Affibody-Drug Conjugates Targeting the Human Epidermal Growth Factor Receptor-3 Demonstrate Therapeutic Efficacy in Mice Bearing Low Expressing Xenografts

The outcome of clinical trials evaluating drugs targeting the human epidermal growth factor receptor 3 (HER3) has been poor, with primary concerns related to lack of efficacy. HER3 is considered a difficult target since its overexpression on tumors is relatively low and there is normal expression in many different organs. However, a significant number of patients across different cancer indications have overexpression of HER3 and the development of novel modalities targeting HER3 is therefore warranted. Here, we have investigated the properties of affibody-based drug conjugates targeting HER3. The HER3-targeting affibody molecule Z_HER3 was fused in a mono- and bivalent format to an engineered albumin-binding domain (ABD) for in vivo half-life extension and was coupled to the cytotoxic drug DM1 via a non-cleavable maleimidocaproyl (mc) linker. In vivo, a moderate uptake was observed for [^99mTc]Tc-labeled Z_HER3-ABD-Z_HER3-mcDM1 in HER3 expressing BxPC3 tumors (3.5 ± 0.3%IA/g) at 24 h after injection, and clearance was predominately renal-mediated. Treatment of mice with BxPC3 human pancreatic cancer xenografts showed that a combination of Z_HER3-ABD-Z_HER3-mcDM1 and its cytostatic analog Z_HER3-ABD-Z_HER3 was efficacious and superior to treatment with only Z_HER3-ABD-Z_HER3, providing tumor growth inhibition and longer median survival (90 d) in comparison to monotherapy (68 d) and vehicle control (49 d). Z_HER3-ABD-Z_HER3-mcDM1 was found to be a potent drug conjugate for the treatment of HER3-expressing tumors in mice.