What is Tumor Hypoxia?

Tumor hypoxia, or low oxygen concentration, is a result of the disordered vasculature found in all solid tumors and represents a compelling target for anti-cancer intervention. Conventional anti-cancer therapies typically target actively dividing cells in close proximity to the vasculature. But solid tumors also have zones in which the vasculature is disordered and chaotic. There are many irregularities and abnormalities in tumor vasculature including a wide variation in the distance between the blood vessels that carry oxygen and other vital nutrients to tissues. In normal tissues, the distance between blood vessels is carefully regulated by several processes. However, in tumors the growth of malignant cells is unregulated and some cells or regions literally outgrow their blood supply, leading to severe deficiencies in the perfusion of oxygen and nutrients. Also, the blood vessels in tumors are often abnormal with “blind-ends” and temporary occlusions. Together, the abnormalities in tumor vasculature and the exaggerated intercapillary spacing lead to distinctive microenvironments that do not exist in normal tissues and to which neither oxygen nor many traditional anti-cancer agents can penetrate. Furthermore, cells in these hypoxic zones are relatively quiescent which renders them refractory to most cancer chemotherapies that target the rapidly proliferating cells which are the hallmark of cancer. And finally, it has been demonstrated that cells kept under a hypoxic constraint accumulate genetic mutations which can ultimately lead to resistance to the same drugs that so effectively targeted their progenitors and which otherwise enhance their metastatic potential, ultimately contributing to treatment relapse. TH-302 has been designed to be selectively activated within these hypoxic cells, killing them as well as those in close proximity via a bystander effect. Preclinical as well as early clinical data suggest that TH-302 exerts significant anti-tumor activity both alone as well as in combination with other therapies that target the rapidly proliferating cells found in normally oxygenated compartments of solid tumors.

A New Class of Drug — TH-302

TH-302 is an anticancer agent in clinical development at Threshold. Preclinically, it is preferentially activated under hypoxic conditions and has demonstrated potent anticancer activity in many preclinical cancer models. TH-302 is converted selectively to the drug’s active form, dibromo isophoramide mustard, a potent DNA alkylator, within hypoxic tumor cells. TH-302 targets levels of hypoxia that are common in tumors but are rare in normal tissues – this is how selective targeting of the tumor occurs. After conversion to the active form of the drug, the more resistant hypoxic cells are exposed to high concentrations of released cytotoxic agent, which can also diffuse into the surrounding oxygenated regions of the tumor, exerting what is referred to as a bystander effect. In this way, TH-302 can kill more of the tumor than can otherwise be accounted for by the hypoxic fraction alone. Because of its selective activation in the hypoxic regions of solid tumors, we believe that TH-302 will be less likely to produce the systemic toxicity caused by most cytotoxic chemotherapies.

In July 2007, we initiated a Phase 1 clinical trial of TH-302, a hypoxia-activated prodrug for the treatment of solid tumors (TH-CR-401). The clinical trial was designed with an initial accelerated titration design followed by a standard dose escalation schema. The trial completed the dose escalation phase and after having established a maximally tolerated dose or “MTD” of 575 mg/m2, has been expanded to include the enrollment of additional patients for whom responses were observed during the dose escalation phase. In addition, following a protocol amendment, the company is also evaluating a less frequent dosing regimen in an attempt to achieve even higher doses of TH-302 with an acceptable safety profile. From the monotherapy results seen to date, the dose limiting toxicity or “DLT” for TH-302 appears to be skin and mucosal toxicity, resulting in rash and, in some cases, blistering. Overall, TH-302 appears to be well tolerated with minimal hematologic toxicity. This is important from the perspective of increasing our confidence that TH-302 is behaving like a true prodrug, selectively releasing its toxic warhead in the tumors as opposed to the systemic circulation where it would be expected to be toxic to the bone marrow. Furthermore, the lack of hematologic toxicity also suggests that TH-302 should combine well with other established chemotherapies whose dose limiting toxicites (“DLT”) tend to be hematologic. With respect to efficacy, one patient with refractory small cell lung cancer metastatic to the liver had a partial response at their initial response assessment with a greater than 45% reduction in target lesion diameters. The patient, who had received two cycles of TH-302, was being followed by RECIST (Response Evaluation Criteria In Solid Tumors) criteria. Another patient with metastatic melanoma received 2 cycles of TH-302 and experienced a greater than 50% reduction in target lesion diameters. Other anti-tumor activity observed include one patient with non-small cell lung cancer with tumor shrinkage lasting over 6 months. Upon completion of the dose escalation phase of the trial, additional small cell lung cancer, non-small cell lung cancer, and metastatic melanoma patients were enrolled at the MTD of TH-302. 2 of the 3 patients who have thus far been assessed, one each with small cell lung cancer and metastatic melanoma, have achieved RECIST-criteria partial responses. This expansion study remains underway.

In August 2008, we initiated an additional clinical trial of TH-302 (TH-CR-402). The clinical trial design includes three separate treatment arms that will each examine TH-302 in combination with one of the following chemotherapeutic agents: gemcitabine, docetaxel or pemetrexed. Approximately 50 patients with advanced solid tumors are planned to enroll in the Phase 1/2, open-label, dose-escalation portion of the clinical trial. Up to six patients per dose level will participate in the dose escalation phase of the trial.

Once the maximum tolerated dose (MTD) has been reached, the Phase 2 portion of the trial will enroll an additional 12 patients at the MTD within each treatment arm as follows: gemcitabine in advanced pancreatic cancer patients, docetaxel in patients with castrate-resistant prostrate cancer (CRPC) and pemetrexed in patients with non-small cell lung cancer. Initial results from this study indicate that TH-302 can be safely combined with each of the 3 established chemotherapy agents, albeit at doses that are still being escalated toward the monotherapy MTD. Early activity signals look promising with RECIST-criteria partial responses observed in several different tumor types, including non-small cell lung, pancreatic, melanoma, ovarian, esophageal, and anal cancers. Of course, since these are uncontrolled combination studies, it is not possible to discern the contribution of TH-302 itself to each of these responses. Although in comparison to historical benchmark response data for each of the established chemotherapy agents, it is hoped that these data will be sufficiently robust to guide the subsequent design of controlled, Phase 2 and/or Phase 3 registrational trials.

In September 2008, we initiated an additional clinical trial of TH-302 (TH-CR-403). The clinical trial design will examine TH-302 in combination with doxorubicin. Approximately 36 patients with metastatic and/or advanced unresectable soft tissue sarcoma are planned to enroll in the clinical trial at various sites in the United States. The dose escalation phase of the trial will enroll up to 24 patients. The primary objective of the dose escalation component of the study is to establish the maximum tolerated dose (MTD) and dose limiting toxicities of TH-302 in combination with doxorubicin. Patients for whom no effective therapy is available or for whom monotherapy with doxorubicin is considered standard therapy are eligible for the trial. Patients who successfully complete one treatment cycle without evidence of significant treatment-related toxicity or progressive disease may continue to receive treatment for up to six cycles. Once the MTD has been reached, the Phase 2 portion of the trial will enroll an additional 12 patients at the MTD. The primary objective of the dose expansion component of the study is to make a preliminary assessment of the efficacy of TH-302 in combination with doxorubicin as measured by the progression-free rate at 6 months in patients previously untreated with chemotherapy. The secondary endpoints of the trial include response, duration of response, progression-free rate at 3 months, progression-free survival, overall survival and various safety parameters. Tumor response will be evaluated at baseline and every six weeks using the Response Evaluation Criteria In Solid Tumors (RECIST). Early activity signals look promising with RECIST-criteria partial responses observed in 2 of the 4 patients who have been assessed to date. The remaining 2 patients achieved stable disease into their 3rd or 4th cycles of treatment.