“The things we hate about ourselves aren't more real than things we like about ourselves.” Ellen Goodman

Monday, March 2, 2009

Rat poison and the F1 driver - the warfarin story

The anticoagulant warfarin actually started life as rat poison. Chemically, it is derived from a natural plant product, coumarin. The way it acts is by inhibiting the enzyme Vitamin K epoxide reductase, and in so doing reduce formation of various Vit K dependent clotting factors.

So what's the deal about F1 drivers?

Well... like F1 drivers, the physician using warfarin needs to keep his eye on the road. Too little warfarin, and there is inadequate therapeutic anticoagulation; too much warfarin and the patient may suffer a catastrophic bleed. Fortunately, he has a way to do this. The Prothrombin Time and other derived measures such as the International Normalized Ratio (INR) provide a heads up to the physician about how much anticoagulation has been provided for the patient. By keeping his eye on the INR, the physician can adjust the dose of warfarin to provide just the right range on anticoagulation the patient needs. This is important, because the warfarin requirements for every patient differ, and the warfarin dose needs to be 'individualized'.

More recently, various other biomarkers enable the physician to make educated guesses about the dosage requirement for the patient. These are genetic markers related to the rate of metabolic degradation of warfarin through cytochrome P450 2C9 (not many such problems in our Chinese population) and the genetically reduced sensitivity of the Vitamin K epoxide reductase C1 subunit (VKORC1). However, using these genetic biomarkers only provide an improved starting dose. Once the race car engine starts, the F1 driver will still have to manage the therapeutic process through keeping a close eye on the INR.

There have been many discussions about the genotyping of patients prior to dosing with warfarin. I have no doubt to its usefulness in helping us to understand the patient a lot better. But becasue there is already a good efficacy marker (the INR) for us to titrate the patients dosing against, the improved starting point may only be of theoretical benefit. I think most of the benefit will come in situations when you need to deliver very fast anticoagulation. Where time is not on an essence, genotyping would likely not be a cost effective option.

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