Proteolysis targeting chimeras (PROTACs) have recently emerged as promising therapeutic agents for cancer therapy. However, their clinical application is considerably hindered by the poor membrane permeability and insufficient tumor distribution of PROTACs. Here we proposed a nanoengineered targeting strategy to construct a self-assembled affibody-PROTAC conjugate nanomedicine (APCN) for tumor-specific delivery of PROTACs. As proof of concept, a hydrophobic PROTAC MZ1 (a bromodomain-containing protein 4 degrader) was selected to couple with a hydrophilic affibody Z_HER2:342 (an affinity protein of human epidermal growth factor receptor 2, HER2) via a smart linker containing disulfide bond to form an amphiphilic Z_HER2:342-MZ1 conjugate. It spontaneously self-assembled into nanoparticles (Z_HER2:342-MZ1 APCN) in water. Upon the excellent targeting property of Z_HER2:342 and HER2 receptor-mediated endocytosis, Z_HER2:342-MZ1 APCN was accumulated in tumor sites and internalized by cancer cells effectively in vitro. Under the intracellular high level of glutathione (GSH), Z_HER2:342-MZ1 APCN released MZ1 to specifically degrade bromodomain-containing protein 4 (BRD4) and subsequently induced BRD4 deficiency-mediated apoptosis of cancer cells. By the tail-vein injection, Z_HER2:342-MZ1 APCN showed the outstanding tumor-specific targeting ability, drug accumulation capacity, enhanced BRD4 degradation and antitumor efficacy in vivo for an HER2-positive SKOV-3 tumor model. Such an affibody mediated nanoengineered strategy would facilitate the application of PROTACs for targeted cancer therapy.

Affibody is a new class of small non-immunoglobulin affinity proteins that possesses high affinity at the picomole level to several tumor overexpressed receptors. Owing to the simple framework, affibody is flexible for modification with payload, but the relatively low molecular weight of this construction simultaneously results in short half-life time which hinders its application in cancer therapy. In this work, we prepared a nanomedicine self-assembled from the conjugate of affibody (Z_{PDGFRβ:09591}, PDGFRβ: platelet-derived growth factor receptor β) with monomethyl auristatin E (MMAE) through cathepsin B cleavable dipeptide linker for targeted colorectal cancer therapy. The nanoscale characteristics of Z_{PDGFRβ:09591}-MMAE affibody-drug conjugate nanomedicine (Z_{PDGFRβ:09591}-M ADCN) resulted in enhanced pharmacokinetics, improved drug accumulation, and promoted biosecurity performance than those of free drugs. As a result, Z_{PDGFRβ:09591}-M ADCN exhibited excellent antitumor efficacy with tumor inhibition rates (TIR) over 99.0% in PDGFRβ-positive tumor models with small solid tumors (~ 150 mm³) or large established tumors (~ 500 mm³), indicating that Z_{PDGFRβ:09591}-MMAE ADCN is promising for the clinic application in colorectal cancer therapy.