Targeted delivery of methotrexate to epidermal growth factor receptor-positive brain tumors by means of cetuximab (IMC-C225) dendrimer bioconjugates

We have constructed a drug delivery vehicle that targets the epidermal growth factor receptor (EGFR) and its mutant isoform EGFRvIII. The monoclonal antibody, cetuximab, previously known as C225, which binds to both EGFR and EGFRvIII, was covalently linked via its Fc region to a fifth-generation (G5) polyamidoamine dendrimer containing the cytotoxic drug methotrexate. As measured by mass spectrometry and UV/vis spectroscopy, the resulting bioconjugate, designated C225-G5-MTX, contained 12.6 molecules of methotrexate per unit of dendrimer. Specific binding and cytotoxicity of the bioconjugate was evaluated against the EGFR-expressing rat glioma cell line F98(EGFR), Using a competitive binding assay, it was shown that the bioconjugate retained its affinity for F98(EGFR) cells, with a 0.8 log unit reduction in its EC50. Only cetuximab completely inhibited binding of the bioconjugate, which was unaffected by methotrexate or dendrimer. Cetuximab alone was not cytotoxic to F98(EGFR) cells at the concentration tested, whereas the IC50 of the bioconjugate was 220 nmol/L, which was a 2.7 log unit decrease in toxicity over that of free methotrexate. The biodistribution of C225-G5-MTX in rats bearing i.c. implants of either F98(EGFR) or F98(WT) gliomas was determined 24 hours following convection enhanced delivery of I-125-labeled bioconjugate. At this time, 62.9 +/- 14.7% ID/g tumor was localized in rats bearing F98(EGFR) gliomas versus 11.3 +/- 3.6% ID/g tumor in animals bearing F98(WT) gliomas, thereby showing specific molecular targeting of the tumor. The corresponding radioactivity of normal brain from the F98(EGFR) tumor-bearing right and non-tumor-bearing left cerebral hemisphere were 5.8 +/- 3.4% and 0.8 +/- 0.6% ID/g, respectively. Based on these results, therapy studies were initiated in F98(EGFR) glioma-bearing rats. Animals that received C225-G5-MTX, cetuximab, or free methotrexate had median survival times of 15, 17, and 19.5 days, respectively, which were not statistically different from each other or untreated control animals. Our results, which are both positive and negative, show that specific molecular targeting is but one of several requirements that must be fulfilled if an antibody-drug bioconjugate will be therapeutically useful.