Targeted Inhibition of PI3K and MEK in Combination With Radiation Therapy in UT15: A Murine Model of Head and Neck Squamous Cell Carcinoma

Abstract

To study the effectiveness of dual targeted PI3K and MEK inhibition alone and in combination with radiotherapy in HNSCC models as a novel clinical treatment strategy. MTT assays were used to establish growth inhibition and schedule of RT with Buparlisib (BKM120) a pan PI3K-inhibitor. Clonogenic survival assays were used to assess the impact of BKM120 alone, Binimetnib (MEK162), a MEK1/2-inhibitor, alone, and both drugs in combination. Subcutaneous xenografts were established in female NIH III HO mice using the UT15 cell line. Tumors grew to 200-400 mm3 before treatment: +BKM120 (10mg/kg oral gavage), +MEK162 (5mg/kg oral gavage), or combined +BKM120 (10mg/kg), +MEK162 (5mg/kg oral gavage) ± RT. In radiation treated cohorts, a sub-curative 45 Gy dose (3 Gy/day 5 days a week) was delivered 4 hours before drug treatment. The primary endpoint was tumor regrowth at 90 days post treatment. For UT15 cell line the IC50 concentration for BKM120 was 0.5 μM and for MEK162 was above 10 μM. Delivery of BKM120 (0.5 μM) 4 hours post radiotherapy produced the greatest reduction in cell viability compared to radiation alone 100% to 70%. Clonogenic survival assay confirmed UT15 to be more sensitive to MEK162 than BKM120 with survivals of 40% at 0.1 μM versus 90% at 0.2 μM, respectively. In the UT15 flank tumor model, RT alone, (BKM120 & MEK162), RT+ BKM120, RT+ MEK162, and RT+ (BKM120 & MEK162) had slower growth rates compared to controls (p=0.004). There was no difference in growth rates between the experimental arms. The time needed to reach twice normalized volume from the start of treatment is listed in Table 1. Western analysis confirmed the targeted agents were acting as expected based on their mechanism of action. When assessing tumor regrowth past the 90 days post-treatment it did appear that RT+ BKM120 cohort never surpasses the 3x normalized treatment volume raising the possibility of permanent change in tumor growth and viability. However, the standard error of the means (SEM) becomes increasing variable and strong conclusions cannot be drawn at this time. In previous experiments, there was a significant difference in growth rates between RT alone and RT+ MEK162 ± BKM120, primarily driven by the combination of RT and MEK162. In our UT15 model, radiation in combination with PI3K and/or MEK inhibition did not significant delay tumor growth rates. There was, however, a suggestion of permanent tumor change, in UT15, after 90 days in particular with PI3K blockade. This is contrary to our UT14 model where MEK inhibition produced the greatest delay in growth patterns. Further experiments with quantitative PCR and other agents are ongoing to determine if this difference is related to varying expression levels or differences in drug efficacy.Abstract 3377; Table 1Treatment cohortsMean Time to 2x volume (days)SEMControl12.11.05RT alone85.75.60BKM12060.33.44MEK16261.15.25BKM120 & MEK16275.911.7RT + BKM120988.78RT + MEK1621017.97RT + (BKM120 & MEK162)87.16.09 Open table in a new tab