Background: Enzymatic degradation of hyaluronan (HA) is a novel strategy to target the desmoplastic stroma of pancreatic cancer. PEGPH20, a pegylated form of recombinant human hyaluronidase PH20, is an investigational drug in clinical trials. Preclinical studies demonstrate that sustained HA removal by PEGPH20 inhibits tumor growth and enhances chemotherapeutic activity in HA-rich xenografts and genetically engineered mouse tumor models. Ph1 PEGPH20 monotherapy studies show increased tumor perfusion by DCE-MRI, metabolic partial responses by FDG-PET, and stromal remodeling in tumor biopsies from selected advanced cancer patients (pts). Methods: This was a dose-escalation study to find the recommended Ph2 dose of PEGPH20 in combination with Gem in pts with Stage IV previously untreated pancreatic cancer. Pts received Gem at 1000 mg/m² IV qwk for Wks 1-7 plus PEGPH20 at 1, 1.6, or 3 μg/kg IV twice a week for Wks 1-4 and qwk for Wks 5-7. Wk 8 was a rest week. Thereafter, PEGPH20 + Gem were given qwk for 3 wks in 4-wk cycles. Serial plasma samples were collected and hyaluronidase activity measured by an ultrasensitive assay to assess PEGPH20 exposure. Plasma HA catabolites were measured by quantitative HPLC to assess PD. Results: 28 pts were enrolled. Plasma PEGPH20 concentrations were proportional to dose, and kinetics were well-characterized by a 2-compartment PK model. Estimates for clearance (0.5-2 mL/hr/kg) were consistent with long t_1/2 (1-2 days) previously seen with single-dose PEGPH20 monotherapy. Wks 1 and 4 PK profiles were similar, suggesting no changes to PEGPH20 clearance mechanisms after multiple doses or effects of Gem on PEGPH20 exposure. Most pretreatment plasma HA levels were <1 μg/mL and increased in a time- and dose-dependent manner after dosing. Circulating HA concentration was >500 µg/mL in several pts given 3 μg/kg PEGPH20. Conclusions: PEGPH20 plasma levels can be predicted using a linear PK model and circulating HA catabolites can be used as a quantitative measure of PEGPH20 PD. Results are consistent with the mechanism of action of hyaluronidase and support further study of PEGPH20 with anticancer agents. Clinical trial information: NCT01453153.