Process Development and GMP Production of a Conjugate Warhead: Auristatin F-HPA-Ala/TFA (XMT-1864/TFA)

An efficient, large-scale manufacturing process is described for XMT-1864/TFA (1-TFA), an auristatin F derivative, used as a novel, highly potent, cytotoxic warhead in Mersana's oncology antibody–drug conjugate platforms. The process achieves high diastereomeric purity and controls the impurities with all intermediates readily isolated by crystallization or precipitation in high yield and purity. Protecting groups were selected to ensure tolerability, scalability, and stability of the intermediates under various solution-phase peptide coupling conditions. Crystallization of the final product was developed to remove specified impurities and provide a high-purity active warhead molecule for use in the bioconjugation processing. The convergent synthesis involving six non-GMP steps and five GMP steps has been carried out in multiple cGMP productions on 1-kg scale to produce 1-TFA in >98% chemical purity and <1% total diastereomeric contamination with ∼50% overall yield for the GMP steps.