Affibody-Drug Conjugates\nTargeting the Human Epidermal\nGrowth Factor Receptor‑3 Demonstrate Therapeutic Efficacy in\nMice Bearing Low Expressing Xenografts

The outcome of clinical trials evaluating drugs targeting\nthe human\nepidermal growth factor receptor 3 (HER3) has been poor, with primary\nconcerns related to lack of efficacy. HER3 is considered a difficult\ntarget since its overexpression on tumors is relatively low and there\nis normal expression in many different organs. However, a significant\nnumber of patients across different cancer indications have overexpression\nof HER3 and the development of novel modalities targeting HER3 is\ntherefore warranted. Here, we have investigated the properties of\naffibody-based drug conjugates targeting HER3. The HER3-targeting\naffibody molecule Z<sub>HER3</sub> was fused in a mono- and bivalent\nformat to an engineered albumin-binding domain (ABD) for <i>in\nvivo</i> half-life extension and was coupled to the cytotoxic\ndrug DM1 <i>via</i> a non-cleavable maleimidocaproyl (mc)\nlinker. <i>In vivo</i>, a moderate uptake was observed for\n[<sup>99m</sup>Tc]Tc-labeled Z<sub>HER3</sub>-ABD-Z<sub>HER3</sub>-mcDM1 in HER3 expressing BxPC3 tumors (3.5 ± 0.3%IA/g) at 24\nh after injection, and clearance was predominately renal-mediated.\nTreatment of mice with BxPC3 human pancreatic cancer xenografts showed\nthat a combination of Z<sub>HER3</sub>-ABD-Z<sub>HER3</sub>-mcDM1\nand its cytostatic analog Z<sub>HER3</sub>-ABD-Z<sub>HER3</sub> was\nefficacious and superior to treatment with only Z<sub>HER3</sub>-ABD-Z<sub>HER3</sub>, providing tumor growth inhibition and longer median survival\n(90 d) in comparison to monotherapy (68 d) and vehicle control (49\nd). Z<sub>HER3</sub>-ABD-Z<sub>HER3</sub>-mcDM1 was found to be a\npotent drug conjugate for the treatment of HER3-expressing tumors\nin mice.