SALICYLATE-INDUCED HYPOPROTHROMBINEMIA

Potentiation of warfarin anticoagulation associated with topical methyl salicylate

OBJECTIVE

To report a case of international normalized ratio (INR) elevation resulting from the administration of topical methyl salicylate in a patient whose INR was previously stable while she received warfarin anticoagulation.

CASE SUMMARY

A 22-year-old white woman presented with an INR of 12.2 after applying a topical pain-relieving gel to her knees daily for eight days. The potentiation of the warfarin anticoagulation was attributed to the low-dose methyl salicylate contained in the product.

DISCUSSION

Methyl salicylate is systemically absorbed through the skin in measurable amounts, and may increase warfarin action by affecting vitamin K metabolism or by displacing warfarin from protein-binding sites. While several investigators have reported this interaction with use of high-dose methyl salicylate, this case indicates that a significant interaction can occur with the use of lower topical doses of methyl salicylate as well.

CONCLUSIONS

Healthcare providers and patients taking warfarin must be aware of the potential hazard of using topical methyl salicylate in combination with warfarin.

Adverse effect of topical methylsalicylate ointment on warfarin anticoagulation: an unrecognized potential hazard

The adverse effect of topical methylsalicylate ointment on warfarin anticoagulation is studied in 11 patients. All patients had an abnormally elevated international normalized ratio after significant usage of topical methylsalicylate ointment as obvious from both the clinical history and a positive blood level of salicylate. Out of the 11 patients, 3 had bleeding manifestation; 2 with bruises and 1 with gastrointestinal bleeding. It is concluded that topical methylsalicylate ointment should be prescribed with care to patients on warfarin and excessive usage is to be avoided since potentially dangerous drug interaction could occur.

Interactions of analgesics with other drugs

Antipyretic analgesics, such as salicylates, acetaminophen, and pyrazolones, are often given concomitantly with a variety of other drugs. Drug interactions that occur at the receptors are known as pharmacodynamic interactions; alterations in absorption (bioavailability), distribution (plasma protein-binding), and elimination (renal excretion, hepatic metabolism) are termed pharmacokinetic interactions. For example, antacids and food both delay the absorption of analgesics. Highly protein-bound drugs (such as phenylbutazone, phenytoin, or warfarin) can compete with the common binding sites of salicylates. Hepatic elimination of salicylates can be influenced by drugs such as beta-blockers and cimetidine. Clinically important interactions involving salicylates, acetaminophen, and other antipyretic analgesics are discussed.

The interaction of salicylate and vitamin K in synthesis of Factor II (prothrombin)

The effects of sodium salicylate on synthesis of a vitamin K-dependent plasma coagulation protein, Factor II (prothrombin) was studied using the isolated rat liver perfused for 10 hours in vitro. Cumulative synthesis of Factor II was measured by a standard coagulation assay, by activation with E. carinatus venom, and by rocket immunoelectrophoresis. When sodium salicylate 5 mg or 25 mg was added to the liver perfusate (volume 100 ml) at the outset of the perfusion, cumulative synthesis of both coagulation activity and immunoreactive protein was significantly less than that seen in control perfusions containing no salicylate. The inhibitory effect of salicylate was prevented by pretreatment of rat liver donors with supplemental vitamin K injected 24 hours before sacrifice. Although some interaction between salicylate and vitamin K was apparent from these experiments, the results from vitamin K-deficient rat liver donors were quite different from those containing salicylate. There was no assayable Factor II coagulation activity produced in 10 hours of perfusion of vitamin K-deficient rat livers, but cumulative synthesis of immunoreactive Factor II was quite comparable to that seen in control perfusions.

Hypoprothrombinemia in naproxen overdose

Transient prolongation of the prothrombin time was observed in the setting of a 10-g overdose of naproxen. The patient reported was previously healthy, without chronic liver disease, bleeding disorders, or malnutrition. The most likely mechanism for this effect is direct inhibition of the synthesis of vitamin-K-dependent clotting factors, possibly via production of "abnormal" prothrombin.

On the mechanism of salicylate-induced hypothrombinaemia

The mechanism of salicylate-induced hypothrombinaemia has been investigated in the rabbit. Administration of methyl salicylate produced a significant decrease in prothrombin complex activity, in the activity of clotting factors II, VII and X but no significant change in factor V activity. Metabolic studies with [3H]vitamin K1 showed that salicylate increased the plasma concentration ratio of [3H]vitamin K1-epoxide: [3H]vitamin K1. The results are consistent with the concept that salicylate produces its anticoagulant effect, like the coumarin anticoagulants, by interruption of the physiologically important vitamin K1-epoxide cycle at the epoxide reductase.