Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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358 HIV AND AIDS<br />
Adverse effects<br />
The adverse effects of the nebulized route include cough <strong>and</strong><br />
bronchospasm, pre-administration of a nebulized β 2 -agonist<br />
minimizes these effects.<br />
Intravenous route adverse effects include:<br />
• hypotension <strong>and</strong> acidosis (due to cardiotoxicity) if given<br />
too rapidly;<br />
• dizziness <strong>and</strong> syncope;<br />
• hypoglycaemia due to toxicity to the pancreatic β-cells,<br />
producing hyperinsulinaemia;<br />
• nephrotoxicity (rarely irreversible);<br />
• pancreatitis;<br />
• reversible neutropenia;<br />
• prolongation of the QTc interval.<br />
Pharmacokinetics<br />
Pentamidine is administered parenterally. The t 1/2 is six hours<br />
<strong>and</strong> it is redistributed from plasma by tissue binding. Renal<br />
excretion is low (5% of dose). Nebulized therapy yields lung<br />
concentrations that are as high or higher than those achieved<br />
after intravenous infusion.<br />
Drug interactions<br />
Pentamidine inhibits cholinesterase. This suggests potential<br />
interactions in enhancing/prolonging the effect of suxamethonium<br />
<strong>and</strong> reducing that of competitive muscle relaxants, but it<br />
is not known whether this is of clinical importance.<br />
Alternative regimens for treating PCP are summarized in<br />
Table 46.4.<br />
Table 46.4: Alternative regimens for treating PCP<br />
Alternative PCP treatment<br />
Trimethoprim, in two<br />
divided doses plus<br />
dapsone, daily<br />
Primaquine, p.o. <strong>and</strong><br />
clindamycin, i.v. for 11 days<br />
<strong>and</strong> then p.o. for 10 days<br />
Atovaquone (a<br />
hydroxynaphthoquinone),<br />
for 21 days<br />
Additional comments<br />
Oral therapy for 21 days, used in<br />
mild to moderate PCP. Check<br />
glucose-6-phosphate<br />
dehydrogenase<br />
Used in mild to moderate PCP.<br />
Check glucose-6-phosphate<br />
dehydrogenase<br />
Oral therapy used in mild to<br />
moderate PCP. Blocks protozoan<br />
mitochondrial electron transport<br />
chain <strong>and</strong> de novo pyrimidine<br />
synthesis. Side effects include<br />
nausea, vomiting, rash <strong>and</strong><br />
hepatitis<br />
TOXOPLASMA GONDII<br />
PYRIMETHAMINE AND SULFADIAZINE<br />
Use<br />
This combination is the first-line therapy for cerebral <strong>and</strong> tissue<br />
toxoplasmosis. Pyrimethamine is given as an oral loading<br />
dose followed by a maintenance dose, together with sulfadiazine.<br />
Treatment is continued for at least four to six weeks<br />
after clinical <strong>and</strong> neurological resolution, <strong>and</strong> for up to six<br />
months thereafter. Folinic acid is given prophylactically to<br />
reduce drug-induced bone marrow suppression.<br />
Mechanism of action<br />
Sulfadiazine acts as a competitive inhibitor of dihydropteroate<br />
(folate) synthase (competing with p-aminobenzoic acid) in folate<br />
synthesis. Pyrimethamine is a competitive inhibitor of dihydrofolate<br />
reductase, which converts dihydrofolate to tetrahydrofolate.<br />
Together they sequentially block the first two major steps in<br />
the synthesis of folate in the parasite. Their selective toxicity is<br />
due to the fact that humans can utilize exogenous folinic acid<br />
<strong>and</strong> dietary folate, whereas the parasite must synthesize these.<br />
Adverse effects<br />
The major toxic effects of the combination are:<br />
• nausea <strong>and</strong> vomiting;<br />
• fever <strong>and</strong> rashes which may be life-threatening<br />
(Stevens–Johnson syndrome);<br />
• bone marrow suppression, especially granulocytopenia;<br />
• hepatitis;<br />
• nephrotoxicity, including crystalluria <strong>and</strong> obstructive<br />
nephropathy.<br />
Pharmacokinetics<br />
Oral absorption of pyrimethamine is good (90%). It undergoes<br />
extensive hepatic metabolism, but approximately 20% is<br />
recovered unchanged in the urine. It has a long plasma t 1/2<br />
(35–175 hours). Because of its high lipid solubility it has a<br />
large volume of distribution, <strong>and</strong> achieves CSF concentrations<br />
that are 10–25% of those in plasma.<br />
Sulfadiazine is rapidly <strong>and</strong> completely absorbed after oral<br />
administration. However, there is substantial first-pass hepatic<br />
metabolism. The mean plasma t 1/2 is ten hours. Cerebrospinal<br />
fluid concentrations are 70% of those in plasma. Clearance is a<br />
combination of hepatic metabolism <strong>and</strong> renal excretion, with<br />
50% of a dose being excreted in the urine, so dose reduction is<br />
needed in patients with renal failure.<br />
Drug interactions<br />
These are primarily due to sulfadiazine (Chapter 43) <strong>and</strong> the<br />
combined bone marrow suppressive effect of pyrimethamine<br />
with other antifolates.<br />
An alternative anti-toxoplasmosis regimen consists of<br />
pyrimethamine in combination with clindamycin with folinic<br />
acid as above. Newer therapies for cerebral toxoplasmosis as