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Sulfadimethoxine/ Ormetoprim (Primor®)

By: Dr. Nicholas Dodman

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  • Sulfadimethoxine and ormetoprim are two different antibiotics compounded into one drug. Sulfadimethoxine is a long-acting sulphonamide and ormetoprim is a diaminopyrimidine structurally related to trimethoprim.
  • These antibiotic medications potentiate each other as they block two sequential steps in bacterial folic acid synthesis, thus inhibiting bacterial thymidine synthesis. Sulfadimethoxine blocks the conversion of para-aminobenzoic acid to dihydrofolic acid by inhibiting the enzyme dihydrofolate synthetase. Ormetoprim blocks the conversion of dihydrofolic acid to tetrahydrofolic acid by inhibiting dihydrofolate reductase. The net effect is that of a potentiated sulfa whose action is not merely bacteriostatic but bactericidal.
  • Potentiated sulfas have a broad spectrum of antimicrobial activity. Susceptible Gram-positive bacteria include: most Streptococci, most Staphylococci, and Nocardia. Susceptible Gram-negative microorganisms include: most Enterobacteriaceae spp. In addition, potentiated sulfas are effective against certain intracellular parasites, notably Coccidian spp. and Toxoplasma gondii.
  • In dogs, sulfadimethoxine is not appreciably metabolized by the liver and renal excretion accounts for the majority of its elimination. Reabsorption of this drug from the renal tubules accounts for its long half-life (therapeutic levels are supposed to be maintained over 24 hours).
  • This is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

    Brand Names and Other Names

  • This drug is registered for use in dogs, chicken, ducks, partridges, and salmonids (salmon and trout)
  • Human formulations: None
  • Veterinary formulations: Primor® (Pfizer) and brand name farm animal preparations

    Uses of Sulfadimethoxine + Ormetoprim

  • Treatment of skin and soft tissue infections (wounds and abscesses) in dogs caused by strains of Staphylococcus aureus and Escherichia coli
  • Treatment of urinary tract infections caused by Escherichia coli, Staphylococcus spp., and Proteus mirabilis susceptible to ormetoprim-potentiated sulfadimethoxine.

    Precautions and Side Effects

  • Sulfadimethoxine + ormetoprim should not be used in animals with known hypersensitivity or allergy to either drug or other sulfonamides.
  • This combination should also be avoided in patients with liver problems, thyroid disease, or blood dyscrasias.
  • Safety in pregnant animals has not been documented and laboratory evidence points to possible teratogenicity.
  • As with other potentiated sulfas, keratoconjunctivitis is a possible sequel to the use of this combination.
  • In addition, toxic hepatopathy, seizures, vomiting, diarrhea, fever, hemolytic anemia, hematuria, polydipsia/polyuria, urticaria, and polyarthritis are all possible side effects.

    Drug Interactions

  • Augmented sulfas, in general, inhibit the metabolism of phenytoin, augmenting its pharmacologic effects and side effects. Sulfonamides can displace methotrexate from plasma protein binding sites, increasing free methotrexate concentrations.
  • Other drug interactions occur when augmented sulfas are coadministered with anticoagulants (prothrombin time increased), cyclosporine (decreased therapeutic efficacy of cyclosporine), and diuretics (increased incidence of thrombocytopenia with purpura in humans).
  • In addition, augmented sulfas may increase the hypoglycemic response to sulfonureas and may increase serum levels of zidovuldine.

    How Sulfadimethoxine + Ormetoprim is Supplied

    Tablets sizes include:

  • 120 milligrams (100 milligrams of sulfadimethoxine and 20 milligrams of ormetoprim)
  • 240 milligrams (200 milligrams of sulfadimethoxine and 40 milligrams of ormetoprim)
  • 600 milligrams (500 milligrams of sulfadimethoxine and 100 milligrams of ormetoprim), or
  • 1200 milligrams (1,000 milligrams of sulfadimethoxine and 200 milligrams of ormetoprim).

    Dosing Information

  • In dogs, the usual dose is 27.5 mg per pound (55 mg/kg) orally for the first day and then 13.75 mg per pound (27.5 mg/kg) orally once daily until the end of the course of treatment.
  • In cats, there is no published dose for cats
  • Treatment should be continued until the patient is asymptomatic for 48 hours but should not exceed a total of 21 consecutive days [to avoid hypothyroidism]. Adequate water intake should be maintained during the treatment period.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse or prevent the development of resistance.

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