Are Antibody-drug conjugates really "magic bullets"?
People in my lab (Mark and Jay, I'm talking about you guys) have finally found my blog and were wondering why I haven't posted in a while. Well the only excuse I could come up with was to blame it on my controlled addiction to twitter. Typing out <150 characters is a lot easier than long(ish) blog posts. The wider (sciencier) audience makes it all more the more attractive. Then again, I am not going to completely give up on this blog yet. So here it is -
Recently, I had the pleasure to listen to John Lambert, CSO of Immunogen, a biotech leader in Antibody-drug conjugate (ADC) development. They already have a breast cancer, ADC drug (Kadcyla) in the market with the help of Genentech. I am going to let the following video explain the basic concept of why we want drugs conjugated to antibodies -
I wanted to point out a couple of not-so-eye-popping yet crucial factors that were not really clear to me, someone not working in this field.
Simple logic suggests that localized delivery of a cytotoxic drug to cancerous cells/tumors by conjugating it to a target-specific antibody (that may or may not already have intrinsic cytotoxic effect) is the pinnacle of drug therapy. But adding a drug molecule into the equation brings us face-to-face with a whole other monster - Pharmacokinetics!
Typical antibodies have the advantage of being made up of aminoacids just like any other protein in our body and their metabolism is therefore not a major concern during drug development. ADCs, on the other hand, have a cytotoxic drug that gets metabolized into products which requires developers to extensively study their toxicity and their kinetics.
You might wonder, Why is this so important? If these molecules are at the tumor site, and if the metabolic products are toxic, shouldn't they be all the more effective in killing the cancerous cells?
Well, the important thing to keep in mind is that almost all anti-cancer antibodies target specific proteins (HER2, EGFR, etc.) that are highly expressed on the cell-surface of cancer cells. But, these proteins are also expressed (in lower concentrations) on normal cells. Although these ADCs are considered to be "magic bullets", when administered, they move throughout the body (in our bloodstream), attach to both normal and cancer (predominantly) cells and hence can get metabolized throughout the body.
But it is not all gloom and doom for ADC therapies. Due to the targeting effect of the antibodies, the concentration of drug molecules required to have a viable effect is much lower than typical radiation/chemo therapies using the drug by itself. Therefore, they have a similar toxicity profile to that of the chemotoxic drugs, but generally better tolerated by patients.
This to me points us towards the next generation of antibodies that target cancer-exclusive proteins. Although, this is easier said than done. I am also not sure, if any truly exclusive proteins exist in tumour cells. The proteins with altered glycosylation and other post-translational modifications (of which there are quite a few of) might be a better target for developing cancer exclusive antibodies (or even lectins).