Current Medicinal Chemistry - Anti-Infective Agents - Volume 4, Issue 3, 2005

The design and optimisation of the second-generation antibacterial oral streptogramin is reported in terms of semi-synthesis, structure-activity relationships, pharmaco-kinetics properties and antibacterial activities. Our endeavours led to the selection of two new combinations RPR131166/RPR132493 (30/70) and RPR202868/RPR132552 (30/70) whose antibacterial properties will be reported in detail. The overall profile of the latter association, which is currently undergoing clinical development, suggests that it could be useful for the treatment of community-acquired infections.

During recent years, new extended-spectrum β-lactamase-type enzymes (ESBL) have emerged worldwide. The new enzymes, called CTX-M-type enzymes, belong to Ambler class A β-lactamases, and possess a serine in the active center of the molecule. The family of CTX-M enzymes is grouped, on the basis of similarities in amino acid sequences, into five major phylogenetic categories: the CTX-M-1 group, the CTX-M-2 group, the CTX-M-8 group, the CTX-M-25 and the CTX-M-9 group. The designation “CTX-M” refers to potent activity against cefotaxime, an aminotiazolil cephalosporin with a methoximino group; these enzymes have only remanent activity toward ceftazidime, another aminotiazolil cephalosporin with a 2-carboxy-2-oxypropaneimino group instead of the methoximino group. The substrate specificity suggests the existence of an active centre in the enzymes able to provide the amino acids and structural requirements of this selective antibiotic hydrolytic profile. As regards the crystalline structure of the CTX-M enzymes, only that of Toho-1 has so far been resolved. There have recently been increasing numbers of reports in the medical literature describing the appearance of new CTXM- type enzymes that are able to hydrolyze ceftazidime, and indicating that the bacterial strains harboring these enzymes show high levels of resistance to ceftazidime. So far, it has been shown that only two unique amino acid replacements are required to confer this peculiar phenotype of ceftazidime hydrolysis (Asp240-Gly and Pro167-Ser, following the criteria for Ambler numbering). As regards cefotaxime hydrolysis, Ser130-Gly replacement in CTX-M-9 and Arg276-Asn in CTX-M-4 decreases hydrolytic activity against cefotaxime. However, no further studies have been carried out to elucidate which new amino acids are necessary for cefotaxime incorporation and hydrolysis. Our research team is currently investigating this, and we have recently discovered new amino acids with critical activity against cefotaxime. In summary, we report here up-to-date information about structural/functional studies of CTX-M- enzymes as well as a description of the enzymes and their phylogenetic relationships.

The extensive use of antimicrobials in community as well as nosocomial environments during the last half century has created a pressure that is able to select resistant microorganisms, transforming this “evolution” into one of the most dangerous phenomena of the last twenty years. Two different aspects of the same problem have to be examined: the appearance of “new opportunistic multiresistant microorganisms, and the assembly of resistance related genetic elements from heterologous sources in well known pathogens. In both cases, the bacterial response to this selective pressure is the acquisition and spread of a variety of determinants due to mutations of normal cellular genes, acquisition of foreign resistance determinants or a combination of these two genetic mechanisms. All these processes are generally present in contemporary Gram-positive pathogens that have evolved and spread over the last twenty years becoming a special, and perhaps unique, threat for the emergence of resistance in our era. Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, and Enterococci, which are currently isolated, are no longer the same organisms isolated 50 years ago: becoming multiresistant or predominant opportunistic pathogens, they have paid a “biological price” to the use of antibiotics in a short time period. Once established, a resistant strain may persist under selective pressure from numerous antimicrobials, furthermore, some resistances are widespread and others local, but it is still unclear why some bacterial lineages achieve epidemic spread whereas others, that are equally resistant, do not. Many interesting new antibiotics have been developed and recently marketed or are undergoing phase III clinical trials. These drugs, some of which have novel mechanisms of action, may help to counterbalance and, if used appropriately, prevent the spread of resistant bacteria. This review will consider some of these new drugs such as streptogramins, oxazolidinones, the newer fluoroquinolones, ketolides, daptomycin, new glycopeptides, glycylcyclines and the newer cephalosporins.

By the early nineteen seventies the mechanism of inhibition of peptidoglycan biosynthesis by various cell wallactive antibiotics was well established, and the Gram-positive bacterium Staphylococcus aureus had often been used in the studies. From the early days of penicillin it was known that cell wall-active antibiotics are typically bactericidal causing cell death and lysis, a phenomenon that has recently been described as programmed cell death. Uncovering the details of the molecular and cellular events occurring subsequent to inhibition of peptidoglycan biosynthesis, a legitimate aspect of knowledge of the mode of action of these agents, has had to await the advent of the omics era-genomics, proteomics and transcriptomics-over the past several years. Genome-wide transcriptional profiling using DNA microarrays of the response of S. aureus to challenge by cell wall active antibiotics has revealed a cell wall stress stimulon of genes upregulated in their expression by these agents. Several of the genes encode proteins involved in cell wall metabolism and their induction can be regarded as a response of the organism to preserve and repair the compromised cell wall. Expression of a significant number of cell wall stress stimulon member genes is controlled by a two-component regulatory system. Cell wall stress stimulons regulated by two-component systems have also been described in Bacillus subtilis and Listeria monocytogenes, and are probably common to all Gram-positive bacteria. Unfortunately S. aureus has not remained susceptible to cell wall-active antibiotics and methicillin-resistant strains are common, and vancomycinintermediate and-resistant strains have arisen. Interestingly, some member genes of the cell wall stress stimulon have been previously encountered in the context of methicillin-resistance or vancomycin-intermediate resistance. There is a threat of pan-resistant S. aureus strains and new antimicrobial agents are needed. Cell wall biosynthesis remains a viable target for new drugs, and recognition of the transcriptomic signature of the cell wall stress stimulon can be used to indicate a cell wall mode of action. Individual cell wall stress member genes may form the basis of a screen for cell wall-active agents. Finally, agents targeting cell wall stress stimulon member gene expression or proteins might enhance the activity of cell wall-active agents that induce the cell wall stress stimulon.

Farnesyltransferase catalyzing the transfer of a farnesyl residue from farnesylpyrophosphate to the thiol of a cysteine side chain of proteins carrying the C-terminal CAAX-tetrapeptide sequence has been one of the prime targets in the development of novel anti-cancer agents. From numerous farnesyltransferase inhibitors that have been described, several have reached advanced stages of clinical trials. In addition to mammals, farnesyltransferases were also identified in different pathogenic protozoa including Plasmodium falciparum, the causative agent of malaria tropica. Therefore, inhibition of farnesyltransferase has also been suggested as a new strategy for the treatment of parasitic infections. The overall sequence identity of human and P. falciparum farnesyltransferase is considerably low, however, amino acid differences in the active sites are rather small. Only few inhibitors have been assayed against P. falciparum farnesyltransferase; some of them displayed high activity against the isolated enzyme but were only moderately active when assayed against blood stages of P. falciparum. A number of established farnesyltransferase inhibitors or derivatives of them are highly active against the human enzyme or against human cancer cells, but generally displayed only micromolar activity against P. falciparum blood stages. However, there are examples of various inhibitors with nanomolar in vitro activity against P. falciparum. In a mouse model, activity has also been demonstrated in vivo. This may justify further efforts in the development of specific anti-malarial farnesyltransferase inhibitors.