Apoptosis is defined as programmed cell death, or cell suicide, and is a normal part of proper human development. For example, the formation of fingers and toes in the growing fetus requires apoptosis for removal of the tissue between them. Menstruation is the sloughing of the inner lining of the uterus (the endometrium) and also occurs by apoptosis. Also, formation of the proper connections (synapses) between nerve cells in the brain requires that the extra cells be eliminated by apoptosis. Defects in the apoptotic pathway are associated with autoimmune disease such as lupus and rheumatoid arthritis.
When there is damage to the DNA, cells respond by increasing production of p53, which is a protein that triggers apoptosis. Mutations in the p53 gene lead to defective p53 protein, and are often found in cancer cells. This makes sense, because cancer represents uncontrolled growth, which can result from loss of cell death. In fact, radiation and chemotherapy are thought to work in some types of cancer by inducing apoptosis.
There are 3 main ways by which a cell commits suicide by apoptosis.
- signals that arise within the cell
- death triggers that bind to receptors at the cell surface (tumor necrosis alpha, lymphotoxin, Fas ligand, aka FasL)
- dangerous oxygen radicals
There are some cancers that are associated with transforming viruses. These are viruses that trick the body and help infected cells escape apoptosis, creating "transformed" cells.
Examples of such viruses are:
- several human papilloma viruses (HPV) implicated in causing cervical cancer
- Epstein-Barr Virus (EBV), which causes mononucleosis and is associated with some lymphomas
There are also many types of cancer cells that are produced without the help of oncogenic viruses, but still play tricks on the body in order to avoid apoptosis.
- Some B-cell leukemias and lymphomas express high levels of the apoptosis inhibitor Bcl-2.
- Melanoma cells inhibit expression of the apoptosis promoter Apaf-1.
- Lung and colon cancer cells secrete elevated levels of a soluble "decoy" ligand that binds to FasL and clogs it up so that it cannot bind its receptor, Fas. When FasL binding to Fas is blocked, cytotoxic T cells cannot protect the body and kill the cancer cells.
Apoptosis occurs by one of two main cellular pathways: extrinsic and intrinsic. Cell surface receptors play a central role in the extrinsic pathway, specifically subtypes of the TRAIL receptor family called TRAIL-R1 and TRAIL-R2. These subtypes contain so-called "death domains" that trigger cell death --- apoptosis --- when activated. TRAIL-R2 is expressed on a broad range of human cancer cells, while its presence is more limited in normal tissue. By creating an antibody that targets the death domain of TRAIL-R2, the goal is to activate the receptor and promote apoptosis.
HGS-ETR2 is an intravenous antibody that has a high specificity for TRAIL-R2, and appears to be well-tolerated at doses < 10 mg/kg. At a dose of 20 mg/kg, dose-limiting toxicities of infection and elevated enzymes were observed. The maximum tolerated dose is yet to be determined. In a phase I study using HGS-ETR2 in heavily pre-treated patients with advanced solid cancers (sarcoma, melanoma, colorectal, liver, lung),
several patients experienced stable disease. Phase II studies are under development.
HGS-ETR1 is an antibody to the TRAIL-R1 receptor, with phase I, Ib, and II trials all in progress, looking at both single-agent and chemotherapy combinations.
HGS-TR2J is another antibody to TRAIL- R2 that is currently in phase I safety studies.
Abnormally active apoptosis inhibitors prolong a cell's life in an abnormal way, and can help tumor cells escape death from standard chemotherapy and essentially live forever. Survivin is a recently discovered human apoptosis inhibitor that is seen in the growing fetus, but is not generally found in differentiated adult tissues. However, survivin is expressed in human cancers of lung, colon, pancreas, prostate and breast, as well as about 50% of high-grade non-Hodgkin's lymphomas. By inhibiting survivin, the goal is to stop cancer cell "immortality" and thus stop cancer growth. Studies on anti-survivin antibodies are underway.
Bcl-2 is another apoptosis inhibitor that protects cancer cells from being killed by chemotherapy and thus prolongs the life of the cancer cells. It is found in many different types of tumor cells.
Genasense (oblimersen sodium) is a type of antisense therapy, which is a category of targeted therapies aimed at blocking production of the target rather than the target itself. Genasense works by turning off the production of bcl-2, with the goal of lowering bcl-2 levels and – hopefully – allowing standard anti-cancer therapy to kill cancer cells more effectively.
Genta Incorporated and Aventis are pursuing a clinical development program to evaluate Genasense's therapeutic potential, both as a single agent and before standard chemotherapy. Multiple preclinical studies have shown that Genasense synergizes with most types of anticancer treatment, including chemotherapy, radiation therapy, and immunotherapy. Based on preclinical data, Genta has studied or is presently studying the use of Genasense in combination with:
- Taxol ® (Paclitaxel; Bristol Myers Squibb)
- Camptosar ® (Irinotecan; Pfizer, Inc.)
- Gleevec ® (Imatinib Mesylate; Novartis)
- Rituxan ® (Rituximab; Genentech/IDEC)
- Fludara ® (Fludarabine; Berlex Laboratories, Inc)
- Cytoxan ® (Cyclophosphamide; Bristol Myers Squibb, Inc.)
- Taxotere ® (Docetaxel; Aventis Pharmaceuticals, Inc.)
- Mylotarg® (Gemtuzumab ozogamicin; Wyeth-Ayerst, Inc.)
- Cytosine arabinoside
(Courtesy of www.genta.com)
Genasense has received "Fast Track" designation by the US FDA in chronic lymphocytic leukemia (CLL) patients, meaning that the indication represents an unmet medical need. Fast Track designation allows a company to file New Drug Applications (NDA) on a "rolling" basis. Genta Inc. recently filed a NDA to obtain marketing approval for the use of Genasense together with fludarabine plus cyclophosphamide for CLL patients previously treated with fludarabine. The NDA in CLL included a randomized phase III trial of fludarabine plus cyclophosphamide with or without Genasense in 241 patients with advanced CLL previously treated with fludarabine. The primary endpoint of the phase III trial was to measure complete and partial responses (CR, PR). Results of this trial were updated in May 2005 at the American Society of Clinical Oncology meeting:
- addition of Genasense to fludarabine plus cyclophosphamide significantly increased proportion of patients who achieved a CR/PR compared with patients treated with chemotherapy alone (17% vs. 7%, respectively; p = 0.025).
- a greater number of patients who achieved CR/PR had relapsed in the chemotherapy-alone group (63%) compared with patients treated with Genasense (20%)
- duration of CR/PR was significantly longer for patients treated with Genasense (median not reached) plus chemotherapy compared with chemotherapy alone (21 mos, p= 0.035)
Genasense is given either by intravenous (IV) or subcutaneous injection, or by continuous IV infusion using a portable pump for 5-7 days, followed by 1-3 weeks off. Common side effects include decreased red blood cell count and fatigue.
Clusterin is yet another anti-apoptosis, tumor cell protector. It is expressed in prostate, kidney, bladder, ovary, lung, and breast cancers. In fact, higher levels of clusterin expression are seen in higher Gleason-score prostate tumors and following short-course prostate hormonal therapy. In animal and petri-dish studies, high clusterin expression is associated with resistance to standard anti-tumor therapies.
OGX-011 is another type of antisense therapy (see Genasense above) that is being codeveloped by Oncogenex Technologies and Isis Pharmaceuticals. This agent is well-tolerated in phase I trials and a dose of 640 mg can be given either alone or together with standard doses of docetaxel or gemcitabine/cisplatin. OGX-011 demonstrates dose-dependent decreases in clusterin expression. Phase II trials are underway looking at OGX-011 in patients with localized and hormone-refractory prostate cancer, breast cancer, and lung cancer.