PHCM 413 Medicinal Chemistry II Lecture 8 PDF

PHCM 413: Medicinal Chemistry II Lecture 8: Cephalosporins  Introduction The cephalosporins are lactam, broad-spectrum, Penicillinase resistance antibiotics that are closely related both structurally and functionally to the penicillins. The f...

PHCM 413: Medicinal Chemistry II Lecture 8: Cephalosporins  Introduction The cephalosporins are lactam, broad-spectrum, Penicillinase resistance antibiotics that are closely related both structurally and functionally to the penicillins. The first cephalosporin (cephalosporin C) was derived from a fungus obtained in the mid-1940s from sewer waters on the island of Sardinia by an Italian professor Giuseppe Brotzu who noted that the waters surrounding the sewage outlet were periodically cleared of microorganisms. He reasoned that an organism might be producing an antibacterial substance, so he collected samples and isolated a fungus called Cephalosporium acremonium (now called Acremonium chrysogenum). The crude extract from this organism was shown to have antibacterial properties, and in 1948, workers at Oxford University isolated cephalosporin C, but it was not until 1961 that the structure was established by X-ray crystallography. Cephalosporium acremonium  Cephalosporin C The structure of cephalosporin C has similarities to that of penicillin in that it has a bicyclic system containing a four-membered strained -lactam ring, but this time the - lactam ring is fused to a six-membered dihydrothiazine ring. Cephalosporin C is not particularly potent compared with penicillins (1/1000 the activity of penicillin G). Still, the antibacterial activity it does have is more evenly directed against Gram-negative and Gram-positive bacteria. Another in-built advantage of cephalosporin C is its greater resistance to acid hydrolysis and -lactamase enzymes. It is also less likely to cause allergic reactions. Therefore, cephalosporin C was seen as a useful lead compound for the development of further broad- spectrum antibiotics, hopefully with increased potency.  Structure-Activity Relationships of Cephalosporin C Many analogs of cephalosporin C have been made that demonstrate the importance of the -lactam ring within the bicyclic system, an ionized carboxylate group at position 4, and the acylamino side chain at position 7. These results tally closely with those obtained for the penicillins. The strain effect of a 6-membered ring fused to a 4- membered ring is less than for penicillin, but this is partially offset by the effect of the acetyloxy group at The shading indicates positions position 3. This can act as a good leaving group in the for possible modification inhibition mechanism by which cephalosporins inhibit of cephalosporin C. the transpeptidase enzyme. There is a limited number of places where modifications can be made, but there are more possibilities than with penicillins. These are as follows: variations of the 7-acylamino side chain, variations of the 3-acetoxymethyl side chain, and extra substitution at carbon 7.  Structure-Activity Relationships of Cephalosporin C Variation of the 7-acylamino side chain alters antimicrobial activity, whereas the variation of the side chain at position 3 predominantly alters the metabolic and pharmacokinetic properties of the compound. Introduction of a methoxy substitution at C-7 is possible. Deacetylation of cephalosporins occurs metabolically The shading indicates positions to produce inactive metabolites. Metabolism can be for possible modification blocked by replacing the susceptible acetoxy group of cephalosporin C. with metabolically stable groups. A methyl substituent at position 3 is good for oral absorption but bad for activity unless a hydrophilic group is present at the -position of the acyl side chain.  Structure-Activity Relationships of Cephalosporin C Mechanism by which cephalosporins inhibit the transpeptidase enzyme  Synthesis of Cephalosporin Analogs at Position 7 Access to analogs with varied side chains at position 7 initially posed a problem. Unlike penicillins, it proved impossible to obtain cephalosporin analogs by fermentation. Similarly, it was not possible to obtain 7-aminocephalosporinic acid (7-ACA), the core chemical structure (a synthon) for the synthesis of cephalosporin, either by fermentation or by enzymatic hydrolysis of cephalosporin C, thus preventing the semi-synthetic approach analogs to the preparation of penicillins from 6-APA. 7-aminocephalosporinic acid Therefore, a way had to be found to obtain 7-ACA from (7-ACA) cephalosporin C by chemical hydrolysis. -lactam ring. This is no easy task, as a secondary amide has to be hydrolyzed in the presence of a highly -lactam ring. Normal hydrolytic procedures are not suitable, so a special method had to be worked out.  Synthesis of Cephalosporin Analogs at Position 7 The first step of the procedure requires the formation of an imino chloride. This is only possible for the secondary amide group, as ring constraints prevent the -lactam nitrogen from forming a double bond within the -lactam ring. The imino chloride can then be treated with an alcohol to give an imino ether. This functional group is more susceptible to hydrolysis than the -lactam ring. So, treatment with aqueous acid successfully gives the desired 7-ACA, which can then be acylated to give a range of analogs. Synthesis of Cephalosporin Analogs at Position 7 Synthesis of 7-ACA and cephalosporin analogs Mechanism for imino chloride formation  First-Generation Cephalosporins Examples of first-generation cephalosporins include cephalothin, cephaloridine, cefalexin, and cefazolin. Compared to penicillins, first-generation cephalosporins have greater stability to acid conditions and -lactamases and have a good ratio of activity against Gram-positive and Gram-negative bacteria. However, most are poorly absorbed through the gut wall and must be injected and are generally lower in activity. Still, as with penicillins, the appearance of resistant organisms has posed a problem, particularly with Gram-negative organisms. These contain -lactamases which are more effective than the -lactamases of Gram-positive organisms. Steric shields successfully protect cephalosporins from these -lactamases but also prevent them from inhibiting the transpeptidase target enzymes. Cephalothin Cephaloridine cefalexin Cefazolin  First-Generation Cephalosporins Cephalothin A disadvantage with cephalothin is the fact that esterase enzymes readily hydrolyze the acetyloxy group at position 3 to give the less active alcohol. The acetyloxy group is important to the mechanism of inhibition. It acts as a good leaving group, whereas the alcohol is a much poorer leaving group. Therefore, it would be useful if this metabolism could be blocked to prolong activity.  First-Generation Cephalosporins Cephaloridine Replacing the ester in cephalothin with a metabolically stable pyridinium group gives cephaloridine. The pyridine can still act as a good leaving group for the inhibition mechanism but is not cleaved by esterases. Cephaloridine exists as a zwitterion and is soluble in water, but, like most first-generation cephalosporins, it is poorly absorbed through the gut wall and has to be injected.  First-Generation Cephalosporins Cefalexin & Cefazolin Cefalexin has a methyl substituent at position 3, which appears to help oral absorption. A methyl group would normally be bad for activity as it is not a good leaving group. However, the presence of a hydrophilic amino group at the -carbon of the Cefalexin 7-acylamino side chain in cefalexin helps to restore activity, and cephalexin is one of the few cephalosporins that is orally active. The mechanism of absorption through the gut wall is poorly understood, and it is unclear why the 3-methyl group is so advantageous for absorption. Cefazolin Cefazolin is another example of a first-generation cephalosporin.  The Synthesis of 3-methylated Cephalosporins It involves using a penicillin starting material, as shown below. The synthesis, which was first demonstrated by Eli Lilly Pharmaceuticals, involves the oxidation of sulfur followed by an acid-catalyzed ring expansion, where the five-membered thiazolidine ring in penicillin is converted to the six-membered dihydrothiazine ring in cephalosporin.  Second-Generation Cephalosporins (Cephamycins) Cephamycins contain a methoxy substituent at position 7, which has proved advantageous. The parent compound cephamycin C was isolated from a culture of Streptomyces clavuligerus and was the first -lactam to be isolated from a bacterial source. Modifying the side chain of cephamycin C gave cefoxitin, which showed a broader spectrum of activity than most first-generation cephalosporins. This is due to greater resistance to - lactamase enzymes, which may be due to the steric hindrance provided by the methoxy group. Cefoxitin shows good metabolic stability to esterases owing to the presence of the urethane group at position 3, rather than an ester. Urethane group  Second-Generation Cephalosporins (Oximinocephalosporins) The development of oximinocephalosporins has been a major advance in cephalosporin research. These structures contain an iminomethoxy group at the -position of the acyl side chain, which significantly increases the stability of cephalosporins against the - lactamases produced by some organisms (e.g. Haemophilus influenza). The first useful agent in this class of compounds was cefuroxime, which like cefoxitin, has an increased resistance to -lactamases and mammalian esterases. Unlike cefoxitin, cefuroxime retains activity against streptococci and, to a lesser extent, staphylococci.  Third-Generation Cephalosporins Replacing the furan ring of the oximinocephalosporins with an aminothiazole ring enhances the penetration of cephalosporins through the outer membrane of Gram-negative bacteria and may also increase affinity for the transpeptidase enzyme. As a result, third-generation cephalosporins containing aminothiazole ring have a marked increase in activity against these Gram-negative bacteria. Various such structures have been prepared, such as ceftazidime, cefotaxime, ceftizoxime, and ceftriaxone, with different substituents at position 3 to vary the pharmacokinetic properties. They play a major role in antimicrobial therapy because of their activity against Gram- negative bacteria, many of which are resistant to other -lactams.  Fourth-Generation Cephalosporins Cefepime and cefpirome are oximinocephalosporins which have been classed as fourth- generation cephalosporins. They are zwitterionic compounds having a positively charged substituent at position 3 and a negatively charged carboxylate group at position 4. This property appears to radically enhance the ability of these compounds to penetrate the outer membrane of Gram-negative bacteria. They are also found to have a good affinity for the transpeptidase enzyme and a low affinity for a variety of -lactamases.  Fifth-Generation Cephalosporins Ceftaroline fosamil is a fifth-generation cephalosporin that has activity against various strains of MRSA and multi-resistant Streptococcus pneumonia (MDRSP). It acts as a prodrug for ceftaroline, and the 1,3- thiazole ring is thought to be important for its activity against MRSA. O - O O - O O O O N S O N N N S O N N N H H S O N S + H N S H S + N N N O N N S Ceftaroline fosamil P S Ceftaroline NH HO OH H 2N  Resistance to Cephalosporins The activity of a specific cephalosporin against a particular bacterial cell is dependent on the same factors as those for penicillins. i.e., the ability to reach the transpeptidase enzyme, stability to any -lactamases that might be present, and the affinity of the antibiotic for the target. For example, most cephalosporins (with the exception of cephaloridine) are stable to the -lactamase produced by S. aureus and can reach the transpeptidase enzyme without difficulty. Therefore, the relative ability of cephalosporins to inhibit S. aureus comes down to their affinity for the target transpeptidase enzyme. Agents such as cephamycins and ceftazidime have poor affinity, whereas other cephalosporins have a higher affinity. The MRSA organism contains a modified transpeptidase enzyme ( PBP2a ) for which both penicillins and cephalosporins have poor affinity.