International Journal of Molecular Sciences Review The Odd Couple(s): An Overview of Beta-Lactam Antibiotics Bearing More Than One Pharmacophoric Group Margherita De Rosa * , Anna Verdino, Annunziata Soriente and Anna Marabotti * Department of Chemistry and Biology “A. Zambelli”, University of Salerno, 84084 Fisciano (SA), Italy; [email protected] (A.V.); [email protected] (A.S.) * Correspondence: [email protected] (M.D.R.); [email protected] (A.M.); Tel.: +39-089-969553 (M.D.R.); +39-089-969583 (A.M.) Abstract: β-lactam antibiotics are among the most important and widely used antimicrobials world- wide and are comprised of a large family of compounds, obtained by chemical modifications of the common scaffolds. Usually these modifications include the addition of active groups, but less frequently, molecules were synthesized in which either two β-lactam rings were joined to create a single bifunctional compound, or the azetidinone ring was joined to another antibiotic scaffold or another molecule with a different activity, in order to create a molecule bearing two different pharmacophoric functions. In this review, we report some examples of these derivatives, highlighting their biological properties and discussing how this strategy can lead to the development of innovative antibiotics that can represent either novel weapons against the rampant increase of antimicrobial resistance, or molecules with a broader spectrum of action. Keywords: bis-azetidinone derivatives; dual antibiotics; podands; siderophores; bis beta-lactam; bis beta-lactam macrocycles Citation: De Rosa, M.; Verdino, A.; Soriente, A.; Marabotti, A. The Odd 1. Introduction Couple(s): An Overview of β-lactam antibiotics are a broad group of molecules that are naturally produced by Beta-Lactam Antibiotics Bearing different organisms (molds belonging to Penicillium spp. and Cephalosporium spp. for More Than One Pharmacophoric penicillins and cephalosporins, respectively, and bacteria belonging to different species for Group. Int. J. Mol. Sci. 2021, 22, 617. monobactams and carbapenems) (Figure1). https://doi.org/10.3390/ The serendipitous discovery of the first representative of this broad group of molecules, ijms22020617 penicillin G, is generally attributed to Alexander Fleming in 1929 [1], although there are pieces of evidence that, about fifteen years before, an Italian physician, Vincenzo Tiberio, Received: 14 December 2020 Accepted: 6 January 2021 had already identified in molds a “principle with bactericidal action” [2]. Only in 1949, Published: 9 January 2021 however, was the structure of this compound fully revealed, thanks to the pioneering X-ray studies of Dorothy Crowfoot Hodgkin [3]. Another Italian pharmacologist, Giuseppe Publisher’s Note: MDPI stays neu- Brotzu, identified the presence of other compounds with bactericidal activity in crude fil- tral with regard to jurisdictional clai- trates of Cephalosporium cultures in 1945 [4]. The founder of this second class of compounds, ms in published maps and institutio- cephalosporin C, was identified from these filtrates in 1953 and its structure was fully char- nal affiliations. acterized in 1961 [5,6]. The other classes of β-lactam antibiotics, namely carbapenems and monobactams, were discovered more recently [7,8], in an effort to identify new members of this broad group of molecules. All these compounds share a common chemical moiety, i.e., a four-member ring with Copyright: © 2021 by the authors. Li- an amidic function, commonly called “β-lactam ring” or “azetidinone”. In penicillins, censee MDPI, Basel, Switzerland. cephalosporins, and carbapenems, this ring is fused to another 5- or 6-member ring, This article is an open access article whereas in monobactams, the β-lactam ring is monocyclic (Figure1). This moiety is the distributed under the terms and con- mainly responsible for the antibacterial properties of all these molecules, due to their ditions of the Creative Commons At- ability to block the bacterial cell wall synthesis as a result of their covalent binding to tribution (CC BY) license (https:// creativecommons.org/licenses/by/ penicillin-binding proteins (PBPs), which are essential enzymes involved in the terminal 4.0/). steps of the synthesis of peptidoglycan, the main component of the bacterial cell wall [9]. Int. J. Mol. Sci. 2021, 22, 617. https://doi.org/10.3390/ijms22020617 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 617 2 of 21 ity to block the bacterial cell wall synthesis as a result of their covalent binding to penicil- Int. J. Mol. Sci. 2021, 22, 617 2 of 21 lin-binding proteins (PBPs), which are essential enzymes involved in the terminal steps of the synthesis of peptidoglycan, the main component of the bacterial cell wall [9]. Figure 1. CoreCore structures structures of of different different classes classes of ofβ-lactamβ-lactam antibiotics. antibiotics. In penicillins, In penicillins, the theβ-lactamβ-lactam moiety moiety is fused is fused to a five- to a memberfive-member thiazolidine thiazolidine ring; ring;in cephalosporins, in cephalosporins, the β the-lactamβ-lactam moiety moiety is fused is fused to a six-member to a six-member dihydrothiazine dihydrothiazine ring; ring; in car- in bapenems,carbapenems, the theβ-lactamβ-lactam moiety moiety is fused is fused to a topyrroline a pyrroline ring; ring; in monobactams, in monobactams, the β the-lactamβ-lactam ring is ring not isfused not fusedto any to other any ring. In all classes, the β-lactam moiety is highlighted in red; the standard numbering of the nucleus for each class is also other ring. In all classes, the β-lactam moiety is highlighted in red; the standard numbering of the nucleus for each class is reported. also reported. AfterAfter the the so-called so-called “Golden “Golden Age” Age” of antibiotic of antibiotic discovery discovery (from (from the 1940s the 1940sto the 1960s) to the [10],1960s) researchers [10], researchers had to realize had to the realize progressiv the progressivee increase increase of microorganisms of microorganisms that are thatnot susceptiveare not susceptive to the action to the of action these ofdrugs. these This drugs. resistance This resistance is caused is causedby the development by the develop- of manyment ofdifferent many different“escape “escapestrategies”, strategies”, such as the such alteration as the alteration of the target, of the the target, development the devel- ofopment pumps of to pumps export tothe export antibiotics the antibioticsoutside the outside microbial the cell, microbial and in particular cell, and infor particular β-lactam antibiotics,for β-lactam the antibiotics, widespread the diffusion widespread of β diffusion-lactamases, of β enzymes-lactamases, able enzymes to inactivate able totheir in- pharmacophoricactivate their pharmacophoric core by hydrolysis core of by the hydrolysis β-lactam of ring the [11].β-lactam Therefore, ring [during11]. Therefore, the last decades,during the a huge last decades,effort has abeen huge made effort worldwid has beene to made identify worldwide or synthesize to identify new antibiotics, or synthe- ablesize to new encompass antibiotics, the ableantimicrobial to encompass resistance the antimicrobial that has been resistancerecently declared that has by been WHO re- “onecently declaredof the by WHOtop 10 “one global of the toppublic 10 global health public healththreats threats facing facing humanity” humanity” (https://www.who.int/news-room/fact-sheets/deta(https://www.who.int/news-room/fact-sheets/detail/antimicrobial-resistanceil/antimicrobial-resistance; declaration; declara- oftion 13 ofOctober 13 October 2020). 2020). Additionally, Additionally, several several derivatives derivatives of the of natural the natural compounds compounds have been have createdbeen created to modulate to modulate their pharmacokinetic their pharmacokinetic properties properties and/or and/or to widen to their widen spectrum their spec- of action.trum of For action. example, For example, to date, hundreds to date, hundreds of cephalosporin of cephalosporin derivatives derivatives are available are availableand they areand classified they are classifiedinto five “generations” into five “generations” that are characterized that are characterized by different by different ways of waysadmin- of istrationadministration and by andan enhanced by an enhanced microbicidal microbicidal activity activity towards towards Gram-negative Gram-negative bacteria bacteria (while the(while founder the founder molecule molecule is mainly is mainlyactive on active Gram-positive on Gram-positive bacteria) bacteria) [12]. [12]. TheThe total synthesis of β-lactam-lactam antibiotics antibiotics in in the the laboratory laboratory is is difficult difficult because these molecules,molecules, despite despite their their limited limited size, size, are are ofte oftenn extremely extremely complex complex in infunction function and and chirality chiral- [13].ity [ 13Therefore,]. Therefore, the vast the vastmajority majority of the of chemical the chemical modifications modifications applied applied to β-lactam to β-lactam anti- bioticsantibiotics involve involve the functionalization the functionalization of the of pharmacophoric the pharmacophoric core, core, typical typical of each of each class, class, us- ingusing the the total total chemical chemical or or enzyme-assisted enzyme-assisted strategi strategies.es. Each Each of of the the 1, 1, 2, 2, 3, 3, or or 4 4 positions positions on thethe β-lactam-lactam ring ring (Figure (Figure 11)) havehave beenbeen