The structures of pluracidomycins, new carbapenem antibiotics

ThnL, a B12-dependent radical S-adenosylmethionine enzyme, catalyzes thioether bond formation in carbapenem biosynthesis

Complex carbapenems are important clinical antibiotics used to treat recalcitrant infections. Their biosynthetic gene clusters contain three essential B12-dependent radical S-adenosylmethionine (rSAM) enzymes. The majority of characterized enzymes in this subfamily catalyze methyl transfer, but only one is required to sequentially install all methionine-derived carbons in complex carbapenems. Therefore, it is probable that the other two rSAM enzymes have noncanonical functions. Through a series of fermentation and in vitro experiments, we show that ThnL uses radical SAM chemistry to catalyze thioether bond formation between C2 of a carbapenam precursor and pantetheine, uniting initial bicycle assembly common to all carbapenems with later tailoring events unique to complex carbapenems. ThnL also catalyzes reversible thiol/disulfide redox on pantetheine. Neither of these functions has been observed previously in a B12-dependent radical SAM enzyme. ThnL expands the known activity of this subclass of enzymes beyond carbon-carbon bond formation or rearrangement. It is also the only radical SAM enzyme currently known to catalyze carbon-sulfur bond formation with only an rSAM Fe-S cluster and no additional auxiliary clusters.

Chemical labeling strategies for small molecule natural product detection and isolation

Covering: Up to 2020.It is widely accepted that small molecule natural products (NPs) evolved to carry out a particular ecological function and that these finely-tuned molecules can sometimes be appropriated for the treatment of disease in humans. Unfortunately, for the natural products chemist, NPs did not evolve to possess favorable physicochemical properties needed for HPLC-MS analysis. The process known as derivatization, whereby an NP in a complex mixture is decorated with a nonnatural moiety using a derivatizing agent (DA), arose from this sad state of affairs. Here, NPs are freed from the limitations of natural functionality and endowed, usually with some degree of chemoselectivity, with additional structural features that make HPLC-MS analysis more informative. DAs that selectively label amines, carboxylic acids, alcohols, phenols, thiols, ketones, and aldehydes, terminal alkynes, electrophiles, conjugated alkenes, and isocyanides have been developed and will be discussed here in detail. Although usually employed for targeted metabolomics, chemical labeling strategies have been effectively applied to uncharacterized NP extracts and may play an increasing role in the detection and isolation of certain classes of NPs in the future.

Carbapenems: past, present, and future

In this review, we summarize the current "state of the art" of carbapenem antibiotics and their role in our antimicrobial armamentarium. Among the β-lactams currently available, carbapenems are unique because they are relatively resistant to hydrolysis by most β-lactamases, in some cases act as "slow substrates" or inhibitors of β-lactamases, and still target penicillin binding proteins. This "value-added feature" of inhibiting β-lactamases serves as a major rationale for expansion of this class of β-lactams. We describe the initial discovery and development of the carbapenem family of β-lactams. Of the early carbapenems evaluated, thienamycin demonstrated the greatest antimicrobial activity and became the parent compound for all subsequent carbapenems. To date, more than 80 compounds with mostly improved antimicrobial properties, compared to those of thienamycin, are described in the literature. We also highlight important features of the carbapenems that are presently in clinical use: imipenem-cilastatin, meropenem, ertapenem, doripenem, panipenem-betamipron, and biapenem. In closing, we emphasize some major challenges and urge the medicinal chemist to continue development of these versatile and potent compounds, as they have served us well for more than 3 decades.

Recent developments in carbapenems

Carbapenems are beta-lactam antibiotics characterised by the presence of a beta-lactam ring with a carbon instead of sulfone in the 4-position of the thyazolidinic moiety. The first carbapenem to be utilised in therapy was imipenem, the N-formimidoyl derivative of thienamycin. Imipenem is coadministered with cilastatin, an inhibitor of human renal dehydropeptidase I, as imipenem is hydrolysed by this enzyme. Meropenem was the first carbapenem with a 1-beta-methyl group and 2-thio pyrrolidinyl moiety, which renders this antibiotic stable to renal dehydropeptidase I. Other carbapenems for parenteral administration later discovered include biapenem, panipenem, ertapenem, lenapenem, E-1010, S-4661 and BMS-181139. Carbapenems which are orally administered include sanfetrinem, DZ-2640, CS-834 and GV-129606. Carbapenems have an ultra-broad spectrum of antibacterial activity and stability to almost all clinically relevant beta-lactamases. This differentiates them from all other currently available classes of beta-lactam antibiotics. However, Class B beta-lactamases, along with some rare Class A and D enzymes, are able to hydrolyse these antibiotics. Although Class B enzymes are generally chromosomally-encoded (isolated from Stenotrophomonas maltophilia, Aeromonas spp., Bacillus cereus, Bacteroides fragilis, Flavobacterium spp. and Legionella gormanii), plasmid-metallo-beta-lactamases now are appearing in B. fragilis, Pseudomonas aeruginosa, Acinetobacter baumannii and members of Enterobacteriaceae such as Serratia marcescens and Klebsiella pneumoniae. The number of these enzymes compared to the number of other beta-lactamase types is still low, however, it is likely that they will spread due to the increased selective pressure of carbapenem use. The very broad spectrum of antimicrobial activity associated with a good clinical efficacy and a favourable safety profile makes the carbapenems valuable as 'first-line' antibiotics in initial empirical therapy for the treatment of severe infections.

b-Lactamase inhibitors

The use of beta-lactamase inhibitors in combination with a beta-lactamase-susceptible antibiotic is a useful strategy to rescue otherwise good antibiotics from failure. However, recent years have seen a rise in the numbers of beta-lactamases that are insensitive to the available beta-lactamase inhibitors. This review summarizes of the mechanisms of action of the principal types of inhibitors and the ways in which beta-lactamase are thought to develop resistance towards them. Ten general classes of inhibitors are reviewed, especially those of therapeutic importance (clavulanic acid, penam sulfones and carbapenems). Copyright 2000 Harcourt Publishers Ltd.

Detection of novel secondary metabolites

The study of antibiotics and other fermentation products has shown that a seemingly unlimited number of compounds with diverse structures are produced by microorganisms. The continued high rate of discovery of new chemical entities, in the light of the abundance of microbial products already described, is due to creative screening procedures that incorporate such features as the emphasis on unusual microorgnaisms, their special propagation and fermentation requirements, supersensitive and highly selective assays, genetic engineering both for the biosynthesis of new compounds and in the development of screening systems, early in vivo evaluation, improved isolation techniques, modern procedures for structure determination, computer-assisted identification, and an efficient multidisciplinary approach. This review focuses on the genesis and development of the gamut of methodologies that have led to the successful detection of the wide variety of novel secondary metabolites that include antibacterial, antigungal, antiviral and antitumour antibiotics, enzyme inhibitors, pharmacologically and immunologically active agents, products useful in agriculture and animal husbandry, microbial regulators, and other compounds for which no bioactive role has yet been found.

Multicenter in vitro evaluation of SM-7338, a new carbapenem

A new carbapenem, SM-7338, was compared with imipenem, cefotaxime, and ceftazidime at five medical centers. Nearly 6,000 strains were tested by reference methods of the National Committee for Clinical Laboratory Standards, and SM-7338 inhibited the largest percentage of gram-negative bacilli. Its spectrum included all members of the family Enterobacteriaceae (99.7% were susceptible to less than or equal to 4 micrograms/ml), Pseudomonas spp. (but not Xanthomonas maltophilia), and Acinetobacter spp. The potency and spectrum of SM-7338 against the gram-positive organisms were less than those of imipenem and superior to those of ceftazidime. Only the enterococci and some oxacillin-resistant staphylococci were less susceptible to SM-7338 (MICs for 90% of isolates, greater than or equal to 8 micrograms/ml). Organisms resistant to ceftazidime were generally susceptible to SM-7338 and imipenem (76%). However, for one-third of the imipenem-resistant gram-negative bacilli (MICs, greater than 8 micrograms/ml), SM-7338 MICs were less than or equal to 4 micrograms/ml. Some endemic differences in patterns of SM-7338 activity against selected gram-negative species were found among some medical centers.

beta-Lactamase inhibitors

In summary, Table XVI shows the inhibition profiles of representative beta-lactamases from each major class of Richmond and Sykes. Either resistance (R) or sensitivity (S) is given as a general guide to the type of compounds likely to inhibit each class. Thus the (qualitative) statements regarding the effectiveness of clavulanic acid can be taken to represent those for the penam sulfones and similarly for MM4550 and the other olivanic acids, carpetimycins, PS series, and asparenomycins. This can also be said of cloxacillin and the other aromatic carboxamido penicillins. Compounds are also included which are specifically or particularly inhibitory to certain beta-lactamases.