Pharmacology of Beta-Lactams, Tetracyclines and Antimalarials

Posted on Mar 23, 2026 in Biology

Beta-Lactam Antibiotics

A β-lactam is a cyclic amide with four atoms (3 carbons and 1 nitrogen) in its ring, known as the β-lactam ring. β-lactam antibiotics are a class of antibiotics which contain a β-lactam ring and primarily exert their action by inhibiting the synthesis of the bacterial cell wall (bactericidal).

Examples include:

  • Penicillin and its derivatives (penams)
  • Cephalosporins and their derivatives (cephems)

Classification of Penicillins

  • Natural: Penicillin G, Penicillin V
  • Semi-Synthetic: Amoxicillin, Ampicillin, Oxacillin, Cloxacillin, Methicillin

Classification of Cephalosporins

  • 1st Generation: Cefadroxil, Cephalexin
  • 2nd Generation: Cefaclor, Cefprozil, Cefoxitin
  • 3rd Generation: Cefixime, Cefotaxime
  • 4th Generation: Cefepime, Ceftobiprole

Other β-Lactams

  • Monobactams: Aztreonam
  • Carbapenems: Imipenem, Meropenem

Mechanism of Action of β-Lactam Antibiotics

All β-lactam antibiotics are bactericidal. They act by inhibiting the synthesis of the cell wall, which causes the lysis of bacteria. They target Penicillin-Binding Proteins (PBPs), which contain enzymes like transpeptidases and carboxypeptidases. These enzymes are responsible for transpeptidation, the cross-linking between peptide chains to form a rigid and strong cell wall.

Bacteria are hyperosmotic, meaning they absorb water into the cytoplasmic region. Due to this, bacteria swell and have a chance to burst, but the cell wall is normally strong and rigid, providing protection. When β-lactam antibiotics are introduced into the body, they bind with PBPs, inhibiting the action of transpeptidases and the synthesis of the cell wall. This causes lysis and cell death. PBPs vary in their affinity for binding penicillin or other β-lactam antibiotics.

Penicillins

Penicillin was the first antibiotic discovered and reported in 1929 by Alexander Fleming. All penicillins are composed of the 6-aminopenicillanic acid nucleus with a side chain determining the antibacterial spectrum and pharmacological properties.

Commercial production of biosynthetic penicillin depends on Penicillium notatum and Penicillium chrysogenum. Semi-synthetic penicillins are widely used because they are resistant to stomach acid (allowing oral administration) and possess a degree of resistance to penicillinase, a penicillin-destroying enzyme produced by some bacteria.

Nomenclature of Penicillins

Nomenclature is handled by two different systems:

  • Chemical Abstract System (CAS): Numbering starts from the sulfur (S) atom. The sulfur atom is assigned the 1st position and the nitrogen (N) atom is assigned the 4th position. It is called 6-acylamino-2,2-dimethyl-3-carboxylic acid.
  • USP System: Numbering starts from the nitrogen (N) atom. The nitrogen atom is given the 1st position and the sulfur (S) atom is assigned the 4th position. It is called 4-thia-1-azabicyclo[3.2.0]heptane.

Stereochemistry of Penicillins

Three chiral carbon atoms are present in the penicillin molecule. All naturally occurring, synthetic, and semi-synthetic penicillins possess the same absolute configuration at these three centers for antimicrobial activity. The absolute stereochemistry of penicillin is 3S:5R:6R.

Structure-Activity Relationship (SAR) of Penicillins

The penicillin molecule contains a highly strained 4-membered β-lactam ring fused to a 5-membered thiazolidine ring. The β-lactam ring is unstable and responsible for antibiotic potency, while the side chain determines the antibacterial spectrum and pharmacological properties.

  • Position 1: Sulfur is essential for activity. If oxidized to a sulfone or sulfoxide, it improves acid stability but decreases activity.
  • Position 2: The dimethyl group is essential for activity; no substitutions are allowed.
  • Position 3: The carboxylic acid is essential for activity. Changing it to an alcohol or ester decreases activity.
  • Position 4: Nitrogen is essential for activity.
  • Position 5: No substitution is allowed.
  • Position 6: Substitutions are allowed on the side chain (R). An electron-withdrawing group decreases electron density and provides compounds with better acid stability for oral use.
  • Position 7: The carbonyl group of the β-lactam ring is essential.

Uses and Adverse Drug Reactions (ADRs)

Penicillins are used to treat a wide variety of bacterial infections, including pneumonia, respiratory tract infections, meningitis, syphilis, and heart infections. ADRs include diarrhea, fever, body aches, severe skin rashes, itching, nausea, and vomiting.

Cephalosporins

Cephalosporins contain a β-lactam ring fused with a 6-membered dihydrothiazine ring. They contain a 7-aminocephalosporanic acid (7-ACA) nucleus. They have an affinity for PBPs and show resistance to some β-lactamases.

Nomenclature and Stereochemistry

The chemical nomenclature of cephalosporins is slightly more complex than penicillins due to the presence of a double bond. They possess two chiral carbons at C-6 and C-7 (6R, 7R).

Mechanism of Action and Resistance

They have almost the same mechanism as penicillin, binding to PBPs (transpeptidases and carboxypeptidases) to inhibit cell wall synthesis. Bacterial resistance occurs when susceptible cephalosporins are hydrolyzed by β-lactamase enzymes before they reach the PBPs.

SAR of Cephalosporins

  • Position 1: Oxidation of sulfur to sulfoxide or sulfone reduces antimicrobial activity. Replacement of S with O increases antibacterial activity. Replacement of S with a methyl group increases chemical stability.
  • Position 2: No changes allowed.
  • Position 3: An acetoxymethyl group increases activity. CH3 or Cl makes the compound orally active. Aromatic thiols or 5-membered heterocycles increase activity. Saturation of the double bond between C-3 and C-4 decreases activity.
  • Position 4: If the carboxyl group is replaced with an ester group, it enhances bioavailability by forming prodrugs for oral activity.
  • Position 5: Nitrogen is essential for activity.
  • Position 6: No substitution allowed.
  • Position 7: Acylation of the amino group increases activity against Gram-positive bacteria. A phenyl ring or other heterocyclic rings (e.g., thiophene, tetrazole) improve the spectrum of activity. Replacement of hydrogen with an OR group increases antibacterial activity.
  • Position 8: No change allowed.

Adverse Effects of Cephalosporins

Cephalosporins are generally well-tolerated but can be more toxic than penicillin. Effects include pain, diarrhea, and hypersensitivity reactions. They are used for respiratory and urinary tract infections.

SAR of Tetracyclines

  • Ring System: A four-ring linear fused system is essential; reducing the rings inactivates the compound.
  • C-2 Amide (CONH2): Essential for activity. Replacement reduces activity, but derivatives like rolitetracycline improve solubility.
  • C-3 & C-12a Hydroxyls: Crucial for forming the keto-enol system involved in ribosome binding.
  • C-4 Dimethylamino group: Essential for antibacterial activity; removal reduces efficacy.
  • C-6 Position: Modification is fruitful. Removing the 6-OH or 6-methyl group (e.g., doxycycline) increases stability against acids/bases and improves lipophilicity.
  • C-7 Position: Substitution (e.g., nitro, amino) can enhance activity, as seen in minocycline.

Mechanism of Action of Tetracyclines

  1. Transport: Tetracyclines enter the microbial cell via passive diffusion and an energy-dependent active transport system.
  2. Ribosomal Binding: They bind specifically to the 30S subunit of the bacterial ribosome, targeting proteins such as uS7, uS14, and uS3.
  3. Inhibition of Protein Synthesis: Binding blocks the attachment of aminoacyl-tRNA to the A-site (acceptor site) on the mRNA-ribosome complex.
  4. Result: This prevents the addition of new amino acids to the growing peptide chain, resulting in a bacteriostatic effect.

Principles of Prodrug Design

Prodrugs are pharmacologically inactive compounds that, when introduced into the body, are converted into pharmacologically active drugs through metabolism or enzymatic action.

Objectives of Prodrug Design

  • Pharmaceutical: To improve solubility, chemical stability, and organoleptic properties; to decrease irritation or pain after local administration; to reduce pharmaceutical technology problems.
  • Pharmacokinetic: To improve absorption (oral and non-oral); to decrease presystemic metabolism; to increase organ or tissue-selective delivery.
  • Pharmacodynamic: To decrease toxicity and improve the therapeutic index; to design single chemical entities combining two drugs (co-drug strategy).

Classification of Prodrugs

  • Carrier-linked Prodrugs: Bipartite, Tripartite, and Mutual prodrugs.
  • Bioprecursor Prodrugs
  • Polymeric Prodrugs

Antimalarial Drugs

Malaria is caused by the Plasmodium parasite and transmitted by the female Anopheles mosquito. Plasmodium belongs to the class of protozoa known as Sporozoa. The four main species affecting humans are P. falciparum, P. vivax, P. malariae, and P. ovale.

Life Cycle of Plasmodium

  1. An infected female Anopheles mosquito bites a human and releases sporozoites.
  2. Sporozoites infect liver cells and mature into schizonts.
  3. Schizonts rupture and release merozoites.
  4. Merozoites infect red blood cells (RBCs) and mature into schizonts, which rupture to release more merozoites.
  5. Some merozoites undergo sexual erythrocytic stages (gametocytes).
  6. In the mosquito’s stomach, male microgametes penetrate female macrogametes to form zygotes.
  7. The zygote becomes motile (ookinete) and forms oocytes, which grow, rupture, and release sporozoites into the salivary glands.

Classification of Antimalarial Drugs

1. Classification by Chemical Structure

  • Quinoline-related compounds: Chloroquine, amodiaquine, primaquine, mefloquine, quinine, piperaquine.
  • Artemisinin derivatives: Artemisinin, dihydroartemisinin, artemether, artesunate.
  • Antifolate compounds: Pyrimethamine, proguanil, chlorproguanil, trimethoprim, sulfadoxine.
  • Antibiotics: Tetracycline, doxycycline, clindamycin.

2. Classification by Site of Action

  • Blood Schizonticides: Kill asexual parasites in RBCs (e.g., Chloroquine, quinine, artemisinins).
  • Tissue Schizonticides: Kill parasites in the liver (e.g., Primaquine, pyrimethamine).
  • Anti-relapse Drugs: Target dormant hypnozoites in P. vivax and P. ovale (e.g., Primaquine).
  • Gametocytocides: Destroy sexual forms to stop transmission (e.g., Primaquine).