Anti-Biotics (P. Chemistry | Ch-12): D Pharma 1st year Notes

Antibiotics are one of the most important discoveries in the history of medicine. They are chemical substances — produced naturally by microorganisms or synthesized in a laboratory — that kill bacteria or inhibit their growth.

As a D.Pharma student, understanding antibiotics is critical not just for your exams but also for your future role as a pharmacy professional.

PCI ER 2020 Syllabus Coverage: This chapter requires you to study each antibiotic with respect to its IUPAC name, chemical structure (starred compounds*), uses, stability & storage conditions, types of formulations, and popular brand names.

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What Are Antibiotics?

• Antibiotics are medications used to treat bacterial infections.
• They work either by killing bacteria (bactericidal) or by preventing bacteria from multiplying (bacteriostatic).
• Antibiotics are NOT effective against viral infections such as the common cold or flu.

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Classification of Antibiotics

By chemical structure, antibiotics are classified into the following major groups:

Class	Examples	Mechanism of Action
Beta-lactams	Penicillins, Cephalosporins, Carbapenems, Monobactams	Inhibit bacterial cell wall synthesis
Macrolides	Erythromycin, Clarithromycin, Azithromycin	Inhibit protein synthesis
Tetracyclines	Tetracycline, Doxycycline, Minocycline	Inhibit protein synthesis
Aminoglycosides	Gentamicin, Tobramycin, Amikacin	Inhibit protein synthesis
Fluoroquinolones	Ciprofloxacin, Levofloxacin, Moxifloxacin	Inhibit DNA replication
Miscellaneous	Chloramphenicol, Clindamycin	Various mechanisms

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Syllabus Drug List (PCI ER 2020 — Chapter 12)

• Beta-lactam Antibiotics: Penicillin G, Amoxicillin*, Cloxacillin, Streptomycin
• Tetracyclines: Doxycycline, Minocycline
• Macrolides: Erythromycin, Azithromycin
• Miscellaneous: Chloramphenicol*, Clindamycin

(Compounds marked with * require chemical structure knowledge for exams)

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Beta-Lactam Antibiotics

Beta-lactam antibiotics all share a beta-lactam ring in their chemical structure. They work by inhibiting cell wall synthesis in bacteria, ultimately causing bacterial cell death.

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1. Penicillin G (Benzylpenicillin)

IUPAC / Chemical Name: Benzylpenicillin

Classification: Narrow-spectrum Beta-lactam antibiotic

Uses: Penicillin G is effective against infections caused by susceptible gram-positive bacteria, including:

• Streptococcus pneumoniae → strep throat, pneumonia
• Staphylococcus aureus → boils, cellulitis
• Neisseria gonorrhoeae → gonorrhea
• Treponema pallidum → syphilis

Stability and Storage Conditions

• Stable at room temperature
• Must be protected from light and moisture

Types of Formulations

• Oral tablets
• Intramuscular (IM) injections
• Intravenous (IV) injections

Popular Brand Names

• Pfizerpen
• Pen-Vee K
• Bicillin L-A

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2. Amoxicillin* (Structure Important for Exam)

IUPAC / Chemical Name: 6-[D(−)β-amino-p-hydroxyphenyl-acetamido]-3-hydroxy-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

Classification: Broad-spectrum Beta-lactam antibiotic (aminopenicillin group)

Key Features of Chemical Structure: Amoxicillin has the classic beta-lactam + thiazolidine ring system (same as all penicillins) with an additional para-hydroxyphenyl group and a free amino (–NH₂) group at the alpha position of the side chain. These two additions make it broader spectrum than Penicillin G.

Uses: Amoxicillin treats a wide range of bacterial infections:

• Upper respiratory tract infections (sinusitis, bronchitis, pneumonia)
• Lower respiratory tract infections (acute otitis media, chronic bronchitis exacerbations)
• Skin and soft tissue infections
• Urinary tract infections (UTIs)
• Gonorrhea

Stability and Storage Conditions: Stable at room temperature

• Protect from light and moisture

Types of Formulations:

• Capsules
• Tablets
• Oral suspension

Popular Brand Names:

• Amoxil
• Polymox
• Trimox

Exam Tip: Amoxicillin is the most commonly prescribed antibiotic worldwide. Remember the –NH₂ group on the side chain — it’s what makes it broader spectrum than Penicillin G.

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3. Cloxacillin

IUPAC / Chemical Name: (2S,5R,6R)-6-[[3-(2-chlorophenyl)-5-methyl-1,2-oxazole-4-carbonyl]amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

Classification: Narrow-spectrum, penicillinase-resistant Beta-lactam antibiotic (isoxazolyl penicillin group)

Key Feature: Cloxacillin is resistant to beta-lactamase (penicillinase), making it effective against beta-lactamase-producing Staphylococci — bacteria that destroy ordinary penicillins.

Uses:

• Skin infections
• Bone infections (osteomyelitis)
• Respiratory infections
• Gonorrhea
• Sepsis

Stability and Storage Conditions

• Stable at room temperature
• Protect from light and moisture

Types of Formulations

• Capsules
• Tablets
• Injection

Popular Brand Names

• Cloxapen
• Cloxacap

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4. Streptomycin

IUPAC / Chemical Name: O-2-deoxy-2-methylamino-A-L-glucopyranosyl-(1→2)-O-5-deoxy-3-C-formyl-beta-L-lyxopentanofuranoside

Classification: Aminoglycoside antibiotic (Note: Although listed under the Beta-lactam/antibiotic section in the syllabus, Streptomycin belongs to the Aminoglycoside class)

Key Feature: Streptomycin was the first antibiotic effective against tuberculosis (TB). It inhibits protein synthesis by binding to the 30S ribosomal subunit.

Uses:

• Tuberculosis (TB) — first-line drug
• Tularemia
• Plague
• Gram-negative bacterial infections: E. coli, Salmonella

Stability and Storage Conditions

• Stable at room temperature
• Protect from light and moisture

Types of Formulations

• Injection (most common)
• Oral suspension
• Ophthalmic ointment
• Topical cream

Popular Brand Names:

• Plantomycin

Exam Tip: Streptomycin is given only by injection for TB treatment because it is poorly absorbed orally.

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Tetracyclines

Tetracyclines are broad-spectrum antibiotics that work by inhibiting protein synthesis. They bind to the 30S ribosomal subunit, blocking the attachment of aminoacyl-tRNA to the mRNA-ribosome complex.

They are called tetracyclines because of their four fused ring (tetracyclic) structure.

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5. Doxycycline

IUPAC / Chemical Name: (4S,4aR,5S,5aR,6R,12aS)-4-(dimethylamino)-3,5,10,12,12a-pentahydroxy-6-methyl-1,11-dioxo-1,4,4a,5,5a,6,11,12a-octahydrotetracene-2-carboxamide

Classification: Second-generation Tetracycline antibiotic

Uses: Doxycycline is one of the most versatile antibiotics and is used to treat:

• Acne
• Lyme disease
• Malaria (prophylaxis and treatment)
• Rocky Mountain spotted fever
• Typhus

Stability and Storage Conditions

• Stable at room temperature
• Store in a cool, dry place, away from direct sunlight

Types of Formulations

• Capsules
• Tablets

Popular Brand Names

• Vibramycin-D
• Efracea

Exam Tip: Doxycycline is preferred over older tetracyclines because it has better oral bioavailability and a longer half-life, requiring only once or twice daily dosing.

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6. Minocycline

IUPAC / Chemical Name: (2E,4S,4aR,5aS,12aR)-2-(Amino-hydroxy-methylidene)-4,7-bis(dimethylamino)-10,11,12a-trihydroxy-4a,5,5a,6-tetrahydro-4H-tetracene-1,3,12-trione

Classification: Second-generation Tetracycline antibiotic

Key Difference from Doxycycline: Minocycline has an additional dimethylamino group at C-7 position, giving it a broader spectrum and higher lipid solubility, making it penetrate tissues better.

Uses:

• Acne (very commonly used)
• Lyme disease
• Gonorrhea
• Syphilis
• Rocky Mountain spotted fever

Stability and Storage Conditions

• Store at room temperature in a cool, dry place
• Protect from light and moisture

Types of Formulations

• Capsules
• Tablets
• Injectable suspension

Popular Brand Names

• Minocin
• Dynacin
• Solodyn

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Macrolides

Macrolides are characterized by a large macrocyclic lactone ring (14–16 membered) with attached sugars. They inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit, blocking translocation.

They are typically used as alternatives for patients allergic to penicillin.

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7. Erythromycin

IUPAC / Chemical Name: (3R,4S,5S,6R,7R,9R,11R,12R,13S,14R)-6-{[(2S,3R,4S,6R)-4-(Dimethylamino)-3-hydroxy-6-methyloxan-2-yl]oxy}-14-ethyl-7,12,13-trihydroxy-4-{[(2R,4R,5S,6S)-5-hydroxy-4-methoxy-4,6-dimethyloxan-2-yl]oxy}-3,5,7,9,11,13-hexamethyl-1-oxacyclotetradecane-2,10-dione

Classification: First-generation Macrolide antibiotic (14-membered ring)

Uses:

• Strep throat
• Pneumonia
• Ear infections
• Gonorrhea
• Used in penicillin-allergic patients

Stability and Storage Conditions

• Stable at room temperature
• Store in a cool, dry place to prevent degradation

Types of Formulations

• Tablets
• Capsules
• Oral suspension

Popular Brand Names

• E-Mycin
• Ery-Tab
• Erythrocin

Exam Tip: Erythromycin is the prototype macrolide antibiotic — the one from which all other macrolides (azithromycin, clarithromycin) were developed.

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8. Azithromycin

IUPAC / Chemical Name: (2R,3S,4R,5R,8R,10R,11R,12S,13S,14R)-2-ethyl-3,4,10-trihydroxy-3,5,6,8,10,12,14-heptamethyl-15-oxo-11-{[3,4,6-trideoxy-3-(dimethylamino)-β-D-xylo-hexopyranosyl]oxy}-1-oxa-6-azacyclopentadec-13-yl 2,6-dideoxy-3C-methyl-3-O-methyl-α-L-ribo-hexopyranoside

Classification: Second-generation Macrolide antibiotic (15-membered ring — also called an azalide)

Key Difference from Erythromycin: Azithromycin has a 15-membered ring (one nitrogen atom inserted into the lactone ring), unlike erythromycin’s 14-membered ring. This gives it:

• Longer half-life (allows once-daily dosing and short 3–5 day courses)
• Better tissue penetration
• Broader spectrum

Uses:

• Respiratory infections: pneumonia, bronchitis, sinusitis
• Ear infections
• Skin infections

Stability and Storage Conditions

• Stable at room temperature
• Store in a cool, dry place, away from direct sunlight

Types of Formulations:

• Tablets
• Oral suspension
• Intravenous (IV) infusion

Popular Brand Names

• Zithromax
• Sumamed
• Hemomycin

Exam Tip: Azithromycin is popularly known as the “Z-Pack.” Its 5-day course is effective because it accumulates in tissues and releases slowly.

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Miscellaneous Antibiotics

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9. Chloramphenicol* (Structure Important for Exam)

IUPAC / Chemical Name: 2,2-Dichloro-N-[(1R,2R)-1,3-dihydroxy-1-(4-nitrophenyl)propan-2-yl]acetamide

Classification: Miscellaneous broad-spectrum antibiotic

Key Features of Chemical Structure: Chloramphenicol has a unique structure with:

• A para-nitrophenyl group
• A dichloroacetamide group (the two chlorine atoms are characteristic)
• Two hydroxyl (–OH) groups

This structure enables it to bind to the 50S ribosomal subunit, inhibiting protein synthesis.

Uses: Chloramphenicol is a broad-spectrum antibiotic used mainly for serious infections:

• Meningitis
• Typhoid fever
• Ear infections

Note: Due to the risk of serious side effects (aplastic anaemia), Chloramphenicol use is now restricted to life-threatening infections where alternatives are not suitable.

Stability and Storage Conditions

• Stable at room temperature
• Store in a cool, dry place, protected from light

Types of Formulations

• Capsules
• Oral suspension
• Eye drops (very commonly used)
• Injection

Popular Brand Names

• Chloromycetin
• Ak-Chlor

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10. Clindamycin

IUPAC / Chemical Name: methyl 7-chloro-6,7,8-trideoxy-6-{[(4R)-1-methyl-4-propyl-L-prolyl]amino}-1-thio-L-threo-α-D-galacto-octopyranoside

Classification: Lincosamide antibiotic (Miscellaneous group)

Key Feature: Clindamycin inhibits protein synthesis by binding to the 50S ribosomal subunit — same target as macrolides. It is particularly effective against anaerobic bacteria and is a key drug for skin and soft tissue infections.

Uses

• Skin infections: acne, impetigo
• Respiratory infections: pneumonia, bronchitis
• Genital infections: pelvic inflammatory disease (PID), gonorrhea
• Bone and joint infections

Stability and Storage Conditions

• Stable at room temperature up to 25°C (77°F)
• Store in a cool, dry place, protected from light

Types of Formulations

• Capsules
• Tablets

Popular Brand Names

• Cleocin
• Dalacin

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Quick Revision Table — All Chapter 12 Antibiotics

Drug	Class	Mechanism	Key Use	Brand Name
Penicillin G	Beta-lactam	Cell wall inhibition	Strep throat, Syphilis	Pfizerpen
Amoxicillin	Beta-lactam	Cell wall inhibition	UTI, Respiratory infections	Amoxil
Cloxacillin	Beta-lactam (penicillinase-resistant)	Cell wall inhibition	Staph skin/bone infections	Cloxapen
Streptomycin	Aminoglycoside	30S ribosome inhibition	Tuberculosis	Plantomycin
Doxycycline	Tetracycline	30S ribosome inhibition	Acne, Malaria, Lyme disease	Vibramycin-D
Minocycline	Tetracycline	30S ribosome inhibition	Acne, Gonorrhea	Minocin
Erythromycin	Macrolide	50S ribosome inhibition	Strep throat, Pneumonia	E-Mycin
Azithromycin	Macrolide (Azalide)	50S ribosome inhibition	Respiratory infections	Zithromax
Chloramphenicol	Miscellaneous	50S ribosome inhibition	Typhoid fever, Meningitis	Chloromycetin
Clindamycin	Lincosamide	50S ribosome inhibition	Anaerobic infections, Acne	Cleocin

FAQ – Antibiotics