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Zorluk: OrtaNeoplastic Processes

A 1919-year-old male of West African descent is admitted to the inpatient oncology unit for the initiation of induction chemotherapy after being diagnosed with bulky Burkitt lymphoma. His baseline renal function and electrolyte panels are within normal limits. Thirty-six hours after starting a cytotoxic regimen containing cyclophosphamide, doxorubicin, vincristine, and methotrexate, he reports severe nausea, muscle twitching, and significantly reduced urine output.

An electrocardiogram shows peaked T waves. Laboratory evaluation reveals the following:

TestResult
Potassium6.76.7 mEq/L
Calcium6.26.2 mg/dL
Phosphorus8.88.8 mg/dL
Uric acid16.416.4 mg/dL
Creatinine3.13.1 mg/dL

The attending physician plans to prescribe a recombinant urate oxidase enzyme to rapidly lower the patient's uric acid levels. Which of the following patient factors represents an absolute contraindication to this specific pharmacologic intervention?

  1. Glucose-6-phosphate dehydrogenase deficiencyCevap
  2. B
    Acute oliguric renal failure
  3. C
    Concurrent administration of allopurinol
  4. D
    Severe, symptomatic hypocalcemia
  5. E
    Heterozygosity for factor V Leiden

Cevap

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an absolute contraindication to rasburicase therapy.
The correct answer is Glucose-6-phosphate dehydrogenase (G6PD) deficiency. The patient has Tumor Lysis Syndrome (TLS) and requires rapid lowering of uric acid levels. The physician plans to use rasburicase, a recombinant urate oxidase. This enzyme facilitates the conversion of uric acid into allantoin, an inactive and highly soluble metabolite. However, this biochemical reaction also produces hydrogen peroxide (H2O2H_2O_2), a potent oxidizing agent. Red blood cells rely entirely on the G6PD enzyme to generate NADPH, which is necessary to maintain a supply of reduced glutathione to neutralize oxidative stressors like H2O2H_2O_2. In a patient with G6PD deficiency, administering rasburicase leads to overwhelming oxidative stress, resulting in severe hemolytic anemia and methemoglobinemia. Therefore, G6PD deficiency is an absolute contraindication to rasburicase, and testing is recommended in high-risk populations prior to administration.

Adım Adım Çözüm

1
Identify the patient's primary clinical syndrome.
The patient has developed Tumor Lysis Syndrome (TLS), characterized by hyperkalemia, hyperphosphatemia, hypocalcemia, hyperuricemia, and acute kidney injury following chemotherapy for a rapidly proliferating tumor.
Recognizing the syndrome contextualizes the need for aggressive metabolic management.
2
Determine the mechanism of action of the planned medication.
The planned drug is rasburicase, a recombinant urate oxidase that catalyzes the oxidation of uric acid to the highly soluble compound allantoin.
Understanding the pharmacodynamics is necessary to identify potential adverse effects and contraindications.
3
Identify the toxic byproducts of the enzymatic reaction.
The conversion of uric acid to allantoin by urate oxidase produces hydrogen peroxide (H2O2H_2O_2) as a metabolic byproduct.
The byproduct is responsible for the specific absolute contraindication associated with this drug.
4
Correlate the toxic byproduct with patient-specific risk factors.
Patients with Glucose-6-phosphate dehydrogenase (G6PD) deficiency cannot produce adequate NADPH to reduce glutathione. They are unable to neutralize the H2O2H_2O_2 oxidative stress, leading to life-threatening hemolysis and methemoglobinemia.
This establishes why G6PD deficiency must be ruled out (often highly relevant in patients of West African descent) before administering rasburicase.

Anahtar Kavram

Rasburicase (recombinant urate oxidase) produces hydrogen peroxide (H2O2H_2O_2) during the breakdown of uric acid, making it absolutely contraindicated in patients with G6PD deficiency due to the risk of severe hemolysis and methemoglobinemia.
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