Antibiotics

The term antibiotic is originated from the word antibiosis [‘against life’].

They are compounds obtained from various species of microorganisms such as bacteria, fungi, actinomycetes.

They can suppress the growth of other microorganisms and eventually may destroy them.

The difference amongst them may be physical, chemical, pharmacological properties, antibacterial spectra, the mechanism or mode of action.

They have made it possible to cure the diseases caused by pneumonia, tuberculosis and meningitis and save the lives of millions of people around the world.

Classification Based on Mechanism of Action

(1). Agents that inhibit the synthesis of bacterial cell wall: (eg., penicillins, cephalosporins (similar structures); others are cycloserine, vancomycin, bracitracin and imidazole (antifungal)

(2). Agents that act directly on the cell membrane of the microorganisms, affecting permeability, and leading to leakage of intercellular compounds (eg., polymyxin, polyene antifungal agents, nystacin and amphotericin B which bind to cell wall sterols)

(3). Agents that affect the function of 30s and 50s ribosomal subunits to cause reversible inhibition of protein synthesis: tetracyclines, erythromycins, chloramphenicol and clindamycin.

(4). Agents that bind 30s ribosomal subunit and alter protein synthesis:

(5). Agents that affects nucleic acid metabolism:

Classification Based on Chemical structure

(1). β- lactum antibiotics

(2). Amino glycoside antibiotics

(3). Tetracycline antibiotics

(4). Polypeptide antibiotics

(5). Macrolide antibiotics

(6). Lincomycins

(7). Other antibiotics

MODIFIED PEPTIDES: PENICILLINS, CEPHALOSPORINS, EXAMPLES OF β-LACTAM ANTIBIOTICS

PenicillinsThe penicillins are the oldest of the clinical antibiotics, but are still the most widely used. The first of the many penicillins to be employed on a significant scale was penicillin G (benzylpenicillin), obtained from the fungus Penicillium chrysogenum by fermentation in a medium containing corn-steep liquor.

Penicillins contain a fused β-lactam-thiazolidine structure, which has its biosynthetic origins in a tripeptide, the components of which are l-aminoadipic acid (formed in β-lactam-producing organisms from lysine via piperideine-6-carboxylic acid), l-cysteine and l-valine.

CEPHALOSPORINS -EXAMPLE OF β-LACTAM ANTIBIOTICSCephalosporin C is produced commercially by fermentation using cultures of a high-yielding strain of Acremonium chrysogenum (formerly Cephalosporium acremonium). Initial studies of the antibiotic compounds synthesized by Cephalosporium acremonium identified penicillin N (originally called cephalosporin N) as the major component, with small amounts of cephalosporin C. In contrast to the penicillins, cephalosporin C was stable under acidic conditions and also was not attacked by penicillinase (β-lactamase). Antibacterial activity was rather low, however, and the antibiotic was poorly absorbed after oral administration. However, the structure offered considerable scope for side-chain modifications, more so than with the penicillins, since it has two side-chains, and this has led to a wide variety of cephalosporin drugs, many of which are currently in clinical use.

Other β-LactamsThe fused β-lactam skeletons found in penicillins and cephalosporins are termed penam and cephem respectively variants containing the basic β-lactam ring system are also found in nature. Although these are derived from amino acid precursors, in contrast to the penicillins and cephalosporins, they are not usually the products of NRPS enzymes. Of particular importance is the clavam or oxapenam fused-ring system typified by clavulanic acid from Streptomyces clavuligerus.

Clavulanic acid is usually combined with a standard penicillin, e.g. with amoxicillin (as co-amoxyclav) or with ticarcillin, to act as a suicide substrate and provide these agents with protection against class A β-lactamases, thus extending their effectiveness against a wider range of organisms.

The naturally occurring monobactams show relatively poor antibacterial activity, but alteration of the side-chain, as with penicillins and cephalosporins, has produced many potent new compounds. Unlike those structures, the non-fused β-lactam ring is readily accessible by synthesis, so all analogues are produced synthetically. The first of these to be used clinically is aztreonam.

Gentamicins- Example of Amino glycoside antibiotics- interfere with protein biosynthesis by acting on the smaller 30S subunit of the bacterial ribosome. Amino glycoside antibiotics are produced in culture by strains of Streptomyces and Micromonospora. Compounds obtained from Streptomyces have been given names ending in -mycin, whilst those from Micromonospora have names ending in -micin. 2-Deoxystreptamine-containing Antibiotics Gentamicin is a mixture of antibiotics obtained from Micromonospora purpurea. Fermentation yields a mixture of gentamicins A, B, and C, from which gentamicin C is separated for medicinal use. This is also a mixture, the main component being gentamicin C1 (50–60%), with smaller amounts of gentamicin C1a and gentamicin C2. These three components differ in respect to the side-chain in the purpurosamine sugar. Gentamicin is clinically the most important of the aminoglycoside antibiotics, and is widely used for the treatment of serious infections, often in combination with a penicillin when the infectious organism is unknown.

OH O

HOH

OH

HNMe2

OOH

CONH2

OH

Tetracycline, a valuable antibiotic is a polyketide which is produced on a large scale from Streptomyces aureofaciens

O

O

HOOH

O

OH

O desosamine

O mycarose

Erythromycin, a macrolide, is a polyketide produced by Streptomycete is an important antibiotic.

Polymyxins- Examples of polypeptide AntibioticsThe polymyxins are a group of cyclic polypeptide antibiotics produced by species of Bacillus. Polymyxins A–E were isolated from Bacillus polymyxa, though polymyxin B and polymyxin E were both subsequently shown to be mixtures of two components.A polypeptide mixture called colistin isolated from Bacillus colistinus was then found to be identical to polymyxin E. PolymyxinB and colistin (polymyxin E) are both used clinically. These antibiotic mixtures respectively contain principally polymyxin B1 with small amounts of polymyxin B2, or predominantly polymyxin E1 (≡ colistin A) with small amounts of polymyxin E2 (≡ colistin B) (Figure 7.24).

Vancomycin, a glycopeptide antibiotic produced in cultures of Amycolatopsis orientalis (formerly Streptomyces orientalis) and has activity against Gram-positive bacteria, especially resistant strains of staphylococci, streptococci, and enterococci. Vancomycin is not absorbed orally, so must be administered by intravenous injection. The antibiotic may cause toxicity to ear and kidneys.

The penicillins are complex mixtures of closely related compounds isolated from the beers resulting from fermentation of the molds, Penicillium notatum and Penicillium chrysogenum.

The active compounds proved to be amides of 6-aminopenicillanic acid ( 6-APA) (1) with differing acyl groups attached to the amine of the β-lactam ring.

6-APA possesses weak but definite antibacterial properties, and its structure represents the minimum structural requirements for the characteristic bioactivity of the penicillins.

(R)

N(S)

S

COOH

(R)CH3

CH3

H

O

H2NH

(1). 6-aminopenicillanic acid(6-APA)

The incorporation of an acyl side chain (2), particularly one containing an aryl residue, increases potency by 50 to 200-fold.

Thus 6-APA (1) is the penicillin pharmacophore, and the amide function serves to modulate the kind and intensity of antimicrobial activity.

All the clinically useful analogs that have reached the marketplace have flowed from this basic fact or have resulted from the adventitious discovery of analogous substances from natural sources.

N

S

COOH

CH3

CH3

H

O

HN

H

(2). Acylated 6-APA)

O

R

N

S

COOH

CH3

CH3

H

O

HN

H

(2). Acylated 6-APA)

O

R

H2C

OH2C

O CH

CH3

O CH

CH2CH3

OHC

R =

R =

R =

R =

R =

Benzylpenicillin (Penicillin G)

Phenoxymethylpenicillin (Penicillin 5)

Phenethicillin

Propicillin

Phenbencillin

pent-2-enylpenicillinP-hydroxybenzylpencillin

n-heptylpenicillin

n-amyllpenicillin

Commercial production of benzylpenicillin (penicillin G)

by fermentation of selected high-yielding strains of Penicillium chrysogenum in the presence of phenylacetic acid. Though wild-type Penicillium chrysogenum cultures produced about 70 mg/L , commercial strains can yield more than 50 g/L . Benzylpenicillin was the earliest commercially available member of the penicillin group of antibiotics, and it still remains an important and useful drug for the treatment of many Gram-positive bacteria, including streptococcal, pneumococcal, gonococcal, and meningococcal infections.

Benzylpenicillin is destroyed by gastric acid; thus, it is not suitable for oral administration and is best given as intramuscular or intravenous injection of the water-soluble sodium salt.

Decomposition under acidic conditions leads to formation of penicillic acid and/or penicillenic acid, depending on pH . The β-lactam ring is opened by a mechanism in which the side-chain carbonyl participates, resulting in formation of an oxazolidine ring.

At higher pH values, benzylpenicillin suffers simple β-lactam ring opening and gives penicilloic acid (Figure 7.38). The strained β-lactam (cyclic amide) ring is more susceptible to hydrolysisthan the unstrained side-chain amide function, since the normal stabilizing effect of the lone pair from the adjacent nitrogen is not possible due to the geometric restrictions .

At higher pH values, benzylpenicillin suffers simple β-lactam ring opening and gives penicilloic acid . The strained β-lactam (cyclic amide) ring is more susceptible to hydrolysis than the unstrained side-chain amide function, since the normal stabilizing effect of the lone pair from the adjacent nitrogen is not possible due to the geometric restrictions .

Supplementation of the fermentation medium with acids other than phenylacetic acid was used to provide structurally modified penicillins, though the scope was limited to a series of monosubstituted acetic acids by the specificity of the fungal enzymes involved in activation of the acids to their coenzyme A esters. The most important new penicillin produced wasphenoxymethylpenicillin (penicillin V), a result of adding phenoxyacetic acid to the culture (Figure 7.40). This new penicillin had the great advantage of being acid resistant, since the introduction of an electron-withdrawing heteroatom into the side-chain inhibits participation of the side-chain carbonyl in the reaction shown in Figure 7.38. Thus, penicillin V is suitable for oral administration, and still has particular value for respiratory tract infections and tonsillitis.

Penicillin, destroys bacteria by inhibiting the enzyme that synthesizes bacterial cell walls

Bacteria develop resistance to penicillin by secreting penicillinase, an enzyme thatdestroys penicillin by hydrolyzing its -lactam ring before the drug can interfere withbacterial cell wall synthesis.

PENICILLIN AND DRUG RESISTANCEPenicillin contains a strained β-lactam ring. The strain in the four-membered ring increases the amide’s reactivity. It is thought that the antibiotic activity of penicillin results from its ability to acylate a CH2OH group of an enzyme that is involved in the synthesis of bacterial cell walls. Acylation inactivates the enzyme, and actively growing bacteria die because they are unable to produce functional cell walls. Penicillin has no effect on mammalian cells because mammalian cells are not enclosed by cell walls. To minimize hydrolysis of the b-lactam ring during storage, penicillins are refrigerated.

Bacteria that are resistant to penicillin secrete penicillinase, an enzyme that catalyzes the hydrolysis of the b-lactam ring of penicillin. The ring-opened product has no antibacterial activity.

PENICILLINS IN CLINICAL USEMore than 10 different penicillins are currently in clinical use. They differ only in the group (R)attached to the carbonyl group. In addition to their structural differences, the penicillins differ in the organisms against which they are most effective. They also differ in their resistance to penicillinase. For example, ampicillin, a synthetic penicillin, is clinically effective against bacteria that are resistant to penicillin G, a naturally occurring penicillin. Almost 19% of humans are allergic to penicillin G.

The techniques used for the production of penicillins have been: (1) natural fermentation (in which the penicillin-producing fungus is allowed to grow on a

variety of complex natural nutrients from which it selects acids for incorporation into the side chain)

(2) biosynthetic production (in which the fermentation medium is deliberately supplemented with unnatural precursors from which the fungus selects components for the synthesis of "unnatural" penicillins), (3) semisynthetic production (in which 6-aminopenicillanic acid obtained by a process involving fermentation, and suitably activated acids are subsequently reacted chemically to form penicillins with new side chains) (4) total synthesis (potentially the most powerful method for making deep-seated structural modifications but which is at present unable to compete economically with the other methods).

Penicillin V is a semisynthetic penicillin that is in clinical use. It is not a naturally occurring penicillin; nor is it a true synthetic penicillin because chemists don’t synthesize it. The Penicillium mold synthesizes it after the mold is fed 2-phenoxyethanol, the compound it needs for the side chain.

Feeding of phenylacetic acid in the fermentation led to a simplification of the mixture of penicillins produced by the fungus due to preferential uptake of this acid and its incorporation into benzylpenicillin,

Inclusion of the appropriate acids in the culture medium thus afforded, respectively, phenoxymethylpenicillin ( penicillin V), phenethicillin , propicillin and phenbencillin.

The most important new penicillin produced was phenoxymethylpenicillin (penicillin V), a result of adding phenoxyacetic acid to the culture

These modifications served to increase stability of the lactam bond towards hydrolysis and thus conferred some degree of oral activity.

The nature of the penicillin derivatives accessible by this "feeding" route was severely limited by the fact that the acylating enzyme of the Penicillium molds would accept only those carboxylic acids which bore at least some resemblance to its natural substrates.

A breakthrough in this field was achieved by the finding that rigid exclusion of all possible side-chain substrate! from the culture medium afforded 6-APA as the main fermentation product.

Supplementation of the fermentation medium with acids other than phenylacetic acid was used to provide structurally modified penicillins, though the scope was limited to a series of monosubstituted acetic acids by the specificity of the fungal enzymes involved in activation of the acids to their coenzyme A esters. The most important new penicillin produced wasphenoxymethylpenicillin (penicillin V), a result of adding phenoxyacetic acid to the culture (Figure 7.40). This new penicillin had the great advantage of being acid resistant, since the introduction of an electron-withdrawing heteroatom into the side-chain inhibits participation of the side-chain carbonyl in the reaction shown in Figure 7.38. Thus, penicillin V is suitable for oral administration, and still has particular value for respiratory tract infections and tonsillitis.

Structure Elucidation of Penicillins- Chemical method

Strategy : Acid hydrolysis of penicillins gave two compounds in equimolar ratio, an amine (pencillamine)and an aldehyde (penilloaldehyde)and one carbon was lost as CO2

NH2

OH

O

Cl

OCl

+ HN

OH

O

+

O

Cl(CH3CO)2O

ON

O

NaOH

H2S

azalactone

S N

COOH

H2O

BoilSH

COOH

NHCOCH3SH

COOH

NH2

2,5,5-trimethyl-2-thiazoline-4-carboxylic acid

DL-pencillamine

(1) HCl (boil)

(2) pyridine

For a detailed mechanism, refer to I.LFinar (Vol 2. p.866)

DL-Valiine

DL-penicillamine was initiallyconverted into the formyl derivative which was then resolved by treating with brucine to form diastereomers which were seperated to get D-pencillamine, which was identical with the degradation product of naturalpencillin.

H2O/H+

R Cl

O

+H2N OCH2CH3

OCH2CH3

HNR

O

H

O

Penilloaldehyde

Structure of Penilloaldehyde

HNR

O

H

O

H2O/H+

R OH

O

+H2N

H

O

All penicillins on vigorous hydrolysis gave substituted acetic acid and aminoacetaldehyde

Synthesis of penilloaldehyde

HNR

O

H

O

Penaldic acidCOOH

HNR

O

H

O

+ CO2

Penilloaldehyde

Hydrolysis of Penicillins yielded D-penicillamine, penilloaldehyde and CO2 .

At this point it was explained that the formation of CO2 is from an unstable β- keto acid named penaldic acid.

Penilloic acid

-CO2

H2O/HgCl2Penicillamine + Penilloaldehyde