Oligonucleotide pharmacokinetics and delivery

Oligo administration during many clinical trials entails direct i.v. infusion, often over a course of several hours. S.c. and, in particular, intradermal administration is usually also associated with high bioavailability.

Oligos bind various serum proteins, including serum albumin (as well as a range of heparin-binding and other proteins that commonly occur on many cell surfaces). Targeting of naked oligos to specific cell types, therefore, is not possible. Following administration, these oligos tend to distribute rapidly to many tissues, with the highest proportion accumulating in the liver, kidney, bone marrow, skeletal muscle and skin. They do not appear to cross the blood-brain barrier. Binding to serum proteins provides a repository for these drugs and prevents rapid renal excretion.

The precise mechanism(s) by which oligos enter cells is not fully understood. Most are charged molecules, sometimes displaying a molecular mass of up to 10-12 kDa. Receptor-mediated endocytosis appears to be the most common mechanism by which charged oligos, such as phosphorothioates, enter most cells. One putative phosphorothioate receptor appears to consist of an 80 kDa surface protein, associated with a smaller 34 kDa membrane protein. However, this in itself seems to be an inefficient process, with only a small proportion of the administered drug eventually being transferred across the plasma membrane.

Uncharged oligos appear to enter the cell by passive diffusion, as well as possibly by endocytosis. However, elimination of the charges renders the resultant oligos relatively hydrophobic, thus generating additional difficulties with their synthesis and delivery.

Attempts to increase delivery of oligos into the cell mainly centre on the use of suitable carrier systems. Liposomes, as well as polymeric carriers (e.g. polylysine-based carriers), are gaining most attention in this regard. Details of such carriers have already been discussed earlier in this chapter.

An alternative system, which effectively results in the introduction of antisense oligonucleotides into the cell, entails application of gene therapy. In this case, a gene, which when transcribed yields (antisense) mRNA of appropriate nucleotide sequence, is introduced into the cell by a retroviral or other appropriate vector. This approach, as applied to the treatment of cancer and AIDS, is being appraised in a number of trials.

Oligos, including modified oligos, appear to be ultimately metabolized within the cell by the action of nucleases, particularly 3'-exonucleases. Breakdown metabolic products are then mainly excreted via the urinary route.

Even phosphorothioate oligos display serum and tissue half-lives of less than a day. As a consequence, continuous or frequent i.v. infusions are required for product administration. Some progress has been reported in the development of second-generation phosphorothioate oligos with improved pharmacokinetic characteristics. The most promising development entails the modification of the ribose sugar found in the repeat nucleotide structure. Attachment (at the T position) of methyl (—CH3) or methoxy ethyl (—CH2CH2OCH3) groups increases product stability, as well as product potency (by enhancing binding affinity for RNA). However, these changes also abrogate the product's ability to activate RNaseH, a primary mechanism of inducing its antisense effect. This, in turn, may be overcome by the more recent development of chimaeric phosphorothioate oligos, in which 2'-modified sugar nucleotides are placed only at the ends of the molecule, leaving a nuclease-compatible gap in the middle.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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