Threshold Pharmaceuticals, Inc. is a clinical-stage biotechnology company. The Company is engaged in the discovery and development of therapeutic and diagnostic agents that selectively target tumor cells for the treatment of patients living with cancer. It is developing two therapeutic product candidates based on hypoxia-activated prodrug technology: evofosfamide and tarloxotinib. Evofosfamide is designed as a prodrug that is activated under the extreme hypoxic conditions commonly found in tumors, but not in healthy tissues. Tarloxotinib is under investigation in two Phase II proof-of-concept trials: one for the treatment of patients with mutant EGFR-positive, T790M-negative advanced NSCLC progressing on an EGFR TKI, and the other for patients with recurrent or metastatic squamous cell carcinomas of the head and neck. [18F]-HX4 is an investigational Positron Emission Tomography imaging agent for hypoxia to identify and quantify the degree of hypoxia in tumors in vivo.