Medicinal Chemistry of β-lactams & Tetracyclines

Medicinal Chemistry I SPH2153 Medicinal Chemistry of β-lactams & Tetracyclines Antibiotics

By: Dr. Rasha Saad

History of penicillin's In 1877 Pasteur and Joubert discovered that certain moulds could produce toxic substances which killed bacteria. Unfortunately, these substances were also toxic to humans and of no clinical value. However, they did demonstrate that moulds could be a potential source of antibacterial agents. In 1928, Fleming noted that a bacterial culture which had been left several weeks open to the air had become infected by a fungal colony

Introduction  β-lactams & Cephalosporin's are Antibacterial agents which inhibit cell wall synthesis  There are two major classes of drug which act in this fashion—penicillins and cephalosporins. We shall consider penicillins first..

Site of antibacterial action

Structure of penicillin  Penicillin contains a highly unstable-looking bicyclic system consisting of a fourmembered (3-lactam ring fused to a fivemembered thiazolidine ring.  The acyl side-chain (R) varies, depending on the make up of the fermentation media. For example, corn steep liquor was used as the medium when penicillin wasfirst mass-produced in the United States and this gave penicillin G (R=benzyl).

Structure of penicillin The overall shape of the molecule is like a half-open book, The skeleton of the molecule suggests that it is derived from the amino acids cysteine and valine.

Penicillin analogues  One method of varying the side-chain is to add different carboxylic acids to the fermentation medium; for example, adding phenoxyacetic acid (PhOCH2CO2H) gives penicillin V.  6-APA is now produced by hydrolysing penicillin G or penicillin V with an enzyme (penicillin acylase) (Fig. 10.22) or by chemical methods . These are more efficient procedures than fermentation.

Properties of penicillin G The properties of benzyl penicillin are summarized below. • Active versus Gram-positive bacilli (e.g. staphylococci, meningitis, and gonorrhoea) and many (but not all) Gramnegative cocci. • Non-toxic! This point is worth emphasizing. • The penicillins are amongst the safest drugs known to medicine. • Not active over a wide range (or spectrum) of bacteria Ineffective when taken orally. Penicillin G can only be administered by injection. It is ineffective orally since it breaks down in the acid conditions of the stomach. • Sensitive to all known (3-lactamases. These are enzymes produced by penicillin resistant bacteria which catalyse the degradation of penicillins. • Allergic reactions are suffered by some individuals.

Structure-activity relationships of penicillins

Structure-activity relationships of penicillins  The strained B-lactam ring is essential.  The free carboxylic acid is essential.  The bicyclic system is important (confers strain on the p-lactam ring—the greater the strain, the greater the activity, but the greater the instability of the molecule to other factors).  The acylamino side-chain is essential (except for thienamycin, see later).  Sulfur is usual but not essential.  The stereochemistry of the bicyclic ring with respect to the acylamino side-chain is important. The results of this analysis lead to the inevitable conclusion that very little variation

Problems facing B-lactam ring

1. Ring strain The bicyclic system in penicillin consists of a four-membered ring and a five membered ring. As a result, penicillin suffers large angle and torsional strains. Acid-catalysed ring opening relieves these strains by breaking open the more highly strained four-membered lactam ring

Tackling the problem of acid sensitivity It can be seen that countering acid sensitivity is a difficult task. Nothing can be done about the ring opening since the p-lactam ring is vital for antibacterial activity. Without it, the molecule has no useful biological activity at all.

Penicillin V (Fig. 10.28) has an electronegative oxygen on the acyl side-chain with the electron withdrawing effect required.

2. Penicillin sensitivity to B-lactamases p-Lactamases are enzymes produced by penicillin-resistant bacteria which can catalyse the reaction shown in Fig. 10.30—i.e. the same ring opening and deactivation of penicillin which occurred with acid hydrolysis

Tackling the problem of B-lactamase sensitivity The strategy is to block the penicillin from reaching the penicillinase active site. One way of doing that is to place a bulky group on the side-chain. This bulky group can then act as a 'shield' to ward off the penicillinase and therefore prevent binding (Fig. 10.31).

Oxacillin, Cloxacillin, and Flucloxacillin Further work eventually got round the problem of acid sensitivity by incorporating into the side-chain a five-membered heterocycle which was designed to act as a steric shield and also to be electron withdrawing

These compounds (oxacillin, cloxacillin, and flucloxacillin) are acid-resistant and penicillinase-resistant, and are also useful against Staph. aureus infections. The only difference between the above three compounds is the type of halogen substitution on the aromatic ring.

3. Narrow spectrum of activity One problem has cropped up in everything described so far; most penicillins show a poor activity against Gram-negative bacteria. There are several reasons for this resistance.

Permeability barrier It is difficult for penicillins to invade a Gram-negative bacterial cell due to the make up of the cell wall. Gramnegative bacteria have a coating on the outside of their cel wall which consists of a mixture of fats, sugars, and proteins (Fig. 10.34). The coating can act as a barrier in various ways.

The only way in which penicillin can negotiate such a barrier is through protein. channels in the outer coating

Tackling the problem of narrow activity spectrum These changes were again confined to variations in the side chain and gave the following results • Hydrophobic groups on the side-chain (e.g. penicillin G) favour activity against Gram-positive bacteria, but result in poor activity against Gram-negative bacteria. • If the hydrophobic character is increased, there is little effect on the Gram-positive activity, but what activity there is against Gram-negative bacteria drops even more. • Hydrophilic groups on the side-chain have either little effect on Gram-positive activity (e.g. penicillin T) or cause a reduction of activity (e.g. penicillin N) (Fig.10.35). However, they lead to an increase in activity against Gram-negative bacteria.

Tackling the problem of narrow activity spectrum • Enhancement of Gram-negative activity is found to be greatest if the hydrophilic group (e.g. NH2, OH, CO2H) is attached to the carbon, alpha to the carbonyl group on the side-chain.

Class I broad-spectrum antibiotics— ampicillin and amoxycillin (Beechams 1964)

Ampicillin is the second most used penicillin in medical practice. Amoxycillin differs merely in having a phenolic group. It has similar properties, but is better absorbed through the gut wall.

Class II broad-spectrum antibiotics— carbenicillin Carbenicillin has an activity against a wider range of Gram-negative bacteria than ampicillin. It is resistant to most penicillinases and is also active against the stubborn Pseudomonas aeruginosa.

The mechanism of action of penicillin's and cephalosporin's

The mechanism of action of penicillins and cephalosporins The wall is a peptidoglycan structure (Fig. 10.59). In other words, it is made up of peptide units and sugar units. It is this final cross-linking reaction which is inhibited by penicillins and cephalosporins, such that the cell wall framework is not meshed together (Fig. 10.60). As a result, the wall becomes 'leaky'.

http://www.you tube.com/watc h?v=qBdYnRhd WcQ

Cephalosporins Discovery and structure of cephalosporin C The second major group of

(3-lactam antibiotics to be discovered were the cephalosporins. The first cephalosporin was cephalosporin C—isolated in 1948 from a fungus obtained from sewer waters on the island of Sardinia. The structure of cephalosporin C (Fig. 10.41) has similarities to that of penicillin in that it has a bicyclic system containing a four-membered (3lactam ring. However, this time the (3-lactam ring is fused with a sixmembered dihydrothiazine ring.

Properties of cephalosporin C The properties of cephalosporin C can be summarized as follows. • Difficult to isolate and purify due to a highly polar side-chain. • Low potency (one-thousandth of penicillin G). • Not absorbed orally. • Non-toxic. • Low risk of allergenic reactions. • Relatively stable to acid hydrolysis compared to penicillin G. • More stable than penicillin G to penicillinase (equivalent to oxacillin). • Good ratio of activity against Gram-negative bacteria and Gram-positive

bacteria.

Structure-activity relationships of Cephalosporin C Many analogues of Cephalosporin C have been made and the structure-activity relationship (SAR) conclusions are as follows. • The (3-lactam ring is essential. • A free carboxyl group is needed at position 4. • The bicyclic system is essential. • The stereochemistry of the side-groups and the rings is important. There are only a limited number of places where modifications can be madeThose places are: 1. • the 7-acylamino side-chain; 2. • the 3-acetoxymethyl side-chain; 3. • substitution at carbon 7.

Cephalosporin's Classification • Cephalosporins – 1st Generation : Cephalexin, Cefazolin – 2nd Generation : Cefoxitin, Cefuroxime, Cefotetan – 3rd Generation : Cefotaxime, Ceftriaxone, Ceftazidime – 4th Generation : Cefepime

2nd Generation & 3rd Generation

Novel B-lactam antibiotics Clavulanic acid (Beechams 1976) Clavulanic acid (Fig. 10.53) was isolated from Streptomyces clavuligerus by Beechams(1976). It has weak and unimportant antibiotic activity. Now it is used in combination with traditional penicillins such as amoxycillin (Augmenting This allows the amount of amoxycillin to be reduced and also in creases the spectrum of activity.

Antibacterial agents which impair protein synthesis Examples of such agents are the rifamycins which act against RNA, and the aminoglycosides, tetracyclines, and chloramphenicol which all act against the ribosomes

Tetracyclines  The tetracyclines as a whole have a broad spectrum of activity and are the most widely prescribed form of antibiotic after penicillins.  They are so named for their four (“tetra-”) hydrocarbon rings (“cycl-”) derivation (“-ine”). To be specific, they are defined as "a subclass of polyketides

Spectrum: -The tetracyclines are broad-spectrum antibiotics. -They are active against the following microorganisms: 1_ gram-positive and gram-negative bacteria 2_ spirochetes 3_ mycoplasmas, 4_ rickettsiae, 6_ Candida albicans

Tetracycline According to source divided into: Naturally occurring Tetracycline Chlortetracycline

Semi-synthetic

Oxytetracycline

Doxycycline, Lymecycline

Demeclocycline

Meclocycline

Methacycline Minocycline, Rolitetracycline

Tetracycline According to duration of action divided into: 1. Short-acting (Half-life is 6-8 hrs) Tetracycline 3. Long-acting (Half-life is 16 hrs or more) Doxycycline Minocycline Tigecycline

Chlortetracycline

Oxytetracycline 2. Intermediate-acting (Half-life is ~12 hrs) Demeclocycline Methacycline