The epidermal growth factor receptor (EGFr) as a target for in situ radiation therapy

In situ radiation therapy traditionally involves the use of a monoclonal antibody (mAb) directed against a specific tumor-associated antigen and labeled with α-particle emitter such as 131-I. An alternative strategy is to use a low molecular weight peptide rather than a mAb as the carrier molecule. Also, recent evidence shows that radioactive elements that emit Auger electrons may be useful for inducing receptor/cell-specific cytotoxicity. Auger electrons provide low energy emissions (<10-20 keV). Although they have a short range in tissue (a few mm), Auger electrons have a high rate of energy deposition that is comparable to high linear energy transfer radiation such as -particles. Human epidermal growth factor (hEGF) is a natural peptide ligand for EGFr, which is frequently overexpressed in breast cancer. EGF is rapidly internalized and translocated to the cell nucleus following binding to EGFr. We are developing a strategy of EGF conjugated to an Auger electron-emitting radionuclide, 111-In, as a treatment for EGFr-overexpressing breast cancers. This strategy has several advantages over the mAb approach, as EGF is an endogenous peptide and should not be immunogenic. Also, its small molecular size should facilitate extravasation and tumor penetration. We have shown that 111In-hEGF is highly and selectively radiotoxic to MDA-MB-468 human breast cancer cells overexpressing EGFr but was not radiotoxic to MCF-7 breast cancer cells with a 100-fold lower level of EGFr expression. We have also demonstrated that 111-In-hEGF was greater than 80-fold more potent on a molar concentration basis at inhibiting the growth of MDA-MB-468 breast cancer cells than paclitaxel (IC50 70 pM vs. 6 nM respectively) and greater than 400-fold more potent than doxorubicin (IC50 20 nM). We have evaluated the therapeutic efficacy of 111-In-hEGF in athymic mice implanted subcutaneously with MDA-MB-468 breast cancer xenografts. Tumour growth was strongly inhibited following administration of five weekly doses (0.5 mCi per dose) of 111-In-hEGF. Based on these promising preclinical data, 111-In-hEGF has been formulated for human use and we have designed a clinical trial to test this agent in patients with metastatic breast cancer