Purpose: To assess the preclinical efficacy, clinical safety and efficacy, and MTD of palbociclib plus nab-paclitaxel in patients with advanced pancreatic ductal adenocarcinoma (PDAC)., Experimental Design: Preclinical activity was tested in patient-derived xenograft (PDX) models of PDAC. In the open-label, phase I clinical study, the dose-escalation cohort received oral palbociclib initially at 75 mg/day (range, 50‒125 mg/day; modified 3+3 design; 3/1 schedule); intravenous nab-paclitaxel was administered weekly for 3 weeks/28-day cycle at 100‒125 mg/m 2 . The modified dose-regimen cohorts received palbociclib 75 mg/day (3/1 schedule or continuously) plus nab-paclitaxel (biweekly 125 or 100 mg/m 2 , respectively). The prespecified efficacy threshold was 12-month survival probability of ≥65% at the MTD., Results: Palbociclib plus nab-paclitaxel was more effective than gemcitabine plus nab-paclitaxel in three of four PDX models tested; the combination was not inferior to paclitaxel plus gemcitabine. In the clinical trial, 76 patients (80% received prior treatment for advanced disease) were enrolled. Four dose-limiting toxicities were observed [mucositis ( n = 1), neutropenia ( n = 2), febrile neutropenia ( n = 1)]. The MTD was palbociclib 100 mg for 21 of every 28 days and nab-paclitaxel 125 mg/m 2 weekly for 3 weeks in a 28-day cycle. Among all patients, the most common all-causality any-grade adverse events were neutropenia (76.3%), asthenia/fatigue (52.6%), nausea (42.1%), and anemia (40.8%). At the MTD ( n = 27), the 12-month survival probability was 50% (95% confidence interval, 29.9-67.2)., Conclusions: This study showed the tolerability and antitumor activity of palbociclib plus nab-paclitaxel treatment in patients with PDAC; however, the prespecified efficacy threshold was not met., Trial Registration: Pfizer Inc (NCT02501902)., Significance: In this article, the combination of palbociclib, a CDK4/6 inhibitor, and nab-paclitaxel in advanced pancreatic cancer evaluates an important drug combination using translational science. In addition, the work presented combines preclinical and clinical data along with pharmacokinetic and pharmacodynamic assessments to find alternative treatments for this patient population., Competing Interests: M. Hidalgo reports grants from Pfizer during the conduct of the study; personal fees and other from BMS, InxMed, Champions; personal fees from MinKi, Velavigo, Oncomatrix; other from Nelum outside the submitted work; and M. Hidalgo is an independant director in BMS. R. Garcia-Carbonero reports personal fees from AAA, Advanz Pharma, Amgen, Bayer, BMS, HMP, Ipsen, Merck, Midatech, MSD, Novartis, Pharma Mar, Pierre Fabre, Servier and grants from BMS, MSD, Pfizer outside the submitted work. W.A. Messersmith reports other from Pfizer during the conduct of the study. I. Garrido-Laguna reports personal fees from SOTIO, Kanaph, Jazz, OncXer; grants from Novartis (to institution), Bayer (to institution), Bristol Myers-Squibb (to institution), Pfizer (to institution), MedImmune (to institution), Lilly (to institution), Incyte (to institution), GlaxoSmithKline (to institution), Tolero Pharmaceuticals (to institution), BridgeBio Pharma (to institution), Jacobio (to institution), Repare Therapeutics (to institution), and Sumitomo Dainippon Pharma Oncology (to institution) outside the submitted work. E. Borazanci reports other from Pfizer during the conduct of the study; other from BMS, Minneamrita Therapeutics, Merck, Helix Biopharma, and Biontech outside the submitted work; and reports consultancy with Vivacitus (self), TD2 (self), and Nanology (self). M. Malumbres reports grants and personal fees from Pfizer and grants from Eli Lilly outside the submitted work. D.J. Shields is an employee of Pfizer Inc. and holds shares in the company. J.E. Grossman reports other from Agenus outside the submitted work. X. Huang reports personal fees from Pfizer, Inc outside the submitted work. J.-F. Martini reports personal fees from Pfizer Inc and other from Pfizer Inc outside the submitted work. Y. Yu reports other from Pfizer Inc. outside the submitted work. K. Kern reports other from Pfizer Inc outside the submitted work. T. Macarulla reports personal fees from Ability Pharmaceuticals SL, Amgen, Aptitude Health, Basilea Pharma, Baxter, BioLineRX Ltd, Celgene, Eisai, Ellipses, Genzyme, Hirslanden/GITZ, Imedex, Ipsen Bioscience, Inc, Janssen, Lilly, Marketing Farmacéutico & Investigación Clínica, S.L, MDS, Medscape, Novocure, Paraxel, PPD Development, Polaris, QED Therapeutics, Roche Farma, Scilink Comunicación Científica SC, Surface Oncology, and Zymeworks; personal fees and other from AstraZeneca, Incyte, Sanofi-Aventis, Servier outside the submitted work. No disclosures were reported by the other authors., (© 2022 The Authors; Published by the American Association for Cancer Research.)