Liposomal delivery
PharmTech:
When the liposomally packaged siRNA drug enters the cell, how is it activated at the appropriate time to perform the appropriate
function? What features of the liposome enable the drug to travel to the appropriate place in the body?
Tracy (Alnylam):
Once LNPs cross into the cell, they are at first contained within an endosomal vesicle. The cationic or ionizeable lipid
component of the LNP is designed to promote disruption of the endosomal membrane to promote release of the siRNA into the
cytoplasm of the cell. Once the siRNA is in the cytoplasm, it is able to associate with the cellular RNAi machinery to enable
RNA interference for the desired target.
Houston (MDRNA):
DiLA2-based liposomes are designed to carefully balance several properties to enable successful delivery of the siRNA in the cytoplasm.
Key properties include: the ability to interact with a cell to initiate internalization, or endosomal uptake; fusion with
the endosome to enable endosomal escape; and release of the siRNA in the cytoplasm. These features can be designed into a
specific delivery component(s), or they can be engineered into the liposome structure. Interaction with a cell surface leading
to endosomal internalization is characteristically driven by nonspecific properties and is primarily manifested by a slight
cationic charge on the liposome surface. Endosomal release is typically achieved by fusion of the liposome with the endosomal
membrane, and requires pH-responsive characteristics. The interaction of the liposome and the endosomal membrane, in response
to the pH gradient in the endosome, is usually based on the pKa of the liposome components and can be designed into the DiLA2 molecule. Ultimate release within the cytoplasm is also achieved by charge properties, again the pKa, that allows for neutrality
and loss of the electrostatic attraction between a positively charged component and the negatively charged siRNA.
On the horizon
PharmTech:
Looking forward, do you foresee having the ability to target various tissues and areas of the body?
Tracy (Alnylam):
Yes. We and others have made significant progress over the last few years in developing delivery systems for siRNA and in
understanding mechanisms of siRNA delivery. The progress made in the field in a short time has given us great optimism that
we will continue to significantly broaden the target cells and tissues addressable by RNAi delivery technology.
Houston (MDRNA):
The ability to target organs besides the liver via systemic delivery is a central goal for MDRNA. Given the nature of the liver to filter out particles from the systemic circulation,
it will be a matter of biasing delivery away from the liver and into other organs. Physical/chemical characteristics such
as size and apparent charge, and careful choice of targeting ligand may allow for this biasing and effective distribution
to other tissue.
For our bladder program, dosing occurs via direct instillation into the bladder; this is more of a topical delivery approach with the intent to localize and maintain
the siRNA within the bladder tissue. The goal for local delivery to other sites such as lung or skin, would focus on maximal
uptake at the site of administration, and the formulation would need to be tailored to provide the characteristics most appropriate
for the target cell type.
Thompson (Quark):
Our near-term, newer programs focus on opportunities where we can obtain effective delivery of the siRNAs without complex
formulations. We are concentrating on disease indications such as the lung, eye, and kidney where we can get effective delivery
and activity of these molecules without any sort of complicated formulation. We are, however, looking at formulations, and
we will consider those further downstream, but at the moment it is more cost-effective (i.e., from a cost-of-goods perspective)
to use nonformulated forms of the drug. Looking forward, we at Quark are working on noninvasive routes of administration (e.g.,
inhalation or topical) so that patients can self-administer the drug without involving injection.
|
