A reader expressed disappointment that our recent listing of “Some Warfarin Drug Interactions”1 did not include acetaminophen. Perhaps it should have. Acetaminophen can increase the anticoagulant effect of warfarin, particularly with continued use, but it does so inconsistently. The mechanism of this interaction has not been established, but may be related to an acetaminophen metabolite inhibiting vitamin K-epoxide reductase, the target for warfarin’s anticoagulant effect.2

Patient susceptibility varies, possibly on a genetic basis; occasional use of acetaminophen generally has little or no effect on the international normalized ratio (INR) in patients on chronic warfarin therapy, but in some, even a few grams of the drug may cause a dramatic increase in INR. One study in healthy subjects found no effect of acetaminophen 4 g per day for 2 weeks, while another study in patients with the same acetaminophen dose for the same period of time found a moderate increase in INR.3,4 It might be prudent to monitor INR in patients on chronic warfarin therapy more closely than usual when they take more than 2 g per day of acetaminophen for more than a few days.

1. Pharmacogenetic-based dosing of warfarin. Med Lett Drugs Ther 2008; 50:39.
2. HH Thijssen et al. Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle. Thromb Haemost 2004; 92:797.
3. D Kwan et al. The effects of acetaminophen on pharmacokinetics and pharmacodynamics of warfarin. J Clin Pharmacol 1999; 39:68.
4. I Mahe et al. Paracetamol: A haemorrhagic risk factor in patients on warfarin. Br J Clin Pharmacol 2005; 59:371.

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Warfarin sodium (Coumadin, and others) and other coumarin anticoagulants prevent thrombosis, but patient response is highly variable and overanticoagulation can lead to hemorrhage. Genotyping patients for single nucleotide polymorphisms (SNPs) that affect coumarin metabolism and sensitivity may help clinicians estimate the therapeutic warfarin dose. The FDA has added a note to warfarin labeling recommending lowrange doses for patients with such genetic variations. Commercial tests for these variants are now available and cost about $500 per test.

Genetic Test for Carbamazepine-Induced Stevens-Johnson Syndrome Carbamazepine (Tegretol, Carbatrol, Equetro, and others), which is now used to treat not only epilepsy but also trigeminal neuralgia and manic episodes in patients with bipolar disorder,1 is a known cause of Stevens-Johnson syndrome (SJS). The incidence of carbamazepine-induced SJS in countries with mainly white populations is 1 to 6 per 10,000 new users of the drug, but Asian patients have a 10-fold higher incidence of this reaction. An association has been found between the human leukocyte antigen (HLA)-B*1502 allele and carbamazepine-induced SJS in a Chinese population.2 This allele occurs almost exclusively in Asians.3,4 The FDA is now recommending that Asian patients be tested for genetic susceptibility to carbamazepine-induced SJS before starting the drug. The genetic test should be available in most clinical chemistry labs.

1. Extended-release carbamazepine (Equetro) for bipolar disorder. Med Lett Drugs Ther 2005; 47:27.
2. WH Chung et al. Medical genetics: a marker for Stevens- Johnson syndrome. Nature 2004; 428:486.
3. C Lonjou et al. A marker for Stevens-Johnson syndrome. . . : ethnicity matters. Pharmacogenomics J 2006; 6:265.
4. A Alfirevic et al. HLA-B locus in Caucasian patients with carbamazepine hypersensitivity. Pharmacogenomics 2006; 7:813.

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The FDA has approved a new genetic test to identify patients who may be at increased risk of severe toxicity when treated with the cancer chemotherapy drug irinotecan (Camptosar). The Invader UGT1A1 Molecular Assay (Third Wave Technologies) detects the UGT1A1*28 allele, a variation in the uridine diphosphate glucuronosyltranferase 1A1 (UGT1A1) gene. The FDA recently revised the safety labeling for irinotecan, recommending that the dosing of irinotecan be reduced for patients who are homozygous for the UGT1A1*28 allele.