Sulfachlorpyridazine Sodium

Sulfonamide Antibiotic Systemic Antibacterial

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 25 mg/kg (combined with trimethoprim at 5 mg/kg) q12h Duration: 5-7 days or until 48 hours after clinical signs resolve
Notes: Administer with water to prevent crystalluria. Ensure adequate hydration.
Cat PO 25 mg/kg (combined with trimethoprim at 5 mg/kg) q12h Duration: 5-7 days or until 48 hours after clinical signs resolve
Notes: Monitor for signs of toxicity, especially in cats with renal impairment.
Horse PO 25 mg/kg (combined with trimethoprim at 5 mg/kg) q12h Duration: 5-7 days
Notes: Use with caution in horses with renal or hepatic disease.
Cattle PO 25 mg/kg (combined with trimethoprim at 5 mg/kg) q12h Duration: 3-5 days
Notes: Observe withdrawal times for meat and milk.
Sheep and Goats PO 25 mg/kg (combined with trimethoprim at 5 mg/kg) q12h Duration: 3-5 days
Notes: Extra-label use; observe withdrawal times.
Rabbit PO 30 mg/kg (sulfachlorpyridazine alone) q24h Duration: 5-7 days
Notes: Use with caution; monitor for gastrointestinal disturbances.
Poultry PO (drinking water) 0.04% solution (400 mg/L) for 5-7 days Continuous Duration: 5-7 days
Notes: Ensure fresh solution daily; observe withdrawal times for eggs and meat.

Clinical Indications & Species Uses

General Indications
  • Treatment of infections caused by sulfonamide-susceptible organisms, especially when combined with trimethoprim.
Dog (Canine)
  • Treatment of bacterial infections caused by susceptible organisms, including urinary tract infections, respiratory infections, skin and soft tissue infections, and enteric infections.
Cat (Feline)
  • Treatment of bacterial infections caused by susceptible organisms, including urinary tract infections, respiratory infections, and skin infections.
Horse (Equine)
  • Treatment of respiratory tract infections, urinary tract infections, and other susceptible bacterial infections.
Cattle (Bovine)
  • Treatment of bacterial pneumonia, enteric infections (e.g., colibacillosis, salmonellosis), and foot rot caused by susceptible organisms.
Small Ruminants (Sheep / Goat)
  • Treatment of respiratory infections, enteric infections, and other susceptible bacterial infections.
Rabbit & Small Mammals
  • Treatment of bacterial infections, particularly respiratory and enteric infections, but use is limited due to potential toxicity.
Avian & Poultry
  • Treatment of colibacillosis, fowl cholera, and other susceptible bacterial infections; however, use is limited due to withdrawal times and regulatory restrictions.

Pharmacology & Mechanism of Action

Drug Class: Sulfonamide Antibiotic | Pharmacological Group: Systemic Antibacterial

Mechanism of Action: Sulfachlorpyridazine is a short-acting sulfonamide that competitively inhibits dihydropteroate synthase, an enzyme involved in the bacterial synthesis of folic acid. By mimicking para-aminobenzoic acid (PABA), it blocks the conversion of PABA to dihydropteroate, thereby inhibiting the production of tetrahydrofolate, which is essential for bacterial nucleic acid synthesis. This bacteriostatic action is effective against susceptible gram-positive and gram-negative organisms. In combination with trimethoprim, it provides a synergistic bactericidal effect by sequentially blocking folate metabolism.

Pharmacodynamics: Sulfachlorpyridazine exhibits a broad spectrum of activity against many veterinary pathogens, including E. coli, Salmonella spp., Pasteurella spp., Staphylococcus spp., and Streptococcus spp. Its activity is time-dependent, and efficacy is enhanced when combined with a diaminopyrimidine such as trimethoprim. The drug is generally bacteriostatic at therapeutic concentrations, but bactericidal when combined with trimethoprim. Resistance can develop via mutations in the target enzyme or increased production of PABA.

⚑ Pharmacokinetics Summary

Absorption: Sulfachlorpyridazine is rapidly and well absorbed from the gastrointestinal tract in most species. Oral bioavailability is high, typically exceeding 80% in monogastric animals. In ruminants, absorption may be slower and less complete due to ruminal degradation.
Distribution: The drug is widely distributed throughout the body, achieving therapeutic concentrations in most tissues and fluids, including pleural, peritoneal, and synovial fluids. It crosses the blood-brain barrier to some extent, especially in the presence of inflammation. Protein binding is moderate, ranging from 20-60% depending on species.
Metabolism: Sulfachlorpyridazine undergoes hepatic metabolism, primarily via acetylation and hydroxylation. The extent of metabolism varies by species; dogs and cats acetylate less than ruminants. Metabolites may retain some antibacterial activity.
Excretion: Excretion is primarily renal, with both glomerular filtration and tubular secretion contributing. A significant portion is excreted unchanged in the urine, which is why sulfonamides are useful for urinary tract infections. Biliary excretion also occurs but is minor.
Half-Life: The elimination half-life is short, approximately 2-4 hours in dogs, 2-5 hours in cats, and 1-3 hours in cattle and horses. In ruminants, the half-life may be slightly longer due to enterohepatic recycling.
Bioavailability: Oral bioavailability is high (80-100%) in monogastric animals; in ruminants, it may be reduced due to ruminal degradation.
Protein Binding: 20-60% depending on species and concentration.

Available Formulations & Strengths

Oral Tablet (combination with trimethoprim) Sulfachlorpyridazine 200 mg / Trimethoprim 40 mg; 400 mg / 80 mg; 800 mg / 160 mg (PO)
Oral Suspension Sulfachlorpyridazine 100 mg/mL (PO)
Injectable Solution Sulfachlorpyridazine 400 mg/mL (IV, IM)
Soluble Powder (for drinking water) Sulfachlorpyridazine 50% (500 mg/g) (PO (drinking water))

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to sulfonamides
  • Severe hepatic or renal impairment
  • Pregnancy (especially near term) unless benefits outweigh risks
  • Use in animals with blood dyscrasias
  • Concurrent use with certain drugs that may increase toxicity (e.g., methotrexate)
Warnings & Clinical Precautions:
  • Ensure adequate water intake to prevent crystalluria and urolithiasis.
  • Use with caution in animals with renal or hepatic disease.
  • Prolonged use may lead to superinfection with resistant organisms.
  • In food animals, observe withdrawal times.
  • May cause photosensitivity in some species.
  • In horses, use caution as sulfonamides can cause diarrhea and colitis.
  • In cats, monitor for signs of toxicity (e.g., anorexia, vomiting, anemia).

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Hypersensitivity reactions (skin rashes, fever)
  • Crystalluria (if hydration is inadequate)

Serious / Severe:

  • Blood dyscrasias (thrombocytopenia, leukopenia, agranulocytosis)
  • Hepatotoxicity
  • Nephrotoxicity
  • Stevens-Johnson syndrome (rare)
  • Keratoconjunctivitis sicca (dry eye) in dogs

Rare:

  • Folate deficiency leading to anemia
  • Polyneuropathy
  • Photosensitization

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Trimethoprim Synergistic antibacterial effect; sequential blockade of folate synthesis. Moderate (beneficial)
Methenamine May increase risk of crystalluria due to acidification of urine. Moderate
Warfarin Sulfonamides may potentiate the anticoagulant effect by displacing warfarin from protein binding sites. High
Methotrexate Increased risk of methotrexate toxicity due to displacement from protein binding and reduced renal excretion. High
Phenytoin Sulfonamides may inhibit the metabolism of phenytoin, leading to increased phenytoin levels and toxicity. Moderate
Antacids containing magnesium or aluminum May reduce absorption of sulfonamides. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Anorexia
  • Vomiting
  • Diarrhea
  • Crystalluria
  • Hematuria
  • Renal failure
  • CNS signs (ataxia, seizures) in severe cases
  • Bone marrow suppression

Emergency Treatment Protocol: Discontinue the drug immediately. Provide supportive care including fluid therapy to enhance renal excretion and prevent crystalluria. Monitor renal function and blood counts. In severe cases, consider alkalinization of urine to increase solubility of sulfonamides. Symptomatic and supportive treatment is essential.

Food Animal Withdrawal Times

πŸ₯© Meat: 7 daysπŸ₯› Milk: 4 daysπŸ₯š Eggs: 5 days

Withdrawal times may vary by country and formulation; always follow label or regulatory guidelines. For extra-label use, consult a veterinarian.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25Β°C) in a dry place.

Light Sensitivity: Light-sensitive β€” protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep container tightly closed. Do not freeze oral suspensions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs, cats, and horses in some formulations; not approved for food animals in the US. In other countries, approval may vary.

Extra-Label (Off-Label) Use: Extra-label use in food animals is permitted under AMDUCA with a valid veterinary-client-patient relationship, but requires extended withdrawal times and must not be used in feed. In the US, sulfachlorpyridazine is not approved for use in food animals; therefore, extra-label use is subject to strict regulations.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Sulfachlorpyridazine is a short-acting sulfonamide often used in combination with trimethoprim to enhance efficacy and reduce the development of resistance. It is particularly useful for urinary tract infections due to high renal excretion. When using this drug, ensure adequate hydration to minimize the risk of crystalluria. Monitor for adverse effects, especially in cats and animals with renal or hepatic impairment. In food animals, strict adherence to withdrawal times is essential. Consider culture and sensitivity testing before initiating therapy to ensure susceptibility.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)