Assessment of dimethylxanthine formation from caffeine in healthy adults: comparison between plasma and saliva concentrations and urinary excretion of metabolites.

Caffeine (CA), paraxanthine (PX), theobromine (TB) and theophylline (TP) were determined in plasma (i.e. total concentrations), ultrafiltrate of plasma (free concentrations) and saliva, by isocratic high performance liquid chromatography (HPLC), 0-24 h after a 200 mg oral load of CA in 10 healthy adults. Total metabolism of CA was established by determination of urinary metabolites, 24 h after CA ingestion, by gradient HPLC and capillary electrophoresis. Saliva concentrations of CA, PX, and TP were lower than plasma concentrations (p < 0.001), whereas TB concentrations in plasma and saliva were similar. Saliva concentrations of CA, PX, TB and TP were higher than the free plasma concentrations (p < 0.001). The area under the plasma concentration-time curve (AUC) showed that PX accounted for 63 +/- 13% of the dimethylxanthines in plasma, while TB accounted for 27 +/- 15% and TP for 10 +/- 2.6%. In contrast, the urine analyses showed that 78 +/- 11% of the excreted metabolites were metabolized through the PX pathway, 14 +/- 8% through the TB pathway and 9 +/- 4% through the TP pathway. The percentage of the AUC for PX, TB and TP in plasma was similar to the percentage of each dimethylxanthine excreted unmetabolized in urine. The percentages of the AUC for PX and TB were correlated to the percentages of PX (r = 0.78, p < 0.001) and TB (r = 0.88, p < 0.001) in urine. The AUC for PX in plasma was lower than (p < 0.001) and correlated to the total PX pathway value (r = 0.95, p < 0.001) and the value for PX plus its specific metabolites in urine. The AUC for TB in plasma was higher than (p < 0.001) and correlated to the total TB pathway value (r = 0.93, p < 0.001) and the value for TB plus its specific metabolites in urine. The AUC for TP in plasma was similar to both the TP pathway value and the value for TP plus its specific metabolites in urine. It is concluded that the AUC for dimethylxanthines in plasma underestimates the formation of PX, overestimates the formation of TB and gives a similar formation of TP from CA, as judged from the urinary metabolites formed through the PX, TB, and TP pathways.