Purpose: Pemetrexed is an antimetabolite that is structurally similar to methotrexate. Because nonsteroidal anti-inflammatory drugs (NSAID) impair methotrexate clearance and increase its toxicity, we evaluated the pharmacokinetics and toxicity of pemetrexed when coadministered with aspirin or ibuprofen in advanced cancer patients. Experimental Design: In two independent, randomized, crossover drug interaction studies, cancer patients with a creatinine clearance (CrCl) z60 mL/min received an NSAID (aspirin or ibuprofen) with either the first or the second dose of pemetrexed (cycle 1 or 2). Pemetrexed (500 mg/m 2 ) was infused i.v. on day 1 of a 21-day cycle, and all patients were supplemented with oral folic acid and i.m. vitamin B 12 . Aspirin (325 mg) or ibuprofen (400 mg; 2 Â 200 mg) was given orally every 6 hours, starting 2 days before pemetrexed administration, with the ninth and final dose taken 1 hour before infusion. Pemetrexed pharmacokinetics with and without concomitant NSAID treatment were compared for cycles 1and 2. Results: Data from 27 patients in each study were evaluable for the analysis of pemetrexed pharmacokinetics. Coadministration of aspirin did not alter pemetrexed pharmacokinetics; however, ibuprofen coadministration was associated with a 16% reduction in clearance, a 15% increase in maximum plasma concentration, and a 20% increase in area under the plasma concentration versus time curve but no significant change in V ss compared with pemetrexed alone. No febrile neutropenia occurred in any patient, and no increase in pemetrexed-related toxicity was associated with NSAID administration. Conclusions: Pemetrexed (500 mg/m 2 ) with vitamin supplementation is well tolerated and requires no dosage adjustment when coadministered with aspirin (in patients with CrCl z60 mL/min) or ibuprofen (in patients with CrCl z80 mL/min).Pemetrexed is a novel antifolate that inhibits three folatedependent enzymes involved in purine and pyrimidine synthesis: thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyl transferase (1). In preclinical studies, pemetrexed was effective against some methotrexateresistant cell lines (1, 2), suggesting that its mechanism of action may differ from that of classic antifolates. The drug has shown antitumor activity in a range of solid malignancies, including mesothelioma, non -small cell lung, bladder, head and neck, breast, cervical, colorectal, pancreatic, and gastric cancers (3, 4). Pemetrexed has received regulatory approval for the treatment of malignant pleural mesothelioma in combination with cisplatin and as single-agent therapy for second-line treatment of non -small cell lung cancer (5 -7).Pharmacokinetic evaluations for three phase I dose-escalation trials showed that pemetrexed is f80% protein bound, with rapid plasma distribution and elimination phases, and exhibits linear pharmacokinetics over a broad range of doses (0.2-838 mg/m 2 ). The steady-state volume of distribution (V ss ) of pemetrexed is small (16 L), suggesti...