Cephalosporins Overview and Generations
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Questions and Answers

What is the role of the acetyloxy group in the mechanism of inhibition?

  • It increases oral absorption.
  • It enhances β-lactamase stability.
  • It is a good leaving group. (correct)
  • It acts as a strong binding agent.
  • Which cephalosporin is known for being poorly absorbed through the gut wall?

  • Cefaclor
  • Cephaloridine (correct)
  • Cefuroxime
  • Cephalexin
  • How does the protonated –NH2 group contribute to cephalosporin stability?

  • It enhances oral absorption.
  • It improves the stability of the β-lactam ring. (correct)
  • It prevents hydrolysis of β-lactam.
  • It acts as an effective leaving group.
  • Which generation of cephalosporins shows increased activity against Gram-negative bacteria?

    <p>Third generation</p> Signup and view all the answers

    What effect does replacing the furan ring with an aminothiazole ring have?

    <p>It enhances penetration through the outer membrane.</p> Signup and view all the answers

    What is a major disadvantage of Cefuroxime regarding Pseudomonas aeruginosa?

    <p>It is ineffective against it.</p> Signup and view all the answers

    Which feature of Cefotaxime contributes to its reduced activity?

    <p>Metabolically vulnerable acetoxy group.</p> Signup and view all the answers

    Why are cephalosporins with a 3-substitution pattern favored for oral activity?

    <p>They stabilize the β-lactam ring.</p> Signup and view all the answers

    What is the structure of cephalosporins characterized by?

    <p>A bicyclic system including a four-membered β-lactam ring</p> Signup and view all the answers

    Which characteristic is true of first generation cephalosporins?

    <p>They primarily target Gram (+) bacteria.</p> Signup and view all the answers

    What modification is NOT commonly associated with cephalosporin synthesis?

    <p>Nucleophilic addition at carbon 3</p> Signup and view all the answers

    How do third generation cephalosporins compare to first generation ones?

    <p>They exhibit less activity against staphylococci.</p> Signup and view all the answers

    Which cephalosporin generation adds activity against Pseudomonas aeruginosa?

    <p>Fourth generation</p> Signup and view all the answers

    What is a drawback of cephalothin (Cefalotin)?

    <p>Rapid hydrolysis of its acetyloxy group</p> Signup and view all the answers

    What type of activity do second generation cephalosporins have?

    <p>Improved activity against Gram (-) while retaining Gram (+)</p> Signup and view all the answers

    What method is commonly used for achieving semisynthetic modifications of cephalosporins?

    <p>Acylation of the 7-amino group</p> Signup and view all the answers

    Which agent is co-administered with imipenem to protect it from hydrolytic metabolism?

    <p>Cilastatin</p> Signup and view all the answers

    What modifies the antibacterial activity spectrum of imipenem?

    <p>Substituents at the 3-position</p> Signup and view all the answers

    What unique feature of doripenem enhances its potency against Pseudomonas species?

    <p>4-β-methyl group</p> Signup and view all the answers

    Which of the following drugs is NOT classified as a carbapenem?

    <p>Aztreonam</p> Signup and view all the answers

    What is the primary consequence of imipenem being metabolized by renal dehydropeptidase-I?

    <p>Shorter half-life and nephrotoxicity</p> Signup and view all the answers

    What is the FDA-approved indication for the combination of meropenem and vaborbactam?

    <p>Complicated urinary tract infection (cUTI)</p> Signup and view all the answers

    How does the α-oriented methyl group at C-2 affect aztreonam?

    <p>Stabilizes it against β-lactamases</p> Signup and view all the answers

    Which carbapenem has a lower incidence of nephrotoxicity due to intrinsic stability in hydrolysis?

    <p>Doripenem</p> Signup and view all the answers

    What characteristic makes the discussed pharmacological agent suitable for once daily administration?

    <p>It is 95% serum bound.</p> Signup and view all the answers

    Which feature contributes to the agent's ability to penetrate the outer membrane of Gram-negative bacteria?

    <p>Z-methoxyimine moiety at C-7.</p> Signup and view all the answers

    What type of activity does Ceftaroline fosamil exhibit?

    <p>Activity against methicillin-resistant Staphylococcus aureus.</p> Signup and view all the answers

    What is a primary advantage of the discussed cephalosporin regarding β-lactamase?

    <p>Greater β-lactamase stability.</p> Signup and view all the answers

    Which cephalosporin is classified as a second generation?

    <p>Cefaclor</p> Signup and view all the answers

    Which of the following cephalosporins is an oximinocephalosporin derivative?

    <p>Cefuroxime</p> Signup and view all the answers

    Which statement is true regarding the characteristics of carbapenems?

    <p>They combine features of β-lactam antibiotics and β-lactamase inhibitors.</p> Signup and view all the answers

    What is a notable limitation of carbapenems?

    <p>Inability to be absorbed from the gut.</p> Signup and view all the answers

    Study Notes

    Cephalosporins

    • Second major group of β-lactam antibiotics
    • First cephalosporin (cephalosporin C) derived from a fungus in the mid-1940s
    • Bicyclic system containing a four-membered β-lactam ring fused to a six-membered dihydrothiazine ring
    • Act similarly to penicillins
    • Modifications can be made at different positions on the molecule
    • Semisynthetic modifications of the 7-ACA nucleus include acylation of the 7-amino group, nucleophilic substitution or reduction of the acetoxy group, and extra substitution at carbon 7

    Generations of Cephalosporins

    • First generation: Primarily active against Gram (+) bacteria, not significantly active against Gram (-) bacteria, lack activity against Pseudomonas aeruginosa, ineffective against methicillin-resistant Staph.aureus
    • Second generation: Retain anti-Gram (+) activity and add better anti-Gram (-) activity
    • Third generation: Less active against staphylococci than first generation, more active against Gram(-) bacteria than first or second generation drugs
    • Fourth generation: Antibacterial spectrum like third generation drugs, but also includes activity against Pseudomonas aeruginosa and some enterobacteria resistant to third generation cephalosporins, also more active against some Gram (+) organisms

    Cephalothin (Cefalotin)

    • Short duration of action due to rapid hydrolysis of the acetyloxy group at C-3 by esterase enzyme
    • Acetyloxy group is important for the mechanism of inhibition and acts as a good leaving group

    Cephaloridine

    • Replacing the acetoxy group with a metabolically stable pyridinium group
    • Pyridine can still act as a good leaving group for the inhibition mechanism, but is not cleaved by esterases
    • Exists as a zwitterion and is poorly absorbed through the gut wall, requires injection

    Cephalexin

    • Contains a 3-CH3 group which helps oral absorption but is bad for activity as it is not a good leaving group
    • Electron-withdrawing character of the protonated –NH2 group improves the stability of the β-lactam leading to orally active drugs
    • Good oral activity, broad spectrum activity (G +ve) & some Gram-negative bacilli, poor β-lactamase stability, poor ability to penetrate cerebrospinal fluids, lacks activity against Pseudomonas aeruginosa

    Cephradine

    • Dihydro-analog of cephalexin and undergoes aromatization in the body producing cephalexin

    Cefaclor

    • 3-substitution pattern and electron-withdrawing character of the protonated –NH2 improves the stability of the β-lactam ring, making it orally active
    • Less active against Gram-negative bacteria than other second-generation cephalosporins

    Cefuroxime

    • Contains a syn (Z) methoxyimino substituent which provides β-lactamases stability due to its steric effect
    • Z-oxime is much more stable than the E-oxime
    • Wide spectrum of activity, useful against organisms resistant to penicillin, poor activity against Pseudomomnas aeruginosa due to lack of drug penetration

    Third Generation Cephalosporins

    • Replacing the furan ring of Cefuroxime with an aminothiazole ring enhances penetration through the outer membrane of Gram-negative bacteria and increases affinity for the transpeptidase enzyme (PBPs)
    • Increase in activity against Gram-negative bacteria
    • Variety of structures with different substituents at position 3 to vary pharmacokinetic properties

    Cefotaxime

    • Metabolically vulnerable acetoxy group attached to C-3, losing about 90% of its activity when hydrolyzed
    • Greater anti–Pseudomonas aeruginosa activity

    Ceftriaxone

    • Metabolically stable, activating and highly acidic 3-thio-2-methyl-1,2-dihydro-1,2,4-triazine-5,6-dione at C-3 that forms an enolic sodium salt at normal pH, making the commercial product a disodium salt
    • 95% serum bound and exhibits an extended serum profile, suitable for once daily administration
    • Greater anti–Pseudomonas aeruginosa activity
    • First-line drug for meningitis due to its best CSF penetration and coverage for bacteria causing meningitis

    Fourth Generation Cephalosporins

    • Contain Z-methoxyimine moiety at C-7 in addition to quaternary ammonium group at C-3, enhancing water solubility, ability to penetrate the outer membrane, and broader anti-Gram-negative activity, active vs.Gram +ve cocci, good affinity for the transpeptidase enzyme, greater β-lactamase stability, excellent anti–Pseudomonas aeruginosa activity
    • Contain “Ceft” + “ol” in their name
    • Contain 5-amino-1,2,4-thiadiazole moiety in the acyl side chain

    Ceftazidime

    • Contains a 3-thio-2-methyl-1,2-dihydro-1,2,4-triazine-5,6-dione at C-3, metabolically stable, activates and is highly acidic, leading to the formation of an enolic sodium salt at normal pH and making commercial product a disodium salt

    Ceftaroline fosamil

    • Activity against strains of methicillin-resistant Staphylococcus aureus (MRSA) and multi-resistant Streptococcus pneumonia (MDRSP)
    • Acts as a prodrug for ceftaroline

    Carbapenems

    • Potent with an extraordinarily broad range of activity against Gram-positive and Gram negative bacteria, including P.aeruginosa
    • High resistance to β-lactamases
    • Combine the functional features of the best β-lactam antibiotics and β-lactamase inhibitors
    • Poor metabolic and chemical stability, not absorbed from the gut
    • Subjected to hydrolytic inactivation by renal dehydropeptidase-I (DHP-I), leading to an unacceptably short t1/2 in vivo and higher incidence of nephrotoxicity

    Thienamycin

    • N-formimidoyl derivative
    • Administration by IM injection or by IV infusion
    • Metabolized by renal dehydropeptidase-l (DHP-1) which deactivates thienamycin through hydrolysis of the β-lactam, producing kidney toxic metabolites

    Imipenem

    • Substituents at the 3-position affect the spectrum of antibacterial activity by influencing penetration into bacteria
    • Administered by IM injection or by IV infusion

    Meropenem

    • More active against Gram-negative and anaerobic bacteria than imipenem, but less active against Gram-positive species
    • Contains an extra methyl group at C-4 which conveys intrinsic resistance and stability to hydrolysis by renal DHP-1, leading to a lower incidence of nephrotoxicity and allowing for single agent administration
    • Administered by IV injection or infusion

    Doripenem

    • Newest of the approved carbapenems
    • Also contains the 4-β-methyl group, which confers stability toward renal DHP-1, so it is given as a single agent
    • Similar spectrum to imipenem and meropenem, considered more potent against Pseudomonas species

    Monobactams

    • Sulfamic acid moiety attached to the β-lactam ring resembles COOH in penicillins
    • Strong electron-withdrawing character of the sulfamic acid group makes the β-lactam bond more subjected to hydrolysis
    • α-oriented methyl group at C-2 is associated with stability of aztreonam toward β-lactamases

    Aztreonam

    • Shows good activity against Gram-negative bacteria and is resistant to β-lactamases
    • Often used for patients with penicillin allergies

    Key Points to Remember

    • Second generation cephalosporins offer improved Gram (-) activity over first generation
    • Third generation cephalosporins offer even greater Gram (-) activity, but reduced Staphylococcal activity
    • Fourth generation cephalosporins add activity against Pseudomonas aeruginosa and some resistant enterobacteria
    • Carbapenems are potent against Gram (+) and Gram (-) bacteria, including P.aeruginosa, and are resistant to β-lactamases
    • Imipenem is the most commonly used carbapenem, but is metabolized by renal DHP-1, leading to kidney toxicity, requiring co-administration of cilastatin to protect it from degradation
    • Meropenem is more stable than imipenem due to a methyl group at C-4, allowing for single agent administration and reducing nephrotoxicity
    • Doripenem is the newest carbapenem with a similar spectrum to imipenem and meropenem, but is considered more potent against Pseudomonas species
    • Monobactams have a distinct structure with a β-lactam ring and a sulfamic acid moiety
    • Aztreonam is the only approved monobactam, effective against Gram (-) bacteria, and resistant to β-lactamases

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    Description

    This quiz explores the structure, history, and classifications of cephalosporins, a major group of β-lactam antibiotics. It covers their generational differences, including their effectiveness against various types of bacteria. Test your understanding of cephalosporin modifications and their clinical significance.

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