Lapatinib in combination with radiation diminishes tumor regrowth in HER2+ and basal-like/EGFR+ breast tumor xenografts

Abstract

Purpose: To determine whether lapatinib, a dual epidermal growth factor receptor (EGFR)/HER2 kinase inhibitor, can radiosensitize EGFR+ or HER2+ breast cancer xenografts.

Methods and materials: Mice bearing xenografts of basal-like/EGFR+ SUM149 and HER2+ SUM225 breast cancer cells were treated with lapatinib and fractionated radiotherapy and tumor growth inhibition correlated with alterations in ERK1 and AKT activation by immunohistochemistry.

Results: Basal-like/EGFR+ SUM149 breast cancer tumors were completely resistant to treatment with lapatinib alone but highly growth impaired with lapatinib plus radiotherapy, exhibiting an enhancement ratio average of 2.75 and a fractional tumor product ratio average of 2.20 during the study period. In contrast, HER2+ SUM225 breast cancer tumors were highly responsive to treatment with lapatinib alone and yielded a relatively lower enhancement ratio average of 1.25 during the study period with lapatinib plus radiotherapy. Durable tumor control in the HER2+ SUM225 model was more effective with the combination treatment than either lapatinib or radiotherapy alone. Immunohistochemical analyses demonstrated that radiosensitization by lapatinib correlated with ERK1/2 inhibition in the EGFR+ SUM149 model and with AKT inhibition in the HER2+ SUM225 model.

Conclusion: Our data suggest that lapatinib combined with fractionated radiotherapy may be useful against EGFR+ and HER2+ breast cancers and that inhibition of downstream signaling to ERK1/2 and AKT correlates with sensitization in EGFR+ and HER2+ cells, respectively.

Fig. 1

Lapatinib-mediated radiosensitization of SUM149 basal-like/epidermal growth factor receptor-positive (EGFR+) breast cancer xenografts. (A) SUM149 tumors were grown to a tumor diameter of 10 mm, treated with lapatinib (30 or 100 mg/kg twice daily at 6-h intervals) or vehicle for total of five treatments within 2.5 days. EGFR was immunoprecipitated and analyzed by Western blotting with anti-phosphotyrosine antiserum. (B) Tumors grown to 100 mm³ and treated with either lapatinib (100 mg/kg twice daily) or vehicle for 10 days starting at Day –10 and radiotherapy at 2 Gy/fraction administered for 3 consecutive days starting at Day –4. Tumor volume changes were normalized to baseline (Day –10) and plotted over time for each treatment group. C = vehicle control; L = lapatinib; R = radiotherapy; L+R = lapatinib plus radiotherapy. (C) Tumor growth rates = the slopes of growth curves during study duration for each treatment group. (D) Synergy assessment = the ratio of expected/observed average fractional tumor volumes (FTV) during study duration for lapatinib plus radiotherapy.

Fig. 2

Lapatinib-mediated radiosensitization of SUM225 HER2+ breast cancer xenografts. (A) Tumors were treated as described in Fig. 1, and tumor volume changes normalized to baseline (Day –10) and plotted over time for each treatment group. C = vehicle control; L = lapatinib; R = radiotherapy; L+R = lapatinib plus radiotherapy. (B) Tumor growth rates = the slopes of growth curves for study duration for each treatment group.

Fig. 3

Radiosensitization by lapatinib correlates with inhibition of ERK1/2 in EGFR+/basal-like cells and with AKT in HER2+ breast cancer cells. (A) Tumors from basal-like/EGFR+ SUM149 xenografts were processed for immunohistochemistry with phosphorylated ERK1/2 antiserum and quantified from mice treated with lapatinib, radiotherapy, lapatinib plus radiotherapy, or vehicle control. (B) Sample immunohistochemistry staining of SUM149 tumors with phosphorylated ERK1/2 serum at 400×. Similarly, tumors from HER2+ SUM225 xenografts were processed for immunohistochemistry with phosphorylated AKT antiserum and (C) quantified. (D) Sample immunohistochemistry of SUM225 tumors with phosphorylated AKT serum at 400×. Open arrows indicate areas of increased staining; solid arrows, areas of reduced staining.