Question 3

Created on Thu, 05/28/2015 - 18:35
Last updated on Wed, 12/23/2015 - 02:40
Pass rate: 60%
Highest mark: ?

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Compare and contrast the pharmacology of ceftriaxone, gentamicin and meropenem.

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College Answer

Ceftriaxone: vial with yellow water soluble powder for reconstitution; only administered parenterally, 33-66% excreted unchanged in urine, no active metabolites, 85-95% protein bound, elimination half life 6-9 hours (> 36 hours with severely impaired renal function), usual dosage 0.5 to 2g IV 12 or 24 hourly; 3rd generation cephalosporin antibiotic, inhibits cell wall synthesis, covers most gram negative rods (except Pseudomonas), and Gram positive cocci (except Methicillin Resistant, and group D streptococci); adverse reactions uncommon, but include overgrowth of non-susceptible organisms, and occasional haematologic, renal and hepatic adverse effects.

Gentamicin: ampoule with 80 mg/2 mL; only administered parenterally, excreted almost entirely by glomerular filtration, elimination half life 2-3 hours, no active metabolites, usual dosage 1 mg/kg tds or up to 5 mg/kg as daily dose, careful monitoring of blood levels required, especially if renal impairment (trough level not > 2 mcg/mL); aminoglycoside antibiotic, inhibits protein synthesis, covers most gram negative rods (including pseudomonas, but variability from hospital to hospital); serious adverse reactions include oto- and renal toxicity, potentiated by other oto- and nephro-toxins, prolongation of neuromuscular blockade may occur, other reactions uncommon.

Meropenem: vial with water soluble powder for reconstitution; only administered parenterally, 70% excreted unchanged in urine (requiring reduction of dosage if significant renal impairment), plasma binding 2%, elimination half life 1 hour, no active metabolites, usual dosage 500mg to 2g every 8 hours; carbapenem antibiotic, inhibits cell wall synthesis, active against a broad spectrum of aerobic and anaerobic bacteria (including Gram positive cocci and Gram negative rods, but excluding MRSA, Enterococcus faecium, Stenotrophomonas and many Pseudomonas); serious adverse reactions are rare, but include overgrowth of non- susceptible organisms, and occasional haematologic, gastrointestinal and hepatic adverse effects.


I refuse to believe that I would have lost marks in this question by not commenting on the colour of ceftriaxone powder.

Here is a tabulated answer with slightly less detail than the college provides.






3rd gen cephalosporin




IV – 1g; up to 3g/day

IV; daily dosing 5-7mg/kg

IV 1-2g – up to 6g/day


Cleared both renally (unchanged) and via biliary excretion (33%)

Cleared renally; rapidly and unchanged

Cleared renally

Mechanism of action

Inhibition of bacterial cell wall synthesis by binding to penicllin-binding proteins

Inhibition of bacterial protein synthesis by binding to the 50S subutnit of the bacterial ribosome

Inhibition of bacterial cell wall synthesis by binding to penicllin-binding proteins



Infections by susceptible gram-positive organisms;
Some gram-negative cover

Infections by susceptible gram-negative organisms;
Little role in treatment of gram-positive infections (with the exception of enterococcal endocarditis)

Infection by suceptible gram-positive and gram negative aerobic and anaerobic organisms; broad cover


Seizures in overdose
Bone marrow suppression in chronic high-dose use
Rapid development of microbial resistance

Accumulation in renal failure increases risk of toxicity
Potentiates neuromuscular blockade

Seizures in overdose