(549e) Isolation of Linker Intermediates for Antibody-Drug Conjugates By Magnesium Complexation

Due to properties of polyethylene glycols (PEGs), the modification of small molecules with PEG-chains can improve water solubility, stability, and oral bioavailability for pharmaceutical applications.¹ The synthetic route of a PEG4-containing linker, used in antibody-drug conjugates (ADCs), required a 7-step telescoped reaction with multiple chromatographic purifications because of the inability to crystallize or precipitate the PEG-containing molecules. Additionally, an intermediate of the linker had high viscosity, which hindered unit operations for material feeding and transferring upon process scale-up and prevented isolation of the stable intermediate in solid form to enable effective storage and transportation. To overcome these challenges, a new process was developed upon previous research, showing the formation of solid complexes from compounds that contain PEG-chains and acid functionalities by chelation of magnesium chloride (MgCl₂).^{2, 3} However, the initial attempts to apply MgCl₂ complexation to this particular intermediate resulted in severe stickiness of the complex, which prevented practical application of the technique for process scale-up.

Herein, we utilized fundamental knowledge of chemical engineering to understand the complexation and to improve physical properties of the ‘intermediate-MgCl₂ complex’ (Fig.1a) for the development of a scalable process. Analysis of reaction kinetics revealed that the particle size of MgCl₂ (Fig.1b) and reaction temperature are the key parameters to complete the complexation without formation of the sticky crust, which inhibits recovery of the complex from the reactor. As a result, the use of milled MgCl₂ improved the physical properties of the solid complex and led a successful 100 L-scale process at 93 ± 2 % of recovery. Furthermore, various analytical technologies were used to elucidate the molecular structure of the complex. Elemental analysis, NMR DOSY experiments, and direct-infusion mass spectrometry supported the presence of complexes mixture with molar ratios of 1:1, 1:2 (majority), and 2:3 of the PEG4-containing intermediate to magnesium (Fig.1c). Accordingly, this isolated complex served as a stable intermediate with improved chemical purity and material handling, resulting in the enhanced scalability of the production process and the effective transportation of the material into the downstream processes. In downstream processes, pH-adjustment of the complex and extraction allow for the removal of MgCl₂ from the intermediate to continue the synthesis of the ADC linker.

References

(1) Turecek, P. L.; Bossard, M. J.; Schoetens, F.; Ivens, I. A. PEGylation of Biopharmaceuticals: A Review of Chemistry and Nonclinical Safety Information of Approved Drugs. Journal of Pharmaceutical Sciences 2016, 105 (2), 460-475.

(2) Noto, V. D.; Münchow, V.; Vittadello, M.; Collet, J. C.; Lavina, S. Synthesis and characterization of lithium and magnesium complexes based on [EDTA][PEG400]2 and [EDTA]3[PEG400]7. Macromolecular Chemistry and Physics 2002, 203 (9), 1211-1227.

(3) Zheng, B.; Li, J.; Pathirana, C.; Qiu, S.; Schmidt, M. A.; Eastgate, M. D. Complexation of Polyethyleneglycol Containing Small Molecules with Magnesium Chloride as a Purification and Isolation Strategy. Organic Process Research & Development 2021, 25 (10), 2270-2276.