With three Antibody Drug Conjugates (ADCs) currently on the market, and more than 40 in clinical trials, there is much interest in research and investment towards these rapidly growing, promising cancer treatments. This was clearly evident at this year’s American Association of Cancer Research (AACR) meeting held April 1 – 5 in Washington DC, where a large number of posters and oral sessions focused on the advancement and innovation of antibody-drug conjugates.
During the session “From Chemistry to the Clinic: Part 3: Advances in Antibody Drug Conjugates,” held Saturday, April 1, 2017, Patrick J. Burke, PhD., Principal Scientist at Seattle Genetics presented the company’s development work towards increasing potency of ADCs with a monomethyl auristatin E (MMAE) payload through higher drug loading by use of a polyethylene glycol (PEG)ylated glucuronide linker system.
Today, the majority of antibody-drug conjugates in development include functionalities that are hydrophobic. This results in increased plasma clearance. As a result, there has been significant interest in generating new linkers that compensate for this potential liability.
Burke detailed some of the difficulties with current MMAE linker technologies, and how their lead PEGylated glucuronide-MMAE linker that uses a self-stabilizing maleimide and a PEG12 side chain is addressing these issues, potentially enabling highly potent and homogenous ADCs.
While the linker technology that has enabled antibody-drug conjugates like gemtuzumab ozogamicin (Mylotarg®; Pfizer), brentuximab vedotin (Adcentris®; Seattle Genetics) and ado-trastuzumab emtansine (Kadcyla®; Genentech/Roche), and has been pivotal in establishing ADCs as viable treatment options, there are many limitations to these technologies.
The conjugation of the drug payload to residues like lysine and cytosine, for example, leads to a high degree of heterogeneity in the product. Additionally, these ADCs are hydrophobic in nature, and this hydrophobicity can have a negative impact on the pKa of the conjugates, resulting in limited number of drugs per antibody, which is often capped at a drug-to-antibody ratio of 4 (DAR4) or less. There is also a limited set of chemical functions with the groups that are currently being used to attach a cleavable type of linker system. 
PEGylated glucuronide-MMAE linker
Burke described three ongoing projects at Seattle Genetics designed to solve these issues, most notable of which was their efforts to increase ADC potency with the PEGylated glucuronide-MMAE linker system that can enable a higher DAR. With current vcMMAE ADCs, conjugating more copies of vcMMAE to an antibody is more potent in vitro, but the results are not the same in vivo, where higher loaded conjugates are cleared more rapidly. With this new linker system, it is possible to harness more potency as well as create more homogenous ADCs in vivo.
To accomplish this, the glucuronide-MMAE linker system incorporates a carboxylic acid trinol motif that makes it more hydrophilic. Alone, this linker isn’t enough to make DAR8 ADCs, which is why Seattle Genetics has incorporated a PEG polymer. The PEG was incorporated in a side chain configuration was found to be ideal, and different PEG sizes (2, 8, 12, and 24) were tested for any changes in efficiency.
Burke presented these data comparing PEGylated and non-PEGylated constructs as well as PEG size efficacy. As expected, the constructs that lacked PEG cleared most rapidly and severely decreased exposure, while an increase in exposure was seen in a stepwise fashion up until a PEG of 8. In tolerability studies with the same constructs at a dose of 15mg/kg, only animal groups that were given PEG sizes sufficient to preserve the pharmacokinetics (PK) properties of the conjugates survived, whereas survival was not seen in animals dosed with insufficient PEG.
Importantly, this linker was then compared to vcMMAE in an experiment looking at a CD90+ model. This data shows that when both constructs were loaded with their ideal DAR (4 for vcMMAE and 8 for the PEGylated glucuronide-MMAE linker), animals treated with 3mg/kg of vcMMAE saw a brief tumor delay, while those given PEGylated glucuronide –MMAE conjugate at the same dose saw a complete remission. When the glucuronide-MMAE linker was given at lower dose of 1mg/kg, the results were similar to the 3mg/kg of vcMMAE, leading to the conclusion that PEGylated glucuronide –MMAE conjugates show a three-fold potency increase when compared to vcMMAE. 
This presentation confirmed previous work presented during the 2014 annual meeting of the AACR. Work presented at this meeting demonstrated that incorporation of a discrete PEG24 polymer into the cleavable β-glucuronide-MMAE linker system either increased or decreased the plasma clearance of the resulting antibody-drug conjugate. This result depended on the configuration of the PEG. For example, when inserted into the drug-linker as a stretcher unit between the maleimide and the cleavage site, PEG24 elicited increased ADC plasma clearance. In contrast, incorporation of the PEG24 as a side chain from a modified lysine residue adjacent to the maleimide resulted in ADCs with slower clearance.
Moving forward, a PEG of up to 12 with this side chain configuration has proven to be ideal for SGNC48A, an ADC that is targeted toward CD48, and which is expected to go from preclinical to Phase I clinical trials this year. Since CD48 is present on 90% of multiple myeloma cancer cells, this technology has the potential to make a huge impact in this difficult to treat patient population. Clinical trials on SGNC48A will be updated in our ADC drug map as progress goes on.