Correlate dose with pharmacology or toxicity.

The lack of phase1 drug metabolism in the liver radically alters the pharmacokinetics of compounds administered to HRN™ mice. The total exposure of compounds is greatly increased as the rate of metabolism and hence clearance is reduced. An example of this is shown below which shows the serum concentration of midazolam in both wild type (green dots) and HRN™ (red dots) mice. It is clear that both the maximum plasma concentration and the area under the curve (AUC) is greatly increased in the HRN™mouse compared to control mice following a single oral administration of the drug (2.5mg/Kg).  The increased peak concentration and duration of exposure to a compound is useful in determining the in vivo efficacy and/or toxicity of compounds.

Pharmacokinetics of midazolam in normal and HRN mice.                                                                        Plasma concentration of midazolam following single oral administration  (2.5mg/Kg) to normal (green dots) and HRN™ (red dots) mice. Both the maximum concentration and total exposure (area under the curve) are dramatically increased in the HRN™.

Determine site of metabolism

The pharmacokinetics of midazolam also demonstrates that the HRN™ can be used to determine whether the compound is metabolised in the liver. As the shape of the PK curve in the above figure does not ‘level out’ at a particular steady state concentration (as would be the case with pentobarbital), it demonstrates there must be metabolism and clearance of the compound. As there is no CYP activity in the liver, this demonstrates that this compound is metabolised in other tissues. In this case it is known that midazolam is metabolised by CYP3A4 located in the GI tract, but this unique feature of the HRN™ model can enable a rapid determination of the site of metabolism.

Determine optimal route of administration

The ability of the HRN™ to distinguish between different sites of metabolism enables it to be used as an elegant tool to determine the optimal route of administration of compounds. An example of this is where the pharmacokinetics of a test compound in development as an oral medicament was measured in normal and HRN™ mice. When the compound was administered orally there was no difference in the pharmacokinetics between normal and HRN™ mice. However, when the compound was administered intravenously there was a profound difference in the peak plasma level and total exposure. Taken together these experiments demonstrate that the compound is being metabolised in both the liver and GI tract, but that the metabolism in the GI tract is significant, and is the primary site of metabolism if the drug is administered orally. An oral route of administration is therefore not advisable for this compound, as the majority of the compound would be broken down before it reached the bloodstream. Studies comparing routes of administration in both HRN™ and normal mice can deliver useful results that can determine the optimal route of administration.