Trastuzumab

Trastuzumab (INN; trade name Herceptin) is a monoclonal antibody that interferes with the HER2/neu receptor.

The HER receptors are proteins that are embedded in the cell membrane and communicate molecular signals from outside the cell to inside the cell, and turn genes on and off. The HER proteins regulate cell growth, survival, adhesion, migration, and differentiation—functions that are amplified or weakened in cancer cells. In some cancers, notably some breast cancers, HER2 is over-expressed, and, among other effects, causes breast cells to reproduce uncontrollably.

Antibodies are molecules from the immune system that bind selectively to different proteins. Trastuzumab is an antibody that binds selectively to the HER2 protein. When it binds to defective HER2 proteins, the HER2 protein no longer causes cells in the breast to reproduce uncontrollably. This increases the survival of people with cancer. However, cancers usually develop resistance to trastuzumab.

The original studies of trastuzumab showed that it improved survival in late-stage (metastatic) breast cancer, but there is controversy over whether trastuzumab is effective in earlier stage cancer. Trastuzumab is also controversial because of its cost, as much as $100,000 per year, and while certain private insurance companies in the U.S. and government health care systems in Canada, the U.K. and elsewhere have refused to pay for trastuzumab for certain patients, some companies have since accepted trastuzumab treatment as a covered preventative treatment.

Trastuzumab was originally developed in mice, as a mouse antibody. Because humans have immune reactions to mouse proteins, it was later developed into a human (humanized) antibody. Because the antibodies were produced from one cell that was grown into a clone of identical cells, it is called a monoclonal antibody.

Trastuzumab is also being studied for use with other cancers. It has been used with some success in women with uterine papillary serous carcinomas that overexpress HER2/neu.

Mechanism of action
The HER2 gene (also known as HER2/neu and ErbB2 gene) is amplified in 20-30% of early-stage breast cancers, which makes it overexpressed. Also, in cancer, HER2 may send signals without mitogens arriving and binding to any receptor, making it overactive.

The HER2 pathway promotes cell growth and division when it is functioning normally, however it has checkpoints and the process is regulated so that cells do not proliferate too rapidly. In some cancer cells the pathway is exploited to promote rapid cell growth and proliferation and hence tumor formation. The EGF pathway includes the receptors HER1 (EGFR), HER2, HER3, and HER4; ligand binding and homo/heterodimerization is required to initiate the pathway. HER2 does not bind ligands directly but it is the preferred dimer partner for the other receptors that have ligands bound. The pathway initiates the MAP Kinase pathway as well as the PI3 Kinase/AKT pathway, which in turn activates the NF-κB pathway. All of these downstream pathways can be used to initiate anti-apoptosis and cell proliferation signals. In cancer cells the HER2 protein can be expressed up to 100 times more than in normal cells (2 million versus 20,000 per cell). This overexpression leads to strong and constant proliferative signaling and hence tumor formation. Overexpression of HER2 also causes deactivation of checkpoints, allowing for even greater increases in proliferation.

HER2 extends through the cell membrane, and carries signals from outside the cell to the inside. In healthy people, signaling compounds called mitogens arrive at the cell membrane, and bind to the outside part of other members of the HER family of receptors. Those bound receptors then link (dimerize) with HER2, activating it. HER2 then sends a signal to the inside of the cell. The signal passes through different biochemical pathways. This includes the PI3K/Akt pathway and the MAPK pathway. These signals promote invasion, survival and growth of blood vessels (angiogenesis) of cells.

Normally, when cells divide, they go through a mitosis cycle, with checkpoint proteins that keep cell division under control. Some of the proteins that control this cycle are called cdk2 (CDKs). These CDKs are inhibited by other proteins. One of those proteins is the inhibitor p27Kip1. Normally, p27Kip1 moves from the cytoplasm to the nucleus, to keep the cycle under control. When HER2 always sends signals, p27Kip1 doesn't move to the nucleus, but accumulates in the cytoplasm instead. This is caused by phosphorylation by Akt.

Trastuzumab is a humanized monoclonal antibody that binds to the domain IV of the extracellular segment of the HER2/neu receptor. Cells treated with trastuzumab undergo arrest during the G1 phase of the cell cycle so there is reduced proliferation. It has been suggested that trastuzumab induces some of its effect by downregulation of HER2/neu leading to disruption of receptor dimerization and signaling through the downstream PI3K cascade. P27Kip1 is then not phosphorylated and is able to enter the nucleus and inhibit cdk2 activity, causing cell cycle arrest. Also, trastuzumab suppresses angiogenesis by both induction of antiangiogenic factors and repression of proangiogenic factors. It is thought that a contribution to the unregulated growth observed in cancer could be due to proteolytic cleavage of HER2/neu that results in the release of the extracellular domain. Trastuzumab has been shown to inhibit HER2/neu ectodomain cleavage in breast cancer cells.

Experiments in laboratory animals indicate that antibodies, including trastuzumab, when bound to a cell, induce immune cells to kill that cell, and that such antibody-dependent cell-mediated cytotoxicity is an important mechanism of action.

There may be other undiscovered mechanisms by which trastuzumab induces regression in cancer.

Predicting response to therapy
Trastuzumab reverses the effects of an overactive HER2 receptor. If the breast cancer doesn't have overactive HER2 receptors, trastuzumab will have no beneficial effect (and may cause harm). Doctors use laboratory tests to find out whether HER2 is overexpressed.

In the routine clinical laboratory, the most commonly employed methods are immunohistochemistry (IHC) and either silver, chromogenic or fluorescent in situ hybridisation (SISH/CISH/FISH). Alternatively, HER-2 amplification can be detected by virtual karyotyping of formalin-fixed paraffin embedded tumor. Virtual karyotyping has the added advantage of assessing copy number changes throughout the genome, in addition to HER-2 status. In addition numerous PCR-based methodologies have also been described.

Routine HER-2 status is performed by IHC, there are two FDA-approved commercial kits available; Dako HercepTest and Ventana Pathway. These are highly standardised, semi-quantitative assays which stratify expression levels into; 0 (<20,000 receptors per cell, no visible expression), 1+ (~100,000 receptors per cell, partial membrane staining, < 10% of cells overexpressing HER-2), 2+ (~500,000 receptors per cell, light to moderate complete membrane staining, > 10% of cells overexpressing HER-2), and 3+ (~2,000,000 receptors per cell, strong complete membrane staining, > 10% of cells overexpressing HER-2). The presence of cytoplasmic expression is disregarded. Treatment with trastuzumab is indicated in cases where HER-2 expression has a score of 3+. However, IHC has been shown to have numerous limitations, both technical and interpretative, which have been found to impact on the reproducibility and accuracy of results, especially when compared with ISH methodologies. It is also true, however, that some reports have stated that IHC provides excellent correlation between gene copy number and protein expression.

Fluorescent in situ hybridization (FISH) is viewed as being the “gold standard” technique in identifying patients who would benefit from trastuzumab. It is, however, expensive and requires a fluorescent microscope and an image capture system. The main expense involved with CISH is in the purchase of FDA-approved kits, and as it is not a fluorescent technique it does not require specialist microscopy and slides may be kept permanently. Comparative studies between CISH and FISH have shown that these two techniques show excellent correlation. The lack of a separate chromosome 17 probe on the same section is an issue with regards to acceptance of CISH. As of June 2011 Hoffman-La Roche has obtained FDA approval for the INFORM HER2 Dual ISH DNA Probe cocktail developed by Ventana Medical Systems. The DDISH (Dual-chromagen/Dual-hapten In-situ hybridization) cocktail uses both HER2 and Chromosome 17 hybridization probes for chromagenic visualization on the same tissue section. The detection can be achieved by using a combination of ultraView SISH(silver in-situ hybridization) and ultraView Red ISH for deposition of distinct chromgenic precipitates at the site of DNP or DIG labeled probes.

Currently the recommended assays are a combination of IHC and FISH, whereby IHC scores of 0 and 1+ are negative (no trastuzumab treatment), scores of 3+ are positive (trastuzumab treatment), and score of 2+ (equivocal case) is referred to FISH for a definitive treatment decision. Industry best practices indicate the use of FDA-cleared Automated Tissue Image Systems by laboratories for mechanized and automated processing of specimen, thereby reducing process variability, avoiding equivocal cases, and enhancing probability of Trastuzumab therapy.

Impact
Trastuzumab has had a "major impact in the treatment of HER2-positive metastatic breast cancer". The combination of Trastuzumab with chemotherapy has been shown to increase both survival and response rate, in comparison to Trastuzumab alone.

It is possible to determine the "erbB2 status" of a tumour, which can be used to predict efficacy of treatment with trastuzumab. If it is determined that a tumour is overexpressing the erbB2 oncogene and the patient has no significant pre-existing heart disease, then a patient is eligible for treatment with trastuzumab. It is surprising that although trastuzumab has great affinity for the receptor and the fact that such a high dose can be administered (due to its low toxicity) 70% of HER2+ patients do not respond to treatment. In fact resistance is developed rapidly by treatment, in virtually all patients. It is suggested that a mechanism of resistance is the lack of p27Kip1 translocation to the nucleus in some strains, enabling cdk2 to induce cell proliferation.

Some recent clinical trials have found trastuzumab reduces the risk of relapse in breast cancer patients by 50% when given in the adjuvant setting (i.e. after breast cancer surgery, before the cancer has spread any further) for one year.

In one British trial this translated as follows: 9.4% of those on the drug relapsed as opposed to the 17.2% of those not on trastuzumab. In this study, almost five out of six patients would not have developed a recurrence during the study whether or not they received trastuzumab, and almost one in ten patients relapsed despite it and therefore received no apparent benefit from the treatment. Only one patient in 13 received positive benefit in terms of cancer recurrence. However, these numbers solely consider the risk of a cancer recurrence and do not account for morbidity and mortality (sickness and death) due to the treatment's side effects.

Even among the 20% of first-time breast cancer patients for whom trastuzumab is an appropriate treatment, the actual net benefits are not overwhelming when viewed in terms of all-cause mortality. All-cause mortality helps balance a reduced risk of death from cancer against the increased risk of death from a treatment's side effects. Repeated, large-scale studies show that it is usually necessary to treat between 25 and 100 patients to prevent a single death during the next two to four years. For each life saved, between ten and 25 patients will develop heart disease; despite effective treatments, some of these patients will die from heart disease. For example, in the N9831 (arm C) and NSABP B31 joint analysis, approximately two patients died of excess heart disease or other complications for every three lives saved by reducing breast cancer. The excess heart disease induced by the drug explains why it is necessary to treat up to 100 cancer patients to save a single life during a two-year study period.

There was debate in 2006 as to whether these benefits may have been over-stated.

The media have sometimes misrepresented trastuzumab as a "cure all" or "wonder drug" and this has caused confusion amongst women with breast cancer about whether they should be receiving the drug or not.

Optimal duration of adjuvant trastuzumab
The optimal duration of adjuvant trastuzumab is currently unknown. One year of treatment is generally accepted as the ideal length of therapy based on current clinical trial evidence that demonstrated the superiority of one year treatment over none. However, a small Finnish trial also showed similar improvement with nine weeks' of treatment over no therapy. Due to the lack of direct head-to-head comparison in clinical trials, it is unknown whether a shorter duration of treatment may be just as effective (with less side effects) than the current accepted practice of treatment for one year. Debate about treatment duration has become a relevant issue for many public health policy makers due to the high financial costs involved in the administration of this treatment for one year. Some countries with a taxpayer funded public health system, such as New Zealand, have opted to only fund for nine weeks of adjuvant therapy as a result. Current clinical trials are in progress hoping to answer this question by directly comparing short versus long duration of therapy.

Side effects
One of the significant complications of trastuzumab is its effect on the heart. Trastuzumab is associated with cardiac dysfunction in 2-7% of cases. As a result, regular cardiac screening with either a MUGA scan or echocardiography is commonly undertaken during the trastuzumab treatment period.

Approximately 10% of patients are unable to tolerate this drug because of pre-existing heart problems; physicians are balancing the risk of recurrent cancer against the higher risk of death due to cardiac disease in this population. The risk of cardiomyopathy is increased when trastuzumab is combined with anthracycline chemotherapy (which itself is associated with cardiac toxicity).

History
The biotech company Genentech gained FDA approval for trastuzumab in September 1998. The drug was jointly developed with UCLA. At Genentech, the antibody was first discovered by scientists including Dr. Axel Ullrich and Dr. H. Michael Shepard. At UCLA's Jonsson Cancer Center, Dr. Dennis Slamon subsequently worked on trastuzumab's development. A book about Dr. Slamon's work, was made into a television film called Living Proof, that premiered in 2008.

Costs
Trastuzumab costs about US$70,000 for a full course of treatment, Trastuzumab brought in $327 million in revenue for Genentech in the fourth quarter of 2007. Genentech refuses to give details to explain the high costs.

Australia has negotiated a lower price of A$50,000 per course. Recently there has been controversy in New Zealand and the UK about public health funding of this drug in the adjuvant setting due to its high cost and perceived limited overall survival (though not breast cancer-free survival). The campaign ran by cancer victims to get the governments to pay for their treatment has gone to the highest levels in the courts and the cabinet to get it licensed against the judgment of the regulator. After a sustained campaign from cancer sufferers, the Ontario Ministry of Health in July 2005 decided that it would pay for treatments with trastuzumab and two other new and controversial anti-cancer drugs.

Since October 2006 trastuzumab has been made available for Australian women with early stage breast cancer via the Pharmaceutical Benefits Scheme. This is estimated to cost the country over A$470 million for 4–5 years supply of the drug.

When trastuzumab was introduced in the UK there was considerable variation in availability between different geographical regions. Scotland was first to make the drug widely available.