In non-small cell lung cancer, the translocation of the ALK gene with EML4 results in a fusion protein that activates signaling pathways, enabling the survival and proliferation of cancer cells.
Adapted from Shaw AT, Solomon B. Targeting anaplastic lymphoma kinase in lung cancer [published online ahead of print February 2, 2011]. Clin Cancer Res. 2011;17(8):2081-2086. doi: 10.1158/1078- 0432.CCR-10-1591.
In 2007, rearrangements of the anaplastic lymphoma kinase (ALK) gene were identified in patients with lung cancer. There has been stunning progress in the intervening years as these preclinical findings were translated into the availability of an FDA-approved therapeutic ALK inhibitor, crizotinib (Xalkori).
The dizzying pace continues as the molecular mechanisms of resistance to crizotinib have already begun to be unraveled and a second generation of ALK inhibitors is under development to overcome these issues. The recent identification of ALK abnormalities in numerous other tumor types besides lung cancer has expanded the potential of these drugs for treating other challenging cancer types.
The ALK gene encodes a receptor tyrosine kinase (RTK) that sits upstream of and transmits signals through a number of other important kinases, including phosphatidylinositol- 3-kinase (PI3K) and Janus kinase (JAK). Under normal circumstances, the ALK receptor is expressed predominantly in the central nervous system (CNS), small intestine, and testes. Its normal function in humans is not fully understood as yet, though it is believed to play an important role in the development of the brain.
The ALK gene was first determined to be a cancer-causing oncogene in 1990. It is now understood that ALK becomes oncogenic in a number of different ways: via mutations in the ALK gene that alter its function, via overexpression of the ALK protein, or via fusion with other genes through a process known as chromosomal translocation. Aberrant expression of the ALK gene through any of these means causes a cell to become cancerous due to inappropriate activation of ALK signaling, driving many of the cancer hallmarks, including cell proliferation and survival, through activation of the downstream pathways mentioned previously.
Activating mutations in the ALK gene have been identified in patients with neuroblastoma, the most common childhood cancer; the majority of patients with familial neuroblastoma and around 10% of patients with sporadic neuroblastoma have ALK mutations, most of which are found in the tyrosine kinase domain. Most prolific is the F1174 mutation, which is also correlated with a significantly worse patient outcome.
ALK protein overexpression is also common in neuroblastoma patients, identified in around 90% of cases. The percentage of cells in a tumor sample that are positive for ALK protein expression has been shown to correlate with patient outcome; the higher the level of ALK overexpression, the worse patient overall survival and disease- free survival. Recent reports have suggested that ALK overexpression is also a common feature of inflammatory breast cancer (IBC), prompting clinicians to begin enrolling patients with IBC in a phase I ALK inhibitor trial.
This illustration depicts the components of a chromosome and a gene, as well as the process of translocation, which plays a role in some ALK-driven cancers.
Illustration courtesy of The Cancer Genome Atlas/National Cancer Institute
The most common ALK abnormality found in cancer is chromosomal translocation. Translocation leads to the rearrangement of genetic information with a chromosome when parts of that chromosome break off and fuse with other chromosomes, or flip around (a process called inversion) and join with a different part of the same chromosome. This results in parts of one gene becoming fused to parts of a completely different gene, and the expression of fusion proteins. The most common fusion partner for ALK is the nucleophosmin (NPM) gene; NPM-ALK occurs in approximately 80% of cases of anaplastic large cell lymphoma (ALCL; the cancer type from which the ALK gene actually derives its name).
Another important fusion partner for ALK is EML4 (echinoderm microtubule-associated protein-like 4). This fusion is found in between 3% and 7% of all cases of non-small cell lung cancer (NSCLC), and thus far is believed to be unique to this kind of cancer. Although EML4-ALK is found in a seemingly small number of lung cancer patients, because lung cancer is so prevalent (the leading cause of death in men and the second-leading cause in women), it is likely that up to 10,000 cases of lung cancer a year in the United States will be ALK-positive. At least six other fusion partners have also been identified in many different tumor types, spanning both hematological malignancies and solid tumors.
The development of ALK inhibitors, designed to interfere with ALK signaling, have revolutionized the treatment of lung cancer. Compared with the decades invested in developing other kinase inhibitors, there has been an extremely rapid progression from discovery of ALK alterations in patients with NSCLC to FDA approval of crizotinib, the first ALK inhibitor for the treatment of patients with ALK-positive NSCLC.
The speed of these advances is due mostly to prior experience with other kinase inhibitors in NSCLC. Clinicians were able to avoid repeating costly mistakes and immediately directed ALK inhibitor research at a specific group of patients with NSCLC with confirmed ALK positivity, against which the inhibitors were most likely to work.
Crizotinib is a dual inhibitor of c-Met and ALK, and was initially being investigated as an inhibitor of the former. When NSCLC cell lines with ALK abnormalities showed sensitivity to crizotinib, researchers seized the opportunity to open ongoing crizotinib trials, aimed at examining its anti-Met activity, to patients with ALK gene rearrangements. Extremely promising phase II results led to accelerated approval of crizotinib by the FDA. Crizotinib has progressed into several phase III trials and a number of other ALK inhibitors are also in early stages of development (Above).
In spite of promising results, patients taking crizotinib frequently relapse due to acquired changes in tumor cell biology that make them unresponsive to crizotinib. The molecular mechanisms of this resistance are now beginning to be understood. Analyses of tumor samples from patients who relapsed indicated that the cells within their tumors had developed numerous different kinds of alterations that drove crizotinib resistance. These included mutations within the ALK kinase domain, which blocked crizotinib, as well as activation and amplification of kinases downstream of ALK, including EGFR and KIT, which allowed ALK to be bypassed.
Several different resistance-conferring mutations in the ALK kinase domain have been identified, including the L1196M mutation, which appears to be a so-called gatekeeper mutation, much like the T790M mutation in EGFR that confers resistance to EGFR inhibitors. Others included G1202R, S1206Y, and the insertion of a threonine at amino acid 1151 (1151Tins).
Different mutations have been shown to generate differing degrees of resistance, with 1151Tins and L1196M mutant cells being significantly more resistant to crizotinib. It is currently unclear with what frequency each type of resistance mutation occurs and, adding further to the complexity, there may be multiple different mutations present within the different cells that make up a single tumor. Therefore, in the future, it will be important for secondary biopsy and molecular evaluation of tumor samples to occur at the time of disease progression.
In response to the issue of crizotinib resistance, researchers are already developing a new generation of ALK inhibitors that demonstrate activity against even cells with the resistance-conferring mutations identified thus far, such as L1196M (Page 2). Since other RTKs are frequently activated as a mechanism of acquired resistance, the combination of ALK inhibitors with other RTK inhibitors against kinases that lie downstream of ALK is also being evaluated as a possible strategy, (eg, EGFR, KIT, mTOR). Research presented at the 2011 American Society of Clinical Oncology meeting reported that the ALK inhibitor X-396 displayed synergistic antitumor activity in vitro when combined with the mTOR inhibitor rapamycin.
Heat-shock protein (Hsp) 90 inhibitors are also undergoing trials in ALK-positive patients. ALK is a client of the Hsp90 chaperone, meaning that Hsp90 may shelter aberrant forms of the ALK protein from destruction by normal mechanisms within the cell; therefore, Hsp90 inhibition would be predicted to facilitate destruction of these mutant proteins. Other strategies include the development of an anti-ALK antibody, which could be used as monotherapy or in combination with ALK inhibitors. ALK antibodies are currently in the very early stages of preclinical development.