LAUSR

Background IGF-1R is overexpressed in a substantial number of breast cancer cases. When phosphorylated, IGF-1R activates the PI3K and MAPK pathways. In preclinical studies, IGF-1R overexpression has been described to be associated with tamoxifen resistance. Expression of activated IGF-1R has not been established as a marker for endocrine resistance in the clinic. We tested the value of p-IGF-1R to predict adjuvant tamoxifen benefit in IGF-1R-positive tumors from ER+ breast cancer patients and its possible association with PI3K/MAPK activation. We also carried out cell line experiments to illustrate a potential causal link between IGF-1R activation status, PI3K/MAPK pathway activation and tamoxifen resistance. Methods Primary tumor blocks from 563 ER+ (stage I-III) postmenopausal patients previously randomized between tamoxifen (1 to 3 years) vs. no adjuvant therapy (IKA trial 1982-1993; Beelen et al, Breast Cancer Res, 2014) were recollected. Immunohistochemistry scoring on tissue microarrays included PTEN, p-IGF-1R, p-AKT(Thr308), p-AKT(Ser473) and p-p70S6K(Thr389) by cytoplasmic intensity (0-3), of p-4EBP1(Ser65) and p-ERK1/2(Thr202/204) by percentage of tumor cells with positive nuclear staining and of p-S6RP(Ser235/236) by percentage of tumor cells with positive membranous staining. Informative data on p-IGF-1R staining was available for 364 IGF-1R-positive tumors. Multivariate Cox models including standard prognostic factors were used to assess hazard ratios (HR) for recurrence-free interval for tamoxifen treatment, p-IGF-1R status (negative vs. positive) and their interaction. MCF-7 and T47D cell lines were used to validate the clinical findings. IncuCyte cell proliferation experiments were performed with various IGF-1R-related growth stimulating and inhibiting conditions. Western blots were carried out on cells under various growth conditions to analyze whether activation of IGF-1R would be associated with PI3K/MAPK pathway activation. Results Patients having tumors without p-IGF-1R expression derived significant benefit from tamoxifen (HR 0.4290, 95% CI 0.2356 - 0.7813; p = 0.00566), while those having tumors with p-IGF-1R expression had no benefit (HR 0.8051, 95% CI 0.2643 – 2.453; p = 0.70). The p for interaction was not significant ( p = 0.32106). Tumors positive for p-IGF-1R had more expression of (phospho)proteins downstream of the PI3K/MAPK pathways. These results were supported by Western blots from the cell lines examined under various growth conditions. In both cell lines, linsitinib (a dual IGF-1R and insulin receptor inhibitor) was able to block IGF-1R signaling, preventing activation of the PI3K and MAPK pathways and abrogating cell proliferation in the presence of tamoxifen. Conclusions Postmenopausal breast cancer patients with p-IGF-1R-positive tumors appear to derive no benefit from adjuvant tamoxifen. Tumors with p-IGF-1R expression were associated with PI3K/MAPK pathway activation. In breast cancer cell lines with activated IGF-1R signaling, addition of linsitinib to endocrine therapy can restore sensitivity. This combination might be an interesting treatment option for tamoxifen-resistant patients. Citation Format: Kruger DT, Alexi X, Opdam M, Schuurman KG, Sanders J, van der Noort V, Boven E, Zwart W, Linn SC. Phosphorylation of insulin-like growth factor-1 receptor (IGF-1R) is associated with tamoxifen resistance by activating the PI3K/MAPK pathway [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-08-06.