A bioanalytical investigation on the exquisitely strong in vitro potency of the EGFR-DNA targeting type II combi-molecule ZR2003 and its mitigated in vivo antitumour activity.

ZR2003 is a type II of combi-molecule designed to target DNA and the epidermal growth factor receptor (EGFR) without requirement for hydrolysis. In human tumour cell lines cultured as monolayers, it showed 6.5-35 fold greater activity than Iressa. Further evaluation in 3D organ-like multilayer aggregates showed that it could block proliferation at submicromolar level. However, despite the superior potency of ZR2003 over Iressa in vitro, its activity xenograft models was not significantly different from that of Iressa. To rationalize these results, we determined the tumour concentration of both ZR2003 and Iressa in vivo and more importantly in vitro in multicellular aggregates. The results showed that in A431 and 4T1 xenografts, the level of ZR2003 absorbed in the tumours were consistently 2-fold less than those generated by Iressa. Likewise, in the multicellular aggregates model, the penetration of ZR2003 was consistently lower than Iressa. In serum containing media, the level of extractable or free ZR2003 was also inferior to those of Iressa. The results from this bioanalytical study, suggest that the discrepancy between the in vitro and in vivo potency of ZR2003 when compared with Iressa, may be imputed to its significantly lower tumour concentration.