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Biologics Elimination – How Does Structural Modification Change it?

Let’s talk about chimerization as a part of structural modification. And you have seen this slide before. Here is the immunoglobulin structure of the antibody the constant region if we trim it down to a smaller size, or we can use it one fragment or we can simply use the variable region of the antibody structure. That is to say, that we reduce the structure. Make the size small. So that the antibody will not be detected by the host system. So that they get to stay in the host system for longer to bring out bring about the effect. But chimerization does little to anything else, except modifying the structure of the antibody to reduce immunogenicity. Now pegylation does more than chimerization.
It is a covalent bounding of PEG, polyethylene glycol to the protein or to the peptide. By doing so, it shields the antigenic or immunogenic epitopes. Epitopes mean the locals or the site. It also shields the receptor-mediated uptake by the RES system that is the reticulo-endothelial system which is a major site for phagocytosis. It prevents degradation by proteolytic enzymes. It increases the molecular size of the polypeptide, and therefore reduces the renal filtration. Polyethylene glycol being a polyalcohol also increased water solubility, making it less adsorbent to the container wall.
Overall, it prolongs the half-life of the biologics. Let’s look at one example of pegylation. Here y axis the PT. PT is the prothrombin time that measures the effect. If you were to look at the native urokinase, urokinase is a biologics as well. If you look at the native urokinase, concentrations are… I mean the prothrombin times or kind of flat. However, if you were to look at the pegylated urokinase, you can see the prothrombin times are up there, at least for the first four hours. Another example for pegylation, or to look at the effect of pegylation, here you have percent of the dose injected through this is the IP route.
By the way, this is gallium now gallium is not a protein. However, this illustrate the effect of pegylation very well also. So IV administration for the free gallium and for the liposomal preparation of gallium, but none pegylated. After pegylation, the liposomal product then you can tell percent of the IV dose recovered is way up there. So this tells you that pegylation does a good job to the recovery of the dose after IV. This is a list of pegylated products. And I will not talk about these products here but wait until the very last lecture of this miniseries. However, I will simply look at two products.
One is Neulasta, which is granulocyte colony-stimulating factor, and pegasys, which is an interferon alpha product for the treatment of hepatitis. Now Pegasys, you can even tell by the name that it is a pegylated product. The other approach to structure modification is glycosylation. Glycosylation is the covalent bounding of carbohydrate or the glycans to the protein molecule. By doing so, it stabilized the protein molecule. It decreases circulatory turnover. It increases absorption, improves distribution and decreases clearance rate. And again, overall effect is a prolongation of half-life so that the biologics would stay in the system for longer to bring out the effect. And this is a list of glycosylated products.
Again I will not talk about this individual product until the very last lecture. What you simply look at, Darbepoetin alfa. Now this is an epotent, prolonged epotent, which is derived or extracted from the Chinese hamster over itself through recombinant technology. And this one is glycosylated at the nitrogen position. And by doing so, the half-life is prolonged by three times. In fact, let’s look at this graph to show the effect of glycosylation. Here is the concentration, and this is the epoetin without glycosylation. And the top curve is glycosylated epoetin. And as you can tell the half-life increased by about three times.
And therefore, we will be able to reduce the dosing frequency from two to three times a week to once a week subcutaneous injection. So that is a tremendous benefit for the patient, because of reduced dosing frequency.

Small molecule drugs are primarily cleared either through hepatic metabolism or renal excretion. In contrast, biologics are eliminated mainly via intracellular proteolytic degradation which occurs throughout the body, or receptor mediated clearance. Renal excretion is largely limited due to the molecular size, however, with certain exceptions. The other elimination pathway is through the formation of anti-drug antibodies (ADA), i.e., immunogenic response against the biologic that trigger proteolytic elimination via the reticuloendothelial system (RES). Structural modifications are undertaken to impede elimination by chimerization, glycosylation or pegylation.

Following SC absorption and distribution, biologics are subsequently eliminated by metabolism or excretion. Biologics are generally characterized with a short half-life; therefore, prolongation of residence time in the host system is desired for therapeutic benefit. This can be achieved through structural modifications such as chimerization, pegylation and glycosylation.

Chimerization works through reduction of immunogenic response, while the other two mechanisms involve reduced immunogenicity and protected proteolytic metabolism. The most notable pegylation example is Neulasta (pegylated filgrastim), with 10 times increase in half-life. Aranesp (glycosylated epoetin) has a three times increase in half-life. The beneficial effects on reduced frequency of administration and better patient compliance for these biologics cannot be overstated.

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Pharmacotherapy: Understanding Biotechnology Products

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