Background: PDA is characterized by invasiveness and therapeutic resistance in part due to a desmoplastic stroma and an immunosuppressive microenvironment (Provenzano PP, Hingorani S. Br J Cancer 2013). PD1/PD-L1 inhibitors have no single agent activity in PDA, except for pts with mismatch repair defects. There is high need to overcome resistance to immune targeted therapies and develop biomarkers for pts selection. Stromal HA poses a physical barrier and protects tumor cells from immune surveillance (Kultti A, et al Biomed Res Int 2014). By remodeling the tumor stroma, PEGPH20 allows infiltration of cytotoxic T lymphocytes, and improves delivery of chemotherapy and PD1/PD-L1 antibodies (Singha NC, et al Mol Cancer Ther 2015). mPDA pts refractory to 1^st line therapy have median overall survival (OS) of 6 mos. We hypothesize that stroma remodeling with PEGPH20 sensitizes PDA to immune therapy, and stroma and immunologic biomarkers will identify pts most likely to benefit. In this trial we will evaluate the efficacy, safety and translational biomarkers of PEGPH20 plus pembrolizumab in HA-high refractory mPDA. Methods: Eligible pts have ECOG PS 0-1, ≤ 2 prior therapies for mPDA, life expectancy ≥ 12 wks, able/willing to have tumor biopsies at baseline and after 6 wks of treatment. PEGPH20 dosing is 3 µg/kg iv QW and pembrolizumab 200 mg iv Q3W (2-4 hrs after PEGPH20 on wk 1) in 3-wk cycles. All pts receive prophylactic low molecular weight heparin. Primary endpoint: progression-free survival (PFS). Secondary endpoints: safety, OS, response rates. Translational endpoints: flow cytometry of peripheral and intratumoral immune cells, T-cell receptor sequencing, immune transcriptome, immune subsets IHC, circulating cytokines, serial plasma and tumor HA levels. For the primary endpoint of PFS, with a sample size of 31 evaluable pts, a one-sided α-level of 0.05, assuming 12 mos of accrual and 6 mos of follow-up, this study has 80% power to detect a difference between the null hypothesis median PFS 3 mos, versus the alternative hypothesis median PFS 6 mos. The study was activated in May 2019 and is open to accrual; 6 pts were enrolled as of 24Sept 2019. Clinical trial information: NCT03634332