Protein Disulfide Isomerase or PDI is the founding member of a large family of thiol isomerases that are highly concentrated in the endoplasmic reticulum. PDI serves an essential role in protein folding. In particular by both forming and reducing disulfide bonds, it enables proteins to obtain an optimal conformation. However, PDI can escape ER retention and be trafficked to cellular membranes and extracellularly. This extracellular PDI has been invoked in many pathological processes including thrombosis.

Dr. Flaumenhaft’s group became interested in PDI in 2008 following the publication of two papers showing that an inhibitory antibody against PDI prevents thrombus formation in mouse models. His laboratory subsequently performed a high throughput screen to identify inhibitors of PDI. Interestingly, the most potent inhibitor was rutin, a compound found in high abundance in commonly consumed foods such as vegetables, fruits, and teas. Evaluation of rutin analogs demonstrated an entire family of flavonoid quercetins that inhibit PDI. 

Testing of flavonoid quercetins, such as rutin and isoquercetin, in animal models demonstrated that they inhibited thrombus formation in vivo. Based on these preclinical data and in collaboration with Jeffrey Zwicker, MD, clinical studies were initiated to evaluate isoquercetin as an antithrombotic. Phase I studies showed that, compared to pre-ingestion plasmas, plasmas obtained from patients following isoquercetin ingestion inhibited PDI activity and decreased platelet-dependent thrombin generation.

These results enabled the initiation of the Cancer Associated Thrombosis-Isoquercetin (CAT-IQ) study, a phase II study to evaluate the role of PDI inhibition with isoquercetin in reducing the hypercoagulable state of cancer. Patients with advanced cancers were enrolled and received either 500 mg or 1000 mg isoquercetin over a 56-day period. Blood samples were obtained prior to and after this study period and a bilateral lower limb venous ultrasound was performed at the end of the study. The primary end points were D-dimer reduction at 56 days and major hemorrhage. Primary VTE, including proximal DVT and PE, was a secondary end point. Analysis of blood samples showed that patients taking the 500 mg isoquercetin dose showed a 46% decrease in PDI activity and a 31% decrease in platelet-dependent thrombin generation, without effects on D-dimer or soluble P-selectin levels, two known predictors of cancer thrombosis. The isoquercetin 1000 mg dose, however, showed a 73% decrease in PDI activity, a 20% decrease in D-dimer (comparable to that seen with FXa inhibitors), a 66% decrease in platelet-dependent thrombin generation, and a 68% decrease in soluble P-selectin. No primary VTE were observed with either the 500 mg or 1000 mg isoquercetin doses. In addition, there were no major hemorrhages with either dose. Toxicities included several grade I bleed/bruises and GI symptoms commonly associated with chemotherapies that patients were receiving concurrently. This study provides proof of concept for the inhibition of PDI by isoquercetin for thromboprophylaxis and sets the stage for further studies and additional development.

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