Receptor Tyrosine Kinase “Hijacking” in Glioblastoma

Francis Collins, director of the National Institutes of Health’s Human Genome Research Institute, wrote in “Brave New Pharmacy” (Time Magazine, June 2001) that a new era of drug discovery was coming. As he put it, this new era is one where “if you understand the genetic basis of a disease, then you can predict what protein it produces and set about developing a drug to block it.”

One success story is Trastuzumab, an antibody that targets the outer domain of HER-2 (Human Epidermal Growth Factor Receptor 2, also known as ErbB-2). Researchers found HER-2 overexpressed in 15-30% of breast cancers. However, targeting other ErbB receptors found in cancer hasn’t been as successful.

ErbB1 (also known as Epidermal Growth Factor Receptor, or EGFR) has also been found overexpressed in a variety of tumors. EGFR is a 170,000 dalton transmembrane glycoprotein with built-in tyrosine kinase activity, and its family members include EGFR, ErbB2 (HER-2), ErbB3, and ErbB4.

The main binding partner for EGFR is epidermal growth factor (EGF), a 53-amino acid polypeptide. Other EGF family members — including transforming growth factor a (TGF-a), amphiregulin, heparin-binding EGF, β-cellulin, neuregulin, and epiregulin — can also bind EGFR with high affinity. Once bound, these proteins trigger EGFR dimerization, internalization, and auto-phosphorylation — setting off signaling events involved in proliferation, migration, survival, and angiogenesis. Since EGFR signaling drives numerous mitogenic effects, EGFR overexpression and/or gain-of-function mutations (EGFRvIII) can promote oncogenic transformation.

Researchers have developed EGFR inhibitors to treat cancers caused by EGFR up-regulation. These include breast, colorectal, head and neck, non-small cell lung, pancreatic, renal cell, squamous cell, and thyroid cancers. EGFR inhibitors work in two ways:

• Protein-tyrosine-kinase (PTK) inhibitors bind to the tyrosine kinase domain inside the cell
• Monoclonal antibodies bind to EGFR’s extracellular component, blocking substrates from binding the receptor and preventing EGFR activation

These drugs include Iressa (Gefitinib), Tarceva (Erlotinib), Erbitux (Cetuximab), Tykerb (Lapatinib), Vectibix (Panitumumab), and Caprelsa (Vandetanib).

However, cancer cells can resist anti-EGFR therapy through several genetic mechanisms:

• Acquiring or selecting for secondary EGFR mutations
• Gaining additional mutations in effectors that constitutively activate signaling pathways downstream of EGFR
• Developing co-occurring amplified or mutated RTKs that bypass the EGFR pathway

Researchers have found EGFR mutations in gliomas, non-small cell lung cancer, and breast and ovarian cancer. These mutations reduce the response to EGFR therapy, most likely because they cause conformational changes that affect the intracellular domains involved in ATP binding sites. These mutations may also overwhelm the contribution of other signaling pathways for cell survival, making cancer cells more dependent on the EGFR signaling pathway to survive.

In the March issue of Cancer Discovery, a team of researchers identified a unique mechanism by which glioblastoma (GBM) cells develop resistance to anti-EGFR therapy. For the first time, they demonstrated that an EGFR-dependent cancer can escape targeted therapy by developing dependence on a different, non-amplified, non-mutated RTK.

Specifically, they showed that GBMs with EGFR mutations evade EGFR tyrosine kinase inhibitors (TKI) by transcriptionally de-repressing platelet-derived growth factor receptor β (PDGFRβ). Cell lines, patient-derived tumor cultures, and xenotransplants showed that the persistently active EGFR mutation (EGFRvIII) suppressed PDGFRβ expression through mTORC1- and ERK-dependent mechanisms. But EGFR TKI treatment de-repressed PDGFRβ, causing the tumors to become “addicted” to this non-amplified, non-mutated RTK for continued growth — and resistant to targeted treatment.

Tumor tissue from GBM patients in a phase II clinical trial for an EGFR TKI (Lapatinib) revealed a reciprocal relationship between PDGFRβ activation and EGFRvIII. Tissue analysis from one patient, taken before and after therapy, showed that Lapatinib treatment significantly reduced EGFR activation while increasing PDGFRβ expression. This supports the researchers’ in vitro and in vivo data showing that pharmacologically inhibiting EGFR causes RTK switching to PDGFRβ signaling.

We have the targets and we have the drugs, but RTK inhibitors have delivered unfulfilled promises. Acquired drug resistance remains a significant challenge for personalized cancer therapy. Even when researchers can identify druggable RTK mutations in patients, along with second-site mutations, non-genetic adaptive resistance mechanisms can still “rewire” a tumor’s circuitry. This happens through pathway crosstalk and the release of inhibitory feedback loops.

To develop more effective kinase cancer drugs, scientists need to combine RTK inhibitors with other agents (chemotherapy, radiation, other small molecules, etc.). They also need to target multiple tumor-promoting signaling pathways — either through drug combinations or a single multi-targeted compound.

Further reading:

Akhavan D, Pourzia AL, Nourian AA, Williams KJ, Nathanson D, Babic I, Villa GR, Tanaka K, Nael A, Yang H, Dang J, Vinters HV, Yong WH, Flagg M, Tamanoi F, Sasayama T, James CD, Kornblum HI, Cloughesy TF, Cavenee WK, Bensinger SJ, Mischel PS. De-repression of PDGFRβ transcription promotes acquired resistance to EGFR tyrosine kinase inhibitors in glioblastoma patients. Cancer Discovery. 2013 Mar 27. [Epub ahead of print]

Deric L. Wheeler, Emily F. Dunn, and Paul M. Harari. Understanding resistance to EGFR inhibitors—impact on future treatment strategies. Nature Reviews Clinical Oncology. 2010 September; 7(9): 493–507.

James Perry, Masahiko Okamoto, Michael Guiou, Katsuyuki Shirai, Allison Errett, and Arnab Chakravarti. Novel Therapies in Glioblastoma. Neurology Research International. Volume 2012 (2012), Article ID 428565, 14 pages