Technology
For many of the known small intracellular messengers phosphate
groups are characteristic, for instance the inositol phosphates.
Under physiological conditions the phosphate groups are partially
ionized rendering the molecules sufficiently hydrophilic to prevent
them from leaking out of the cell they are generated in. This
fact is an important pre-requisite to permit individual responses
of single cells to external stimuli. Unfortunately, the same principle
applies when external doses of messenger molecules are applied
to cells in order to artificially stimulate signaling pathways.
These experiments scientists would love to do without disrupting
the plasma membrane, as it is unavoidable when electroporation,
saponification or microinjection techniques are used.
To circumvent this problem, unpolar masking group are frequently
employed in modern drug development. These groups should be stable
in the extracellular space but have to be designed in a way that
endogenous enzymes inside the cell deprotect the charged groups.
This approach is usually called a prodrug approach in Medicinal
Chemistry. A large amount of work has been done to increase the
efficacy and bioavailability of antiviral drugs (e.g. AZT) or
some penicillins. Membrane-permeant derivatives of cyclic nucleotide
signaling molecule like cAMP and cGMP showed enormous increases
in efficacy over the charged molecules and these compounds became
commercially available recently.
For inositol polyphosphates the task appeared to be particularly
challenging, due to the relative large number of phosphate groups
and the neighboring vicinal hydroxy groups. Scientists from the
groups of Roger Tsien and Carsten Schultz were successful in masking
the phosphates of many inositol polyphosphates and phosphoinositides
with acyloxymethyl ester groups and the hydroxy groups with butyrates.
The resulting fully protected derivatives were able to readily
penetrate plasma membranes and were shown to generate the original
molecules inside cells within minutes after administering the
compounds. The acyloxymethyl esters are supposed to be cleaved
much more readily than the butyrates and this should prevent scrambling
of the phosphates. The lipophilicity of the phosphate masking
group determines the overall lipophilicity of the prodrug molecule.
Higher membrane penetration and loss in solubility have to be
considered by chosing the optimal masking group. If there are
already highly lipophilic groups in the molecule as is the case
in phospholipids, acetoxymethyl esters on the phosphates will
suffice. For the unsubstituted inositol polyphosphates more lipophilic
groups like propionoxymethyl ester groups showed increased cellular
responses in several studies.
For more details, please refer to:
C. Schultz, Prodrugs of Biologically Active Phosphate Esters.
Bioorg. & Med. Chem. 11, 885-898 (2003)