Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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328 ANTIBACTERIAL DRUGS<br />
Enterococcus faecalis, but not (when used alone) against other<br />
streptococci. They synergize with penicillins in killing<br />
Streptococcus faecalis in endocarditis. Aminoglycosides are used<br />
in serious infections including septicaemia, sometimes alone<br />
but usually in combination with other antibiotics (penicillins<br />
or cephalosporins). Gentamicin is widely used <strong>and</strong> has a<br />
broad spectrum, but is ineffective against anaerobes, many<br />
streptococci <strong>and</strong> pneumococci.<br />
Tobramycin is probably somewhat less nephrotoxic than<br />
gentamicin. Amikacin is more effective than gentamicin for<br />
pseudomonal infections <strong>and</strong> is occasionally effective against<br />
organisms resistant to gentamicin. It is principally indicated<br />
in serious infections caused by Gram-negative bacilli that are<br />
resistant to gentamicin. Topical gentamicin or tobramycin<br />
eye drops are used to treat eye infections.<br />
Mechanism of action<br />
These drugs are transported into cells <strong>and</strong> block bacterial protein<br />
synthesis by binding to the 30S ribosome.<br />
Adverse effects<br />
These are important <strong>and</strong> are related to duration of therapy <strong>and</strong><br />
trough plasma concentrations. They are more frequent in the<br />
elderly <strong>and</strong> in renal impairment. Therapeutic monitoring is<br />
performed by measuring plasma concentrations before dosing<br />
(trough) <strong>and</strong> at ‘peak’ levels (usually at an arbitrary one hour<br />
after dosing). Eighth nerve damage is potentially catastrophic<br />
<strong>and</strong> is often irreversible. Acute tubular necrosis <strong>and</strong> renal failure<br />
are usually reversible if diagnosed promptly <strong>and</strong> the drug<br />
stopped or the dose reduced. Hypersensitivity rashes are<br />
uncommon. Bone marrow suppression is rare. Exacerbation of<br />
myasthenia gravis is predictable in patients with this disease.<br />
Pharmacokinetics<br />
Aminoglycosides are poorly absorbed from the gut <strong>and</strong> are<br />
given by intramuscular or intravenous injection. They are<br />
poorly protein bound (30%) <strong>and</strong> are excreted renally. The halflife<br />
is short, usually two hours, but once daily administration<br />
is usually adequate. This presumably reflects a post-antibiotic<br />
effect whereby bacterial growth is inhibited following clearance<br />
of the drug. In patients with renal dysfunction, dose<br />
reduction <strong>and</strong>/or an increased dose interval is required.<br />
Cerebrospinal fluid (CSF) penetration is poor.<br />
Drug interactions<br />
Aminoglycosides enhance neuromuscular blockade of nondepolarizing<br />
neuromuscular antagonists. Loop diuretics<br />
potentiate their nephrotoxicity <strong>and</strong> ototoxicity.<br />
CHLORAMPHENICOL<br />
Uses<br />
Chloramphenicol has a broad spectrum of activity <strong>and</strong> penetrates<br />
tissues exceptionally well. It is bacteriostatic, but is<br />
extremely effective against streptococci, staphylococci,<br />
H. influenzae, salmonellae <strong>and</strong> others. Uncommonly it causes<br />
aplastic anaemia, so its use is largely confined to life-threatening<br />
disease (e.g. H. influenzae epiglottitis, meningitis, typhoid fever)<br />
<strong>and</strong> to topical use as eyedrops.<br />
Mechanism of action<br />
Chloramphenicol inhibits bacterial ribosome function by<br />
inhibiting the 50S ribosomal peptidyl transferase, thereby preventing<br />
peptide elongation.<br />
Adverse effects<br />
These include:<br />
1. haematological effects – dose-related erythroid suppression<br />
is common <strong>and</strong> predictable, but in addition aplastic<br />
anaemia occurs unpredictably with an incidence of<br />
approximately 1:40 000. This is irreversible in 50% of cases.<br />
It is rarely, if ever, related to the use of eyedrops.<br />
2. grey baby syndrome – the grey colour is due to shock<br />
(hypotension <strong>and</strong> tissue hypoperfusion).<br />
Chloramphenicol accumulates in neonates (especially if<br />
premature) due to reduced glucuronidation in the<br />
immature liver (see Chapter 10).<br />
3. other – sore mouth, diarrhoea, encephalopathy <strong>and</strong> optic<br />
neuritis.<br />
Pharmacokinetics<br />
Chloramphenicol is well absorbed following oral administration<br />
<strong>and</strong> can also be given by the intramuscular <strong>and</strong> intravenous<br />
routes. It is widely distributed <strong>and</strong> CSF penetration is<br />
excellent. It mainly undergoes hepatic glucuronidation, but in<br />
neonates this is impaired.<br />
Drug interactions<br />
Chloramphenicol inhibits the metabolism of warfarin,<br />
phenytoin <strong>and</strong> theophylline.<br />
MACROLIDES<br />
Macrolide antibiotics (e.g. erythromycin, clarithromycin,<br />
azithromycin) have an antibacterial spectrum similar, but not<br />
identical to that of penicillin. Distinctively, they are effective<br />
against several unusual organisms, including Chlamydia,<br />
Legionella <strong>and</strong> Mycoplasma.<br />
ERYTHROMYCIN<br />
Uses<br />
Uses include respiratory infections (including Mycoplasma<br />
pneumoniae, psittacosis <strong>and</strong> Legionnaires’ disease), whooping<br />
cough, Campylobacter enteritis <strong>and</strong> non-specific urethritis.<br />
Erythromycin is a useful alternative to penicillin in penicillinallergic<br />
patients (except meningitis: it does not penetrate the<br />
CSF adequately). It is useful for skin infections, such as lowgrade<br />
cellulitis <strong>and</strong> infected acne, <strong>and</strong> is acceptable for patients<br />
with an infective exacerbation of chronic bronchitis. It is most<br />
commonly administered by mouth four times daily, although<br />
when necessary it may be given by intravenous infusion.