Herein we demonstrate that conjugation of a next generation maleimide (NGM) to engineered cysteines in a THIOMAB™ antibody delivers a THIOMAB™ antibody-drug conjugate (TDC) with a drug loading of ca. 2. This TDC is highly stable in blood serum conditions, selective and potent towards HER2 expressing cell lines and meets the current criteria for optimised antibody-drug conjugates (ADCs).Antibody-drug conjugates (ADCs) have emerged as targeted therapeutics against cancer by combining the selectivity of the antibody component with the potency of cytotoxic drugs. There are two FDA-approved ADCs already on the market (Adcetris™ and Kadcyla™) and over 40 additional ADCs currently undergoing clinical trials.
1-4Classical methods of conjugation for producing ADCs include lysine modication 5 and cysteine modication via reduction of solvent accessible disulde bonds followed by reaction with maleimides.6 It has been recognised that both methods generate heterogeneous mixtures of ADCs with different drug loadings and varied sites of attachment, resulting in sub-optimal therapeutic indices and higher clearance rates.7-13 Formation of higher drug-to-antibody ratio species (DAR > 4) have narrower therapeutic indices. 1,7,9,12,14 The latter method also results in the loss of structural disulde bonds which can lead to poor stability.
7,9,14The next generation of conjugation methods has aimed to reduce heterogeneity by focusing on site-selectivity. Chemoenzymatic methods have been used for ADC synthesis such as glycan modication, 15-17 glutamine amidation 18 and cysteine oxidation to aldehyde tags.19 Site-selective disulde modica-tion with small molecules has been achieved using nextgeneration maleimides (NGMs),
20-27 pyridazinediones (PDs)28-32 and bis-sulfones. 33,34 Site-specic reactivity has also been accomplished through incorporation of non-natural aminoacids into proteins for bioorthogonal conjugation.
10,35-39The THIOMAB™ antibody technology platform uses sitedirected mutagenesis to incorporate cysteines into the antibody. Conjugation to a classical maleimide bearing the drug of choice affords a THIOMAB™ antibody-drug conjugate (TDC) with full control over site reactivity and a precise drug-toantibody ratio (DAR).12,40-42 The technology also avoids issues with some chemoenzymatic methods that can result in over oxidation of aminoacids 43 and aldehyde tag conversion to unreactive gem-diols.44 TDCs have been demonstrated to improve therapeutic indices and pharmacodynamics over ADCs prepared by classical cysteine and lysine modication.
11,12,42Maleimides are rapid thiol-selective conjugation reagents.45,46 However, the succinimide conjugates thus formed can undergo retro-Michael reactions and scavenging with albumin, 47 resulting in premature drug release during blood circulation.10,41,48 N-Alkyl succinimide conjugates can be stabilised against retro-Michael reactions by hydrolysis to succinamic acid derivatives, 49-51 which resulted in ADCs with improved pharmacokinetics and in vivo efficacy. 41,51,52 Most Nalkyl t...