The MEK-mitogen-activated protein kinase (MAPK) signal transduction pathway is involved with numerous cellular processes including cell growth and differentiation. Phosphorylation of MAPK (pMAPK) by MEK results in activation of this pathway. In various solid tumors, the MEK-MAPK pathway is constitutively active; therefore inhibition of this pathway may provide a therapeutic strategy for treating cancer. The objective of this study was to determine the extent and duration of inhibition of pMAPK in selected normal tissues in rats following single oral or intravenous (IV) doses of the novel MEK inhibitor, PD0325901. Male Sprague-Dawley rats (9/group) received either single oral (PO) or IV doses of PD0325901 at 10, 30, or 100 mg/kg (60, 180, and 600 mg/m(2), respectively). Controls received vehicle alone which was aqueous 0.5% hydroxypropylmethyl-cellulose/0.2% Tween 80 for PO dosing and 20% beta-cyclodextran sulfobutyl ether in water (w:v) for IV dosing. Animals (3/group/day) were euthanized on Days 2, 3, and 4, at approximately 24, 48, and 72 h after dosing, respectively. The effects on pMAPK in liver and lung were determined by Western blot analysis and compared with plasma PD0325901 levels. Satellite animals (6/dose/route) received single PO or IV doses and serial blood samples were collected for determination of toxicokinetic parameters of PD0325901 and its major metabolite. In general, systemic exposure to PD0325901 was comparable between routes of administration due to high PO bioavailability (56-109%). Plasma area under the concentration-time curve values of the pharmacologically inactive carboxylic acid metabolite ranged from 18 to 40% of PD0325901. Clinical signs of toxicity occurred at 100 mg/kg PO or IV, indicating the maximum-tolerated dose had been achieved. On Day 2, pMAPK was inhibited 57-95% in liver and 86-99% in lung at all doses, irrespective of route of administration. On Day 3, lung pMAPK remained inhibited 75-91% at all IV doses and by 88% after the 100-mg/kg PO dose. Liver pMAPK remained inhibited 79 and 91% on Day 3 after 100 mg/kg by IV and PO doses, respectively. On Day 4, liver pMAPK was still inhibited 66% after the 100-mg/kg PO dose. The EC(50) and EC(90) plasma drug levels for inhibition of lung pMAPK were calculated to be 20 and 99 ng/ml, respectively. Liver pMAPK levels were inhibited at least 50% at plasma PD0325901 concentrations > or =50 ng/ml. In conclusion, single PO or IV doses of PD0325901 resulted in dose-dependent inhibition of pMAPK in liver and lung. Inhibition of pMAPK in liver was comparable between routes of administration at < or =30 mg/kg, whereas inhibition of pMAPK in lung occurred for a longer duration following IV administration. Measurement of pMAPK in normal tissues served as a means for assessing the pharmacologic activity of PD0325901 and should be included in toxicity studies to evaluate toxicity-pharmacology relationships.