Introduction: Although gefitinib (ZD1839; IRESSATM), a specific EGF-R tyrosine kinase inhibitor (TKI), has been shown to possess anti-tumour activity in a range of cancer types including androgen refractory prostate cancer, de novo and acquired resistance to the inhibitor has been reported clinically (Ranson, The Oncologist 2001; 58 (suppl 2A):114-22). In this study, we examined the involvement of the IGF1-R in the acquisition of resistance to gefitinib in the DU145 model of prostate cancer. Methods: The DU145 human prostate cancer cell line was continuously exposed to 1mM gefitinib. Alterations in signalling pathways were assessed using immunocytochemical, Western blotting and/or RTPCR techniques. Matrigel invasion assays were performed and cell proliferation was assessed by evaluating anchorage dependent growth. Cell sensitivity to the specific IGF1-R TKI, AG1024, (1-10mg/ml) was also measured. Results: Continuous exposure of the DU145 cells to 1mM gefitinib resulted in an approximate 60% initial growth inhibition after 10 days treatment. The surviving cells however, showed slow but steady increases in growth over the following 2 months, reaching a stable growth rate which far exceeded that of the parental cell line after a total of 3 months. This gefitinib-resistant subline designated DU145/TKI-R, demonstrated reduced EGF-R expression and negligible basal phosphorylated EGF-R activity. Compared with the parental DU145 cells, the DU145/TKI-R cells showed i) an increased production of IGFII mRNA, ii) elevated levels of both total and phosphorylated IGF1-R and PKCd, and iii) a marked increased sensitivity to growth inhibition by the IGF1-R inhibitor AG1024. Significantly, while 1mM gefitinib initially reduced the invasion of the DU145 cells through matrigel by 40%, the DU145/TKI-R cells overcame this inhibition. Furthermore, the migratory activity of the DU145/TKI-R cells was substantially reduced (45%) by treatment with AG1024 (10mM).\ud Conclusion: In prostate cancer cells, acquired resistance to gefitinib is associated with increased signalling through the IGF1-R, which also plays a role in the invasive capacity of the gefitinib-resistant phenotype. Thus, therapeutic strategies that target the IGF1-R may increase the efficacy and duration of response to gefitinib
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