ET-743: A Review of Recent Results

by

Margaret von Mehren, MD

Fox Chase Cancer Center

Philadelphia, PA 19111

ET-743: Mechanisms of Action

Ecteinascidin-743 (ET-743, Yondelis^TM, Pharma Mar/Johnson & Johnson) is a novel chemotherapeutic agent identified from a screen of marine organisms (Figures 1 and 2).  

Figure 1: Ecteinascidia turbinata, the sea squirt, growing in its natural habitat. ET-743 is isolated from this organism. Photo used with permission of Pharma Mar USA, Inc.

This agent derived from a Caribbean tunicate Ecteinascidia turbinata is a tetrahydroisoquinolone alkaloid. Alkylating drugs bind to DNA and disrupt its function.  ET-743 has a unique method of action that differs from other alkylating agents. It binds the DNA helix along the minor groove and causes the DNA structure to be bent (Ref. 1). ET-743 attaches to specific sequences of DNA that contain guanine, one of the building blocks of DNA (Ref. 2). This blocks the cell from completing the process of cell division (Ref. 3), disrupts the organization and assembly of the cells skeleton (Ref. 4), and inhibits the function of topoisomerase I, an enzyme important in the process of DNA replication (Ref. 5).

Figure 2: Chemical diagram of the structure of ET-743. There are 3 subunits of the molecule labeled A, B, and C important for how the drug works. Subunits A and B are responsible for binding to DNA. The binding of ET-743 to DNA changes the normal structure of the molecule, causing it to bend. In addition, the carbinolamine group functions to alkylate the DNA, which is the substitution of one chemical group for an active hydrogen atom in the DNA.  When such a substitution occurs, DNA strands are cross-linked and can not replicate, thus causing the cell to die. Subunit C interacts with other molecules, such as transcription factors. Click on the figure to enlarge it.

ET-743 also interferes with the normal binding of proteins to DNA that function to activate genes (Ref. 6).  The MDR1 gene is one of the genes that ET-743 can inhibit the activation. 

When MDR1 is activated, it produces a protein that pumps some types of chemotherapy out of cancer cells, decreasing the ability of these drugs to kill the cancer cell. When the MDR1 pump is present in high amounts it leads to resistance to chemotherapy (Ref. 7), and therefore ET-743 may prevent the development of chemotherapy resistance because it causes less MDR1 protein to be made. This is supported by experiments that show ET-743 can increase the levels of doxorubicin and vincristine in cells with over-expression of MDR1 and by the ability of ET-743 to decrease the amount of MDR1 in a cell line (Ref. 8). However, in another study MDR1 was suggested to be one of the ways in which cancer cells become resistant to ET-743 (Ref. 9).  Further testing has found that not all ET-743 resistant cell lines have increased levels of MDR1 and not all cell lines with increased levels of MDR1 are resistant to ET-743 (Ref. 8). A chondrosarcoma cell line made resistant to ET-743 was found to have changes in its skeleton structure and to make less collagen compared to the parent cell line (Ref. 10). ►

Surprisingly, tumor cell lines that do not have the ability to repair damage to DNA are resistant to the effects of ET-743. Researchers have proposed that the binding of ET-743 in normal cells traps proteins responsible for this process. The structure formed not only inhibits the normal repair process but without the trapped proteins, ET-743 is less toxic to the cell (Ref 11, Ref. 12). Micro array studies, a method to determine differences in gene expression between two DNA samples, have shown that following treatment with ET-743, cells will have changes in genes involved with DNA damage response, transcription and signal transduction (Ref. 13). Although much is known, there is not a complete understanding at this time of all the effects of ET-743 and how it causes cancer cells to die.

Pre-clinical Studies of ET-743 in Sarcomas

ET-743 was tested in many bony and soft tissue sarcoma cell lines before clinical testing (Ref. 14, Ref. 15), which is unusual in drug development. Li and colleagues tested ET-743 in human fibrosarcoma, mesenchymal chondrosarcoma, liposarcoma, hemangiopericytoma, mixed mesodermal and malignant fibrous histiocytoma (MFH) cell lines.  In this study the fibrosarcoma and MFH cell lines were the most sensitive to the drug, which is of interest as fibrosarcomas are clinically not very sensitive to chemotherapy. ET-743 was found to be more effective than other agents used in sarcoma therapy such as methotrexate, doxorubicin, etoposide and paclitaxel. The presence of commonly altered proteins that may decrease response to chemotherapy such as p53, pRb, and overexpression of MDM2 were not found to affect the therapeutic benefit of ET-743.  In addition, ET-743 did not affect the levels of proteins that can increase cell survival.  This same group studied the effects of ET-743 in conjunction with doxorubicin, trimetrexate, or paclitaxel on fibrosarcoma, liposarcoma and breast cancer cell lines (Ref. 16, Ref 18). They determined that the combination of ET-743 with doxorubicin lead to enhanced activity.  The sequencing of drugs was also important with greater activity seen when ET-743 was given before doxorubicin or paclitaxel. The combination of ET-743 with trimetrexate lead to decreased activity compared to the effects of either drug alone.  Additional studies testing the combination of doxorubicin with ET-743 in rhabdomyosarcoma and fibrosarcoma cell lines found the combination to be more effective that either agent alone (Ref. 17). In particular, the combination of the drugs when given simultaneously was shown to have significant activity in a fibrosarcoma cell line resistant to doxorubicin. In addition, ET-743 has been studied in combination with cisplatin, a drug commonly used in osteosarcomas (Ref. 19). In a variety of cell lines, including a rhabdomyosarcoma cell line, the combination of ET-743 with cisplatin was more effective than either agent alone. Of particular interest to this audience is the work by Scotlandi and colleagues that evaluated ET-743 in Ewing’s sarcoma and osteosarcoma cell lines.  ET-743 was found to have effects against bone sarcoma cell lines as well as some cell lines made resistant to methotrexate, cisplatin, or adriamycin. In vitro treatment of Ewing’s sarcoma cell lines to ET-743 for 24-72 hours caused cell death. Lastly, the combination of ET-743 with a novel antiangiogenic agent lead to increased death of tumor cells and fewer blood vessels in a chondrosarcoma model (Ref. 20).  

Phase I Clinical Trials

The primary purpose of a Phase I clinical trial is to determine the maximum tolerated dose that can be given safely without severe side effects.  Phase I studies of ET-743 have tested a variety of schedules.  The first phase I study assessed evaluated ET-743 as a 24-hour continuous infusion repeated every 21 days (Ref. 21).  Doses from 50 to 1800 micrograms/m² were tested.  The most common side effects that limited an increase in dose were low neutrophil and platelet counts.  In addition, the majority of patients were noted to have elevations of their liver function tests, and patients with underlying liver abnormalities were found to be more likely to have severe side effects.  The maximum tolerated dose was 1500 micrograms/m² for patients that had received prior chemotherapy.  Three patients had tumor shrinkage, including patients with osteosarcoma and liposarcoma, and four patients with progressing soft tissue sarcomas at the time of study entry had stable disease for 3 or more months.  Activity in sarcomas using this schedule was reported separately including 12 patients from the phase I trial above as well as an additional 17 patients treated on a compassionate use program (Ref. 22).   This report included 25 soft tissue, 3 osteogenic and 1 Ewing’s sarcoma patients treated at 1200, 1500 or 1800 micrograms/m².  The majority of patients had large tumor volumes and they were considered resistant to chemotherapy, including anthracyclines, standard first line therapy for soft tissue sarcoma (STS) in the adjuvant and metastatic disease setting.   Two patients each with soft tissue sarcoma and osteosarcoma had significant tumor shrinkage with an additional 12 patients having disease stabilization for 2.8-15 months.  Median duration of response was 10.5 months with median duration of disease stabilization being 5.2 months. 

A phase I trial has also looked at a 72-hour continuous infusion repeated every 21 days. It evaluated 600, 900, 1200, and 1050 micrograms/m² with the latter dose level recommended for further studies.  The highest dose level was complicated by significant changes in liver function tests, low platelet and white blood cell counts as well as a syndrome of muscle breakdown called rhabdomyolysis that lead to renal failure (Ref. 23). A patient with epitheliod mesothelioma was noted to have an improvement in metabolic tumor activity on PET scan. This study enrolled 4 patients with leiomyosarcoma and 2 with Gastrointestinal Stromal Tumors (GIST), none of whom responded.  Lengthening the time of drug infusion from 1 hour to 72 hours is associated with fewer effects on blood counts (23), however an increase in dose results in increasing liver toxicity (Ref. 24). More recently a phase I trial using a daily times five schedule has been reported (Ref. 25). The maximum tolerated dose (MTD) for this schedule was 325mg/m². There were 5 patients with STS enrolled on this study with a patient with uterine leiomyosarcoma experiencing greater than 25% decrease in tumor volume. Ovarian and primary peritoneal cancers have also shown evidence of response to ET-743.

In all the schedules examined the most common toxicities were neutropenia, thrombocytopenia, and transaminitis (liver function abnormalities).   Severe toxicities resulting in death however have been seen.  An analysis of 5 phase I trials looking at parameters associated with toxicity revealed that patients in whom liver alkaline phophatase increased following treatment with ET-743 was associated with increase risk of severe side effects (Ref. 26).  Protocols now incorporate frequent sampling of liver function tests between cycles of therapy.  In addition, preclinical studies in rats, the animal model with the greatest risk for liver side effects from drugs, showed that animals that received dexamethasone had decreased liver toxicity (Ref. 27).  Dexamethasone is now a standard premedication for patients receiving ET-743. 

Ongoing phase I clinical trials are testing the combination of ET-743 with doxil, taxotere and cisplatin are ongoing. Further testing of anthracyclines in combination with ET-743 in metastatic sarcoma is under development. A phase I trial has been completed by the Children’s Oncology Group evaluating ET-743. The results of that trial have not been published to date.

Pharmacokinetic studies

Pharmacokinetic studies are done to determine drug levels in a patient’s blood, urine or stool following injection of a drug.  It allows investigators to understand how a drug is distributed in the body and to determine how and how long it takes for the drug to be eliminated from the body.  These studies have revealed some interesting data for ET-743.  Pharmacokinetic studies revealed significantly lower drug levels in patients with GIST compared with other STS (Ref. 28).  Patients with retroperitoneal primary tumors had significantly higher drug levels compared to patients with tumors arising at other primary sites.  Patients with elevated liver alkaline phosphatase levels, but not elevated serum hepatic enzymes have had statistically higher drug levels (Ref. 28).  Severe toxicities were more frequent in patients with abnormal liver function tests at baseline or those with liver metastases.  The addition of dexamethasone has decreased toxicity and is associated with a decrease in the drug level in blood.  The relevance of the differences in achieved drug levels in different disease types is intriguing, yet it is unclear if increasing the dose in patients with lower serum levels would be feasible or if an increase would lead to more patients with tumor responses. 

Phase II Clinical Trials

Phase II clinical trials test a new drug or combination of drugs and look to see what the efficacy of the drug is in a specific disease setting.  The 24-hour infusion schedule has been evaluated in STS in patients that have metastatic disease.  For chemotherapy naïve patients, objective response rates were noted in 14% with an additional 14% with stable disease (Ref. 290.  The 12-month progression free survival in these patients was 18% and 49% of patients were alive with disease at one year (Ref. 29). 

Two phase II trials from the US and France enrolled patients previously treated with single agent doxorubicin and ifosfamide or the combination (Ref. 30, Ref. 31), with a group of patients with extensive pretreatment in the French study. In these two studies objective responses were noted in 8% and 4% of patients respectively, with one complete response in a patient with liposarcoma.  Responses were also seen in leiomyosarcoma, with minor responses in endometrial sarcoma and fibrosarcoma. Sixty percent of patients with responsive disease or disease stabilization in the French trial were resistant to anthracyclines and/or ifosfamide. French investigators reported 24% of the patients were without progression of their disease at 6 months, with 30% of patients alive at 2 years.  The American trial reported 9% of patients were without progression of disease at one year, however 53% of patients remained alive with disease at one year.  Partial and minor responses were reported in uterine leiomyosarcomas, fibrosarcoma, and endometrial uterine sarcoma, with stable disease in renal sarcoma, alveolar sarcoma, and neurogenic sarcoma.  Although response rates have been low in these trials, these studies looked at patients that had progressed following standard therapies and were found to have prolonged progression free survivals. This may be due to a change in growth kinetics of tumors once on ET-743 (Ref. 32). Based on the activity seen in phase I and II trials, an ongoing phase II randomized trial is comparing the 24-hour infusion schedule to a 3-hour weekly infusion every three weeks out of four in patients with metastatic leiomyosarcoma and liposarcoma that have progressed following anthracycline and ifosfamide chemotherapy. 

A phase II trial in patients with previously treated osteogenic sarcoma using the 24-hour infusion schedule demonstrated some activity (Ref. 33). Patients in this study were younger than in most of the other previously reported studies, with the average being 18 years old, but ranging from 12-67. The patients had all received prior standard therapy for their disease and the majority had also received chemotherapy for recurrent disease. The median number of prior chemotherapies was 5.  Treatment was associated with the expected toxicities, but drug levels in patients that had severe toxicity during the first cycle of therapy were found to be increased compared to the levels in patients without these severe side effects. There were no patients with complete or partial responses, however 3 patients had minor responses, with a decrease in tumor size of 49%, 36% and 25%. Overall, in this heavily pretreated group, the median time to disease becoming worse was only 1.3 months. A phase II study found no clinical activity in patients with GIST, with only 2 of 20 patients having stable disease for 4 and 10 months (Ref. 28). 

Currently ongoing phase II trials are evaluating the efficacy of ET-743 in advanced breast, ovarian, endometrial and prostate cancers.  An upcoming study coordinated by the Children’s oncology group will be opening, testing ET-743 in recurrent or refractory soft tissue sarcomas and Ewing’s family tumors for patients diagnosed before the age of 21.

Conclusions

ET-743 is a unique drug in many respects. It was identified as a potential drug by screening sea organisms.  Its method of killing cells is complex but clearly is different from other known anticancer drugs.  We still have more to do to understand which tumors will benefit from treatment with ET-743. Based on the clinical data from the studies already done, ET-743 has shown that it is active in a group of tumors that do not have many active agents. The information on the activity in soft tissue sarcomas is real and clearly some patients are benefiting from this novel compound. The upcoming phase II study in pediatric patients with sarcomas will extend our information and provide further data as to its role in the treatment of Ewing’s sarcoma in particular. 

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