Study reveals complex mechanism underlying resistance to targeted lung cancer drugs
A new class of drugs that block the epidermal growth factor receptor (EGFR) on lung cancer cells may get around the growing problem of resistance to targeted therapy drugs like Iressa and Tarceva. In a report to appear in Proceedings of the National Academy of Science, researchers from Massachusetts General Hospital (MGH) Cancer Center and colleagues from Wyeth Pharmaceuticals research division describe finding how drugs called irreversible EGFR inhibitors apparently avoid resistance and may offer patients longer term remission. The study has received early online release on the PNAS website.
"These irreversible inhibitors form an unbreakable bond with the EGFR molecule," says Daniel Haber, MD, PhD, director of the MGH Cancer Center and senior author of the PNAS paper. "The initial group of EGFR inhibitors can fall off the receptor, but once the irreversible inhibitors bind, the receptor is permanently out of commission." The researchers also discovered a new mechanism underlying the development of resistance, one which the new drugs may be able to avoid.
EGFR inhibitors, like Iressa (gefitinib) and Tarceva (erlotinib), are used to treat advanced non-small-cell lung cancer (NSCLC), the leading cause of cancer deaths in the U.S. In 2004 MGH researchers found that a particular set of mutations in the EGFR molecule could identify patients whose tumors would respond to Iressa, which early clinical trials indicated was effective in less than 15 percent of patients. The identified mutations magnified the cells' response to growth factor, fueling tumor growth, but they also increased tumors' sensitivity to Iressa. Subsequent research found that similar mutations were associated with sensitivity to Tarceva.
Although results for those patients who did respond to these EGFR inhibitors were often rapid and dramatic, they were also short lived, lasting for an average of 6 to 8 months. Researchers at several centers attributed this resistance to a secondary mutation in a different part of the EGFR molecule found in some but not all resistant patients. These secondary mutations are similar to mutations associated with acquired resistance to Gleevec and other targeted cancer therapy drugs. The current study was designed to further investigate those mutations and the mechanisms underlying treatment resistance.
The researchers first conducted a detailed genetic analysis of tumor samples from two patients whose cancer had recurred after initially responding to Iressa treatment. The initial sensitizing mutation was found in all recurrent tumor cells from both patients, but the resistance-associated secondary mutation was found in only 20 percent of cells from one patient and virtually no cells from the other patient, confirming that other resistance mechanisms must exist.
The researchers then started with Iressa-sensitive NSCLC cells and created multiple lines of cultured Iressa-resistant tumor cells. None of these resistant lines acquired a secondary mutation in EGFR, but they appeared to have changes in the intricate way the receptor receives chemical signals and transmits them into the cell.
"These cells go through a complex process of internalization, in which the receptors take up their ligand [signalling molecule] on the cell surface, move inside the cell, release the ligand and then move back to the cell membrane,"says Haber, a professor of Medicine at Harvard Medical School. "We're starting to appreciate that a very slight change in the regulation of this process can have a great impact, and much of this had been predicted by several researchers studying EGFR regulation."
In what they consider their most critical observation, the investigators also found that NSCLC cells that have become resistant to Iressa, through either the secondary EGFR mutation or altered receptor processing, continue to be sensitive to a new class of compounds that bind more tightly and irreversibly to EGFR. Haber and his colleagues believe that the stronger binding of these "irreversible inhibitors" overcomes both the structural change resulting from the secondary EGFR mutation and the changes in receptor processing. Not only are Iressa-resistant NSCLC cells still sensitive to these irreversible inhibitors, but very few NSCLC cells treated with these new drugs ever become resistant to the new drugs.
One of the irreversible EGFR inhibitors used in this study – HKI-272, manufactured by Wyeth – is already in a phase 1 clinical trial. And MGH Cancer Center researchers are planning to lead a multi-institutional phase 2 trial in NSCLC patients whose tumors have sensitizing EGFR mutations but have become resistant to Iressa or Tarceva.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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