Endocrine Abstracts

Tyrosine kinase inhibitors have previously been shown to inhibit DNA synthesis in human pituitary adenomas (Jones et al JCEM 1997; 82: 2143-7). ZD1839 ('Iressa'), a selective inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase, is currently undergoing clinical trials in human cancers. We have investigated the effects of ZD1839 and EGF in the human pituitary cell line HP75 (derived by SV40 transfection of a human pituitary silent gonadotrophinoma) and in surgically removed human pituitary adenomas. Cells were incubated with EGF, with and without ZD1839, for 72 hours and cell growth was assessed using Alamar blue. EGF (20 ng/ml) produced no significant effect on growth of HP75 cells. ZD1839 inhibited growth with an IC (sub)50(/sub) of 10 micromolar; high doses induced cell death. In the human pituitary adenomas, mixed responses to ZD1839 were observed: some tumours had a similar response to the HP75 cells but others showed no response, even at high doses (50 micromolar). The lack of EGF response and impaired IC50 for ZD1839 by HP75 compared with cells known to have wild-type EGFR led us to look for a truncated form of the EGFR. EGFRvIII loses part of its extracellular domain, is unable to bind EGF and is constitutively activated. We have identified EGFRvIII in HP75 cells using a specific antibody (clone DH8.3) for this truncated form. Staining was predominantly granular and cytoplasmic with some membranous staining. NR6M cells, transfected to express EGFRvIII only, were used as the positive control. In culture, NR6M cells responded to ZD1839, demonstrating an IC(sub)50(/sub) of 15 micromolar.

We have identified, for the first time, the presence of the EGFRvIII in human pituitary cells, which may account for the high ZD1839 IC50 and absence of EGF stimulation of growth in HP75 cells.

'Iressa' is a trademark of the AstraZeneca group of companies