Medicinal Chemistry II / lecture 5 3rd stage/ 2nd semester 4 / 4 /2019 Antibiotics Medicinal Chemistry II / lecture 5 3rd stage/ 2nd semester 4 / 4 /2019

introduction An antibiotic is a type of antimicrobial substance active against bacteria and is the most important type for fighting bacterial infections. Antibiotic medications are widely used in the treatment and prevention of such infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses.

Classification of Antibiotics According to Their Mechanism of Action

β-LACTAM ANTIBIOTICS Antibiotics that possess the β-lactam (a four-membered cyclic amide) ring structure are the dominant class of agents currently used for the chemotherapy of bacterial infections. The first antibiotic to be used in therapy, penicillin (penicillin G or benzyl penicillin), and a close biosynthetic relative, phenoxymethyl penicillin (penicillin V), remain the agents of choice for the treatment of infections caused by most species of Gram-positive bacteria. The discovery of a second major group of β-lactam antibiotics, the cephalosporins, and chemical modifications of naturally occurring penicillins and cephalosporins have provided semisynthetic derivatives that are variously effective against bacterial species known to be resistant to penicillin, in particular, penicillinase-producing staphylococci and Gram- negative bacilli.

Mechanism of Action In addition to a broad spectrum of antibacterial action, two properties contribute to the unequaled importance of β- lactam antibiotics in chemotherapy: *a potent and rapid bactericidal action against bacteria in the growth phase. *a very low frequency of toxic and other adverse reactions in the host. The uniquely lethal antibacterial action of these agents has been attributed to a selective inhibition of bacterial cell wall synthesis. Specifically, the basic mechanism involved is inhibition of the biosynthesis of the peptidoglycan that provides strength and rigidity to the cell wall.

Structure of Peptidoglycan layer Peptidoglycan is a carbohydrate composed of alternating units of NAMA and NAGA. The NAMA units have a peptide side chain The cross linking reaction is catalyzed by a class of transpeptidases known as penicillin binding proteins.

β-lactams mimic the structure of the D-Ala-D-Ala link and bind to the active site of PBPs, disrupting the crosslinking process. The β-lactam binds to Penicillin Binding Protein (PBP) PBP is unable to crosslink peptidoglycan chains The bacteria is unable to synthesize a stable cell wall The bacteria is lysed

BETA-LACTAM ANTIBIOTICS (inhibitors of cell wall synthesis) Their structure contains a beta-lactam ring. The major subdivisions are: Penicillins whose official names usually include or end in “cillin” Cephalosporins which are recognized by the inclusion of “cef” or “ceph” in their official names. Carbapenems (e.g. meropenem, imipenem) Monobactams (e.g. aztreonam) Beta-lactamase inhibitors (e.g. clavulanic acid, sulbactam).

At first, β-lactam antibiotics were mainly active only against Gram-positive bacteria, yet the development of broad- spectrum β-lactam antibiotics active against various Gram- negative organisms has increased their usefulness.

THE PENICILLINS Penicillin nucleus consists of: Thiazolidine ring (Ring A) - Sulphur containing with COOH Beta lactam ring (Ring B) – (Broken by Beta-lactamase) Side chain is attached at position – 6- (NHCOR) Side chains attached through amide linkage. (Broken by Amidase) • Beta Lactam ring is broken by: • Penicillinase (Beta Lactamase), and by gastric acid. • Resultant Product is Penicilloic acid with No anti-bacterial Activity The penicillin molecule contains three chiral carbon atoms (C-3, C-5, and C-6).

Chemical Degradation The main cause of deterioration of penicillin is the reactivity of the strained lactam ring, particularly to hydrolysis. The course of the hydrolysis and the nature of the degradation products are influenced by the pH of the solution. Thus, the β-lactam carbonyl group of penicillin readily undergoes nucleophilic attack by water or (especially) hydroxide ion to form the inactive penicilloic acid, which is reasonably stable in neutral to alkaline solutions but readily undergoes decarboxylation and further hydrolytic reactions in acidic solution. In strongly acidic solutions (pH < 3), penicillin undergoes a complex series of reactions leading to various inactive degradation products.

Acid-catalyzed degradation in the stomach contributes strongly to the poor oral absorption of penicillin. Thus, efforts to obtain penicillins with improved pharmacokinetic and microbiological properties have focused on acyl functionalities that would minimize sensitivity of the β-lactam ring to acid hydrolysis while maintaining antibacterial activity. Substitution of an electron-withdrawing group in the α position of benzylpenicillin markedly stabilizes the penicillin to acid-catalyzed hydrolysis.

STRUCTURAL ACTIVITY RELATIONSHIP

Bacterial Resistance Some bacteria, in particular most species of Gram-negative bacilli, are naturally resistant to the action of penicillins. Other normally sensitive species can develop penicillin resistance The best understood and, probably, the most important biochemical mechanism of penicillin resistance is the bacterial elaboration of enzymes that inactivate penicillins. Such enzymes, which have been given the nonspecific name penicillinases, are of two general types: - β-lactamases and acylases. By far, the more important of these are the β-lactamases, enzymes that catalyze the hydrolytic opening of the β-lactam ring of penicillins to produce inactive penicilloic acids.

Products Penicillin G (Benzylpenicillin) Penicillin V benzylpenicillin is noted to possess effectiveness mainly against G +ve organisms. Given parenterally, Acid unstable Narrow spectrum Penicillin V EFFECTIVE AGAINST: Gram positive, less effective against Gram negative bacteria Can be given orally, resistant to gastric acid

Amino-Penicillins Ampicillin Effective against: Gram positive + Gram negative bacteria Broad spectrum Can be given orally and parenterally Prone to beta-lactamase Amoxicillin

Methicillin Methicillin sodium is particularly resistant to inactivation by the penicillinase found in staphylococci cannot be used orally.

Piperacillin/ Anti Pseudomonal Penicillin EFFECTIVE AGAINST: • Gram positive +Gram negative (more) CHARACTERISTICS: • Extended Spectrum • Should be given parenterally (not absorbed orally) *Piperacillin+Tazobactam

Gram negative + Limited Gram positive Carbenicillin Gram negative + Limited Gram positive Ticarcillin Mainly gram negative bacteria particularly Pseudomonas aeruginosa

β-lactamase inhibitors Resemble β-lactam antibiotic structure. Has negligible antibacterial activity. Bind to β-lactamase and protect the antibiotic from destruction Most successful when they bind the β-lactamase irreversibly Three important in medicine: Clavulanic Acid Sulbactam Tazobactam

Non β-lactam β-lactamase inhibitor Avibactam is a non β-lactam β-lactamase inhibitor  developed by Actavis  jointly with AstraZeneca. A new drug application for avibactam in combination with ceftazidime (branded as Avycaz) was approved by the FDA on February 25, 2015, for treating complicated UTI  and complicated intra-abdominal infections caused by antibiotic resistant-pathogens, including those caused by multi-drug resistant Gram- negative bacterial pathogens.

Vaborbactam  is a non β-lactam β-lactamase inhibitor discovered by Rempex Pharmaceuticals. It is not effective as an antibiotic  by itself. In the United States, the combination drug meropenem/vaborbactam (Vabomere) is approved by the FDA in August 2017 for complicated UTI and pyelonephritis

CARBAPENEMS Carbapenems are a class of beta-lactam antibiotics with a broad spectrum of antibacterial activity. They have a structure that renders them highly resistant to beta lactamases. *Imipenem – Broad spectrum, covers Gram-positive, Gram-negative (including ESBL-producing strains), Pseudomonas an anaerobes *Meropenem – Less seizure-inducing potential, can be used to treat CNS infections * Ertapenem – Lacks activity vs. Acinetobacter and Pseudomonas – Has limited activity against penicillin-resistant pneumococci

Imipenem Ertapenem

Monobactam Aztreonam: Monobactams are drugs with a monocyclic β-lactam ring. They are relatively resistant to beta-lactamases and active aganist Gram-negative rods (including Pseudomonas and Serratia). They have no activity against Gram-positive bacteria or anaerobes. Examples: Aztreonam (parenteral), Tigemonam (oral). Aztreonam: β-Lactamase resistance is like that of ceftazidime, which has the same isobutyric acid oximinoacyl group.

Cephalosporins Cephalosporins are similar to penicillins but have a 6 member dihydrothiazine ring instead of a 5 member thiazolidine ring. These has been conventionally classified into five generations First-generation cephalosporins are predominantly active against Gram-positive bacteria, and successive generations have increased activity against Gram-negative bacteria 2nd generation have a greater gram-negative spectrum while retaining some activity against gram-positive bacteria. Third-generation drugs exhibit the lest activity against gram- positive bacteria, but most potent activity against gram-negative bacteria

β-Lactamase Resistance The susceptibility of cephalosporins to various lactamases varies considerably with the source and properties of these enzymes. Cephalosporins are significantly less sensitive than all but the β-lactamase–resistant penicillins to hydrolysis by the enzymes from S. aureus and Bacillus subtilis. The “penicillinase” resistance of cephalosporins appears to be a property of the bicyclic cephem ring system rather than of the acyl group. Despite natural resistance to staphylococcal β-lactamase, the different cephalosporins exhibit considerable variation in rates of hydrolysis by the enzyme.

Beta–lactam Resistance What is Resistance? Drug resistance refers to unresponsiveness of a microorganism to an antimicrobial agent. Drug resistance are of two types: ---Natural Resistance ---Acquired Resistance

Natural Resistance: Acquired Resistance: Some microbes have always been resistant to certain anti- microbial agent. They lack the metabolic process or the target side that is affected by particular drug. E.g: Gram negative bacilli are normally unaffected by Penicillin G. M. tuberculosis is insensitive to Tetracyclines. This type of resistance does not pose significant clinical problem. Acquired Resistance: It is the development of resistance by an organism which was sensitive before due to the use of antimicrobial agent over a period of time. This can happen with any microbe and is a major clinical problem. However, the development of resistance is dependent on the microorganism as well as the drug.

Beta–lactamases

Extended spectra Beta-Lactamase (ESBL) ESBLs are enzymes that mediate resistance to extended-spectrum (third generation) cephalosporins (e.g., ceftazidime, cefotaxime, and ceftriaxone) and monobactams (e.g., aztreonam) but do not affect cephamycins (e.g., cefoxitin and Cefotetan) or carbapenems (e.g., meropenem or imipenem).

Metallo Beta-lactamase New delhi metallo-beta lactamase1 (NDM1) Resistant against broad spectrum of beta-lactam antibiotics These include the antibiotics of the carbapenem family. This class of β-lactamases is characterized by the ability to hydrolyze carbapenems (carbapenemase) and by its resistance to the commercially available β-lactamase inhibitors but susceptibility to inhibition by metal ion chelators. The most common bacteria that make this enzyme are Gram negative such as Escherichia coli and Klebsiella pneumoniae , Pseudomonas aeroginosa.

AMINOGLYCOSIDES Aminoglycosides are so named because their structures consist of amino sugars linked glycosidically. All have at least one aminohexose, and some have a pentose lacking an amino group (e.g., streptomycin, neomycin, and paromomycin). Although the aminoglycosides are classified as broadspectrum antibiotics, their greatest usefulness lies in the treatment of serious systemic infections caused by aerobic Gram-negative bacilli. The aminoglycosides act directly on the bacterial ribosome to inhibit the initiation of protein synthesis. They bind to the 30S ribosomal subunit to form a complex that cannot initiate proper amino acid polymerization.

All aminoglycoside antibiotics are absorbed very poorly (less than 1% under normal circumstances) following oral administration, and some of them (kanamycin, neomycin, and paromomycin) are administered by that route for the treatment of GI infections. Because of their potent broadspectrum antimicrobial activity, they are also used for the treatment of systemic infections. Their undesirable side effects, particularly ototoxicity and nephrotoxicity, have restricted their systemic use to serious infections or infections caused by bacterial strains resistant to other agents.

Structure–Activity Relationships

Ring I is crucially important for characteristic broad spectrum antibacterial activity, and it is the primary target for bacterial inactivating enzymes. Amino functions at 6 ` and 2 ` are particularly important as kanamycin B (6 ` -amino, 2 ` -amino) is more active than kanamycin A (6-amino, 2 ` -hydroxyl), which in turn is more active than kanamycin C (6 ` -hydroxyl, 2 ` -amino). Few modifications of ring II (deoxystreptamine) functional groups are possible without appreciable loss of activity in most of the aminoglycosides. The 1-amino group of kanamycin A can be acylated (e.g., amikacin), however, with activity largely retained. Ring III functional groups appear to be somewhat less sensitive to structural changes than those of either ring I or ring II.

Despite improvements in antibacterial potency and spectrum among newer naturally occurring and semisynthetic aminoglycoside antibiotics, efforts to find agents with improved margins of safety have been disappointing. The potential for toxicity of these important chemotherapeutic agents continues to restrict their use largely to the hospital environment.

Pharmacokinetics Highly polar basic drugs: poor oral BA Administered parenterally or applied locally Poorly distributed and poorly protein bound Do not undergo any significant metabolism Nearly all IV dose is excreted unchanged in urine Dose adjustment is needed in renal insufficiency

Products

Streptomycin Neomycin It is considered one of the most useful antibiotics for the treatment of GI infections, dermatological infections, and acute bacterial peritonitis. Also, it is used in abdominal surgery to reduce or avoid complications caused by infections from bacterial flora of the bowel.

Amikacin The synthesis formally involves simple acylation of the 1- amino group of the deoxy streptamine ring of kanamycin A with L-AHBA (amino hydroxybutyric acid). The remarkable feature of amikacin is that it resists attack by most bacteria-inactivating enzymes and, therefore, is effective against strains of bacteria that are resistant to other aminoglycosides, including gentamicin and tobramycin. Kanamycin Amikacin

Gentamicin it has a broad spectrum of activity against many common pathogens, both Gram-positive and Gram-negative. Sisomicin Its antibacterial potency and effectiveness against aminoglycoside- inactivating enzymes resemble those of gentamicin. Sisomicin also exhibits pharmacokinetics and pharmacological properties similar to those of gentamicin.

Plazomicin  is a next generation aminoglycoside antibacterial derived from sisomicin by appending a hydroxy-aminobutyric acid (HABA) substituent at position 1 and a hydroxyethyl substituent at position 6‘. It was developed by Achaogen, to treat serious bacterial infections due to multidrug-resistant Enterobacteriaceae, including carbapenem-resistant Enterobacteriaceae and was approved by the US FDA on June 25, 2018

Plazomicin (ZEMDRI) is approved by the FDA for adults with complicated urinary tract infections (cUTI), including pyelonephritis, caused by Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, or Enterobacter cloacae, in patients who have limited or no alternative treatment options. Zemdri is an intravenous infusion, administered once daily. The FDA declined approval for treating bloodstream infections due to lack of effectiveness.

Post antibiotic effect Aminoglycosides exhibit concentration dependent killing. They also possess significant Post-antibiotic effect. Single daily dosing at least as effective as and no more toxic than multiple dosing.

Mechanism of resistance Synthesis of plasmid mediated bacterial transferase enzyme: Inactivate aminoglycosides ↓ transport into bacterial cytosol alteration of receptor protein on 30 S ribosomal unit by mutation: prevents attachment