to provide resistance to cefotaxime, ceftriaxone, ceftazidime and aztreonam and to some extent to C-3′ quaternary ammonium cephalosporins. However, MIC values for cefpirome or cefepime to TEM-type enzyme producing Enterobacteriaceae are < 8 mg/L, but these compounds are less stable to the hydrolysis by SHV-type enzymes (MIC values < 16 mg/L) except for SHV-5 (MIC 2-4 mg/L) 5. Substitution by catechol or pyridone moities gives rise to a new wave of cephalosporins which are stable to hydrolysis by these enzymes, such as RU 59863 6.
NOVELTIES IN THE FIELD OF MECHANISM OF ACTION
The effectiveness of cephalosporins against Gram-negative bacilli is due to a combination of the ability to penetrate the outer membrane, the stability to β-lactamase hydrolysis in the periplasmic space, and affinity for penicillin binding proteins (PBPs).
Nikaido et al. 7, using proteoliposome, demonstrated that cefclidin, cefpirome and cefepime penetrated the porin channel of Escherichia coli and E. cloacae more rapidly than did ceftazidime. Yoshimura et al. 8 showed that the C-7-β methoxyimino residue decreases the rate of penetration of monoanionic cephalosporins by a factor of > 10, through the outer membrane into the OmpF porin of E. coli. Non-2-amino-5-thiazolyl quaternary ammonium cephalosporins are hydrolysed at low concentrations (≤10 μM) of type 1 β-lactamases 9. Type 1 β-lactamases hydrolyse these cephalosporins (cefpirome, cefepime, cefclidin) more slowly than did the earlier compounds at low substrate concentrations.
Cefpirome, cefepime and cefclidin share low affinities or high Km values for type 1 β-lactamases and would be expected to be hydrolysed slowly in vivo 10,11. It has been shown that cefpirome, cefepime, cefclidin, and ceftazidime share a high affinity for PBP 3 of E. coli K-12 12.
CLASSIFICATION OF C-3′ quaternary AMMONIUM CEPHALOSPORINS
Cephalosporins belong to a complex class of β-lactam antibiotics. Numerous classifications have been proposed: chemical 13, pharmacokinetic 14, biological 15 and microbiological 15.
Within the microbiological classification, cephalosporins could be divided into six groups. Groups III, IV and V represent the broad-spectrum cephalosporins, while the C-3′ quaternary ammonium cephalosporins belong to group IV. This group of compounds can be divided into two subgroups, according to the position (C-3 or C-7) of the quaternary ammonium chain (Figure 3).
Group IV-1 C-7 quaternary ammonium cephalosporins
Few compounds have been designed in this subgroup. Three derivatives of this series, L-640876, L-642946, and L-652813 bind preferentially to PBP 2 of E. coli.
Group IV-2 C-3′ quaternary ammonium cephalosporins
These derivatives have been divided into two classes according to the aryl substituent at C-7: 2-amino-5-thiazolyl or 5-amino-2-thiadiazolyl nucleus.
Group IV-2A 2-amino-5-thiazolyl derivatives
Numerous compounds were described but only cefpirome and cefepime were introduced into clinical practice. All these compounds contain a syn alkoxyimino moiety at C-7. Cefpirome contains a cyclopentenopyridinium group at the C-3′ position. Cefepime has a N -methylpyrrolidinium group at the C-3′ position. Cefoselis (FK 037) is currently under development in Japan and cefquinone (HR 111) is used in veterinary medicine.
Group IV-2B 5-amino-2-thiadiazolyl derivatives
Two compounds are under development in Japan: cefclidin, which contains a 4-carbamoyl-1-quinuclidinium moiety. Cefluprenam has an original chemical structure with a fluoromethoxyimino moiety at C-7 and a propenyl side chain at C-3. Cefozopran with an imidazolopyridium was recently introduced in clinical practice.
DEFINITION OF GROUP IV CEPHALOSPORINS
Group IV covers molecules that differ in their chemical structure and antibacterial activity. Cephems, which belong to group IV possess two or more of the following characteristics: the presence of a C-3′ quaternary ammonium side chain and are zwitterionic compounds; a broad antibacterial spectrum including P. aeruginosa; good antibacterial activity against Enterobacteriaceae isolates producing type 1 β-lactamases such E. cloacae, S. marcescens or Citrobacter freundii. They have to fulfil the definition of group III 14,16.
NOVELTIES IN ANTIBACTERIAL ACTIVITIES
All the C-3′ quaternary compounds introduced for clinical use or under development possess the same antibacterial activity as cefotaxime, but they differ by adding antipseudomonal activity 17,18. There are differences, however, among these compounds: cefclidin is twice as active as ceftazidime against P. aeruginosa, but cefepime and cefpirome are two-fold less active than ceftazidime against P. aeruginosa (Figure 4).
Compared to ceftazidime these compounds are more active against Enterobacteriaceae producing type 1 β-lactamases such as S. marcescens, C. freundii, E. cloacae, E. coli, Providencia and M. morganii (MIC90 8 mg/L) which are resistant to ceftazidime (Table 1) 18. Studies have shown that cefpirome generates significant β-lactamase induction less frequently than ceftazidime against E. cloacae. Cefpirome has been shown to select derepressed mutants in vitro and in vivo to a lesser extent than ceftazidime 17.
Table 1. In vitro activity of C-3′ quaternary ammonium cephalosporin against Enterobacteriaceae
Against S. pneumoniae, the most active compound within the C-3′ quaternary ammonium cephems was cefpirome, whatever the resistance phenotype to penicillin G of the pneumococci isolates 19. Against penicillin-resistant isolates, cefepime and cefozopran exhibit equivalent activity to cefotaxime. Cefluprenam was weakly active and cefclidin is inactive (Figure 5).
Against S. aureus isolates susceptible to oxacillin, cefpirome was the most active compound. It was three times more active than cefozopran and cefluprenam and eight-times more active than cefepime. Ceftazidime was poorly active and cefclidin was inactive (Figure 6).
C-3′ quaternary ammonium cephalosporins belong to group IV of the microbiological classification. Their chemical structure is innovative and the azolium part as well as the 2-amino-5-thiazolyl ring and the oxime residue, constitute the backbone of all novel series of cephalosporin derivatives. Due to the C-3′ azolium part they have an original mechanism of action. Furthermore, they have overcome bacterial resistance by stability to and low affinity for cephalosporinases.
Among the available C-3′ quaternary ammonium cephalosporins, cefpirome followed by cefozopran have a well-balanced antibacterial spectrum including Gram-positive cocci and Gram-negative bacilli. The activity against enterococci is more controversial. Cefpirome exhibits in vitro activity against E.faecalis but not against E. faecium. Cefepime exerts an overall higher activity than cefluprenam except against S. aureus. Cefclidin is the most active derivative against Gram-negative bacilli, but it is ineffective against Gram-positive cocci. Cefoselis shares the same antibacterial activity as cefozopran.
The main aim of the preparation of these molecules was for microbiological advantages. All of them show approximately the same pharmacokinetic profile. They have an apparent elimination half-life of two hours and are mainly eliminated unchanged in urine.