2- Tetracyclines Classification Broad spectrum bacteriostatic group of antibiotics Classification 1- Short acting (6-8 hrs) e.g., Tetracycline (Tetracid) & Oxytetracycline (Oxytetracid) 2- Intermediate acting (12 hrs) e.g., Democycline & methacycline 3- Long acting (15-18 hrs) e,g., Doxycycline (Vibramycin) & Minocycline (Minocin)

Pharmacokinetics 1- Incompletely absorbed orally Affected by food except doxycycline & minocycline Absorption is decreased by milk, Ca++, Mg++, Fe++ & Al+++ 2- Distributed all over the body Pass BBB & placenta Concentrated at site of calcification (bone & teeth)

Mechanism of action 3- Metabolized in liver by conjugation 4- Excretion Mainly renal Doxycycline is excreted in bile (Preferred for patients with impaired renal function) Mechanism of action Inhibit bacterial protein synthesis by binding to 30S ribosomal subunit

Resistance to tetracyclines 1- Decreased intracellular concentration by: Impaired influx Increased efflux by an active transport pump 2- Production of proteins that interfere with tetracycline-ribosome binding 3- Enzymatic degradation of tetracyclines by acetylation

Therapeutic uses 1- Drug of choice in treatment of infection caused by: Mycoplasma e.g., Pneumonia Chlamydia e.g., Urethritis Rickettsia e.g., Typhus fever 2- Treatment of gonorrhea and syphilis in penicillin allergic patients 3- Treatment of acne & amebiasis 4- Eradication of Helicobacter pylori 5- Prevention of malaria 6- Topically for skin & eye infections

Adverse effects 1- Teeth and bone abnormalities Tetracycline binds with Ca++ in teeth and bones causing: A- Yellow brown discoloration of teeth and enamel hypoplasia B- Retardation in bone growth Contraindicated in pregnancy, lactation and in children less than 8 years

Drug interactions 2- GIT disturbances Nausea, vomiting, epigastric pain & diarrhea 3- Outdated tetracycline (after expiry date) renal tubular dysfunction (Fanconi syndrome) 4- Superinfections Enterocolitis, vaginal or oral candidiasis 5- Hypersensitivity Drug interactions Antacids (e.g., Mg hydroxide & Ca carbonate) bind tetracycline in the intestine & reduce its absorption into the body

B- Antibiotics bind to 50S ribosome 1- Macrolides (Big molecule) Erythromycin, Clarithromycin & Azithromycin are examples of macrolides Erythromycin (Erythrocin, Erythrocid) Pharmacokinetics Absorbed orally but acid sensitive, so used as enteric coated or as stearates Distributed all over the body but does not cross BBB Metabolized in liver & excreted in bile

Spectrum Mechanism of action Effective against most Gram +ve & some Gram –ve organisms Corynebacterium, Mycoplasma & Chlamydia Mechanism of action Bacteriostatic at usual dose but bactericidal in large dose Inhibits protein synthesis by binding to 50S subunit of the bacterial ribosome

Resistance to erythromycin Reduced permeability of the cell membrane or active efflux Production of esterase enzyme that hydrolyzes erythromycin Modification of the ribosomal binding site by plasmid encoded methylase enzyme Bacteria that develop resistance to erythromycin are resistant to other macrolides as well

Therapeutic uses 1- Drug of choice in: Corynebacterial infections e.g., Diphtheria Chlamydial infections e.g., Urethritis 2- Alternative to: Penicillins in patients allergic to penicillin Tetracyclines in Chlamydial infection during pregnancy

Adverse reactions Drug interactions Epigastric pain & diarrhea Reversible ototoxicity Impaired liver function Hypersensitivity reactions Drug interactions Erythromycin inhibits cytochrome P450 metabolism of theophylline, warfarin, carbamazepine & terfenadine Toxic concentrations

Clarithromycin (Klacid, Claribiotic) Similar to erythromycin but: More acid stable Longer duration of action (twice/day) More effective against H. influenza & H. pylori Less GIT irritation

Azithromycin (Zithromax, Zithrocin) Similar to erythromycin but: More acid stable Longer duration of action (once/day) Less active against staphylococci & streptococci More active against H. influenza No enzyme inhibition or drug interaction More expensive

2- Ketolides e.g., Telithromycin (KeteK) Semisynthetic derivatives of erythromycin Acid stable and can therefore be taken orally Binds to the subunit 50S of the bacterial ribosome Effective against macrolide resistant organisms Adverse effects include visual disturbances and cardiac arrhythmia

3- Clindamycin (Dalacin-C) It is a lincosamide & structurally related to macrolides Pharmacokinetics Absorbed orally & parenterally Distributed all over the body but not CSF, concentrated in bone & teeth Metabolized in liver & excreted in bile

Antibacterial activity Binds to 50S ribosomal subunit & inhibits protein synthesis Active against Gram +ve cocci & many anaerobes Therapeutic uses 1- Bone & teeth infections 2- Locally in acne

Adverse effects 1- Enterocolitis (treated by vancomycin) 2- Liver function impairment 3- Intestinal disturbances 4- Allergy & skin rash