Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 451

Linkage name Substituent (R)

O

O

CH 2 Phosphorothioate

-S -

BASE

O

Methylphosphonate

-CH 3

O R

P

Linkage

O

Methylphosphotriester

-O-CH 3

O

CH 2

BASE

Ethylphosphotriester

-O-CH 2 -CH 3

O

-NH-CH 3

Alkylphosphoramidate

Figure 14.15 Major types of modifi cation potentially made to an oligo's phosphodiester linkage in order to
increase their stability or enhance some other functional characteristic. The native phosphodiester link is
shown to the left

Some progress has been made to overcome such diffi culties, and continued progress in the area
is expected to render the next generation of oligos more therapeutically effective.
Native oligonucleotides display a 3´-5´ phosphodiester linkage in their backbone
(Figure 14.15). These are sensitive to a range of nucleases naturally present in most extracellular
fl uids and intracellular compartments. The half-life of native oligonucleotides in serum is only of
the order of 15 min, and oligoribonucleotides are less stable than oligodeoxynucleotides. Selec-
tive modifi cation of the native phosphodiester bond can render the product resistant to nuclease
degradation.
Modifi cation usually entails replacement of one of the free (non-bridging) oxygen atoms of
the phosphodiester linkage with an alternative atom or chemical group (Figure 14.15). Most
commonly, the oxygen has been replaced with a sulfur atom and the resultant phosphorothioates
display greatest clinical promise. Phosphorothioate-based oligos, 'S-oligos', display increased
resistance to nuclease attack while remaining water-soluble. They are also easy to synthesize
chemically and they display a biological half-life of several hours. Most antisense oligos cur-
rently assessed in clinical trials are S-oligos, as is the sole antisense agent approved for general
medical use to date (Vitravene, Box 14.3). Further modifi ed oligos may well improve product
pharmacokinetic and pharmacodynamic properties, as alluded to towards the end of the next
section.