The purpose of our study was to develop a pharmacokinetic model to quantify the intracellular 5-fluorouracil (5-FU) concentration in liver metastases, which is expected to be closely correlated to therapy response. In addition, the influence of the biomodulator folinic acid on the action of 5-FU in the metastases was investigated.

After i.v. application of 5-FU labeled with the positron emitter fluorine-18 (5-[18F]FU), the kinetics of the regional 5-[18F]FU/uptake was measured dynamically with positron emission tomography over 120 min in 14 patients with a total of 27 liver metastases from colorectal adenocarcinoma. Activity-time curves were evaluated in the metastases, the normal liver tissue, as well as in the aorta and analyzed by a six-compartment model. The catabolic breakdown of 5-FU to α-fluoro-β-alanine (FBAL) in the normal liver tissue was modeled to separate the catabolites from the cytostatic agent 5-[18F]FU and the active 5-[18F]fluorodeoxyuridine nucleotides.

With our model, all measured activity-time courses could be described adequately with only small interindividual variations in parameters connected with liver and blood. Extrahepatic clearance of 5-FU was estimated as 0.66 ± 0.33 liters/min, whereas the hepatic clearance was 0.52 ± 0.25 liters/min. The Michaelis-Menten parameters describing the nonlinear conversion of 5-FU to FBAL were Km = 11.3 ± 6.4 µmol and Vmax = 147.1 ± 130.7 µmol/min. The maximum FBAL concentration in the liver was reached between 35 and 65 min after i.v. 5-FU infusion. The most sensitive parameters for therapy monitoring were kin and kout, which characterize the transport in and out of the intracellular volume of the metastases, respectively. Tumor response can only be expected if kin is high and kout is low (“trapping”). These criteria were met by 6 of the 27 metastases, which were identical to those with high values for the area under the intracellular 5-FU concentration curve (AUC5-FUmeta,IC). The parameters kin and kout were also used to investigate the influence of the biomodulating agent folinic acid on drug effect. Five of the six metastases that showed trapping belonged to patients who received folinic acid. With the exception of one patient, however, all patients who received folinic acid had multiple metastases, of which only one was able to trap 5-FU. Because patient response can only be expected when all metastases trap 5-FU, folinic acid showed no effect on the overall clinical response.

With the quantitative modeling approach used, trapping of 5-FU can be assessed noninvasively and on an individual basis. This makes it possible to adjust the dose for each individual patient to optimize the treatment schedule.