Abstract
Background
Protein disulfide isomerase (PDI) is a thiol isomerase secreted by platelets and endothelial cells and is required for thrombus formation. We previously showed that quercetin flavonoids inhibit PDI and block thrombus formation following vascular injury in a mouse model. Quercetins are present in a wide range of fruits and vegetables and epidemiologic data suggest an antithrombotic effect. Initial human studies demonstrated that the oral administration of isoquercetin reduces PDI activity in plasma. To evaluate the antithrombotic strategy of inhibiting PDI, we performed a phase II trial to evaluate the efficacy of isoquercetin to reduce the hypercoagulability of cancer patients at high risk for venous thromboembolism (VTE). In high-risk cancer cohorts with protocol-mandated screening for VTE, the reported incidence of VTE often exceeds 20% within months of initiation of chemotherapy. The risk of major hemorrhage is also higher in patients with advanced cancer which complicates the decision to recommend thromboprophylaxis.
Methods
We performed a phase II study at 11 sites to assess the safety and efficacy of isoquercetin in cancer patients at high risk for VTE. Cohort A received isoquercetin 500 mg daily and Cohort B received 1000 mg isoquercetin daily for 56 days with laboratory assays performed at the beginning and at completion of study. Eligible patients had locally advanced or metastatic pancreatic, non-small cell lung, or colorectal cancer and enrolled within 4 weeks of initiating first or second line chemotherapy. The primary endpoint of the study was a reduction in D-dimer at day 56. Patients were monitored for the development of venous thromboembolism including a protocol-required bilateral lower extremity ultrasound performed at day 56. The primary VTE endpoint was defined as any symptomatic proximal or distal lower extremity DVT, symptomatic pulmonary embolism or fatal PE, or asymptomatic proximal DVT diagnosed by screening compression ultrasound. Radiology images for all suspected VTE were centrally reviewed and adjudicated by an independent committee.
Results
There were no primary VTE endpoints observed in either cohort. Paired plasma samples were analyzed in 25 patients who received isoquercetin 500 mg daily (Cohort A) and 21 patients who received the isoquercetin 1000 mg daily (Cohort B). For the primary D-dimer endpoint, the administration of isoquercetin 1000 mg decreased D-dimer plasma concentrations by a median of 20% equating to a median decrease of 206 ng/ml fibrinogen equivalent units (P<0.001). The administration of isoquercetin 500 mg did not affect plasma D-dimer levels at 56 days (median change +9.90%, P=0.93). In both cohorts we measured an increase in PDI inhibition in plasma at day 56 (cohort A median increase 46%, P<0.001; cohort B median increase 78%, P<0.001). Corroborating the antithrombotic efficacy of the 1000 mg dose, we also observed a significant decrease in circulating soluble P-selectin (median decrease 58%, P<0.001) and platelet-dependent thrombin generation (median decrease 55%, P=0.006) as shown in the Figure. Isoquercetin did not prolong the PT or PTT. There were two grade 1 hemorrhoidal bleeds and no major hemorrhages observed.
Conclusions
The oral flavonoid, isoquercetin, prevented venous thromboembolism in cancer without an observed increase in hemorrhage. These results represent the first evidence in humans that targeting extracellular PDI has antithrombotic efficacy in a high VTE-risk population and implicate PDI-targeted therapies as a new class of antithrombotic therapeutics. (Clinicaltrials.gov #NCT02195232).
Zwicker:Daiichi: Honoraria; Quercegen: Research Funding; Parexel: Consultancy; Incyte: Research Funding.
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