JNTU B.Pharmacy III MEDICINAL CHEMISTRY-III Previous Question Paper (August/September-2021 - R17)

Penicillins are a group of β-lactam antibiotics derived from 6-aminopenicillanic acid (6-APA). They act by inhibiting bacterial cell wall synthesis through the inhibition of transpeptidase enzymes, leading to bacterial lysis.

Classification of penicillins:

1. Natural Penicillins: Acid-labile and narrow spectrum (e.g., Penicillin G, Penicillin V).
2. Penicillinase-resistant Penicillins: Resistant to staphylococcal β-lactamases (e.g., Methicillin, Cloxacillin).
3. Aminopenicillins: Broader spectrum, effective against Gram-negative bacteria (e.g., Ampicillin, Amoxicillin).
4. Carboxypenicillins: Active against Pseudomonas aeruginosa (e.g., Carbenicillin, Ticarcillin).
5. Ureidopenicillins: Extended spectrum, including Pseudomonas (e.g., Piperacillin).

Structure-Activity Relationship (SAR):

1. β-lactam ring: Essential for activity; opening the ring leads to loss of antibacterial potency.
2. Free Carboxylic Acid: The C-3 carboxyl group is essential for activity; it must be free or exist as a prodrug (ester) that is hydrolyzed in vivo.
3. Acylamino side chain: The configuration at the C-6 position is critical. The presence of an electron-withdrawing group at the alpha-position of the side chain increases acid stability (e.g., Penicillin V).
4. Bicyclic system: The fused thiazolidine ring provides the necessary ring strain and spatial orientation for the β-lactam ring to bind to the target enzymes.
5. Stereochemistry: The 5R, 6R configuration is required for biological activity.

Penicillins and cephalosporins are β-lactam antibiotics characterized by a strained four-membered ring, which is essential for their antibacterial activity but also makes them highly susceptible to chemical degradation, particularly in aqueous solutions.

Chemical Degradation of Penicillins:

1. Acid-catalyzed hydrolysis: In acidic conditions (pH < 4), the carbonyl oxygen of the side-chain amide is protonated, facilitating an intramolecular nucleophilic attack by the amide oxygen on the β-lactam carbonyl. This leads to the formation of penillic acid.
2. Base-catalyzed hydrolysis: Hydroxide ions attack the β-lactam carbonyl directly, opening the ring to form penicilloic acid, which is biologically inactive.
3. Penicillenic acid formation: At neutral pH, penicillins can undergo rearrangement to form penicillenic acid, which is a major allergen responsible for hypersensitivity reactions.

Chemical Degradation of Cephalosporins:

1. β-lactam ring opening: Similar to penicillins, the β-lactam ring is susceptible to nucleophilic attack by water or hydroxyl ions, resulting in the formation of inactive cephalosporoic acid derivatives.
2. C-3 substituent degradation: Cephalosporins often possess a leaving group at the C-3 position (e.g., acetoxy group). In aqueous solution, this group can be hydrolyzed to form a hydroxymethyl derivative, which subsequently lactonizes to form an inactive desacetyl-lactone.
3. Isomerization: The double bond in the dihydrothiazine ring can migrate from the Δ3 position to the Δ2 position, resulting in a significant loss of antibacterial potency.

These degradation pathways necessitate the formulation of these drugs as dry powders for reconstitution or the use of buffered solutions to maintain stability.