Tuesday, March 8, 2011

Monoclonal Antibody Therapies

[Note: this is both longer and more technical than most of my posts. I've written it for people who are really interested in the production of drugs like Benlysta. Everyone else should probably skip this one.]

            Humira. Remicade. Benlysta. The list of monoclonal antibody therapy is short, but it’s growing. There are two salient points about these medications that most people are aware of. The first is that when they work they work extremely well. The second is that they are unbelievably, extraordinarily expensive. So much so that some people are unsure that this class of drug is going to go far unless something changes. While I am not for a minute defending pharmaceutical companies’ marking their prices so high (especially in the States), I do think there are valid reasons for the shocking price-tag attached to monoclonal antibody therapies. To explain my reasoning on this, I think it’s necessary to go into how such therapies are developed. Let me say right off the bat that I do not know how companies are currently developing such drugs. But I do know how monoclonal antibodies are produced for research purposes, and I don’t think it’s too much of a stretch to say that the methods are probably somewhat similar.

Step 1: production of an antibody.

            Antibodies are small proteins that are developed by the body to fight off infection. Each antibody can only “grab” onto a very specific shape of molecule. This grabbing is done by the two flexible arms of the antibody. Each antibody has a different set of amino acids, which are the building blocks of any proteins, that make up its flexible arms. Unfortunately, even with our best prediction software we are only just beginning to understand how different amino acid sequences allow proteins to grab certain shapes of molecules. So you can’t just stick a target into a program and be given the exact sequence needed for an antibody that binds that target. Instead you have to find an immune system that will do the job for you.

            When making monoclonal antibodies for research, the first thing to do is to find your target. What molecule do you want to have your antibody grab onto and therefore block? In the case of an autoimmune disease, the idea is generally to block a protein that serves as a signal between different parts of the immune system. Just finding this molecule is a challenge, as the same signal might be used in different ways by different parts of the body. So you have to find a single signal that works in a very specific way.

            Let’s say you have a target molecule now. You know what signal in the immune system you want to block, and you know which protein is responsible for that signal. You also know that if you find an antibody that binds this protein, the protein will be prevented from being used as a signal. So the challenge now is finding that antibody. To do so, scientists will purify a large quantity of the target protein. They then inject that protein into animals of a different species. So a human protein, in the case of drug design, might be injected into rabbits. The idea behind this is that the rabbits’ immune systems will see the protein and say, “Hey, we don’t know what this is, but it isn’t one of our proteins!” The immune systems will think that the protein is coming from an invading pathogen (germ), and will start making random antibodies until they make one that binds the target protein. Theoretically. This works well on paper, but not always in the lab.

            Even if a rabbit’s immune system does make an antibody that binds to the human signal protein, the scientists still have to find the specific rabbit B cell that produces that antibody, which is really not a simple task. To find the B cell, a blood sample is taken from the rabbit. Then all of the B cells are removed from the rest of the blood using laser cytometry. Each and every B cell has to be tested until one is found that produces an antibody against the human protein. This task of separating and testing can take months or years. And even if a B cell is found that is producing antibodies that bind the target protein, the game isn’t over. A single B cell will die relatively quickly. In order to keep your B cell happy and producing antibodies, you have to fuse it to a cancer cell, which has the usual cell death signals switched off. If the two fuse than you have an “immortalized” B cell that will continue to live and produce antibodies.

Producing a Chimaera Antibody:

            Assuming you now have a specific and effective antibody that blocks the signal protein you want to take out, now you have to make it so that the human body doesn’t reject the rabbit antibody. Normally if you stick an antibody from another species into a human, the human’s immune system will think that the  antibody is foreign, and will try to take it out of the blood. So you have to take your very carefully prepared antibody, chop off the parts that are rabbit-specific, and replace those parts with human parts. This is why these therapies are called Chimaera antibodies; they have flexible regions from one species, and non-flexible regions from a human.

Testing:

Now comes testing. Years and years of testing. Not only does the antibody need to bind the target protein, it has to bind it in such a way that the target protein can no longer act as a signal in the immune system. Even more importantly, it is absolutely essential that the antibody not bind to ANYTHING else in the human body. If it binds something other than the target then it could cause damage similar to that found in an autoimmune disease. So years of pre-clinical, clinical and more clinical trials have to be performed. They have to answer first: “Is this drug safe or will it cause humans to go into anaphylactic shock? Only after the drug is found to be safe can the trials move on to address the question, “Does this drug actually work?”

Drug Delivery:

            If the drug is safe and if it works, then it can receive FDA approval. Even then the challenges aren’t over. Monoclonal antibodies are proteins. Proteins are very sensitive to heat or cold, and cannot be taken orally. If an antibody therapy were swallowed, the stomach acid would chew up the antibody before it had a chance to do any good. Also, if you heat up proteins they lose their shape and are worthless. If you freeze them the same thing may or may not happen. So shipping, storing, and delivering such drugs is a royal pain.

            As I said at the beginning, I don’t know if the above method is the exact one used by companies for producing monoclonal antibodies.  One major difference between the protocol described here and that used for Benlysta is that Benlysta is a fully human monoclonal antibody, whereas this protocol describes chimaera antibodies. (Some of the other antibody treatments are chimaera, though). I hope that this is at least close enough to give you an idea of how much work really goes into producing such a product. While monoclonal antibody therapies shouldn’t, in my opinion, run $30,000/year, I do believe that it is reasonable that they would cost significantly more than your average bottle of prednisone or methotrexate.

2 comments:

  1. Hello,

    Thanks for sharing your views about Monoclonal antibody therapies. These antibodies are the body's natural immune defense against invading pathogens. They are produced by specific cells of the immune system termed B cells...

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  2. Of course. Monoclonal antibody therapies are just harnessing antibodies artificially produced in order to hit specific receptors on B cells (or whatever the target is; I'm referring to Benlysta there). This post wasn't meant to explain the natural humoral response to pathogens, just to introduce the basic science behind monoclonal antibody therapies. It would be a great idea for me to do a post on the basics of B cells, humoral vs. cellular immunity, etc., though! (Guess who did her MS in immunology? and is a wee bit of a geek? :)

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