Bioequivalence of endogenous substances facing homeostatic equilibria: an example with potassium

Bioequivalence for highly variable drugs: regulatory agreements, disagreements, and harmonization

Regulatory authorities introduced procedures in the last decade for evaluating the bioequivalence (BE) for highly variable drugs. These approaches are similar in principle but differ in details. For example, the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) recommend differing regulatory constants. The constant suggested by FDA results in discontinuity of the BE limits around the switching variation at 30% observed within-subject variation of the reference product. The regulatory constant of EMA does not have these problems. The Type I error reaches 6-7% around the switching variation with the EMA constant but 16-17% with the FDA constant. Various procedures were recently suggested, especially for the EMA approach, to eliminate the inflation of the Type I error. Notably, the so-called Exact algorithms try to amalgamate the positive features of both EMA and FDA procedures without their negative sides. The computational procedure for the EMA approach is simple and has a straightforward interpretation. The procedure for the FDA approach is based on an approximation, has a bias at small degrees of freedom, and requires a suitable computer program. All regulatory agencies impose a second requirement constraining the point estimate of the ratio of geometric means. In addition, EMA and Health Canada impose an upper limit for applying the recommended procedures. These expectations have psychological motivation and political rationale but no scientific foundations. Their inclusion results in incorrect and misleading interpretation of the principal criterion which involves confidence intervals. Different regulatory authorities expect to apply their approaches either to both AUC and C_max or only to AUC or only to C_max. Rational resolution of the disharmonization is needed.

Synergic development of pharmacokinetics and bioanalytical methods as support of pharmaceutical research

The development of pharmacokinetics led this science to achieve a relevant role in the investigation of new chemical entities for therapeutic application, and has allowed a series of new useful realizations of out of patent drugs like prolonged release and delayed release formulations, therapeutic delivery system (TDS) for drugs to be active in systemic circulation avoiding the first pass effect, orodispersible and effervescent formulations, intramuscular and subcutaneous depot formulations acting over a long period, oral inhalatory systems, and drug association at fixed dose. The above applications had pharmacokinetics as protagonist and have required the support from bioanalytical methods to assay drug concentrations, even in pg·mL(-1) of plasma, that really have paralleled the synergic development of pharmacokinetics.The complexity of the above realizations required specific guidelines from the regulatory authorities, mainly the US FDA and EU EMA, which have normalized and, in most cases, simplified the above applications admitting some waivers of in vivo bioequivalence.However, this review highlights some critical points, not yet focused on by operating guidelines, which need to be clarified by regulatory authorities. One of the most relevant issues is about the planning and conducting bioavailability and bioequivalence trials with endogenous substances, that possess own homeostatic equilibria with fluctuations, in some cases with specific rhythms, like melatonin and female sex hormones. The baseline subtraction required by guidelines to define the net contribute to the exogenous absorbed drug in most cases is a non-solvable problem.

A pharmacokinetics evaluation of a new, low-volume, oral sulfate colon cleansing preparation in patients with renal or hepatic impairment and healthy volunteers

The pharmacokinetics (PK) of an oral sulfate solution (OSS) for bowel cleansing preparation was studied. OSS (30 g of sulfate) was split between 2 doses, 12 hours apart. Safety measures included electrocardiography, vital signs, adverse events, hematology, blood chemistry, and urinalysis. Six adult patients with moderate renal disease (MRD), 6 with mild-moderate hepatic disease (M/MHD), and 6 normal healthy volunteers (NHVs) completed the study. Adverse events were mild to moderate in severity and were mainly limited to headache and expected gastrointestinal symptoms. Serum sulfate levels were highly variable at all times, even after adjusting for baseline. Sulfate was higher in MRD in comparison to the other groups. The C(max) and AUC were higher in the patients, but no statistically significant differences emerged. Sulfate levels returned to predose values within 54 hours after dosing. No electrolyte disturbances occurred. Urinary sulfate excretion was approximately 20% of the dose. OSS was well tolerated. The types and severity of adverse events were similar to those seen in large phase III trials. While patients with MRD had elevated sulfate, the levels were less than those in renal failure and did not alter biochemical parameters that are associated with hypersulfatemia.

The degree of predictivity in pilot studies on six subjects in bioequivalence trials

This study, based on computer simulations, analysed the degree of predictivity of pilot trials on six subjects, with the idea of a further pivotal trial on 18 or more volunteers aimed at assessing bioequivalence. Volunteers enrolled in 10 pivotal bioequivalence trials were considered. For every trial, a thousand bootstrap samples were generated to simulate trials with six subjects, while keeping a balanced design for sequence, period and formulation. Then a standard ANOVA for crossover trials, with 90% confidence intervals for the ratios, was done on C(max) and AUC for each simulated trial. The number of subjects needed to achieve bioequivalence was based on the residual error of the ANOVA. When this number was smaller or equal in the case of bioequivalence, or larger in the case of insufficient evidence to conclude on bioequivalence, to the number of the subjects enrolled in the original trial, the subgroup was considered predictive. Otherwise it was considered non-predictive.The average predictivity index, calculated by dividing the number of predictive findings by the total number of subgroups, in our case 1000, and multiplying the result by 100, was 71.1% for C(max) and 82.9% for AUC. Results of the simulations suggest that pilot trials on six volunteers can be useful for predicting the pool size of volunteers in bioequivalence trials, and for in vivo-in vitro correlation studies in pharmaceutical development strategy.