This phase 1 study in Japanese patients evaluated the safety, pharmacokinetics, and preliminary efficacy of palbociclib, a highly selective and reversible oral cyclin-dependent kinase 4/6 inhibitor, as monotherapy for solid tumors (part 1) and combined with letrozole as first-line treatment of postmenopausal patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer (part 2). Part 1 evaluated palbociclib 100 and 125 mg once daily (3 weeks on/1 week off; n=6 each) to determine maximum tolerated dose. Part 2 evaluated palbociclib maximum tolerated dose (125 mg) plus letrozole 2.5 mg (n=6). Most common treatment-related adverse event was neutropenia (all grades/grade 3/4): 83%/67% (100 mg), 67%/33% (125 mg), 100%/83% (palbociclib plus letrozole); heavier pretreatment with chemotherapy may have resulted in higher neutropenia rates observed with the 100-mg dose. Palbociclib exposure was higher with 125 versus 100 mg (mean area under plasma concentration-time curve over dosing interval [τ]: 1322 vs 547.5 ng·h/mL [single-dose], 2838 vs 1276 ng·h/mL [multiple-dose]; mean maximum plasma concentration: 104.1 vs 41.4 ng/mL [single-dose], 185.5 vs 77.4 ng/mL [multiple-dose]). Half-life was 23 to 26 hours. No drug-drug interactions between palbociclib and letrozole occurred. Four patients had stable disease (≥24 weeks in 1 patient with rectal cancer [100 mg] and 1 with esophageal cancer [125 mg]) in part 1; 2 had partial response, and 2 stable disease (both ≥24 weeks) in part 2. Palbociclib 125-mg dose (schedule 3/1) was tolerated and is the recommended dose for monotherapy and letrozole combination therapy in Japanese patients. A5481010; NCT01684215 This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.

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