A number of cancer-targeting peptide-drug conjugates (PDCs) have been explored as alternatives to antibody drug conjugates (ADCs) for targeted cancer therapy. However, the much shorter circulation half-life of PDCs compared with ADCs in vivo has limited their therapeutic value and thus their translation into the clinic, highlighting the need to develop new approaches for extending the half-life of PDCs. Here, we report a new strategy for targeted cancer therapy of a PDC based on a molecular hybrid between an antihapten antibody and a hapten-labeled PDC. An anticotinine antibody (Ab(cot)) was used as a model antihapten antibody. The anticancer drug SN38 was linked to a cotinine-labeled aptide specific to extra domain B of fibronectin (cot-APT(EDB)), yielding the model PDC, cot-APT(EDB)-SN38. The cotinine-labeled PDC showed specific binding to and cytotoxicity toward an EDB-overexpressing human glioblastoma cell line (U87MG) and also formed a hybrid complex (HC) with Ab(cot) in situ, designated HC[cot-APT(EDB)-SN38/Ab(cot)]. In glioblastoma-bearing mice, in situ HC[cot-APT(EDB)-SN38/Ab(cot)] significantly extended the circulation half-life of cot-APT(EDB)-SN38 in blood, and it enhanced accumulation and penetration within the tumor and, ultimately, inhibition of tumor growth. These findings suggest that the present platform holds promise as a new, targeted delivery strategy for PDCs in anticancer therapy.