**Hydrophilic Monomethyl Auristatin E Derivatives as Novel Candidates for the Design of Antibody-Drug Conjugates**

**Filip S. Ekholm 1,†, Suvi-Katriina Ruokonen 1,†, Marina Redón 1, Virve Pitkänen 2, Anja Vilkman 2, Juhani Saarinen 2, Jari Helin 2, Tero Satomaa 2,\* and Susanne K. Wiedmer 1,\***


Received: 28 October 2018; Accepted: 14 December 2018; Published: 24 December 2018

**Abstract:** Antibody-drug conjugates (ADCs) are promising state-of-the-art biopharmaceutical drugs for selective drug-delivery applications and the treatment of diseases such as cancer. The idea behind the ADC technology is remarkable as it combines the highly selective targeting capacity of monoclonal antibodies with the cancer-killing ability of potent cytotoxic agents. The continuous development of improved ADCs requires systematic studies on the nature and effects of warhead modification. Recently, we focused on the hydrophilic modification of monomethyl auristatin E (MMAE), the most widely used cytotoxic agen<sup>t</sup> in current clinical trial ADCs. Herein, we report on the use of micellar electrokinetic chromatography (MEKC) for studying the hydrophobic character of modified MMAE derivatives. Our data reveal a connection between the hydrophobicity of the modified warheads as free molecules and their cytotoxic activity. In addition, MMAE-trastuzumab ADCs were constructed and evaluated in preliminary cytotoxic assays.

**Keywords:** antibody-drug conjugate; biopharmaceutical; cytotoxicity; hydrophobicity; micellar electrokinetic chromatography
