Threshold Pharmaceuticals, Inc. Release: Preclinical Studies Find That Tarloxotinib May Overcome Resistance To EGFR Tyrosine Kinase Inhibitors

SOUTH SAN FRANCISCO, CA--(Marketwired - November 06, 2015) - Threshold Pharmaceuticals, Inc. (NASDAQ: THLD) today announced new preclinical data demonstrating that tarloxotinib bromide*, or tarloxotinib, may overcome resistance to first- and second- and third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). The data will be reported today in two scientific posters (Abstracts A66 and A67) at the AACR-NCI-EORTC Molecular Targets and Cancer Therapeutics Meeting being held November 5-9, 2015, Boston. Tarloxotinib is Threshold's proprietary hypoxia-activated prodrug of an irreversible EGFR TKI exclusively licensed from the University of Auckland, New Zealand.

The research to be reported at the meeting focuses on preclinical models of EGFR-dependent cancers including non-small cell lung cancer (NSCLC) and squamous cell carcinomas of the head and neck (SCCHN) or skin (SCCS). These types of cancers are currently treated with drugs that block the activity of EGFR to interfere with tumor cell growth, but most tumors ultimately become resistant to therapy, and some do not respond at all.

Scientists are trying to understand the mechanisms underlying EGFR inhibitor resistance and discover new treatment options for patients with EGFR-driven cancers.

At the meeting, Adam Patterson, Ph.D. and Jeff Smaill, Ph.D., of the University of Auckland, New Zealand, will report that switching to low-dose tarloxotinib treatment in laboratory models of NSCLC resulted in significant regression of tumors that were progressing despite ongoing treatment with erlotinib, a first-generation EGFR TKI. These tumor models were heterozygous for EGFR whereby both wild-type (normal) and mutant (abnormal) forms of EGFR were present.

Independent research has shown that persistent wild-type EGFR signaling is associated with TKI resistance(1), and patients with heterozygous EGFR-mutant NSCLC have worse outcomes following EGFR TKI therapy than those with pure mutant-EGFR disease(2).

"Our research supports the hypothesis that persistent wild-type EGFR signaling within the tumor may be an important yet underappreciated mechanism of resistance to TKIs," Patterson said.

To test for the role of wild-type EGFR signaling in TKI resistance, Patterson and colleagues engineered one NSCLC model to have extra copies of the gene for wild-type EGFR. In the original parental heterozygous model, treatment with osimertinib (AZD9291), a third-generation TKI designed to "spare" wild-type EGFR, led to tumor regressions. In contrast, in the genetically engineered model with about 40% more wild-type EGFR, tumors started to regrow after initially responding to osimertinib. Tumor regrowth was brought under control upon switching to tarloxotinib treatment, which resulted in immediate and marked tumor regressions.

"Our preliminary findings suggest that tarloxotinib may be able to overcome wild-type EGFR-driven resistance to TKI therapy," Patterson said. "We believe this is related to the role of hypoxia in driving wild-type EGFR signaling within tumors coupled with the hypoxia-activation of tarloxotinib."

Using special imaging techniques, the team led by Patterson and Smaill were able to show a spatial overlap between hypoxic regions within tumors and EGFR signaling. Similarly, they were able to visualize the areas within a tumor where tarloxotinib released its TKI and found these areas to comprise the hypoxic compartment.

"Through collaboration with Dr. Angus Grey from the University of Auckland, we have for the first time demonstrated the mechanism of action of a hypoxia-activated prodrug in a human tumor model using MALDI Imaging Mass Spectrometry. This promises to be a very important technique for this field moving forward," Patterson said.

The scientists also presented data on tarloxotinib in models of SCCHN and SCCS. Across multiple cancer in vitro cell lines, tarloxotinib's TKI exhibited greater anti-proliferative activity and consistently silenced EGFR signaling to a greater extent than equimolar concentrations of cetuximab, afatinib or dacomitinib. When tested in in vivo models, tarloxotinib was more effective compared to afatinib in controlling SCCS tumor growth, and compared to cetuximab in controlling SCCHN tumor growth. A single dose of tarloxotinib significantly reduced the hypoxic compartment in a SCCHN tumor model.

"Taken all together, the data suggest that preferential activation of tarloxotinib in the hypoxic tumor microenvironment leads to reduction of the hypoxic compartment and effective silencing of EGFR signaling within the tumor," Smaill said. "Tumor-targeted activation of tarloxotinib may limit systemic side effects, and wild-type EGFR shut down may address an apparently important mechanism of TKI resistance, both potentially contributing to better outcomes for patients with EGFR-dependent cancers."

"This important translational work continues to support our ongoing proof-of-concept Phase 2 trials in patients with NSCLC and in patients with SCCHN and SCCS," said Tillman Pearce, MD, Chief Medical Officer at Threshold. "Initial PET imaging using our proprietary hypoxia imaging agent, [18F]-HX4, shows that imaging hypoxia in these tumors is possible. We hope that by combining imaging with response data we can start to determine which patients would benefit most from tarloxotinib therapy. We look forward to having preliminary results from these studies in the first half of 2016."

About Non-Small Cell Lung Cancer
Lung cancer is the most common cause of death from cancer worldwide; an estimated 1.8 million new cases were diagnosed in 2012(3). The most common type of lung cancer, non-small cell lung cancer (NSCLC), accounts for approximately 85 to 90 percent of cases(4). EGFR activating mutations occur in approximately 10 percent of NSCLC cases in Caucasian patients and up to 35 percent in Asian patients(5). Tarceva®, Iressa®, and Gilotrif® are the first- and second-generation EGFR inhibitors currently approved for patients with the EGFR activating mutations. Nearly all patients ultimately progress on these therapies due to a variety of resistance mechanisms.

About Squamous Cell Carcinomas Head and Neck
Most head and neck cancers, which include cancers of the larynx (voice box), throat, lips, mouth, nose, and salivary glands, begin in the squamous cells that line the moist surfaces inside the head and neck, and are therefore referred to as squamous cell carcinomas of the head and neck (SCCHN). SCCHN is diagnosed in approximately 59,000 people in the U.S. annually and is responsible for some 12,000 deaths(6). In the recurrent/metastatic setting, chemotherapy or cetuximab monotherapy is the standard of care with response rates are about ten percent and disease progression occurs within two to three months.(7)

About Squamous Cell Carcinomas of the Skin
Non-melanoma skin cancers typically resulting from chronic sun exposure or other sources of ultraviolet rays are the most common types of cancer. Twenty percent of these skin cancers originate from squamous cells normally present in the outer layers of the skin (SCCS); five percent of SCCS will become locally advanced, recur, or metastasize. In the U.S., approximately 2,000 deaths per year are attributed to SCCS(8). As with SCCHN, SCCS is associated with EGFR overexpression and appear to be responsive to EGFR inhibitor therapy(9).

About Tarloxotinib Bromide
Tarloxotinib bromide, or "tarloxotinib," is a prodrug designed to selectively release a covalent (irreversible) EGFR tyrosine kinase inhibitor under severe hypoxia, a feature of many solid tumors. Accordingly, tarloxotinib has the potential to effectively shut down aberrant EGFR signaling in a tumor-selective manner, thus potentially avoiding or reducing the systemic side effects associated with currently available EGFR tyrosine kinase inhibitors. Tarloxotinib is currently being evaluated in two Phase 2 proof-of-concept trials: one for the treatment of patients with mutant EGFR-positive, T790M-negative advanced non-small cell lung cancer progressing on an EGFR tyrosine kinase inhibitor, and the other for patients with recurrent or metastatic squamous cell carcinomas of the head and neck or skin. Threshold licensed exclusive worldwide rights to tarloxotinib from the University of Auckland, New Zealand, in September 2014.

About [18F]-HX4
[18F]-HX4 [flortanidazole (18F)] is a novel investigational tumor hypoxia tracer developed to potentially identify and quantify the degree of hypoxia in tumors in vivo. Positron emission tomography (PET) is a nuclear medical imaging technique that non-invasively produces a three-dimensional image of functional processes in the entire body. PET is routinely used to inform physicians on diagnosis and treatment of cancer and is used in cancer treatment centers globally. [18F]HX4 has a 2-nitroimidazole "trigger" that is designed to be activated under the extreme hypoxic conditions generally found in tumors but not typically in normal healthy tissue. Clinical data has demonstrated the potential of [18F]HX4 to quantify the degree of hypoxia within different tumors.

About Threshold Pharmaceuticals
Threshold Pharmaceuticals, Inc. is a biotechnology company focused on the discovery and development of drugs targeting tumor hypoxia, the low oxygen condition found in the microenvironments of most solid tumors as well as the bone marrows of some patients with hematologic malignancies. This approach offers broad potential to treat a variety of cancers. By selectively targeting tumor cells, we are building a pipeline of drugs that hold promise to be more effective and less toxic to healthy tissues than conventional anticancer drugs. For additional information, please visit our website (www.thresholdpharm.com).

Forward-Looking Statements
Except for statements of historical fact, the statements in this press release are forward-looking statements within the meaning of Section 21E of the Securities Exchange Act of 1943 and Section 27A of the Securities Act of 1933, including all statements regarding the Company's development activities and clinical development outlook for the two Phase 2 proof-of-concept clinical trials of tarloxotinib; and the ability of Threshold to build and protect the value of tarloxotinib, [18F]-HX4 and its hypoxia related technology. These statements involve risks and uncertainties that can cause actual results to differ materially from those in such forward-looking statements. Potential risks and uncertainties include, but are not limited to: the risks that Threshold's evaluation of tarloxotinib is at an early stage and it is possible that tarloxotinib may not be found to be safe or effective in the ongoing Phase 2 proof-of-concept trials of tarloxotinib or in any other studies of tarloxotinib that Threshold may conduct, and that Threshold may otherwise fail to realize the anticipated benefits of its licensing of this product candidate; the risk that preclinical studies and Phase 1 or 2 clinical trials of our product candidates may not predict the results of subsequent human clinical trials, including the risks that tarloxotinib preclinical and Phase 1 clinical data may not accurately predict whether a safe and effective dose can be attained in the patient populations for tarloxotinib that Threshold is targeting; the ability of Threshold to enroll or complete planned tarloxotinib clinical trials, the difficulty and uncertainty of pharmaceutical product development, including a reliable manufacturing supply and regulatory approval of its clinical assets. Further information regarding these and other risks is included under the heading "Risk Factors" in Threshold's Quarterly Report on Form 10-Q, which has been filed with the Securities and Exchange Commission on July 30, 2015 and is available from the SEC's website (www.sec.gov) and on our website (www.thresholdpharm.com) under the heading "Investors". Notwithstanding changes that may occur with respect to matters relating to any forward looking statements, we undertake no duty to update any forward-looking statement made in this news release.

*Tarloxotinib bromide is the proposed International Nonproprietary Name

References

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