CLINICAL USE


Antimicrobial agent:Toxoplasmosis in AIDS patients (unlicensed indication)Prevention of rheumatic fever

DOSE IN NORMAL RENAL FUNCTION

Loading dose: 2–4 gMaintenance dose: 2–6 g daily in divided doses

PHARMACOKINETICS

  • Molecular weight &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp :250.3; 272.3 (as sodium salt)
  • %Protein binding &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp :20–55
  • %Excreted unchanged in urine &nbsp &nbsp : 80 –

  • Volume of distribution (L/kg) &nbsp &nbsp &nbsp :0.29

  • half-life – normal/ESRD (hrs)&nbsp &nbsp &nbsp :17/Prolonged

    DOSE IN RENAL IMPAIRMENT

    GFR (mL/MIN)

  • 20 to 50 &nbsp &nbsp : Dose as in normal renal function
  • 10 to 20 &nbsp &nbsp : Use 50% of dose and monitor levels
  • <10 &nbsp &nbsp &nbsp &nbsp &nbsp : Use 25% of dose and monitor levels

    DOSE IN PATIENTS UNDERGOING RENAL REPLACEMENT THERAPIES

  • CAPD &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp:Unknown dialysability. Dose as in GFR <10 mL/min
  • HD &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp :Dialysed. Dose as in GFR
  • <10 &nbsp &nbsp &nbsp &nbsp &nbsp : mL/min
  • HDF/high flux &nbsp :Dialysed. Dose as in GFR
  • <10 &nbsp &nbsp &nbsp &nbsp &nbsp : mL/min
  • CAV/VVHD &nbsp &nbsp &nbsp:Dialysed. Dose as in GFR=10–20 mL/min

    IMPORTANT DRUG INTERACTIONS

    Potentially hazardous interactions with other drugs

  • Antibacterials: increased risk of crystalluria with methenamine
  • Anticoagulants: effect of coumarins enhanced
  • Anti-epileptics: antifolate effect and concentration of phenytoin increased
  • Antimalarials: increased risk of antifolate effect with pyrimethamine
  • Antipsychotics: avoid concomitant use with clozapine (increased risk of agranulocytosis)
  • Ciclosporin: reduced levels of ciclosporin; increased risk of nephrotoxicityCytotoxics: increase risk of methotrexate toxicity

    ADMINISTRATION

    Reconstition

    –

    Route

    Oral

    Rate of Administration

    –

    Comments

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    OTHER INFORMATION

    Penetrates into the CSF within 4 hours of oral administration to produce therapeutic concentrations which may be more than half those in the bloodMetabolised in the liver to the acetylated form, with elimination predominantly via the kidneysUrinary excretion of sulfadiazine and its acetyl derivative is dependent on pH; when the urine is acidic about 30% is excreted unchanged in both fast and slow acetylators, whereas when the urine is alkaline about 75% is excreted unchanged by slow acetylatorsCrystalluria may be avoided by adequate hydration and alkalinising the urine to a pH >7.15Blood concentrations of 100–150 micrograms/mL are desirableFor treatment of toxoplasmosis, use sulfadiazine in conjunction with pyrimethamine 25–100 mg daily.
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