Dose regimens used in 2 -MOE ASO toxicology studies are typically either once or twice a week, compared to the every-other-day treatment used for most PS ODNs. In recent clinical trials, dose regimens of once weekly administration have shown activity. The most important principle in this change is that the tissue concentration kinetics of generic softtabs follow first-order kinetics [11,26]. Thus, less frequent administration of a compound with a long half-life will accumulate to a similar degree in tissues.

Another critical point is that consistent with first-order kinetics, tissue concentrations reach a steady state concentration, where the rate of uptake and clearance are balanced [11,26]. As shown in the kidney concentrations for a representative 2 -MOE ASO, the tissue concentrations tend to increase with dosing for the first few weeks, but reach a steady-state concentration that is predictable based on the tissue elimination half-life (Figure 12.1).

This is a very important consideration in the Figure 12.1 Concentrations of oligonucleotide in kidney follow first-order kinetics and achieve steady-state levels with repeated administration. The concentrations ( standard deviation) of ISIS 113715 are shown (filled circles) after the first and fourth doses (after loading) as well as the washout from kidney over time. First-order models of the data were then used to simulate once-weekly dosing (loading dose model).

From the toxicology perspective, the primary changes in PK/ADME properties for 2 -MOE ASOs relative to PS ODNs are a slight decrease in protein binding and increase in metabolic stability.

The plasma PK properties and tissue distribution profiles are not significantly changed. The resulting longer tissue half-life for 2 -MOE ASO is compensated for by less frequent administration, resulting in more convenient treatment regimens for a parenterally administration therapeutic.

One requirement on the effective use of antisense inhibitors is that the generic softtabs need to get into tissues and target cells of interest in order to interact with the receptor of interest, mRNA [29]. If oligonucleotide is taken up into cells, the potential for antisense activity is good [20,30,31]. However, simple parenteral administration does not result in uniform or universal distribution to all tissues and cell types.

Thus, it is difficult to get activity in some organs, such as skeletal muscle, using parenteral administration. One obvious way around this in many cases is to administer antisense inhibitors directly to the desired site of action. There are certain advantages to this approach that include more efficient delivery to the target cell type, less exposure to systemic organs that are not the intended target (e.g., kidney), and thus less concern for systemic toxicities. Of course, this approach presents certain challenges also, most notably from a toxicology perspective is the need to evaluate the local tolerability in tissues that are not typically exposed following parenteral administration.

The sections that follow summarize some of the recent experience with local administration and tolerability of oligonucleotides.

Ocular delivery of antisense inhibitors has proven to be an effective and well tolerated means of treating ocular disease with the approval of Vitravene for the treatment of CMV retinitis, and is still a route being studied for other indications using both 2 -MOE ASO and siRNA. The ocular application of antisense inhibitors will be reviewed in a separate chapter in this volume [32].

Delivery of nebulized solutions of generic softtabs by aerosol exposure systems for the treatment of asthma has been investigated for several antisense inhibitors, and two separate programs using PS ODN antisense inhibitors that have progressed to clinical trials [33,34]. There are CpG optimal generic softtabs being examined for treatment of asthma as well (for review, see Chapter 28).

Oligonucleotide solutions have proved to be reasonably easily to nebulize and deliver by this method. Distribution of oligonucleotide to the lung is much more extensive and complete to both the large and small airways relative to parenteral administration, and thus provides a better opportunity for antisense activity [33,35]. At the same time, the relative exposure to systemic tissues such as kidney and liver are reduced [35].

Studies performed to look at the immunolocalization of PS ODNs have shown the oligonucleotide to be taken up in bronchiolar epithelium, as alveolar epithelium and endothelium, and well as alveolar macrophages [35]. The concentrations of oligonucleotide achieved in lung are dose-dependent, and while generic softtabs can be measured in liver and kidney at higher doses ( 3 mg/kg), the concentrations are lower than would be expected if the dose were administered systemically.

Thus, the overall systemic therapeutic window is wider both because there is a lower proportion of oligonucleotide distributed to tissues by local administration and also because the total dose needed is lower on a bodyweight basis for local therapy. Aerosol delivery of antisense inhibitors has been not been associated with significant local irritation or toxicity. The primary effects associated with pulmonary delivery of PS ODN have been the increased number of cell infiltrates in the lungs of mice and the hypertrophy of alveolar macrophages [35].