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Plasma concentrations of drugs and drug metabolites in bioavailability studies are routinely bioassayed with high performance liquid chromatography (HPLC), gas chromatography (GC), and liquid chromatography coupled with tandem mass spectrometry (LC-MS-MS) methods. In the 1980s and 1990s, HPLC had achieved a relevant role compared to GC and other techniques in pharmacokinetic bioassays. However, in the last few years the LC-MS-MS technique, known as tandem mass spectrometry, has attained a predominant role over all other techniques. This is because it requires a less amount of matrix, possesses better specificity and sensitivity, and requires a shorter operating time. In the experience of the authors, LC-MS-MS in fact requires on average 3-4 fold shorter time to complete a bioequivalence study when performed by one operator than does HPLC. The higher cost of the apparatus and technical assistance of LC-MS-MS are fully compensated by the shorter operating time. Part or most of the HPLC apparatuses have been or are being replaced by LC-MS-MS systems in laboratories operating in clinical and pre-clinical pharmacokinetics and, to a large extent, in those involved in assessing permeability in screening procedures of new chemical entities. This paper analyses the growing development of the LC-MS-MS technique and compares four couples of methods validated with HPLC or GC versus LC-MS-MS, giving analytical details of the two approaches.

This paper reports the results of a pharmacokinetic study involving 24 healthy volunteers and designed to characterise the rate and extent of diclofenac absorption after the administration of a single dose of diclofenac (CAS 15307-86-5) potassium salt 50 mg in sachet (Voltfast) and tablet (Cataflam) formulations. Timed plasma concentrations of diclofenac during a 12-h-period after dosing were measured by means of HPLC with UV detection at 275 nm and a quantification limit of 10 ng/ml; the method was fully validated for pharmacokinetic purposes. These plasma concentrations were used to calculate Cmax, tmax, trapezoidal AUC0-t and AUC0-infinity and t1/2 by means of noncompartmental analysis. Cmax and tmax are the parameters expressing the rate of absorption, whereas the AUCs reflect the extent of absorption. The rate of absorption with the sachets proved to be very fast, reaching peak values at 10 min in seven subjects and at 15 min in the remaining subjects: mean time was 13.68 min, with concentrations at 5 min being 38% of Cmax. The average time to peak concentration with the tablets was 53.10 min. The extent of absorption of the sachets and tablets was similar, with AUC0-infinity values of respectively 1362 and 1214 ng.ml-1.h, and a 90% confidence interval 1.05-1.20. The highly soluble potassium salt of diclofenac was rapidly absorbed, especially in its sachet formulation, and thus appears to be an invaluable analgesic agent that is particularly useful for quick pain relief.

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