R-Limonene metabolism in humans and metabolite kinetics after oral administration

We studied the R-limonene (LMN) metabolism and elimination kinetics in a human in vivo study. Four volunteers were orally exposed to a single LMN dose of 100-130 µg kg^-1 bw. In each case, one pre-exposure and subsequently all 24 h post-exposure urine samples were collected. From two subjects, blood samples were drawn up to 5 h after exposure. The parent compound was analysed in blood using headspace GC-MS. The metabolites cis- and trans-carveol (cCAR), perillyl alcohol (POH), perillic acid (PA), limonene-1,2-diol (LMN-1,2-OH), and limonene-8,9-diol (LMN-8,9-OH) were quantified in both blood and urine using GC-PCI-MS/MS. Moreover, GC-PCI-MS full-scan experiments were applied for identification of unknown metabolites in urine. In both matrices, metabolites reached maximum concentrations 1-2 h post-exposure followed by rapid elimination with half-lives of 0.7-2.5 h. In relation to the other metabolites, LMN-1,2-OH was eliminated slowest. Nonetheless, overall renal metabolite elimination was completed within the 24-h observation period. The metabolite amounts excreted via urine corresponded to 0.2 % (cCAR), 0.2 % (tCAR), <0.1 % (POH), 2.0 % (PA), 4.3 % (LMN-1,2-OH), and 32 % (LMN-8,9-OH) of the orally administered dose. GC-PCI-MS full-scan analyses revealed dihydroperillic acid (DHPA) as an additional LMN metabolite. DHPA was estimated to account for 5 % of the orally administered dose. The study revealed that human LMN metabolism proceeds fast and is characterised by oxidation mainly of the exo-cyclic double bond but also of the endo-cyclic double bond and of the methyl side chain. The study results may support the prediction of the metabolism of other terpenes or comparable chemical structures.