Cephalosporin resistance, tolerance, and approaches to improve their activities

Cephalosporins comprise a β-lactam antibiotic class whose first members were discovered in 1945 from the fungus Cephalosporium acremonium. Their clinical use for Gram-negative bacterial infections is widespread due to their ability to traverse outer membranes through porins to gain access to the periplasm and disrupt peptidoglycan synthesis. More recent members of the cephalosporin class are administered as last resort treatments for complicated urinary tract infections, MRSA, and other multi-drug resistant pathogens, such as Neisseria gonorrhoeae. Unfortunately, there has been a global increase in cephalosporin-resistant strains, heteroresistance to this drug class has been a topic of increasing concern, and tolerance and persistence are recognized as potential causes of cephalosporin treatment failure. In this review, we summarize the cephalosporin antibiotic class from discovery to their mechanisms of action, and discuss the causes of cephalosporin treatment failure, which include resistance, tolerance, and phenomena when those qualities are exhibited by only small subpopulations of bacterial cultures (heteroresistance and persistence). Further, we discuss how recent efforts with cephalosporin conjugates and combination treatments aim to reinvigorate this antibiotic class.

Multiple Roles of Cu(II) in Catalyzing Hydrolysis and Oxidation of β-Lactam Antibiotics

The widely used β-lactam antibiotics such as penicillins and cephalosporins are known to be susceptible to Cu^II-catalyzed hydrolysis at their four-membered β-lactam ring. However, this study elucidates that Cu^II can in fact play multiple roles in promoting the hydrolysis and/or oxidation of β-lactam antibiotics under environmental aquatic conditions (pH 5.0-9.0 and 22 °C), depending on β-lactams' structural characteristics and solution pH. Most significantly, the β-lactam antibiotics that contain a phenylglycine primary amine group on the side chain can undergo direct oxidation by Cu^II via this functional group. On the other hand, the β-lactam ring of penicillins is susceptible to Cu^II-catalyzed hydrolysis, followed by oxidation of the hydrolysis product by Cu^II. In contrast, the β-lactam ring of cephalosporins is susceptible to Cu^II-catalyzed hydrolysis only. Solution pH influences the Cu^II-promoted transformation by affecting the β-lactam and Cu^II complexation through protonation/deprotonation of critical organic functional groups. When Cu^II acts as an oxidant to promote the transformation of β-lactam antibiotics to yield Cu^I, the overall role of Cu^II appears catalytic if the reaction occurs under ambient atmospheric condition, due to quick oxidation of Cu^I by oxygen to regenerate Cu^II. Compared to earlier literature that largely assumed only the hydrolytic catalyst role of Cu^II in promoting degradation of β-lactam antibiotics, the oxidative roles of Cu^II identified by this study mark important contributions to a more accurate mechanistic understanding.

Synthesis of Antibiotics

The synthesis of β-lactams, tetracyclines, and erythromycins as three of the major families of antibiotics will be described herein. We will describe why these antibiotics were the ultimate synthetic targets in the past and how modern synthetic organic chemistry has evolved to address these challenges with new, improved strategies and methods. An additional aspect we would like to highlight here is the fact that these first syntheses had to be particularly creative as most of the modern synthetic methods were not available at that time, or were developed in the course of these syntheses.

The evolving role of chemical synthesis in antibacterial drug discovery

The discovery and implementation of antibiotics in the early twentieth century transformed human health and wellbeing. Chemical synthesis enabled the development of the first antibacterial substances, organoarsenicals and sulfa drugs, but these were soon outshone by a host of more powerful and vastly more complex antibiotics from nature: penicillin, streptomycin, tetracycline, and erythromycin, among others. These primary defences are now significantly less effective as an unavoidable consequence of rapid evolution of resistance within pathogenic bacteria, made worse by widespread misuse of antibiotics. For decades medicinal chemists replenished the arsenal of antibiotics by semisynthetic and to a lesser degree fully synthetic routes, but economic factors have led to a subsidence of this effort, which places society on the precipice of a disaster. We believe that the strategic application of modern chemical synthesis to antibacterial drug discovery must play a critical role if a crisis of global proportions is to be averted.

Fungal Metabolites as Pharmaceuticals

Natural products, their derivatives or compounds based on natural product leads constitute ~50 % of clinically used pharmaceuticals. This review highlights pharmaceuticals currently used in Australia and New Zealand that have their origins in fungal metabolites, discussing the natural products chemistry and medicinal chemistry leading to their application as pharmaceuticals.

Iridium-catalyzed borylation of thiophenes: versatile, synthetic elaboration founded on selective C-H functionalization

Iridium-catalyzed borylation has been applied to various substituted thiophenes to synthesize poly-functionalized thiophenes in good to excellent yields. Apart from common functionalities compatible with iridium-catalyzed borylations, additional functional group tolerance to acyl (COMe), and trimethylsilyl (TMS) groups was also observed. High regioselectivities were observed in borylation of 3-and 2,5-di-substituted thiophenes. Electrophilic aromatic C-H/C-Si bromination on thiophene boronate esters is shown to take place without breaking the C-B bond, and one-pot C-H borylation/Suzuki-Miyaura cross-coupling has been accomplished on 2- and 3-borylated thiophenes.

Recent advances in the chemistry and biology of carbapenem antibiotics

The discovery of the olivanic acids and thienamycin aroused considerable interest amongst medicinal chemists and microbiologists around the world. The susceptibility of these agents to metabolic degradation has, however, been a major obstacle in their development. For many years the only notable success from such intensive research was the combination of imipenem with cilastatin, an inhibitor of the renal dipeptidase enzyme DHP-1. The enormous success of Primaxin for the treatment of a range of life-threatening bacterial infections provided the impetus for the discovery of totally synthetic, non-natural carbapenem derivatives that combine the broad spectrum of antimicrobial activity with stability to enzymatic degradation. This has indeed been realised in the development of meropenem; it possesses the broad spectrum of activity and resistance to beta-lactamases that are embodied in imipenem as well as displaying increased stability to human dehydropeptidases. Most recent research has focused upon the development of carbapenem antibiotics which combine broad spectrum antimicrobial activity and metabolic stability with oral absorption, for the treatment of community-acquired infections. Indeed, the pro-drug esters of the tricyclic carbapenems represent the first significant advance in this respect. However, the increased use of carbapenem antibiotics would undoubtedly accelerate the emergence of carbapenem-hydrolysing enzymes. The ultimate challenge could therefore be the design and synthesis of carbapenem derivatives that are resistant to these metallo-beta-lactamases. Due to the enormous problems encountered in the development of the carbapenem antibiotics, this area of research has, in the past, been described as a battlefield that did not bode well for the future [181]. Primaxin and meropenem proved however that these problems were not insurmountable, and are therefore a testimony to the persistence and dedication of those scientists in their war against bacterial infection.

The interaction of cephaloridine with model membrane systems and rat kidney lysosomes

The antibiotic cephaloridine has been shown to interact with phospholipid structures, using the techniques of ultraviolet difference spectroscopy, surface pressure measurements and liposome models. The results indicate that this interaction is at least partly hydrophobic in nature and help explain the disruptive effects of high concentrations of cephaloridine on both artificial and natural phospholipid structures (lysosomes). Low concentrations of cephaloridine were shown to inhibit a lysosomal membrane-bound phospholipase 2 and it is suggested that such an inhibition may explain the cephaloridine-induced stabilization of rat-kidney lysosomes.

Polymer-bound carbonic anhydrides in N-acylation of 7-aminocephalosporanic acid

Aminocephalosporanic acid tert-butyl ester was reacted with polystyrene-bound mixed carbonic-carboxylic anhydrides to give the corresponding N-acylated derivatives. Clevage of the tert-butyl protecting group with trifluoroacetic acid gave the corresponding cephalosporanic acid.

The simultaneous quantitative determination of cephalothin and cefazolin in serum by high pressure liquid chromatography

Conventional microbiological assay procedures for cephalosporins in serum do not allow the determination of serum concentrations if more than one cephalosporin is present in a single sample. An HPLC procedure has been developed which permits the simultaneous quantitative determination of cefazolin sodium and cephalothin sodium in serum. Reverse phase chromatography using methanol in 0.2 M ammonium acetate as the mobile phase was employed to separate and quantitate the two cephalosporins in a trichloroacetic acid supernatant solution prepared from serum.

Clinical bacteriology of cephazolin

Cephazolin, the latest parenteral cephalosporin produced by substitution of heterocyclic groups on the 7-aminocephalosporanic acid (7-ACA) nucleus, became available for clinical use in the U.K. in 1974. Its history is traced from the original Sardinian mould (1945) through isolation of cephalosporin C (1953) and 7-ACA (1962) to its discovery (1969) and subsequent clinical use. Its mode of actionis examined and its bactericidal nature confirmed, MICs of over 500 common pathogens are used to define its spectrum. The cut-off point for sensitivity and resistance is taken as 20 mg. per l. With 30 mug. discs zone sizes less than or equal to 14mm. indicate resistance. The clinical relevance of MICs and attainable serum and urine levels is examined by relating them to clinical response in 12 patients. A disc study compares cephazolin with 4 other available cephalosporins, namely cephaloridine, cephalothinm cephalexin and cephradine. Many major discrepancies indicate that they are not interchangeable. The laboratory and clinical implications are discussed, including the need for local policy decisions regarding choice of preparation. The change from a parenteral to an oral form should be preceded by a sensitivity test against the causal organism. The validity of using a single representative disc is questioned and the use of phrases such as 'sensitive to the cephalosporins' deprecated. Cephazolin is potent, broad-spectrum and bactericidal and is potentially life-saving in serious hospital infections. The cephalosporins rival the aminoglycosides as 'best-guess' primary choice and are preferable in pregnancy, the puerperium and in pulmonary infections. Within the group cephazolin will seriously challenge or even oust cephaloridine and cephalothin.

Influence of antibiotic stability on the results of in vitro testing procedures

Wick, Warren E. (The Lilly Research Laboratories, Indianapolis, Ind.). Influence of antibiotic stability on the results of in vitro testing procedures. J. Bacteriol. 87:1162-1170. 1964.-Certain antibiotics undergo at least partial degradation under the conditions of in vitro testing procedures. With cephalothin used as an example, experimental evidence is presented to indicate the necessity for re-evaluation of results obtained from in vitro sensitivity testing methods for some antibiotics. The in vitro activity of cephalothin, tetracycline, and chloramphenicol against a variety of gram-negative bacteria is described. Plate counts demonstrate changes in the viable cell population over a 48-hr period in tubes of minimal inhibitory concentration (MIC) tests, with an abrupt rise in MIC value for cephalothin between the 12th and 24th hr. Data obtained by chromatographic methods, showing the degradation of cephalothin for the same time interval, indicated that instability of the antibiotic between the 12th and 24th hr might adversely affect the results obtained from standard 20- to 24-hr in vitro antibiotic sensitivity testing methods. Because repeated administration of an antibiotic to a patient at 4- to 8-hr intervals reinforces the original concentration, a more accurate estimate of antibacterial activity of that antibiotic might preferably be related to this time interval.

Cephalothin, a New Cephalosporin with a Broad Antibacterial Spectrum

Cephalothin is 7-(thiophene-2-acetamido) cephalosporanic acid; it was prepared by N -acylation of the nucleus of cephalosporin C, 7-aminocephalosporanic acid. Cephalothin had a broad spectrum of antibiotic activity that was essentially unaffected by human serum or inoculum level, the activity of penicillinase, or pH variation of the growth medium. In vitro development of resistance by staphylococci could not be demonstrated, but the gram-negative organisms did develop a stepwise type of resistance to the antibiotic. Staphylococci made resistant in vitro to 5-methyl-3-phenyl-4-isoxazole penicillin were also resistant to cephalothin and to 6-(2,6-dimethoxybenzamido) penicillin; however, the mechanism of resistance to each antibiotic may have differed. Some complications involved in the laboratory evaluation methods currently in use in the field of antibiotics are examined.