Current Drug Targets (v.17, #9)

Meet Our Editorial Board Member by Martina Sanderson-Smith (971-971).

From “An Enzyme Able to Destroy Penicillin” to Carbapenemases: 70 Years of Beta-lactamase Misbehaviour by Jean-Marie Frère, Eric Sauvage, Frédéric Kerff (974-982).
As early as 1940, Abraham and Chain described “an enzyme able to destroy penicillin”. In the late 1940's, penicillin-resistant strains of Staphylococcus aureus were found to be a clinical problem. They produced a penicillinase that could hydrolyze the amide bond in the β-lactam ring. Later, an enzyme mediated by an R-factor was isolated from Enterobacteriaceae. Methicillin and cephalosporins, both very poor substrates of the S. aureus enzyme, were found to be sensitive to this new enzyme. Third generation cephalosporins appeared to solve the problem, but further enzymes were selected that exhibited extended spectra and could for instance hydrolyze cefotaxime and/or ceftazidime. The discovery of carbapenems constituted a major advance for our antimicrobial arsenal: they inactivated most of the essential penicillin binding proteins effectively and escaped the activity of nearly all known -β lactamases. However, the metallo-β-lactamases, which had not been recognised as a major danger before 1990, were found to act as effective carbapenemases and started to spread in a worrying way. Moreover, carbapenem-hydrolyzing enzymes were found in each of the 3 classes of active-site serine β-lactamases.

Decoding the Structural Basis For Carbapenem Hydrolysis By Class A β-lactamases: Fishing For A Pharmacophore by Donatella Tondi, Simon Cross, Alberto Venturelli, Maria P. Costi, Gabriele Cruciani, Francesca Spyrakis (983-1005).
Nowadays clinical therapy witnesses a challenging bacterial resistance limiting the available armament of antibiotics. Over the decades strains resistant to all antibiotics have been selected while medicinal chemists were not able to develop agents capable of destroying them or to prevent their extension. In particular, carbapenem-resistant Enterobacteriaceae (CRE), representing one of the most common human pathogens, have been reported with increased frequency since their first identification twenty years ago. The enterobacterial carbapenemases differ from the extended spectrum β-lactamases (ESBL) in their ability to hydrolyze β-lactams, cephalosporins and most importantly monobactams and carbapenems. They are progressively spreading throughout the world, therefore leaving no effective β-lactam to cure bacterial infections. Several BLs-carbapenemase Xray structures have been determined making these enzymes attractive targets for structure-based drug design studies. However, very little has been done so far to powerfully address the inhibitor design issues for this emerging type of BLs. Here, we focus on the structural basis for molecular recognition and for broad spectrum activity of class A carbapenemases: based on available 3-dimensional structural information we identify a theoretical pharmacophoric model as a starting point for the development of needed carbapenemases inhibitors.

Structural and Functional Aspects of Class A Carbapenemases by Thierry Naas, Laurent Dortet, Bogdan I. Iorga (1006-1028).
The fight against infectious diseases is probably one of the greatest public health challenges faced by our society, especially with the emergence of carbapenem-resistant gram-negatives that are in some cases pan-drug resistant. Currently, β-lactamase-mediated resistance does not spare even the newest and most powerful β-lactams (carbapenems), whose activity is challenged by carbapenemases. The worldwide dissemination of carbapenemases in gram-negative organisms threatens to take medicine back into the pre-antibiotic era since the mortality associated with infections caused by these “superbugs” is very high, due to limited treatment options. Clinically-relevant carbapenemases belong either to metallo-β- lactamases (MBLs) of Ambler class B or to serine-β-lactamases (SBLs) of Ambler class A and D enzymes. Class A carbapenemases may be chromosomally-encoded (SME, NmcA, SFC-1, BIC-1, PenA, FPH-1, SHV-38), plasmid-encoded (KPC, GES, FRI-1) or both (IMI). The plasmid-encoded enzymes are often associated with mobile elements responsible for their mobilization. These enzymes, even though weakly related in terms of sequence identities, share structural features and a common mechanism of action. They variably hydrolyse penicillins, cephalosporins, monobactams, carbapenems, and are inhibited by clavulanate and tazobactam. Three-dimensional structures of class A carbapenemases, in the apo form or in complex with substrates/inhibitors, together with site-directed mutagenesis studies, provide essential input for identifying the structural factors and subtle conformational changes that influence the hydrolytic profile and inhibition of these enzymes. Overall, these data represent the building blocks for understanding the structure-function relationships that define the phenotypes of class A carbapenemases and can guide the design of new molecules of therapeutic interest.

B1-Metallo-β-Lactamases: Where Do We Stand? by Maria F. Mojica, Robert A. Bonomo, Walter Fast (1029-1050).
Metallo-β-Lactamases (MBLs) are class B β-lactamases that hydrolyze almost all clinically-available β-lactam antibiotics. MBLs feature the distinctive αβ/βα sandwich fold of the metallo-hydrolase/oxidoreductase superfamily and possess a shallow active-site groove containing one or two divalent zinc ions, flanked by flexible loops. According to sequence identity and zinc ion dependence, MBLs are classified into three subclasses (B1, B2 and B3), of which the B1 subclass enzymes have emerged as the most clinically significant. Differences among the active site architectures, the nature of zinc ligands, and the catalytic mechanisms have limited the development of a common inhibitor. In this review, we will describe the molecular epidemiology and structural studies of the most prominent representatives of class B1 MBLs (NDM-1, IMP-1 and VIM-2) and describe the implications for inhibitor design to counter this growing clinical threat.

β-lactam antibiotics have revolutionized modern medicine, but resistance to these drugs is a major public health crisis. Traditionally, class C β-lactamases were referred to as cephalosporinases due to their substrate preference for this particular class of β-lactams. However, the emergence of AmpC enzymes with extended-spectrum activity (extended-spectrum cephalosporinases or ESACs) is particularly worrisome, especially given that most clinical β-lactamase inhibitors are ineffective against these enzymes. This review summarizes structures of several extended spectrum class C β-lactamases and analyzes the structure-function relationship observed among them.

Structure-Function Relationships of Class D Carbapenemases by Jean-Denis Docquier, Stefano Mangani (1061-1071).
Class D carbapenemases, also known as Carbapenem-Hydrolyzing class D β-Lactamases (CHDLs) are of increasingly high clinical relevance, as they have been found in various important human pathogens, such as Acinetobacter baumannii and Klebsiella pneumoniae and contribute to the evolution of these pathogens towards extensively or totally-drug resistance (XDR/TDR) phenotypes. Essentially two main groups of phylogenetically-related enzymes have been described: one including the acquired OXA-23, OXA-24/40, OXA-51 and OXA-58 enzymes mostly in Acinetobacter baumannii, and the other including the OXA-48-related variants, i.e. OXA-54, OXA-162, OXA-163 and OXA- 181. In this article, the biochemical and structural features of class D carbapenemases will be discussed. Furthermore, the mechanistic hypothesis based on recently obtained crystal structures of the native forms of class D carbapenemases and mutants thereof, in complex with relevant substrates or inhibitors, will be critically reviewed. Finally, the mechanism of inhibition by available inhibitors, some of which are currently in clinical development, will be discussed.

For decades, the available anticancer therapies were mostly based on nonspecific cytotoxic regimens. These cytostatic combinations, while effective in some subpopulations of patients, are often limited by extensive toxicity and/or development of tumor resistance. Although standard chemotherapy still remains a common therapeutic tool in the fight with cancer, immunotherapy increasingly revolutionizes treatment strategy for several hematologic malignancies. For a subset of patients with B-cell lymphoproliferative disease, the introduction of subsequently developed classes of anti-CD20 monoclonal antibodies (mAbs) has resulted in improved overall response rates and, to some extent, patient overall survival. Rituximab, the most thoroughly-explored chimeric mouse anti-human anti-CD20 mAb, has been widely and successfully introduced to oncohematology, but also to other fields of medicine, such as transfusiology or rheumatology. Currently, several new generation anti-CD20 mAbs are undergoing different stages of preclinical and clinical studies of assessment to further improve the outcome and overcome mechanisms of resistance. The nature of the direct mechanisms responsible for the anticancer properties of different classes of anti-CD20 mAbs is still not fully understood. This is reflected in different approaches during the investigation of novel anti-CD20 agents. So far, three classes of anti- CD20 mAb have been described. In this review, we focus on CD20 antigen-targeting therapies both currently available and undergoing preclinical or clinical investigation for B-cell lymphoproliferative malignancies.

MicroRNAs in Breast Cancer: One More Turn in Regulation by Pilar E. Asensio, Eduardo T. Martin, Begona P. Merlo, Estefanía E. Armas, Ana L. Hernández (1083-1100).
MicroRNAs (miRNAs) are small non-coding RNA molecules that critically regulate the expression of genes. MiRNAs are involved in physiological cellular processes; however, their deregulation has been associated with several pathologies, including cancer. In human breast cancer, differently expressed levels of miRNAs have been identified from those in normal breast tissues. Moreover, several miRNAs have been correlated with pathological phenotype, cancer subtype and therapy response in breast cancer. The resistance to therapy is increasingly a problem in patient management, and miRNAs are emerging as novel therapeutic targets and potential predictive biomarkers for treatment. This review provides an overview of the current situation of miRNAs in breast cancer, focusing on their involvement in resistance and the circulating miRNA. The mechanisms of therapeutic resistance regulated by miRNAs, such as the regulation of receptors, the modification of enzymes of drug metabolism, the inhibition of cell cycle control or pro-apoptotic proteins, the alteration of histone activity and the regulation of DNA repair machinery among others, are discussed for breast cancer clinical subtypes. Additionally, in this review, we summarize the recent knowledge that has established miRNA detection in peripheral body fluids as a suitable biomarker. We review the detection of miRNA in liquid biopsies and its implications for the diagnosis and monitoring of breast cancer. This new generation of cancer biomarkers may lead to a significant improvement in patient management.