Knowledge of the fundamental pharmacokinetics of a drug in humans, such as its clearance, volume of distribution and absolute bioavailability can be extremely valuable in the drug development process. In order to obtained these parameters however, the drug must be administered intravenously (IV). This has been a major barrier in the past as expensive and time consuming toxicology and formulation has been necessary.
A new method has recently emerged whereby the drug is 14C labeled and given IV at a very low dose, contemporaneously with the extravascular administration. Plasma samples taken over time are analysed by LC-MS/MS to determine the total systemic drug concentration whilst the IV tracer dose is followed using Accelerator Mass Spectrometry (AMS). The very low dose ameliorates the need for IV safety toxicology and formulation and the contemporaneous dosing regimen removes any issues of concentration-dependent kinetics, which can be problematic with a conventional cross-over design.
The method was first proposed in the 1970s using stable isotopes but because of their natural abundance, there were limitations due to the lack of suitable assay sensitivity. 14C is a rare isotope (ca 0.0000000001%) and AMS is an extremely sensitive analytical technique being able to measure drug concentrations in the low femtogram range. This now makes the acquisition of IV pharmacokinetics in humans routinely available.
The technique has been applied to development drugs as well as those on the market for many years. One such example is fexofenadine which has been available for over a decade, but has never been previously administered to humans IV. Its absolute oral bioavailability is therefore currently unknown, although estimates have been made between 10 and 100%. 14C-Fexofenadine was administered as a 100 ug IV dose over a 120 mg oral dose and plasma samples were analysed for unchanged parent drug by AMS and HPLC-fluorescence. The absolute oral bioavailability of fexofenadine was determined as 35% in humans for the first time and insights were obtained regarding the biliary excretion of this widely used drug.
In another study, nelfinavir, a novel HIV protease inhibitor was administered orally (1250 mg) for 12 days along with 2 IV 14C-doses (1 mg) on days 1 and 11. The oral absolute bioavailability was determined and an assessment of changes of clearance due repeat dosing were examined.
A compellingly powerful technique has therefore emerged where the analytical powers of isotopic AMS analysis are combined with LC-MS/MS. |
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