A Herceptin-DM1 conjugate produced complete regressions in mice bearing human breast cancer where Herceptin alone slowed tumor growth, according to new data presented here today.
Approximately 25% of all breast cancer patients carry extra copies of HER2, a protein that instructs their tumor to grow. Herceptin is a monoclonal antibody that binds to HER2 and suppresses its growth signal.
“We’re looking for ways to enhance the clinical benefit of Herceptin, such as attaching to it agents that might be synergistic, such as DM1, a compound that blocks cell division,” said Ralph H. Schwall, Ph.D., senior scientist at Genentech, Inc., South San Francisco, CA, and lead author of the current study
In this study, Herceptin was chemically bound to DM1 and tested against three different experimental breast cancer systems that have extra copies of HER2. In the first two, which used human breast cancer cells growing in mice, Herceptin-DM1 resulted in the complete disappearance of all tumors, whereas Herceptin as a single agent slowed tumor growth but did not cause regression.
The third experiment used a breast tumor from a transgenic mouse engineered to have high levels of HER2. The growth of this mouse tumor did not respond to Herceptin, but Herceptin-DM1 caused the tumor to shrink by more than 90%, indicating that Herceptin-DM1 can work in a tumor that is resistant to Herceptin. Tumors began to regrow four to six weeks after the final dose of Herceptin-DM1, but regressed again when retreated with Herceptin-DM1.
Genentech manufactures Herceptin and partnered with ImmunoGen, Inc., of Cambridge, MA, to develop the linked molecule. “We’re pursuing additional animal studies to get a realistic safety profile of this conjugate,” Dr. Schwall said.
DM1 is a member of the maytansine family of tubulin-blocking compounds. Tubulin, a central actor in the process of cell division, is needed to ensure that each daughter cell receives the correct amount of chromosomes. Maytansine’s mechanism of action is similar to paclitaxel (Taxol), but it works at concentrations up to 1,000-fold lower.
New Bcr-Abl Inhibitor Shows Superior Potency to Gleevec in Chronic Myelogenous Leukemia Cell Line Although STI571 (Gleevecâ) has produced complete responses in chronic myelogenous leukemia (CML), resistance in some patients highlights the need for other drugs that inhibit its molecular target, Bcr-Abl. According to research by investigators at Memorial Sloan-Kettering Cancer Center and Rockefeller University, a new drug called PD173955 (PD17) demonstrated significantly greater potency than STI571 against Bcr-Abl-containing cell lines and CML patients’ cells. PD17 is a member of a class of tyrosine kinase inhibitors originally synthesized by Parke Davis and shown to be potent inhibitors of src family kinases.
CML is a chronic form of leukemia. In its initial stage, it has only one mutation – a translocation between chromosomes 9 and 22 that results in a Bcr-encoded sequence fused to a truncated c-Abl gene. The resulting Bcr-Abl protein greatly increases c-Abl’s tyrosine kinase activity and is directly responsible for all the clinical manifestations of CML.
In searching for additional inhibitors of Bcr-Abl, Drs. Bayard Clarkson, William Bornmann, and Darren Veach at Memorial Sloan-Kettering Cancer Center (MSKCC), and Dr. John Kuriyan at Rockefeller University found that PD17 and a closely related compound called PD166326 were considerably more inhibitory to Bcr-Abl tyrosine kinase than STI571. PD17, PD16, and several other analogs were synthesized at MSKCC. The three-dimensional structure of PD17 in complex with Abl kinase was determined in Dr. Kuriyan’s laboratory by x-ray crystallography.
PD17 binds to the same ATP binding pocket as STI571, and analysis of the crystal complexes showed that whereas STI571 required that the activation loop be in the closed or inactive conformation before it can bind, it appears that PD17 can bind into both the open (active) and closed conformations of Abl. Researchers believe that this probably accounts for PD17’s increased potency.
PD16 and other analogs are currently being examined in structural modeling studies, kinase assays, and bioassays in Bcr-Abl expressing cells to search for even more active and specific inhibitors. PD17 has also been shown to have markedly greater anti-tumor effects than STI571 in a number of human glioblastoma, neuroblastoma, and sarcoma cell lines, thus having a broader range of activity than STI571.
“Now that we know the three-dimensional structure of the Abl kinase domain and exactly how the specific inhibitors differ in their binding properties at a molecular level, it should be possible to make rational modifications to design and produce even more potent and specific drugs for treatment of leukemia and other types of cancer,” said Dr. Clarkson, head of the Hematopoietic Cell Kinetics Laboratory at MSKCC and senior author of the study.
Although CML has only a single mutation in its initial stage and most solid cancers have multiple mutations, the authors hope that the molecular structural approach to drug design in CML will serve as a paradigm for how to go about developing highly specific drugs for other forms of cancer as well as other diseases.
Experimental drug heightens effect of Paclitaxel in mice bearing human breast cancers
The drug 17-AAG enhanced the activity of paclitaxel (Taxol) against human breast cancer in mice by impeding activation of AKT, a central protein in the transmission of survival signals to cells. The effect was dependent on dose and schedule.
“17-AAG appears to act on pathways in cancer not addressed by approved agents currently in use,” noted David B. Solit, M.D., clinical assistant attending physician in the Department of Medicine, Genitourinary Oncology Section, at Memorial Sloan-Kettering Cancer Center in New York City.
Groups of 10-12 mice received one of three drug protocols: 17-AAG; paclitaxel, an effective compound against breast cancer; or a combination of both drugs. Although 17-AAG demonstrated no activity as a single agent, it significantly improved the performance of paclitaxel alone, reported Dr. Solit. The combined therapy induced a complete response rate of 25%-30% and mean tumor regression of more than 90%, compared with 4% complete response and mean tumor regression of approximately 30% with paclitaxel alone. A weekly schedule of 50 mg/kg of 17-AAG on three consecutive days achieved the best results, but the major effects were seen at a threshold dose of 50 mg/kg on the first day, according to Dr. Solit.
In their research on tumors that would be most sensitive to17-AAG, Dr. Solit and his colleagues pinpointed two characteristics: overexpression of the tyrosine kinase HER2 and dependence on steroid receptors. The fact that many breast cancers have high levels of HER2 and that breast cancer is a steroid receptor-dependent disease directed their attention to a study combining 17-AAG with paclitaxel in this malignancy.
“17-AAG binds to all four members of the heat shock protein 90 family and disrupts their activity,” Dr. Solit explained. These proteins seem to have a role in cellular stress effect and may protect cells from anticancer therapy. Like heat shock proteins, AKT also shields cells from cytotoxic stressors and is highly sensitive to 17-AAG, suggesting a potential for better response to paclitaxel (a tubulin-binding compound) with addition of a drug that neutralizes these two cell protectors.
“We think that AKT may be responsible for creating an effect of 17-AAG with paclitaxel where none existed. Our findings give some support to the idea that AKT is important in HER2-directed anticancer therapy,” Dr. Solit concluded.
Antisense drug plus chemotherapy induces remission in relapsing and resistant leukemia
An experimental drug that blocks a protein associated with resistance to chemotherapy may offer new hope to leukemia patients for whom standard chemotherapy has failed, according to data from a Phase I study presented here. In the study, complete remission of disease occurred in 45% of patients who received Genasense™ in addition to a three-drug chemotherapy regimen.
“This is a very encouraging first step, which suggests that Genasense may have a role in making leukemia patients sensitive to chemotherapy again,” said Guido Marcucci, M.D., principal study investigator and assistant professor at Ohio State University College of Medicine in Columbus.
The study involved 20 patients who had been previously treated for acute lymphocytic or acute myelogenous leukemia and had either not responded to standard chemotherapy or had relapsed afterward. Study participants received a five-day continuous infusion of Genasense, followed by a five-day continuous infusion of the Genasense combined with the chemotherapeutic drugs fludarabine, cytarabine, and filgrastim.
In nine patients, all signs of leukemic cells in the blood and bone marrow disappeared. Remission was sustained for four months on average; two patients stayed in complete remission for more than a year. Typical side effects of chemotherapy such as fever, nausea, and vomiting did not occur at higher-than-expected levels, suggesting that Genasense itself was not causing significant additional toxicity.
Although these results are promising, Dr. Marcucci cautioned that larger studies must be performed to validate the effectiveness of Genasense in overcoming resistance to chemotherapy for advanced leukemia. Trials to assess the activity of Genasense in untreated acute myelogenous leukemia are expected to begin shortly at Ohio State University.
Genasense blocks production of the protein bcl-2. An overabundance of bcl-2 in cancer cells promotes resistance to chemotherapy by thwarting apoptosis, or programmed cell death. An “antisense” drug, Genasense contains a synthetic segment of genetic material that intercepts the chemical message (sense) from the gene carrying instructions for bcl-2, thus preventing production of the protein. It is considered a “modifier,” explained Dr. Marcucci, because it is intended to be used in concert with chemotherapeutic drugs, rather than as a cancer treatment in its own right.