Open questions on bioequivalence: some problems and some solutions

Complex method for calculating total serum calcium concentration

Background

Calcium drugs are analogs of endogenous substances. The baseline concentration of endogenous substances in the body and homeostasis mechanisms of regulation make the bioavailability and bioequivalence (BE) evaluation of these drugs very difficult. The aim of the study was to assess the pharmacokinetic parameters of calcium in the studies of comparative bioavailability combined with the assessment of homeostatic regulation mechanisms.

Methods

The study included 42 healthy male volunteers. We studied the composition of 1000 and 1200 mg of calcium, and 800 and 1000 IU of cholecalciferol. Blood sampling was carried out at 8 a.m. and 0.5, 1, 2, 3, 4, 6, 8 h after administering the drugs. Calcium pharmacokinetic parameters were measured: the area under the curve (AUC) “concentration-time”, and maximum concentration (Cmax) in volunteers. The excretion of calcium in urine in terms of creatinine clearance and parathyroid hormone (PTH) levels was assessed. The maximum excretion of calcium was converted to creatinine (calcium/creatinine). Enzyme-linked immunosorbent assay (ELISA) was used to determine the PTH and the o-cresolphthalein method to quantitatively measure the total calcium serum and urine.

Results

Results indicated that both the AUC and the excretion of calcium are comparable, and the differences are not statistically significant, p>0.05 (the ratio of calcium and creatinine in the urine maintained within physiological levels). The comparable dynamics of PTH concentration is an indirect measure of the amount of absorbed calcium.

Conclusions

The study demonstrates that, along with the standard way of calculating the area under the curve “concentration-time” (total calcium in serum), it is important to assess the calcium excretion in urine in terms of creatinine excretion, as well as focusing on the changes in the PTH levels in volunteers, which could be considered as a marker of the calcium level in the blood.

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 randomized phase l pharmacokinetic study comparing SB4 and etanercept reference product (Enbrel®) in healthy subjects

Aims

SB4 has been developed as a biosimilar of etanercept. The primary objective of the present study was to demonstrate the pharmacokinetic (PK) equivalence between SB4 and European Union ‐sourced etanercept (EU‐ETN), SB4 and United States‐sourced etanercept (US‐ETN), and EU‐ETN and US‐ETN. The safety and immunogenicity were also compared between the treatments.

Methods

This was a single‐blind, three‐part, crossover study in 138 healthy male subjects. In each part, 46 subjects were randomized at a 1:1 ratio to receive a single 50 mg subcutaneous dose of the treatments (part A: SB4 or EU‐ETN; part B: SB4 or US‐ETN; and part C: EU‐ETN or US‐ETN) in period 1, followed by the crossover treatment in period 2 according to their assigned sequences. PK equivalence between the treatments was determined using the standard equivalence margin of 80–125%.

Results

The geometric least squares means ratios of AUC_inf, AUC_last and C_max were 99.04%, 98.62% and 103.71% (part A: SB4 vs. EU‐ETN); 101.09%, 100.96% and 104.36% (part B: SB4 vs. US‐ETN); and 100.51%, 101.27% and 103.29% (part C: EU‐ETN vs. US‐ETN), respectively, and the corresponding 90% confidence intervals were completely contained within the limits of 80–125 %. The incidence of treatment‐emergent adverse events was comparable, and the incidence of the antidrug antibodies was lower in SB4 compared with the reference products.

Conclusions

The present study demonstrated PK equivalence between SB4 and EU‐ETN, SB4 and US‐ETN, and EU‐ETN and US‐ETN in healthy male subjects. SB4 was well tolerated, with a lower immunogenicity profile and similar safety profile compared with those of the reference products.

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.

Novel approach to bioequivalence assessment based on physiologically motivated model

The study was conducted to exemplify an approach capable of obtaining a new insight into bioequivalence (BE) assessment, by the use of a physiologically motivated model. Data from an oral BE study of two piroxicam (PXM) products was used as an example. The BE study was carried out with 24 healthy European subjects according to a two-sequence crossover-randomized design. The test and reference formulations were a PXM generic formulation (LaborMed Pharma, Romania) and Feldene (Pfizer, USA), respectively. Plasma concentrations of PXM were monitored by a validated high-performance liquid chromatography over a period of 144 h after administration. After the structure of the optimal model was selected, parameters that characterized the whole-body disposition behavior of PXM in the subjects were derived. The paired Student's t-test and Wilkoxon's test were performed on the derived parameters. The null hypothesis of no differences in the parameters of the whole-body disposition behavior of PXM related to the test and reference product was not rejected at 5% level of significance. This result suggested that the compared products were bioequivalent and could be used interchangeably in clinical setting. The presented approach might show a new way, worth incorporating in future BE guidelines.

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.

Effects of Food on the Clinical Pharmacokinetics of Anticancer Agents

Pharmacokinetic interactions between food and orally administered drugs involve changes mainly in the absorption and metabolism of a drug, and may have clinical implications. Such interactions, in particular, may be of major clinical significance for cancer chemotherapy since the majority of anticancer agents are toxic, have a low therapeutic index and are administered long term, most often in combination with other cytotoxic agents. The purpose of this review is to compare the pharmacokinetic profiles of various anticancer drugs, including chemopreventive agents that have been examined previously in fasted and fed conditions, and to discuss the underlying basis/mechanisms of food effect in light of a drug's physicochemical and pharmacokinetic properties. Clinical pharmacokinetic parameters such as maximum concentration, area under the concentration-time curve, time to maximum concentration and half-life for each drug are compared in fasted and fed states, and specific dietary recommendations are summarised accordingly. In addition, the effects of food on the metabolite kinetics and pharmacodynamic responses, and the potential role of food effect in the modulation of oral biovariability and multidrug resistance have been extensively discussed. Overall, this comprehensive pharmacokinetic analysis indicates that a broad spectrum of food effects is seen among anticancer agents because of diverse factors regulating each drug's oral bioavailability and its interactions with food. The consideration of such effects is important, as it could lead to more rational pharmacological monitoring and possibly improve the oral chemotherapy of cancer in children, adults and the elderly.

LDL-cholesterol lowering effect of a generic product of simvastatin compared to simvastatin (Zocor) in Thai hypercholesterolemic subjects -- a randomized crossover study, the first report from Thailand

BACKGROUND

It is commonly agreed that people with a high blood LDL-cholesterol will have a higher risk of coronary artery disease (CAD) than people with low blood LDL-cholesterol. Due to the increasingly high costs of medication in Thailand, the government has set up several measures to combat the problem. One of such strategies is to promote the utilization of locally manufactured drug products, especially those contained in the National Drug List. Simvastatin, an HMG-CoA reductase inhibitor, is listed as an essential drug for the treatment of hypercholesterolemia. Here, we reported the study on the LDL-cholesterol-lowering effect of a generic simvastatin product in comparison with the Zocor, in 43 healthy thai volunteers.

METHOD

The generic product tested was Eucor, locally manufactured by Greater Pharma Ltd., Part, Thailand, and the reference product was Zocor (Merck Sharp & Dohme, USA). The two products were administered as 10-mg single oral doses in a two-period crossover design. After drug administration, serial blood samples were collected every 4 weeks for 16 weeks. The major parameter monitored in this study was blood LDL-cholesterol.

RESULT

After taking the drugs for the first 8 weeks, no statistically significant difference was detected in blood LDL-cholesterol between the first (Zocor-treated) and the second (Eucor-treated) groups. After crossover and taking drugs for further 8 weeks, a similar result was obtained, i.e., no significant difference in blood LDL-cholesterol between the first (Eucor-treated) and the second (Zocor-treated) groups was observed. Upon completion of the 16-week study, there was also no statistically significant difference in the changes of all tested blood parameters between the two products (randomized block ANOVA, N = 37). Only minor side effects, mainly dizziness and nausea, were observed in both products.

CONCLUSION

Our study demonstrated no significant differences in the therapeutic effect and safety between the generic and original simvastatin products.

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

Oral administration of endogenous substances in most cases results in negligible net increases in baseline plasma concentrations, associated with high variability. This poses the problem of their bioequivalence. Using the data obtained from a bioequivalence investigation of potassium aspartate (test vs reference formulation), the authors demonstrate the inconsistency of bioequivalence based on plasma concentrations and standard methods. Potassium aspartate was given orally at a dose of 15.8 mmoles to 12 healthy volunteers as test and reference values according to a two-period, two-formulation, two-sequence design. The individual net values of the area under the curve of plasma concentration (AUC) and cumulative urinary excretion (CUE), both obtained with the test formulation as post-dose minus baseline, were multiplied by 2, 3, 4, 5 and 6 and added to the baseline in order to simulate the administration of increasing single doses of the test, assuming dose-linear kinetics. Data generated with the test formulation were compared with original data of the reference according to 90% confidence intervals. With AUC, bioequivalence of test and reference formulations was demonstrated with 1 : 1, 2 : 1 and 3 : 1 test to reference dose ratios. With CUE only the 1 : 1 dose ratio comparison produced bioequivalence. The authors conclude that bioequivalence of endogenous substances conducted with standard procedures in most cases is a useless exercise. With potassium and more generally with drugs cleared via urine, urinary excretion would reflect the extent of absorption more faithfully than AUC.