Physiological Dynamics in the Upper Gastrointestinal Tract and the Development of Gastrointestinal Absorption Models for the Immediate-Release Oral Dosage Forms in Healthy Adult Human

This review is a revisit of various oral drug absorption models developed in the past decades, focusing on how to incorporate the physiological dynamics in the upper gastrointestinal (GI) tract. For immediate-release oral drugs, GI absorption is a critical input of drug exposure and subsequent human body response, yet difficult to model largely due to the complex GI environment. One of the biggest hurdles lies at capturing the high within-subject variability (WSV) of bioavailability measures, which can be mechanistically explained by the GI physiological dynamics. A thorough summary of how GI dynamics is handled in the absorption models would promote the development of mechanism-based oral drug absorption models, aid in the design of clinical studies regarding dosing regimens and bioequivalence studies based on WSV, and advance the decision-making on formulation selection.

Generic lamotrigine extended-release tablets are bioequivalent to innovator drug in fully replicated crossover bioequivalence study

OBJECTIVE

Lamotrigine is a commonly prescribed antiepileptic drug. U.S. Food and Drug Administration (FDA)-funded clinical studies have demonstrated bioequivalence (BE) for generic lamotrigine immediate-release (IR) products in epilepsy patients with generic substitution. To address the potential concerns about the risk of generic-brand substitution of lamotrigine extended-release (ER) products, considering the complexity of controlled release systems and pharmacokinetic variations associated with possible within-subject variability (WSV), this prospective study assessed (1) BE of generic and brand lamotrigine ER products in a fully replicated BE study design in healthy subjects and (2) whether such fully replicated study design and WSV data can better support the approval of generic lamotrigine ER products.

METHODS

This open-label, single-dose, two-treatment, four-period, two-sequence, fully replicated crossover BE study compared generic lamotrigine ER tablet to brand Lamictal XR (200 mg) in 30 healthy subjects under fed conditions. Pharmacokinetics (PK) profiles were generated based on intensive blood sampling up to 144 h.

RESULTS

The two products showed comparable peak plasma concentration (C_max ), area under the concentration-time curve (AUC) from time zero to the last measurable time point (AUC_0-t ) and AUC extrapolated to infinity (AUC_0-inf ), whereas median time to C_max (T_max ) values differed, that is, 10 h for generic and 22 h for brand products, respectively. WSVs for PK metrics were small (~8% of C_max and ~6% of AUC) and similar between these two products. PK simulation predicted equivalent PK measurements of both products at steady state and after brand-to-generic switch, except the first day upon switching. No serious adverse events were reported.

SIGNIFICANCE

The generic lamotrigine ER tablet product demonstrates BE to the brand product in a fully replicated BE study design with healthy subjects, supporting the adequacy of the two-way crossover study design to demonstrate BE and generic-brand substitution of lamotrigine ER products.

GWAS of longitudinal trajectories at biobank scale

Biobanks linked to massive, longitudinal electronic health record (EHR) data make numerous new genetic research questions feasible. One among these is the study of biomarker trajectories. For example, high blood pressure measurements over visits strongly predict stroke onset, and consistently high fasting glucose and Hb1Ac levels define diabetes. Recent research reveals that not only the mean level of biomarker trajectories but also their fluctuations, or within-subject (WS) variability, are risk factors for many diseases. Glycemic variation, for instance, is recently considered an important clinical metric in diabetes management. It is crucial to identify the genetic factors that shift the mean or alter the WS variability of a biomarker trajectory. Compared to traditional cross-sectional studies, trajectory analysis utilizes more data points and captures a complete picture of the impact of time-varying factors, including medication history and lifestyle. Currently, there are no efficient tools for genome-wide association studies (GWASs) of biomarker trajectories at the biobank scale, even for just mean effects. We propose TrajGWAS, a linear mixed effect model-based method for testing genetic effects that shift the mean or alter the WS variability of a biomarker trajectory. It is scalable to biobank data with 100,000 to 1,000,000 individuals and many longitudinal measurements and robust to distributional assumptions. Simulation studies corroborate that TrajGWAS controls the type I error rate and is powerful. Analysis of eleven biomarkers measured longitudinally and extracted from UK Biobank primary care data for more than 150,000 participants with 1,800,000 observations reveals loci that significantly alter the mean or WS variability.

Implementation of multiple statistical methods to estimate variability and individual response to training

HIGHLIGHTS

What are the findings? We implemented five statistical methods in a single study to estimate the magnitude of within-subject variability and quantify responses to exercise training at the individual level.The various proposed methods used to estimate individual responses to training provide different types of information and rely on different assumptions that are difficult to test.Within-subject variability is often large in magnitude, and as such, should be systematically evaluated and carefully considered in future studies to successfully estimate individual responses to training. How might it impact on clinical practice in the future? Within-subject variability in response to exercise training is a key factor that must be considered in order to obtain a reproducible measurement of individual response to exercise training. This is akin to ensuring data is reproducible for each subject.Our findings provide guidelines for future exercise training studies to ensure results are reproducible within participants and to minimize wasting precious research resources.By implementing five suggested methods to estimate individual response to training, we highlight their feasibility, strengths, weaknesses, and costs, for researchers to make the best decision on how to accurately measure individual responses to exercise training.

Proof of Concept in Assignment of Within-Subject Variability During Virtual Bioequivalence Studies: Propagation of Intra-Subject Variation in Gastrointestinal Physiology Using Physiologically Based Pharmacokinetic Modeling

While the concept of ‘Virtual Bioequivalence’ (VBE) using a combination of modelling, in vitro tests and integration of pre-existing data on systems and drugs is growing from its infancy, building confidence on VBE outcomes requires demonstration of its ability not only in predicting formulation-dependent systemic exposure but also the expected degree of population variability. The concept of variation influencing the outcome of BE, despite being hidden with the cross-over nature of common BE studies, becomes evident when dealing with the acceptance criteria that consider the 90% confidence interval (CI) around the relative bioavailability. Hence, clinical studies comparing a reference product against itself may fail due to within-subject variations associated with the two occasions that the individual receives the same formulation. In this proof-of-concept study, we offer strategies to capture the most realistic predictions of CI around the pharmacokinetic parameters by propagating physiological variations through physiologically based pharmacokinetic modelling. The exercise indicates feasibility of the approach based on comparisons made between the simulated and observed WSV of pharmacokinetic parameters tested for a clinical bioequivalence case study. However, it also indicates that capturing WSV of a large array of physiological parameters using backward translation modelling from repeated BE studies of reference products would require a diverse set of drugs and formulations. The current case study of delayed-release formulation of posaconazole was able to declare certain combinations of WSV of physiological parameters as ‘not plausible’. The eliminated sets of WSV values would be applicable to PBPK models of other drugs and formulations.

No Relationships Between the Within-Subjects' Variability of Pain Intensity Reports and Variability of Other Bodily Sensations Reports

Purpose

The subjective nature of pain assessment and its large variance negatively affect patient–health care provider communication and reduce the assay sensitivity of pain clinical trials. Given the lack of an objective gold standard measure, identifying the source (true or error) of the within-subject variability of pain reports is a challenge. By assessing the within-subjects variability of pain and taste reports, alongside with interoceptive measures, the current study is aimed to investigate if the ability to reliably report bodily sensations is a cross-modal characteristic.

Patients and Methods

This prospective study enrolled healthy volunteers from local universities. After consenting, subjects underwent the Focus Analgesia Selection Task (FAST), to assess within-subjects variability of pain reports in response to experimental noxious stimuli; a taste task, which similarly assesses within-subjects variability of tastes (salty and sweet) intensity reports; and the heartbeat perception task, an interoceptive task aimed to assess how accurate subjects are in monitoring and reporting their own heartbeat. In addition, all subjects completed the Multidimensional Assessment of Interoceptive Awareness (MAIA), the Perceived Stress Scale (PSS), and Hospital Anxiety and Depression Scale (HADS). Spearman’s correlations were used to assess relations between all measures.

Results

Sixty healthy volunteers were recruited. Variability of intensity reports of different modalities were independent of each other (P > 0.05 for all correlations). The only correlation found was within modality, between variability of intensity reports of salt and sweet tastes (Spearman’s r = 0.477, P < 0.001). No correlations were found between any of the task results and questionnaire results.

Conclusion

Within-subjects variability of pain reports do not relate to variability of reports of other modalities or to interoceptive awareness. Further research is ongoing to investigate the clinical relevance of within-subjects’ variability of pain reports.

Pharmacokinetic and Safety Profiles of a Fixed-Dose Combination of Amlodipine, Valsartan, and Atorvastatin: A 3-Period Replicate Crossover Study

The objective of study was to compare the pharmacokinetic and safety profiles of a fixed‐dose combination (FDC) formulation of 5/160/20 mg amlodipine/valsartan/atorvastatin with those of separate formulations of a 5/160‐mg amlodipine/valsartan tablet and a 20‐mg atorvastatin tablet. This was a randomized, open‐label, single‐dose, 3‐sequence, 3‐period replicate crossover study with 42 subjects. Serial blood samples for pharmacokinetic assessment were collected up to 72 hours postdose. For establishing bioequivalence (BE) for amlodipine, valsartan, and atorvastatin, a reference‐scaled average BE approach was used if applicable, as well as the conventional limit of 0.80‐1.25. The 90% confidence intervals (CIs) for the geometric mean ratios (GMRs) for the maximum plasma concentration (C_max) and the area under the curve to the last measurable concentration (AUC_t) between the FDC and separate formulations were within the 0.80‐1.25 limit for all analytes but atorvastatin. The estimated within‐subject standard deviation of the log‐transformed values of the separate formulations, the reference intervention, was 0.3804 for the C_max of atorvastatin, being set at 0.7489‐1.3352 for the BE acceptance limit. For both the C_max and AUC_t for atorvastatin, the GMRs lay within 0.80‐1.25, and the 90%CIs for the GMRs were within the BE acceptance limit. This 3‐period replicate crossover study demonstrated the BE of the FDC formulation of amlodipine, valsartan, and atorvastatin and the separate formulations of an amlodipine/valsartan tablet and an atorvastatin tablet. A similar incidence of treatment‐emergent adverse events (TEAEs) was observed in both interventions, and headache was the most common TEAE.

Intratask Variability As a Correlate for DRD4 and SLC6A3 Variants: A Pilot Study in ADHD

Behavioral variability may be an ADHD key feature. Currently used ex-Gaussian/Fast Fourier Transform analyses characterize general distribution and oscillatory/rhythmic components of performance but are unable to demonstrate slow cumulative changes over entire tasks.

OBJECTIVE

To explore how performance of ADHD children and unaffected sibs gradually evolves in relation to genetic variants linked to ADHD.

METHOD

A total of 40 kids (20 ADHD-discordant sib pairs) between 8 and 13 years resolved a visual Go/NoGo with 10% NoGo probability. Variable number tandem repeats (VNTRs) at DRD4 and SLC6A3 were identified following standard protocols. Performance changes were assessed by linear/logistic mixed-effect models.

RESULTS

Models exploring SLC6A3 effects demonstrated less accentuated increments of response time (RT) (p = .046) and cumulative increments in the correct responses to "NoGo" (p = .00027) in 10R/10R participants. Models for DRD4 showed faster decline of correct responses to "Go" (p = .0078) in 2R/7R carriers.

CONCLUSION

Dynamical analysis of attention/inhibition measures may unravel new correlates to DRD4 and SLC6A3 variants.

Fukuda and Babinski-Weil tests: within-subject variability and test-retest reliability in nondisabled adults

The Fukuda Stepping Test and the Babinski-Weil test are clinical assessments that presumably reveal unilateral vestibular hypofunction. However, abnormal performances on both of these two tests have been found in nondisabled individuals. This study compared measures of lateral and longitudinal displacements and body rotation for both tests, as well as the within-subject variability and test-retest reliability of the measures. In addition, correlations between hand and foot dominance and these measures were studied. Fifty young, nondisabled participants performed three trials of the 100-step Fukuda test and three trials of the Babinski-Weil test. The testing session was repeated 7 d later (retest). Lateral displacement, body rotation, and within-subject variability of these two measures were larger on the Fukuda than the Babinski-Weil test. No difference in test-retest reliability was found between the two tests, and a significant correlation was found between body rotation on the Fukuda test and score on the Waterloo Footedness Questionnaire. There may have been smaller variability in results of the Babinski-Weil test because it contains fewer steps (36) than the 100-step Fukuda test. Future research should compare tests with an equal number of steps; but, in the meantime, the Babinski-Weil test seems to have better psychometric properties than the Fukuda test, at least in nondisabled individuals.

An update on the treatment of type 1 and type 2 diabetes mellitus: focus on insulin detemir, a long-acting human insulin analog

Basal insulin analogs are used to minimize unpredictable processes of NPH insulin. Modification of the human insulin molecule results in a slower distribution to peripheral target tissues, a longer duration of action with stable concentrations and thus a lower rate of hypoglycemia. Insulin detemir is a basal insulin analog that provides effective therapeutic options for patients with type 1 and type 2 diabetes. For glycemic control, no significant differences were found in HbA_1c levels compared with NPH and insulin glargine. It is comparable with insulin glargine in significantly reducing rates of all types of hypoglycemia. Clinical studies have demonstrated that detemir is responsible for significantly lower within-subject variability and no or less weight gain than NPH insulin and glargine. Recent pharmacodynamic studies have shown that detemir can be used once daily in many patients with diabetes. Together with patient-friendly injection devices and dose adjustments, it provides a treatment option with the potential to lower the key barriers of adherence to insulin therapy in type 2 diabetes. Recent guidelines for treatment of type 2 diabetes suggest starting intensive therapy of hyperglycemia at an early stage of diabetes and recommend therapeutic options that provide the possibility of reaching HbA_1c goals individually, with a low risk of hypoglycemia or other adverse effects of treatment. The properties of insulin detemir match these requirements.