Brain Metastasis of Patients with Lung Adenocarcinoma: Epidermal Growth Factor Receptor Mutations and Response to Whole-Brain Radiation Therapy

Keywords

brain metastasis, non-small cell lung cancer, epidermal growth factor receptor mutation, treatment, whole brain radiotherapy, epidermal growth factor receptor tyrosine-kinase inhibitors, gefitinib, erlotinib

Outline
Introduction 114
Brain Metastasis and EGFR Mutation 114
Preclinical Study and Mechanism 115
- EGFR and Radiation-Induced DNA Damage 115
- EGFR Mutation and Radiotherapy 115
- EGFR-TKIs Combined with Radiotherapy 116
Clinical study ( Table 11.1 ) 116
- EGFR Mutation and Radiotherapy 116
- EGFR-TKIs Treatment for Brain Metastasis 118
- EGFR-TKIs Combined with Radiotherapy 118
Conclusion 119
References

Introduction

Approximately 1.2 million new cases of lung cancer occur worldwide every year ( ). Most patients with non-small cell lung cancer (NSCLC) are diagnosed at an advanced stage ( ). Brain metastases develop during the course of the disease in ≈30–50% of the NSCLC patients, especially in those with adenocarcinoma ( ). Brain metastasis results in significant morbidity and mortality in NSCLC patients, so the prognoses of these patients are very poor. The overall survival after brain metastasis has occurred is around 1–4 months ( ).

Whole brain radiotherapy (WBRT) plays an important role in the treatment of NSCLC patients with brain metastasis ( ). The recently used regimen for WBRT is usually 30 Gy delivered in 10 fractions over 2 weeks. The effect of WBRT includes improvement of neurologic symptoms, such as headache, nausea or other neurologic deficits, and quality of life ( ). NSCLC patients with brain metastasis who received WBRT had median overall survival ranging from 3 to 6 months ( ). It is uncommon for WBRT to produce complications, particularly acute toxicity. When acute toxicities occur, they are often self-limited. Dementia is the most common late-delayed complication, and it was noted 4–36 months after treatment in approximately 10–15% of the patients who received WBRT ( ).

Epidermal growth factor receptor (EGFR), a transmembrane glycoprotein of the ErbB receptor family, controls cell proliferation, differentiation, anti-apoptosis, angiogenesis, and invasion ( ). After ligand binding, dimerization develops between HER receptors, which triggers tyrosine phosphorylation. Then, several intracellular signaling cascades are activated by EGFR, including: Ras/Raf/MEK/ERK, phophotidylinositol 3-kinase (PI3K)/AKT, Src tyrosine kinases, PLCγ, PKC, and STAT activation and downstream signaling ( ). NSCLC patients with tumors harboring EGFR mutations have a dramatic treatment response to the epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKI), gefitinib and erlotinib ( ). EGFR mutations are frequently associated with lung adenocarcinoma, females, never smokers, and East-Asian ethnicity. L858R mutation and a deletion in exon-19 (del-19) account for 90% of the EGFR mutations ( ).

Because lung adenocarcinoma frequently metastasizes to the brain and has a high EGFR mutation rate, interaction between brain metastasis, radiotherapy and EGFR mutation has been reported in preclinical and clinical studies. This may provide a new clinical treatment concept and modality. The more we understand the issue, the better care we can provide for our patients.

Brain Metastasis and EGFR Mutation

The brain is a frequent metastatic site of lung cancer, especially adenocarcinoma ( ). However, the mechanism by which lung cancer metastasizes to the brain is still unknown. For breast cancer, ErbB2 overexpression is associated with brain metastases and poor prognosis, and ErbB2 amplification is an important risk factor for brain metastasis in breast cancer patients ( ). Overexpression of ErbB2, specifically in lung adenocarcinoma, is correlated with poor prognosis and intrinsic chemoresistance ( ). ErbB2 may phosphorylate downstream molecules and activate PI-3 kinase/AKT pathways. These pathways are also activated in NSCLC with EGFR mutations and inhibited by EGFR-TKIs ( ). report that EGFR mutations were detected in 12 of 19 metastatic lung adenocarcinomas to the brain (63%) and 10 patients had del-19. The authors concluded that EGFR mutations frequently present in brain metastases ( ). EGFR mutations may play an important role in brain metastasis of lung cancer.

Preclinical Study and Mechanism

EGFR and Radiation-Induced DNA Damage

Radiation produces free radicals which cause DNA damage. The most lethal damage is a double-strand break (DSB), which produces chromosomal deletion, translocation, or disorganization ( ), and results in cell cycle arrest, apoptosis, or gene inactivation ( ). For radiation-induced DSB of DNA, there are two mechanisms for repair – homologous recombination and non-homologous end-joining (NHEJ) ( ). NHEJ is pivotal in DSB repair, while homologous recombination is the supportive pathway ( ). Proteins necessary for NHEJ include DNA-PK, DNA IV ligase IV, XRCC4 and Artemis. Among them, DNA-PK is a nuclear kinase, and contains three subunits, a DNA-PK catalytic subunit (DNA-PKc) and two regulatory subunits, Ku70 (70 kDa) and Ku80 (80 kDa). NHEJ is initiated from the binding of Ku70 and Ku80 to DNA breaken ends after radiation damage. They translocate inward along the DNA molecule by one helical turn. Then, DNA-PKcs are recruited and form a stable DNA-PKcs–DSB complex to prevent premature processing and ligation of the DNA ends. DNA-PKcs are phosphorylated and recruit several end-processing enzymes, for example, ligase IV, Rad50 Nbs1and Mre11 ( ). The complex then repairs radiation-induced DNA damage by joining the DNA ends.

EGFR plays an important role in determining radioresponse, and overexpression of EGFR has radioprotective activity. EGFR-mediated radioprotection could be divided into three phases. First, an immediate early phase of DNA repair involves EGFR internalization, phosphorylation and nuclear translocation ( ); then, suppression of DNA damage by induction of apoptosis and cell cycle arrest through the activation of phophotidylinositol 3-kinase and protein kinase B/AKT kinase ( ). Finally, activation of Ras/Raf/MEK/ERK and STAT pathways promotes cell proliferation and rapid repopulation ( ).

EGFR Mutation and Radiotherapy

Although a tumor with EGFR overexpression shows radioresistance, the EGFR mutation has been associated with enhanced radiosensitivity in cell lines and clinical practice ( ). NSCLC cell lines harboring deletions of E746 to A750 or L858R mutations present with a marked radiosensitive response to 1/500 to 1/1000 compared with wild-type EGFR NSCLCs. NSCLCs with the EGFR mutation show delayed DSB repair kinetics, and lack the ability to block DNA synthesis through the G1 cell cycle checkpoint after ionic radiation. Then, radiation to NSCLC cell lines with the EGFR mutation induces obvious apoptosis and development of micronuclei. Exogenous expression of L858R and deletion in E746-A750 in wild-type cell lines also significantly reduces clonogenic survival after radiation ( ).

In comparison with wild-type EGFR, NSCLC cell lines with EGFR mutations have defects in radiation-induced EGFR nuclear translocation, although the cell lines show high level of tyrosine phosphorylation after ionic radiation. Mutant EGFR fails to bind catalytic subunits (DNA-PKcs) and regulatory factors (Ku70 and Ku 80) of DNA-PK, which are the key enzymes of NHEJ. As a result, NSCLC cell lines with EGFR mutations lack the radioprotective function normally present ( ).

EGFR-TKIs Combined with Radiotherapy

Chemotherapy was considered to have a limited effect in the treatment of brain metastasis in NSCLC patients due to the blood–brain barrier (BBB) ( ). Recently, some animal studies suggest that the BBB is disrupted in brain metastasis because the brain metastatic lesions showed obvious enhancement on computed tomography (CT) or magnetic resonance imaging (MRI). However, intracranial drug concentration is still insufficient ( ). EGFR-TKI has low molecular weight and excellent cell penetration ( ). In addition, radiotherapy treatment may also disrupt the BBB ( ). Together, combination treatment with EGFR-TKI and WBRT may have a synergistic effect.

reported that gefitinib could enhance the antitumor activity of ionizing radiation. showed that combination treatment with gefitinib and radiation inhibited not only cell proliferative growth but also tumor angiogenesis. This is associated with cell cycle arrest and enhancement of radiation-induced apoptosis. Gefitinib radiosensitizes NSCLC cells by inhibiting ataxia telangiectasia mutated (ATM) activity and therefore inducing mitotic cell death ( ). Thus, combination treatment with EGFR-TKIs and WBRT may be the treatment of choice for NSCLC patients with brain metastasis.

Clinical study ( Table 11.1 )

EGFR Mutation and Radiotherapy

In clinical observation, retrospectively collected 63 NSCLC patients with brain metastasis who all received WBRT. Patients with EGFR mutations had higher response rates to WBRT compared with those with the wild-type (54% versus 24%; p =0.045). Both the administration of EGFR tyrosine kinase inhibitor ( p =0.034) and harboring the EGFR mutation ( p =0.029) were independently associated with response to WBRT. WBRT produces a significantly good response in NSCLC patients with brain metastasis harboring EGFR mutations. This is consistent with in vitro study of relative radiosensitivity in activating EGFR mutations ( ).

Table 11.1

Summary of Clinical Studies about WBRT and/or EGFR-TKIs for NSCLC Patients with Brain Metastasis

Study Design	Treatment (NSCLC patient number)	Race	AdenoCa (mutation patients)	Brain Tumor Response (%)	PFS	Median Survival
Phase II trial	Gefitinib ( n =41)	European	27 ^#	10	3	5
Retrospective	Gefitinib ( n =14) ^¶	East-Asian	12 ^¶	43
Phase II trial	Gefitinib ( n =40)	East-Asian	40	32	9	15
Retrospective	WBRT ( n =18)	East-Asian	18 (13)	39	—	11.7
	TKI+WBRT ( n =45)		45 (33)	67	—	22.3
			mutation(+):	54		17.3
Phase II trial	Gefitinib+WBRT ( n =21)	East-Asian	14	81	10	13
Retrospective	Erlotinib ( n =69)	European	47 (14)	26.4	2.9	4.3
			mutation(+):	82.4	11.7	12.9
Phase II trial	Gefitinib/erlotinib ( n =23) ^μ	East-Asian	23 (23) ^μ	83	6.6	15.9
Retrospective	Gefitinib ( n =45)	East-Asian	45 (5) *	26.7	4.17	23.4
	Gefitinib+WBRT ( n =45)		45 (7) *	64.4	7.12	14.83
					( p =0.001)	( p =0.002)