Bioavailability studies of drugs with nonlinear pharmacokinetics: II. Absolute bioavailability of intravenous phenytoin prodrug at therapeutic phenytoin serum concentrations determined by double-stable isotope technique

A sensitive HPLC-FLD method for the quantification of alpelisib, a novel phosphatidylinositol 3-kinase inhibitor, in rat plasma: Drug metabolism and pharmacokinetic evaluation in vitro and in vivo

Alpelisib, a novel phosphatidylinositol 3-kinase inhibitor, is an oral anticancer agent approved for the treatment of advanced or metastatic breast cancer. In this study, a sensitive bioanalytical method using high-performance liquid chromatography combined with a fluorescence detector (HPLC-FLD) was developed for the determination of alpelisib in rat plasma. This newly developed method was validated in terms of linearity (1-1,000 ng/mL), precision, accuracy, recovery, matrix effect, and stability according to the US Food and Drug Administration guideline and these parameters were within the acceptable limits. Alpelisib tended to be stable in plasma, urine, simulated intestinal fluid, and buffer with pH > 4.0 for 24 h, but in the pH 1.2 buffer and simulated gastric fluid for up to 4 h only. A study involving intravenous administration of alpelisib in rats showed that the dose-normalized area under the plasma concentration versus time curve (AUC) of alpelisib changed significantly as the dose increased from 1 to 10 mg/kg. Similarly, an oral rat study indicated that the dose-normalized AUC and the fraction of dose that remained in the gastrointestinal (GI) tract changed significantly as the dose increased from 0.5 to 10 mg/kg. These nonlinear (dose-dependent) pharmacokinetics of intravenous and oral alpelisib could be attributed to the saturation of ubiquitous metabolism among most tissues and/or GI absorption processes. To the best of our knowledge, this is the first study to investigate the in vivo nonlinear pharmacokinetics of alpelisib and its possible mechanisms, together with a new HPLC-FLD method to determine alpelisib in biological matrices.

Bioavailability of intravenous fosphenytoin sodium in healthy Japanese volunteers

To compare and evaluate the bioavailability for intravenous fosphenytoin sodium with that of intravenous phenytoin sodium in Japanese subjects. In study 1, healthy Japanese male volunteers received a 30-min infusion of 375 mg fosphenytoin sodium or an equimolar dose of 250 mg phenytoin by a double-blind, crossover method. In study 2, other healthy Japanese male volunteers received a 30-min or 10-min infusion of 563 mg fosphenytoin sodium, followed by a dose of 750 mg after 2 weeks in an unblinded manner. Comparing with 250 mg phenytoin sodium, 375 mg fosphenytoin sodium exhibited lower total plasma phenytoin C max, whereas the geometric mean ratio of the AUC of total and free phenyotoin for fosphenytoin sodium at a dose of 375 mg was very similar to phenytoin sodium at a equimolar dose of 250 mg (AUC0-t ratio: 0.98 and 1.02, respectively). Therefore, fosphenytoin is almost completely converted to phenytoin in subjects. Fosphenytoin sodium was rapidly converted to phenytoin at doses of 375, 563, and 750 mg. The maximum concentration (C max) of total plasma phenytoin increased in a dose-dependent manner. The area under the plasma concentration-time curve (AUC) increased slightly more than proportionally with the administered dose, and clearance (CL) decreased with increasing dose. Pain and other infusion-site reactions were reported by all 12 subjects with phenytoin sodium, whereas very few symptoms were observed with fosphenytoin sodium. In conclusion, fosphenytoin sodium is considered to be a useful substitute for phenytoin sodium with almost no associated injection-site reactions.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

Assessing the bioequivalence of analogues of endogenous substances ('endogenous drugs'): considerations to optimize study design

BACKGROUND

Assessment of the bioequivalence of generic versions of certain reference drugs is complicated by the presence of endogenous levels of said compounds which cannot be distinguished from externally derived compound levels following drug administration. If unaccounted for, the presence of endogenous compound biases towards equivalence in bioequivalence studies of these drugs. Bioequivalence assessments may be complicated further as disposition of the exogenous analogue can be subject to various endogenous processes resulting in nonlinear pharmacokinetics. To overcome these inherent biases a number of different strategies have been employed.

AIMS

To critically review methods used to overcome confounding biases in bioequivalence studies of 'endogenous' drugs.

METHODS

A literature search of the EMBASE and PubMed databases was performed.

RESULTS

The following strategies were identified: ablation/modulation of baseline endogenous substance levels; recruitment of 'substance-deficient' populations; restriction of dietary intake of the relevant substance; standardization of conditions with the potential to affect relevant homeostatic mechanisms; correction for baseline substance levels; and administration of supra-therapeutic drug doses.

CONCLUSIONS

On the basis of this review key study design concepts, intended to optimize the design of future bioequivalence studies of these so-called 'endogenous drugs', are described. The dual stable isotope method, which could be used in a specific context, is also discussed.

The use of isotopes in the determination of absolute bioavailability of drugs in humans

Absolute bioavailability studies in humans are not routinely performed as part of the drug registration process. They tend to be reasonably demanding, not least because toxicology data are required to support intravenous administration of a drug. Moreover, the classical crossover design of an absolute bioavailability study can suffer from artefacts caused by concentration-dependent pharmacokinetics. Many of the problems associated with absolute bioavailability studies can be alleviated using isotopically labelled drugs. Stable isotopes have been used in the performance of absolute bioavailability studies in humans for > 30 years. More recently, the advantages of using radiolabelled drugs have been expanded by using the ultrasensitive technology of accelerator mass spectrometry. Isotopic labelling not only allows for the accurate and efficient determination of absolute bioavailability, but can also provide information on first-pass effects and other pharmacokinetic parameters.

Fosphenytoin: clinical pharmacokinetics and comparative advantages in the acute treatment of seizures

Fosphenytoin is a phosphate ester prodrug developed as an alternative to intravenous phenytoin for acute treatment of seizures. Advantages include more convenient and rapid intravenous administration, availability for intramuscular injection, and low potential for adverse local reactions at injection sites. Drawbacks include the occurrence of transient paraesthesias and pruritus at rapid infusion rates, and cost. Fosphenytoin is highly bound (93-98%) to plasma proteins. Saturable binding at higher plasma concentrations accounts for an increase in its distribution volume and clearance with increasing dose and infusion rate. Fosphenytoin is entirely eliminated through metabolism to phenytoin by blood and tissue phosphatases. The bioavailability of the derived phenytoin relative to intravenous phenytoin is approximately 100% following intravenous or intramuscular administration. The half-life for conversion of fosphenytoin to phenytoin ranges from 7-15 minutes. Faster intravenous infusion rates and competitive displacement of derived phenytoin from plasma protein binding sites by fosphenytoin compensate for the expected conversion-related delay in appearance of phenytoin in the plasma. Unbound phenytoin plasma concentrations achieved with intravenous fosphenytoin loading doses of 100-150 or 50-100mg phenytoin sodium equivalents/min are comparable, and achieved at similar times, to those with equimolar doses of intravenous phenytoin at 50 (maximum recommended rate) or 20-40 mg/min, respectively. The rapid achievement of effective concentrations permits the use of fosphenytoin in emergency situations, such as status epilepticus. Following intramuscular administration, therapeutic phenytoin plasma concentrations are observed within 30 minutes and maximum plasma concentrations occur at approximately 30 minutes for fosphenytoin and at 2-4 hours for derived phenytoin. Plasma concentration profiles for fosphenytoin and total and unbound phenytoin in infants and children closely approximate those in adults following intravenous or intramuscular fosphenytoin at comparable doses and infusion rates. Earlier and higher unbound phenytoin plasma concentrations, and thus an increase in systemic adverse effects, may occur following intravenous fosphenytoin loading doses in patients with a decreased ability to bind fosphenytoin and phenytoin (renal or hepatic disease, hypoalbuminaemia, the elderly). Close monitoring and reduction in the infusion rate by 25-50% are recommended when intravenous loading doses of fosphenytoin are administered in these patients. The potential exists for clinically significant interactions when fosphenytoin is coadministered with other highly protein bound drugs. The pharmacokinetic properties of fosphenytoin permit the drug to serve as a well tolerated and effective alternative to parenteral phenytoin in the emergency and non-emergency management of acute seizures in children and adults.