Brighton researchers' discovery may prolong cancer patients

The success of the original kinase inhibitors had raised hope that drugs that target key kinases underlying other cancers, such as members of the human epidermal growth factor receptor (HER) family, might be similarly efficacious. However, several small-molecule inhibitors of HER family kinases have shown limited efficacy in HER2-driven breast cancers, despite effective inhibition of kinase activity. An article in Nature, 7 Jan 2007, scientists had provided an explanation for this phenomenon: failure to completely inhibit the kinase activity of HER2 allows oncogenic signaling through the kinase-inactive family member HER3 to continue.

Signaling in the HER family, which consists of epidermal growth factor receptor (EGFR), HER2, HER3 and HER4, involves receptor dimerization and transphosphorylation, which leads to the activation of various pathways, including the potentially oncogenic phosphatidyl-inositol 3-kinase (PI3K)/Akt pathway. While these agents are effective at inhibiting EGFR and HER2 phosphorylation in patients' tissues and tumors, Akt activity is not inhibited as might be anticipated in many patients, which could explain the limited clinical activity of the drugs.

Patients with HER2-positive breast cancer being treated with anti-HER2 therapy may be able to prevent or delay resistance to the therapy with the addition of a PI3K kinase inhibitor to their treatment regimens. Dual simultaneous inhibition of both HER2 and PI3K may prolong the use of anti-HER2 therapies in women with breast cancer. Designing 'targeted' anticancer drugs begins with identifying the genes or proteins that are specific to the development of cancer and testing whether blocking those genes or proteins gets rid of thecancer. Genetic (molecular) tests are instrumental in accomplishing this task. 

However, understanding 'targeted' treatments begins with understanding the cancer cell. Every tissue and organ in the body is made of cells. In order for cells to grow, divide, or die, they send and receive chemical messages. These messages are transmitted along specific 'pathways' that involve various genes and proteins in a cell. 

Genetic-based testing examines a single process within the cell or a relatively small number of process. The aim is to tell if there is a theoretical predispostion to drug response. Phenotype analysis not only examines for the presence of genes and proteins but also for their 'functionality' (their interaction with other genes, proteins, and processes occurring within the cell, and for their response to 'targeted' drugs). 

Genetic-based testing involves the use of dead, formaldehyde preserved cells that are never exposed to 'targeted' drugs. Genetic-based tests cannot tells us anything about uptake of a certain drug into the cell or if the drug will be excluded before it can act or what changes will take place within the cell if the drug successfully enters the cell. 

Genetic-based tests cannot discriminate among the activities of different drugs within the same class. Instead, it assumes that all drugs within a class will produce precisely the same effect, even though from clinical experience, this is not the case. Nor can Genetic-based tests tell us anything about drug combinations. 

Phenotype analysis (functional profiling) looks at 'fresh' living cancer cells. It assesses the net result of all cellular processes, including interactions, occurring in real time whencancer cells actually are exposed to specific anti-cancer drugs. It can discriminate differing anti-tumor effects of different drugs within the same class. It can also identify synergies indrug combinations. 

When considering a 'targeted' cancer drug which is believed to act only upon cancer cells that have a specific genetic defect, it is useful to know if a patient's cancer cells do or do not have precisely that defect. Although presence of a 'targeted' defect does not necessarily mean that a drug will be effective, absence of the targeted defect may rule out use of the drug. 

As you can see, just selecting the right test to perform in the right situation is a very important step on the road to personalizing cancer therapy. Sometimes a drug will inhibit the 'target' but not stop the growth of cancer. Not all genes and proteins have a critical role in the survival and growth of cancer cells. 

The are many pathways to altered cellular (forest) function, hence all the different 'trees' which correlate in different situations. Improvement can be made by measuring what happens at the end of all processes (the effects on the forest), rather than the status of the individual trees (pathways/mechanisms). You still need to measure the net effect of all processes, not just the individual molecular (gene/protein) targets.

You can see why laboratory oncologists like Drs. Nagourney and Weisenthal have been interested in this. Nagourney will be presenting some information about the comparisons of various kinases.

Robert A. Nagourney, Paula J. Bernard, Eric Federico, Sophie Nguyen, Steven S. Evans.

Presentation Abstract Number: 3525

Presentation Title: Functional analysis of epidermal growth factor receptor tyrosine kinase inhibitors: A comparison of Afatanib, Lapatinib, and Gefitinib in human tumor primary cultures.

Protein Kinases

Signal transduction is defined as any biochemical communication from one part of the cell to another. It is essential for normal functioning of the cell and is highly regulated. The process begins with a specific protein called a receptor that is bound in the cell surface membrane. The portion of the receptor that faces the exterior of the cell contains a ligand or site that can bind to a signaling molecule. This binding results in the activation of the receptor. The interior portion of the receptor is either a functional enzyme, or can combine with and activate an enzyme.

Receptors for most growth factors are enzymes called tyrosine kinases. Signal transduction can be described as a cascade or reactions, in which a chemical change in one molecule leads to change in another molecule (mostly proteins). The signaling process begins when the enzyme receives a phosphate group from ATP, an energy generating molecule present in the cell. The phosphate group is then transferred to a series of protein kinase molecules in turn. The process continues until an activated molecule enters the nucleus, where it results in the activation of genes responsible for functioning of the cell cycle and cell division.

The cancer state is typically characterized by a signaling process that is unregulated and in a continuous state of activation. This may be due to the action of oncogenes, or genes that code for abnormal proteins that are themselves kinase enzymes or otherwise activate the signaling process. Gene mutations of cancer could also alter the receptor molecule in a manner that it remains active without regulation. The signal transduction pathways are very complex and still not completely understood. All proteins in the pathways are potential candidates for inhibition.

Epidermal growth factor receptors (EGFR) are typical enzyme-linked receptors, with an exterior ligand that binds with a signaling molecule, and an internal tyrosine kinase enzyme site. Drugs are developed to inhibit expression at either of these sites. Iressa binds to the external ligand, and has shown activity against non-small-cell lung cancer, adenocarcinoma and breast cancer. In the case of breast cancer, Iressa inhibits an overactive HER/neu tyrosine kinase. The monoclonal antibody, Erbitux, also binds to and inhibits the external ligand of EGFR. This antibody shows promise for use in patients with head a neck cancer who have developed resistance to chemotherapy.

Since unregulated signal transduction is a primary characteristic of many types of cancers, researchers are very active in the pursuit of inhibitors that can control the process. These drugs promise to become an essential part of the physician's armament against cancer, particularly those cancers that have developed resistance to other forms of treatment.

However, setbacks with Gleevec and Iressa, that specifically target protein kinases, reflect a lack of validated biomarkers. The next classes of signal transduction inhibitors, the vascular endothelial growth factor receptor (VEGFR) also lack validated biomarkers.

What is needed is to test the concept of targeted cancer drugs with biomarkers as pharmacodynamic endpoints, and with the ability to measure multiple parameters in cellular screens now in hand using flow cytometry.

The importance of mechanistic work around targets as a starting point for drug development should be downplayed in favor of a systems biology (cell function analysis) approach were compounds are first screened in cell-based assays, with mechanistic understanding of the target coming only after validation of its impact on the biology.

Gleevec turned out to be one of the first examples of a multi-targeted kinase inhibitor. The lessons learned from the Gleevec experience are that mutant kinase targets are a smoking gun for kinase dependency, resistance reveals tumor heterogeneity, and the conformation of the kinase (active or inactive) may be important when choosing drug leads to take into the clinic. In such molecules, different portions bind to different sites on kinases. Given the heterogeneity of tumors among people with cancer (and even in the same person over time), multiple drugs give clinicians an opportunity to vary dosing in proportion to the specific person's tumor expression profile and the pathways activated in that individual.

The fundamental role of kinases in cancer biology and the success of pioneering therapeutics have prompted intensive efforts to develop kinase inhibitors. However, many of these drugs cry out for validated clinical biomarkers to help set dosage and select people likely to respond.