The clinical efficacy of epidermal growth factor receptor (EGFR)-targeted inhibitors is limited due to resistance mechanisms of the tumor such as activation of compensatory pathways. Crosstalk between EGFR and insulin-like growth factor 1 (IGF-1R) signaling has been frequently described to be involved in tumor proliferation and resistance. One of the attractive features of nanomedicines is the possibility to codeliver agents that inhibit different molecular targets in one nanocarrier system, thereby strengthening the antitumor effects of the individual agents. Additionally, exposure to healthy tissues and related unwanted side-effects can be reduced. To this end, we have recently developed anti-EGFR nanobody (Nb)-liposomes loaded with the anti-IGF-1R kinase inhibitor AG538, which showed promising antiproliferative effects in vitro. In the present study, we have further evaluated the potential of this dual-active nanomedicine in vitro and for the first time in vivo. As intended, the nanomedicine inhibited EGFR and IGF-1R signaling and subsequent activation of downstream cell proliferation and survival pathways. The degree of inhibition induced by the nanomedicine on a molecular level correlated with cytotoxicity in tumor cell proliferation assays and may even be predictive of the response to nanomedicine treatment in tumor xenograft models. Combination therapy with kinase inhibitor-loaded Nb-liposomes is therefore an appealing strategy for inhibiting the proliferation of tumors that are highly dependent on EGFR and IGF-1R signaling.