While in Colorado, I had a chance to hear a remarkable talk by Dr. Peter Senter of Seattle Genetics. In recent years, this company has made quite a splash in the area of antibody/drug candidates (ADCs). One of their more recent accomplishments is FDA-approved brentuximab vedotin (Adcetris) for relapsed Hodgkin lymphoma and anaplastic large cell lymphoma.
If you have any interest in molecules that are toxic, the ADC concept is quite enabling. The idea is that the toxic “payload” is linked to an antibody, and thus specificity is “outsourced” as the antibody part is engineered to have high affinity for the target cell of interest. If a cancer cell is the target, the ADC binds to its cell surface, followed by internalization and payload release as the ADC undergoes degradation inside the cell. As you might imagine, a lot of effort goes into perfecting the payload/antibody conjugation chemistry. The corresponding reactions are fairly simple and typically rely on covalent cysteine modification. The key parameter is temporary stability of the conjugate as one does not want the toxic component to leak out prematurely. One of the recent Nature Biotechnology papers by Seattle Genetics describes a cool solution to the problem of undesired retro-Michael addition of payloads from antibody-drug conjugates. It turns out that planting an amine in the vicinity of the maleimide makes imide hydrolysis way faster. The retro-Michael reaction of the opened form is in turn significantly slower. I like this work because it is not often that I can trace back the inner workings of powerful technology to simple and teachable physical organic chemistry.
I have to note that I do not fully agree with the mechanistic statement made by the authors. They propose that the amine is there to catalyze water addition: “These results are consistent with an intramolecular catalysis mechanism in which the proximal amine promotes the attack of the succinimide carbonyl group by water”. In my view, this is almost certainly not the reason for the observed effect. Unless there is something unusual with the kinetics of imide hydrolysis, the more likely explanation is faster collapse of the tetrahedral intermediate when the amine is placed nearby.
A similar example is a primary or secondary amino group near a disulfide, significantly accelerating the S-S cleavage under neutral conditions, including the reduction and and S-S interchange. In that case, my guess is that RS-NR2 species are involved. This happens only when the amine is 2 or 3 methylenes away, if you acylate the amino or put it farther away, you get quite normal stable disulfide bond.
I think you are right – it is nucleophilic catalysis. Thanks for bringing this up. Good example!
It is interesting that Seattle Genetics has been focusing on -SH and not going for -NH2. The latter has been the favorite target for Immunogen. I was wondering if Seattle Genetics has made any development on homogeneity front.
Not sure. Peter Senter (he is their VP of chemistry) is going to call me tomorrow to discuss various aspects, I can ask.