A combined accelerator mass spectrometry-positron emission tomography human microdose study with 14C- and 11C-labelled verapamil

BACKGROUND AND OBJECTIVE

In microdose studies, the pharmacokinetic profile of a drug in blood after administration of a dose up to 100 μg is measured with sensitive analytical techniques, such as accelerator mass spectrometry (AMS). As most drugs exert their effect in tissue rather than blood, methodology is needed for extending pharmacokinetic analysis to different tissue compartments. In the present study, we combined, for the first time, AMS analysis with positron emission tomography (PET) in order to determine the pharmacokinetic profile of the model drug verapamil in plasma and brain of humans. In order to assess pharmacokinetic dose linearity of verapamil, data were acquired and compared after administration of an intravenous microdose and after an intravenous microdose administered concomitantly with an oral therapeutic dose.

METHODS

Six healthy male subjects received an intravenous microdose [0.05 mg] (period 1) and an intravenous microdose administered concomitantly with an oral therapeutic dose [80 mg] of verapamil (period 2) in a randomized, crossover, two-period study design. The intravenous dose was a mixture of (R/S)-[14C]verapamil and (R)-[11C]verapamil and the oral dose was unlabelled racaemic verapamil. Brain distribution of radioactivity was measured with PET whereas plasma pharmacokinetics of (R)- and (S)-verapamil were determined with AMS. PET data were analysed by pharmacokinetic modelling to estimate the rate constants for transfer (k) of radioactivity across the blood-brain barrier.

RESULTS

Most pharmacokinetic parameters of (R)- and (S)-verapamil as well as parameters describing exchange of radioactivity between plasma and brain (influx rate constant [K(1)] = 0.030 ± 0.003 and 0.031 ± 0.005 mL/mL/min and efflux rate constant [k(2)] = 0.099 ± 0.006 and 0.095 ± 0.008 min-1 for period 1 and 2, respectively) were not statistically different between the two periods although there was a trend for nonlinear pharmacokinetics for the (R)-enantiomer. On the other hand, all pharmacokinetic parameters (except for the terminal elimination half-life [t1/2;)]) differed significantly between the (R)- and (S)-enantiomers for both periods. The maximum plasma concentration (C(max)), area under the plasma concentration-time curve (AUC) from 0 to 24 hours (AUC(24)) and AUC from time zero to infinity (AUC(∞)) were higher and the total clearance (CL), volume of distribution (V(d)) and volume of distribution at steady state (V(ss)) were lower for the (R)- than for the (S)-enantiomer.

CONCLUSION

Combining AMS and PET microdosing allows long-term pharmacokinetic data along with information on drug tissue distribution to be acquired in the same subjects thus making it a promising approach to maximize data output from a single clinical study.

An AMS method to determine analyte recovery from pharmacokinetic studies with concomitant extravascular and intravenous administration

The absolute bioavailability, clearance and volume of distribution of a drug can be investigated by administering a very low dose of the (14)C-drug intravenously along with a therapeutic nonlabeled dose by the extravascular route (typically orally). The total drug concentration is measured by an assay such as LC-MS and the (14)C-drug is measured by accelerator MS (AMS). In another article in this issue, a method validation is proposed where AMS was used as the analytical assay. Part of the validation is to assess the recovery of the analyte being measured as this has a direct impact on its quantification. In this article, a method of internal standardisation is described where the UV response of the nonlabeled analyte, spiked in excess into the matrix being analysed, is used for internal standardization. The method allows for the recovery of analyte to be measured in each individual sample being analysed. It is important to know the recovery of a (14)C-labeled analyte when determining its mass concentration from (14)C:(12)C isotopic ratio data using AMS. A method is reported in this article that utilizes the UV response of the nonlabeled drug for internal standardization, so that the recovery for each individual sample analyzed can be ascertained.

Utilization of DBS within drug discovery: a simple 2D-LC–MS/MS system to minimize blood- and paper-based matrix effects from FTA elute™ DBS

Background: Dried blood spot-based bioanalysis potentially introduces novel matrix effects that need to be eliminated or controlled. Within nonregulatory drug discovery these can be defined as ≤20% and ≤30% for nominal peak area, respectively. Results: Controlling matrix effects for a panel of compounds by simple 1D-HPLC–MS/MS was not achievable and the optimization of 2D-HPLC–MS/MS is reported here. Simple inclusion of a ‘trapping’ stage was not sufficient to improve matrix effects and optimization of the reconstitution solvent, reconstitution volume and injection volume was required for a generic system to be developed. Conclusion: A generic 2D-LC–MS/MS system has been developed that eliminates paper-based matrix effects and eliminates or controls dried blood spot matrix effects for a panel of compounds extracted from FTA Elute™ with methanol.