Sensitive fluorometric method for determination of quercetin in plasma or urine.

Various flavonoids (in Europe, quercitin glucoside and its semisynthetic derivatives, especially) are used in treating varicosity. Nevertheless, we still lack almost all pharmacokinetic data on these substances, mainly because of analytical difficulties. Flavonoids were first measured spectrophotometrically (1) and later fluorometrically by the formation of a borocitric complex (2). Porter et al. (3) reported on the spectrophotometry of soluble metal complexes with flavonoids; 10 ,ug of flavonoid per milliliter of urine was the least amount that they could detect. These authors admitted that besides the lack of sensitivity this method was not very specific. In 1954 Neu synthesized tetraphenyldiboroxide (“Flavognost,” Figure 1) and used this new organic boron reagent to chelate with fiavonoids to form strong fluorophors (4, 5). This technique has not been used to measure quercetin in urine or plasma. We describe here our use of it to determine as little as 0.2 zg of quercetin per milliliter of urine or plasma.