BACKGROUND This phase Ib study evaluated the safety, pharmacokinetics, and activity of enzastaurin either 500 mg once daily (QD) or 250 mg twice daily (b.i.d.) in combination with pemetrexed. METHODS Pemetrexed 500 mg/m(2) with folic acid and vitamin B(12) was given on day 1 every 21 days with enzastaurin 500 mg orally QD starting on day 5 of cycle 1 after a loading dose of 400 mg thrice daily on day 4. To evaluate whether a b.i.d. regimen results in higher enzastaurin exposures, the study was amended. After amendment, in cycle 1, patients received 500 mg enzastaurin QD on days 1-15 without initial loading dose and 250 mg b.i.d. on days 16-30; in subsequent cycles, patients received pemetrexed on day 1 every 21 days with enzastaurin b.i.d. RESULTS Sixty-eight patients (42 preamendment and 26 postamendment) were assessed. Pemetrexed toxicity and pharmacokinetics did not appear to be altered by enzastaurin. Enzastaurin average steady-state plasma concentration (C(av,ss)) decreased by approximately 25% in the presence of pemetrexed. Enzastaurin C(av,ss) were approximately 40% higher in the b.i.d. versus QD regimen. Three patients (4.4%) with thyroid cancer of follicular/papillary type had partial response as defined by RECIST. CONCLUSIONS Pemetrexed plus enzastaurin is well tolerated with preliminary evidence of anticancer activity, particularly in thyroid cancer.