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Hydroxyurea (Mylocel®, Hydrea®)

By: Dr. Nicholas Dodman

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  • Hydroxyurea is a chemotherapeutic agent used to treat certain solid tumors and myeloid tumors.
  • The mechanism of its antineoplastic effect is unknown. However, acting as an antimetabolite, it blocks the ribonucleotide reductase system, interfering with the conversion of ribonucleotides to deoxyribonucleotides. DNA synthesis is blocked while RNA and protein synthesis remains unaffected.
  • Hyroxyurea exhibits increased effectiveness when used in conjunction with irradiation therapy.
  • Absorption of hydroxyurea from the GI tract is excellent (bioavailability 80to 100%). Peak plasma levels are attained in 1 to 2 hours. Peak cerebral spinal fluid (CSF) levels occur in 3 hours. Fifty percent of an administered dose is metabolized by the liver and is subsequently excreted in the urine as urea. Urinary excretion of intact drug in urine accounts for the balance of elimination.
  • Hydroxyurea is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian. It is also a controlled substance
  • Hydroxyurea is not approved for use in animals by the Food and Drug Administration (FDA) but may be legally prescribed by veterinarians as an extra-label drug

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Mylocel® (MGI Pharma), Hydrea® (Bristol-Myers Squib), and generic hydroxyurea.
  • Veterinary formulations: None

    Uses of Hyroxyurea

    As a chemotherapeutic agent in the treatment of the following conditions:
  • Melanoma
  • Chronic myelocytic leukemia
  • Polycythemia
  • Mastocytomas
  • Erythrocytosis
  • Feline hypereosinophilic syndrome

    Precautions and Side Effects

  • Marked bone marrow suppression and severe anemia are contraindications to the use of hydroxyurea. Bone marrow suppression is the most likely side effect: It is heralded by leukopenia, thrombocytopenia, and anemia.
  • Renal toxicity may also occur. Hydroxyurea should be used with caution in patients with pre-existing renal dysfunction.
  • Minor complications of hydroxyurea's use may include skin rash or ulceration, erythema, alopecia, stomatitis, anorexia, vomiting, and diarrhea.
  • Monitoring parameters should include: 1. CBC, including platelet count every 2 weeks until stable, thereafter every 3 months; 2. BUN, serum creatinine, and liver function before therapy and every 3 to 4 months during therapy. If the WBC count falls below 2,500/mm3 or the platelet count falls below 100,000/mm3 treatment should be temporarily interrupted until these parameters climb into an acceptable range. Anemia, if it occurs, can be treated by blood transfusion.

    Drug Interactions

  • Laboratory test interactions: serum uric acid, BUN, and creatinine levels may be increased by hydroxyurea.

    How Hydroxyurea is Supplied

  • Hydroxyurea is available in 1000 mg tablets and 500 mg capsules.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Doses of hydroxyurea vary widely depending on the reason for prescribing.
  • In dogs, the usual dose is 25 to 40 mg per pound (50 to 80 mg/kg) orally every three days.
  • In cats, the usual dose is 12.5 mg per pound (25 mg/kg) orally three times per week.
  • 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.

    ** Some veterinarians may suggest starting treatment at a higher dose rate such as 15 mg per pound (30 mg/kg) orally once a day for one week (for dogs and cats) prior to reducing the dose into the aforementioned range.

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