Human Data on Pharmacokinetic Interactions of Cannabinoids: A Narrative Review

Concomitant use of cannabinoids with other drugs may result in pharmacokinetic drug-drug interactions, mainly due to the mechanism involving Phase I and Phase II enzymes and/or efflux transporters. Cannabinoids are not only substrates but also inhibitors or inducers of some of these enzymes and/or transporters. This narrative review aims to provide the available information reported in the literature regarding human data on the pharmacokinetic interactions of cannabinoids with other medications. A search on Pubmed/Medline, Google Scholar, and Cochrane Library was performed. Some studies were identified with Google search. Additional articles of interest were obtained through cross-referencing of published literature. All original research papers discussing interactions between cannabinoids, used for medical or recreational/adult-use purposes, and other medications in humans were included. Thirty-two studies with medicinal or recreational/adult-use cannabis were identified (seventeen case reports/series, thirteen clinical trials, and two retrospective analyses). In three of these studies, a bidirectional pharmacokinetic drug-drug interaction was reported. In the rest of the studies, cannabinoids were the perpetrators, as in most of them, concentrations of cannabinoids were not measured. In light of the widespread use of prescribed and non-prescribed cannabinoids with other medications, pharmacokinetic interactions are likely to occur. Physicians should be aware of these potential interactions and closely monitor drug levels and/or responses. The existing literature regarding pharmacokinetic interactions is limited, and for some drugs, studies have relatively small cohorts or are only case reports. Therefore, there is a need for high-quality pharmacological studies on cannabinoid-drug interactions.

Therapeutic advantages of the combined use of closantel and moxidectin in lambs parasitized with resistant gastrointestinal nematodes

The serious widespread development of nematode resistance has motivated the use of combined anthelmintic formulations. However, the advantages/disadvantages of the combined use of anthelmintics require further scientific characterization. The goals of the current trial were a) to characterize the pharmacokinetics of closantel (CLO) and moxidectin (MXD) administered both subcutaneously (sc) and orally either separately or co-administered (CLO + MXD) to lambs; b) to compare the nematodicidal activity of both molecules given individually or co-administered to lambs infected with resistant nematodes. Seventy (70) Corriedale lambs naturally infected with multiple resistant gastrointestinal nematodes were involved in the pharmacokinetic and efficacy trials. The animals were allocated into six groups (n = 10) and treated with either CLO, MXD, or with the CLO + MXD combined formulation by both the oral and sc routes. Additionally, an untreated control group (n = 10) was included for the efficacy trial. The efficacy was estimated by the faecal egg count reduction test (FECRT). Higher systemic exposure of both CLO and MXD was observed after the sc compared to the oral administration in lambs. The combined administration of CLO + MXD did not markedly alter their disposition kinetics. At 13 days post-treatment, the administration of both molecules as a single active principle reached efficacy levels ranging between 80% (MXDoral), 84% (CLOoral), 85% (CLOsc), and 92% (MXDsc). The combined oral and sc treatments reached 99% efficacy. No adverse effects were observed after the combined treatment of CLO + MXD, and their co-administration did not show any adverse pharmacokinetic interaction. The combined effect of CLO + MXD successfully restored the maximum efficacy levels, which were not reached by the individual active ingredients.

Quantitative systems pharmacology Model to characterize valproic acid-induced hyperammonemia and the effect of L-carnitine supplementation

Valproic acid (VPA) is a short-chain fatty acid widely prescribed in the treatment of seizure disorders and epilepsy syndromes, although its therapeutic value may be undermined by its toxicity. VPA serious adverse effects are reported to have a significant and dose-dependent incidence, many associated with VPA-induced hyperammonemia. This effect has been linked with reduced levels of carnitine; an endogenous compound involved in fatty acid's mitochondrial β-oxidation by facilitation of its entrance via the carnitine shuttle. High exposure to VPA can lead to carnitine depletion causing a misbalance between the intra-mitochondrial β-oxidation and the microsomal ω-oxidation, a pathway that produces toxic metabolites such as 4-en-VPA which inhibits ammonia elimination. Moreover, a reduction in carnitine levels might be also related to VPA-induced obesity and lipids disorder. In turn, L-carnitine supplementation (CS) has been recommended and empirically used to reduce VPA's hepatotoxicity. The aim of this work was to develop a Quantitative Systems Pharmacology (QSP) model to characterize VPA-induced hyperammonemia and evaluate the benefits of CS in preventing hyperammonemia under both chronic treatment and after VPA overdosing. The QSP model included a VPA population pharmacokinetics model that allowed the prediction of total and unbound concentrations after single and multiple oral doses considering its saturable binding to plasma proteins. Predictions of time courses for 2-en-VPA, 4-en-DPA, VPA-glucuronide, carnitine, ammonia and urea levels, and for the relative change in fatty acids, Acetyl-CoA, and glutamate reflected the VPA induced changes and the efficacy of the treatment with L-carnitine. The QSP model was implemented to give a rational basis for the L-carnitine dose selection to optimize CS depending on VPA dosage regime and to assess the currently recommended L-carnitine rescue therapy after VPA overdosing. Results show that a L-carnitine dose equal to the double of the VPA dose using the same interdose interval would maintain the ammonia levels at baseline. The QSP model may be expanded in the future to describe other adverse events linked to VPA-induced changes in endogenous compounds.

Tissue Drug Concentration

Blood flow enables the delivery of oxygen and nutrients to the different tissues of the human body. Drugs follow the same route as oxygen and nutrients; thus, drug concentrations in tissues are highly dependent on the blood flow fraction delivered to each of these tissues. Although the free drug concentration in blood is considered to correlate with pharmacodynamics, the pharmacodynamics of a drug is actually primarily commanded by the concentrations of drug in the aqueous spaces of bodily tissues. However, the concentrations of drug are not homogeneous throughout the tissues, and they rarely reflect the free drug concentration in the blood. This heterogeneity is due to differences in the blood flow fraction delivered to the tissues and also due to membrane transporters, efflux pumps, and metabolic enzymes. The rate of drug elimination from the body (systemic elimination) depends more on the driving force of drug elimination than on the free concentration of drug at the site from which the drug is being eliminated. In fact, the actual free drug concentration in the tissues results from the balance between the input and output rates. In the present paper, we develop a theoretical concept regarding solute partition between intravascular and extravascular spaces; discuss experimental research on aqueous/non-aqueous solute partitioning and clinical research on microdialysis; and present hypotheses to predict in-vivo elimination using parameters of in-vitro metabolism.

The role of efflux transporters and metabolizing enzymes in brain and peripheral organs to explain drug-resistant epilepsy

Drug‐resistant epilepsy has been explained by different mechanisms. The most accepted one involves overexpression of multidrug transporters proteins at the blood brain barrier and brain metabolizing enzymes. This hypothesis is one of the main pharmacokinetic reasons that lead to the lack of response of some antiseizure drug substrates of these transporters and enzymes due to their limited entrance into the brain and limited stay at the sites of actions. Although uncontrolled seizures can be the cause of the overexpression, some antiseizure medications themselves can cause such overexpression leading to treatment failure and thus refractoriness. However, it has to be taken into account that the inductive effect of some drugs such as carbamazepine or phenytoin not only impacts on the brain but also on the rest of the body with different intensity, influencing the amount of drug available for the central nervous system. Such induction is not only local drug concentration but also time dependent. In the case of valproic acid, the deficient disposition of ammonia due to a malfunction of the urea cycle, which would have its origin in an intrinsic deficiency of L‐carnitine levels in the patient or by its depletion caused by the action of this antiseizure drug, could lead to drug‐resistant epilepsy. Many efforts have been made to change this situation. In order to name some, the administration of once‐daily dosing of phenytoin or the coadministration of carnitine with valproic acid would be preferable to avoid iatrogenic refractoriness. Another could be the use of an adjuvant drug that down‐regulates the expression of transporters. In this case, the use of cannabidiol with antiseizure properties itself and able to diminish the overexpression of these transporters in the brain could be a novel therapy in order to allow penetration of other antiseizure medications into the brain.

Clinical Pharmacokinetics of Cannabinoids and Potential Drug-Drug Interactions

Over the past few years, considerable attention has focused on cannabidiol (CBD) and Δ⁹-tetrahydrocannabinol (THC), the two major constituents of Cannabis sativa, mainly due to the promising potential medical uses they have shown. However, more information on the fate of these cannabinoids in human subjects is still needed and there is limited research on the pharmacokinetic drug-drug interactions that can occur in the clinical setting and their prevalence. As the use of cannabinoids is substantially increasing for many indications and they are not the first-line therapy in any treatment, health care professionals must be aware of drug-drug interactions during their use as serious adverse events can happen related with toxic or ineffective outcomes. The present chapter overview summarizes our current knowledge on the pharmacokinetics and metabolic fate of CBD and THC in humans and discusses relevant drug-drug interactions, giving a plausible explanation to facilitate further research in the area.

Potential Therapeutic Role of Carnitine and Acetylcarnitine in Neurological Disorders

BACKGROUND

Current therapy of neurological disorders has several limitations. Although a high number of drugs are clinically available, several subjects do not achieve full symptomatic remission. In recent years, there has been an increasing interest in the therapeutic potential of L-carnitine (LCAR) and acetyl-L-carnitine (ALCAR) because of the multiplicity of actions they exert in energy metabolism, as antioxidants, neuromodulators and neuroprotectors. They also show excellent safety and tolerability profile.

OBJECTIVE

To assess the role of LCAR and ALCAR in neurological disorders.

METHODS

A meticulous review of the literature was conducted in order to establish the linkage between LCAR and ALCAR and neurological diseases.

RESULTS

LCAR and ALCAR mechanisms and effects were studied for Alzheimer's disease, depression, neuropathic pain, bipolar disorder, Parkinson's disease and epilepsy in the elderly. Both substances exert their actions mainly on primary metabolism, enhancing energy production, through β-oxidation, and the ammonia elimination via urea cycle promotion. These systemic actions impact positively on the Central Nervous System state, as Ammonia and energy depletion seem to underlie most of the neurotoxic events, such as inflammation, oxidative stress, membrane degeneration, and neurotransmitters disbalances, present in neurological disorders, mainly in the elderly. The impact on bipolar disorder is controversial. LCAR absorption seems to be impaired in the elderly due to the decrease of active transportation; therefore, ALCAR seems to be the more effective option to administer.

CONCLUSION

ALCAR emerges as a simple, economical and safe adjuvant option in order to impair the progression of most neurological disorders.

Pharmacokinetics of Subcutaneous Levetiracetam in Palliative Care Patients

Background: Seizure control is challenging in the palliative care setting. Subcutaneous (SC) levetiracetam (LEV) is currently an off-label route of administration and effectiveness, tolerability, and pharmacokinetics studies for this route are scarce. Objectives: This prospective study aimed at evaluating effectiveness and tolerability of SC LEV as well as characterizing its pharmacokinetics. Subjects: Patients (n = 7) who attended the palliative care clinic between September 2018 and January 2019 with diagnosis of seizures, ≥18 years, and in need of SC route of administration were included in the study. Measurements: LEV plasma levels were determined using high-performance liquid chromatography and pharmacokinetic analysis were performed using Monolix 2018R2 (France). pH and osmolality of the three SC infusion solutions were also determined. Results: Seven patients took part in the study. Seizures were controlled in six out of seven patients with doses of 1000 and 3000 mg/day. Adverse effects were mild. pH and osmolality of the SC infusion solutions were within the accepted values reported in the literature. Mean plasma LEV concentrations were 14.4 mg/L (1000 mg/day) and 27.7 mg/L (2000 mg/day). The population clearance (2.5 L/h) and the elimination half-life (10.4 hours) were successfully estimated. Conclusions: Based on this data, SC LEV was effective and well tolerated. Pharmacokinetic parameters for the SC route were successfully determined.

Sex-by-formulation interaction in bioequivalence studies: the importance of formulations and experimental conditions

In a recently published investigation, the authors argued against the likelihood of sex-based subject-by-formulation interactions in bioequivalence studies, i.e. male and female subjects exhibiting different discriminatory potential to detect bioavailability differences between formulations. The researchers performed a strong methodological study showing the increased probability of false-positive findings in exploratory subgroup analysis, a well-known and documented statistical issue. Indeed, the main limitation of assessing a sex-by-formulation interaction in average bioequivalence studies lies in the fact that these clinical trials are not designed for this purpose. In this commentary, we further discuss on why the impact of sex differences in gastrointestinal physiology over in vivo drug dissolution and absorption rate cannot remain hidden behind statistical limitations, particularly when average bioequivalence conclusions could be affected. Regulatory agencies should encourage and support these important issues related to biopharmaceutical quality of drug products in both sexes. In addition, a sex-based analysis of bioequivalence results will enhance the representativeness of conclusions and provide important information regarding formulation performance, thereby promoting the efficacy and safety of generic drugs and reducing consumer risk. The extrapolation of study conclusions from one sex to another is far away from being scientifically proven.

Integration of in vitro biorelevant dissolution and in silico PBPK model of carvedilol to predict bioequivalence of oral drug products

Bioequivalence implementation in developing countries where a high proportion of similar drug products are being marketed has found several obstacles, impeding regulatory agencies to move forward with this policy. Biopharmaceutical quality of these products, several of which are massively prescribed, remains unknown. In this context, an in vitro-in silico-in vivo approach is proposed as a mean to screen product performance and target specific formulations for bioequivalence assessment. By coupling in vitro biorelevant dissolution testing in USP-4 Apparatus (flow-through cell) with physiologically-based pharmacokinetic (PBPK) modeling in PK-Sim® software (Bayer, Germany), the performance of seven similar products of carvedilol tablets containing 25 mg available in the Uruguayan market were compared with the brand-name drug Dilatrend®. In silico simulations for Dilatrend® were compared with published results of bioequivalence studies performed in fasting conditions allowing model development through a learning and confirming process. Single-dose pharmacokinetic profiles were then simulated for the brand-name drug and two similar drug products selected according to in vitro observations, in a virtual Caucasian population of 1000 subjects (50% male, aged between 18 and 50 years with standard body-weights). Population bioequivalence ratios were estimated revealing that in vitro differences in drug release would have a major impact in carvedilol maximum plasma concentration, leading to a non-bioequivalence outcome. Predictions support the need to perform in vivo bioequivalence for these products of extensive use. Application of the in vitro-in silico-in vivo approach stands as an interesting alternative to tackle and reduce drug product variability in biopharmaceutical quality.

L-Carnitine supplementation to reverse hyperammonemia in a patient undergoing chronic valproic acid treatment: A case report

Valproic acid is a broad-spectrum anticonvulsant that has also gained attention in the psychiatric setting. With respect to safety, valproic acid may induce a seemingly rare condition, hyperammonemia, which can induce a wide variety of symptoms ranging from irritability to coma. The proposed mechanism of hyperammonemia involves depletion of carnitine and overproduction of a toxic metabolite, 4-en-valproic acid, both of which impair the urea cycle and thus ammonia elimination. Carnitine is a commonly used antidote for acute intoxication of valproic acid, but is not a therapeutic option for management of chronic adults with adverse effects related to valproic acid. We herein report a case involving a woman with epilepsy who developed hyperammonemia after a change in her anticonvulsant therapy. She reported increased seizures and gastrointestinal disturbances. Her ammonia, valproic acid, 4-en-valproic acid, and carnitine levels were monitored. Her ammonia level was elevated and her carnitine level was at the inferior limit of the population range. She was supplemented with carnitine at 1 g/day. After 1 month, her ammonia level decreased, her carnitine level increased, and her seizures were better controlled. Carnitine supplementation was useful for reversal of her hyperammonemia, allowing her to continue valproic acid for seizure control.

Current State and Future Perspectives in QSAR Models to Predict Blood- Brain Barrier Penetration in Central Nervous System Drug R&D

The Blood-Brain Barrier (BBB) is a physical and biochemical barrier that restricts the entry of certain drugs to the Central Nervous System (CNS), while allowing the passage of others. The ability to predict the permeability of a given molecule through the BBB is a key aspect in CNS drug discovery and development, since neurotherapeutic agents with molecular targets in the CNS should be able to cross the BBB, whereas peripherally acting agents should not, to minimize the risk of CNS adverse effects. In this review we examine and discuss QSAR approaches and current availability of experimental data for the construction of BBB permeability predictive models, focusing on the modeling of the biorelevant parameter unbound partitioning coefficient (Kp,uu). Emphasis is made on two possible strategies to overcome the current limitations of in silico models: considering the prediction of brain penetration as a multifactorial problem, and increasing experimental datasets through accurate and standardized experimental techniques.

Sex Effect on Average Bioequivalence

PURPOSE

Generic formulations are by far the most prescribed drugs. This scenario is highly beneficial for society because medication expenses are significantly reduced after expiration of the exclusivity period conceded to the branded name drug. Correspondingly, these formulations must be adequately evaluated to avoid drug inefficacy and toxicity in the overall patient population. Bioequivalence studies are the only in vivo evaluation that a generic drug must overcome to reach the market. These clinical trials have not been exempt from underrepresentation of female subjects and a lack of sex-based analysis. Frequently, conclusions obtained in men are extrapolated to women. Furthermore, the obtained results are not analyzed to determine sex differences. The aim of this study was to discuss the effect that male and female differences in gastrointestinal physiology can have on bioequivalence conclusions and to show why a sex-based analysis must be conducted in these studies to improve the evaluation of generic drugs.

METHODS

This discussion was based on observed sex differences in product bioavailability discrimination (sex-by-formulation interaction) and on residual variability through an analysis of average bioequivalence data previously reported by other researchers and data collected by our center. Bioequivalence studies of oral formulations, with a 2-period, 2-sequence, 2-treatment random crossover design performed in healthy subjects with at least 6 subjects of each sex, were included. In addition, the bioequivalence conclusion that would have been reached in each study if performed with only 1 sex was estimated.

FINDINGS

The data reveal that differences in both product bioavailability discrimination and residual variability occur with a significant incidence in bioequivalence studies. In either C_max or AUC, a significant sex-by-formulation interaction was present in 1 of 3 reviewed studies, whereas differences in residual variability between sexes were significant for >50% of studies. Moreover, the performed estimations suggest that the reported bioequivalence conclusions were not verified in at least 1 sex for 1 of 3 studies and were not verified in men and in women for 1 of 6 studies.

IMPLICATIONS

This research shows that extrapolation of bioequivalence results from the male population to the female population is not always valid. Bioequivalence studies must therefore be performed with both male and female subjects in similar proportions. Sex-based analysis in bioequivalence can improve study design, enhance the representativeness of conclusions, and provide important information regarding formulation performance, thereby promoting the efficacy and safety of generic drugs.

Complete dataset for 2-treatment, 2-sequence, 2-period efavirenz bioequivalence study conducted with nightly dosing

The efavirenz pharmacokinetic raw data presented in this article was obtained in an average bioequivalence study between a local brand and Stocrin (Merck Sharp & Dohme, purchased from Australia, batch H009175, expiration date November 2013). Dose was administered at night (9:00 p.m.) two hours after food intake. Fourteen healthy subjects, 8 women and 6 men, completed the study. For each subject, 15 data points until 96 h post-administration are included. Subject demographic characteristics and sequences of administration are provided along with individual pharmacokinetic profiles of efavirenz obtained for both formulations after a single oral dose of 600 mg. This data provides information in support of the research article “Sex-by-formulation interaction assessed through a bioequivalence study of efavirenz tablets” [1].

Sex-by-formulation interaction assessed through a bioequivalence study of efavirenz tablets

Although sex-related differences in gastrointestinal physiology have been vastly reported, its impact on drug oral bioavailability and bioequivalence (product discrimination) is often ignored. On this work results from an average bioequivalence study between tablets containing 600mg of the antiretroviral efavirenz (EFV), carried out with 14 healthy subjects (8 female and 6 men) in a randomized 2-period, 2-treatment crossover design, are analyzed from a sex-based approach. Sequences were balanced within each sex group. Considering all subjects, no differences were observed on EFV absorbed amount, as shown by the estimated 90CI of the AUC96 Test/Reference bioequivalence ratio (T/R): 0.950-1.05. However, results were not conclusive due to the 90CI for CMAX T/R was 0.743-1.07. Over this parameter, a significant sex-by-formulation interaction was detected: 90CI CMAX T/R was 0.838-1.36 in women and 0.540-0.920 in men; with a 52% relative difference between point estimates. Formulation differences were therefore evidenced only by male subjects. In vitro dissolution and disintegration tests for both products were carried out in two aqueous media: A) SLS 0.25% and B) HCl/KCl pH1.2. T/R results for dissolution efficiency and tablet disintegration times of formulations in both A and B media were highly correlated with CMAX T/R bioequivalence results observed in women and men respectively, showing that a dissimilar gastrointestinal environment between sexes affected EFV oral absorption. This work shows how sex-by-formulation interaction can affect bioequivalence conclusions. Sex effect on product discrimination should be specially disclosed in bioequivalence studies, mainly for drugs aimed to be given to both sexes.

MDR-1 and MRP2 Gene Polymorphisms in Mexican Epileptic Pediatric Patients with Complex Partial Seizures

Although the Pgp efflux transport protein is overexpressed in resected tissue of patients with epilepsy, the presence of polymorphisms in MDR1/ABCB1 and MRP2/ABCC2 in patients with antiepileptic-drugs resistant epilepsy (ADR) is controversial. The aim of this study was to perform an exploratory study to identify nucleotide changes and search new and reported mutations in patients with ADR and patients with good response (CTR) to antiepileptic drugs (AEDs) in a rigorously selected population. We analyzed 22 samples In Material and Methods, from drug-resistant patients with epilepsy and 7 samples from patients with good response to AEDs. Genomic DNA was obtained from leukocytes. Eleven exons in both genes were genotyped. The concentration of drugs in saliva and plasma was determined. The concentration of valproic acid in saliva was lower in ADR than in CRT. In ABCB1, five reported SNPs and five unreported nucleotide changes were identified; rs2229109 (GA) and rs2032582 (AT and AG) were found only in the ADR. Of six SNPs associated with the ABCC2 that were found in the study population, rs3740066 (TT) and 66744T > A (TG) were found only in the ADR. The strongest risk factor in the ABCB1 gene was identified as the TA genotype of rs2032582, whereas for the ABCC2 gene the strongest risk factor was the T allele of rs3740066. The screening of SNPs in ACBC1 and ABCC2 indicates that the Mexican patients with epilepsy in this study display frequently reported ABCC1 polymorphisms; however, in the study subjects with a higher risk factor for drug resistance, new nucleotide changes were found in the ABCC2 gene. Thus, the population of Mexican patients with AED-resistant epilepsy (ADR) used in this study exhibits genetic variability with respect to those reported in other study populations; however, it is necessary to explore this polymorphism in a larger population of patients with ADR.

Concentration-dependent mechanisms of adverse drug reactions in epilepsy

Antiepileptic drugs can cause some adverse effects ranging from mild to acute and serious ones. The inducing properties of some of them may result in vitamin D, vitamin K and estrogens catabolism and thus risk of fractures or efflux transport overexpression at the blood brain barrier and consequently lack of effect at the action site. Some are responsible for the formation of reactive metabolites, such as arene oxides or atropaldehyde intermediates, in skin, liver and other organs, causing hypersensitivity reactions or can enhance a commonly minor metabolic pathway increasing the formation of toxic metabolites. Drug-induced myopia and other visual problems have also been described with the use of antiepileptic agents. A pharmacological insight of the possible concentration-dependent mechanism involved in these reactions is given in this review and in some cases some preventive measures to revert them.

Usefulness of salivary drug monitoring for detecting efflux transporter overexpression

BACKGROUND

Bioavailability (F) and clearance (CL) are two pharmacokinetic parameters difficult to differentiate from simple plasma measurement when a drug is administered orally. Venous (V) / artery (A) concentration ratio of a drug could be a reliable index of its CL if measurements of plasma concentration were performed during a period of time where the absorption process was not longer operative, then during a pure elimination phase.

OBJECTIVE

A novel subrogate using two protocolized saliva samples sequentially collected (first, S1, and second, S2) was designed in order to replace V and A free plasma drug concentrations, respectively. Two drugs, phenytoin (PHT) and carbamazepine (CBZ), which are well-known for their inducer properties and their dose-dependent clearance variations, were studied taking into account the sex of individuals.

SETTING AND PATIENTS

A multicentre two-phase collaborative study was done. The first phase was performed with healthy volunteers in order to determine salivary pharmacokinetic parameters after single dose administration. Twelve volunteers (6 male and 6 female) received 400 mg of CBZ (2 tablets x 200 mg, immediate release product). Twenty four volunteers (10 male and 14 female) received 100 mg of PHT. The second phase was carried out with epileptic patients under CBZ (11 male 15 female) or PHT (11 male and 11 female) monotherapy, in order to study dose-related and sex-related pharmacokinetic differences.

MAIN OUTCOME MEASURES

In the single dose trials, peaks (Tmax, Cmax) were computed directly from the data. Areas under concentrationtime curves (AUC∞), AUC∞xW (area corrected by weight) and half-lives (t1/2) were calculated. In the case of CBZ, AUCCBZ-10,11- epoxide/AUCCBZ metabolic ratios were also calculated. After multiple dose administration, S1 and S2 trough morning drug concentrations were measured.

RESULTS

Cmax and AUC differed significantly between sexes for the two drugs after single dose administration. Nevertheless, the apparent clearance (CL/F) per unit of body weight did not differ (CBZ) or slightly differed (PHT) between sexes. Higher metabolic ratio for CBZ in women would lead to lower F and therefore lower CL in this gender. In the case of PHT, women would have either lower F or higher CL than men. After multiple dose administration, S1/S2 saliva drug concentration ratio correlated positively with S2 for CBZ, showing that CBZ clearance increases with daily dose. Gender differences were also observed for CBZ-10,11-epoxide concentration, being bioavailability the main parameter responsible for this difference. S1/S2 saliva PHT concentration ratio correlated negatively with S2, showing that PHT clearance diminishes with dose as it has been previously reported. Since a significant difference was found for S1/S2 ratio between male and females, CL is the pharmacokinetic parameter influenced by gender in PHT disposition.

CONCLUSION

S1/S2 saliva drug concentration ratio was sensitive enough for detecting systemic clearance changes. Both CBZ and PHT would modify their bioavailability and clearance by inducing efflux transporter throughout chronic treatments, from the first dose to multiple dose administration.

Sex- and smoke-related differences in gastrointestinal transit of cyclosporin A microemulsion capsules

The aim of this work was to study the effect of the sex and the smoking status on the pharmacokinetics and the bioequivalence assessment of a branded and a generic cyclosporine A microemulsion formulation in soft-gelatin capsule. Sixteen healthy volunteers (eight women and eight men) participated in a CyA bioequivalence study, with nine of the volunteers being smokers. Sandimmun Neoral® (brand formulation; Reference) and Sigmasporin Microral® (generic formulation; Test) were administered under fasting conditions. Pharmacokinetic parameters were calculated through non compartmental analysis. Bioequivalence was declared based on the 90% confidence intervals (90% CI) for the T/R ratio of the geometric means for each parameter. In vitro determination of the capsules opening time was performed in simulated gastric fluid without enzyme with USP Apparatus 2. The extent of absorption was similar between both products for all subjects or each sex-group. The absorption rate was similar for both products when considering all subjects, whereas a significant difference in the TMAX between the two products was observed for the male subjects only, which relates to its slower capsule opening time observed in vitro (12.4 versus 6.0 min). No differences were observed in women that could relate to their slower gastric emptying. Differences in drug exposure were observed between smokers and non-smokers. Sex- and smoke-related differences in the gastrointestinal transit should be considered when the on-set time would be determinant for the treatment success of a drug.

Chronic administration of phenytoin induces efflux transporter overexpression in rats

BACKGROUND

Efflux transporters overexpression has been proposed as one of the responsible mechanism for refractory epilepsy by preventing access of the antiepileptic drug to the brain. In this work we investigated whether phenytoin (PHT), could induce efflux transporters overexpression, at different biological barriers and to evaluate the implication it could have on its pharmacokinetics and therapeutic/toxic response.

METHODS

Forty-two adult females Sprague Dawley divided in five groups were treated with oral doses of 25, 50 and 75mg/kg/6h of PHT for 3 days and two additionally groups were treated with intraperitoneal (ip) doses of 25mg/kg/6h or 100mg/kg/24h. At day 4 PHT plasma concentrations were measured and, obtained several organs, brain, parotid gland, liver and duodenum in which were analyzed for the Pgp expression. At day 4 PHT plasma concentrations were measured and several tissues: brain, parotid gland, liver and duodenum were obtained in order to analyze Pgp expression. In order to evaluate the oral bioavailability of PHT, two groups were administered with oral or intraperitoneal doses of 100mg/kg and plasma level were measured.

RESULTS

An induction of the expression of efflux transporter mediated by phenytoin in a concentration-and-time dependent manner was found when increasing oral and ip doses of phenytoin, One week after the interruption of ip treatment a basal expression of transporters was recovered.

CONCLUSIONS

Overexpression of efflux transporters can be mediated by inducer agents like PHT in a local-concentration dependent manner, and it is reversible once the substance is removed from the body. The recovery of basal Pgp expression could allow the design of dosing schedules that optimize anticonvulsant therapy.

Antiepileptic drugs: Energy-consuming processes governing drug disposition

Diffusion is not the main process by which drugs are disposed throughout the body. Translational movements of solutes given by different energy-consuming mechanisms are required in order to dispose them efficiently. Membrane transportation and cardiac output distribution are two effective processes to move the molecules among different body sites. Gastrointestinal-blood cycling constitutes a supplementary way to regulate the distribution of molecules between the non-hepatic organs and the liver. Any change in the relative supply of drug molecules among eliminating organs could modify their clearance from the body. Either the nonlinear phenytoin (PHT) pharmacokinetic response or the influence that carbamazepine (CBZ) exerts on PHT exposure could be explained throughout their efflux transporter inducer abilities. Cardiac output distribution difference between the individuals might also explain the dual CBZ-over-PHT interaction response. Finally, valproic acid (VPA) pharmacokinetics can be understood by adding to these mechanisms of transportation its ability to cross the mitochondrial membrane of the hepatocyte.

Relative bioavailability and comparative clinical efficacy of different ivermectin oral formulations in lambs

Background

Several oral ivermectin (IVM) formulations for use in sheep are available in the pharmaceutical veterinary market in different countries. All of them are indicated at the same dose rate to treat the gastrointestinal nematodes. However, there is a lack of information on the relative systemic exposure (plasma bioavailability) and clinical efficacy among oral formulations routinely used in sheep. The main goal of the work reported here was to perform a pharmaco-parasitological assessment of three different IVM oral formulations in lambs infected with multiple resistant gastrointestinal nematodes. The comparative drug systemic exposure (IVM plasma concentrations) and nematodicidal efficacies (clinical efficacy) in lambs were determined for a reference (RF) and two different test (T1, T2) IVM oral formulations. One hundred and fifty six (n= 156) healthy Corriedale lambs, naturally infected with multiple resistant gastrointestinal nematodes were allocated into four experimental groups (n=39). Animals in each group received treatment (200 μg/kg) with either the RF, one of the test IVM formulations or were kept as untreated control. Blood samples were collected over 15 days post-treatment (n=8). The IVM plasma concentrations were measured by high performance liquid chromatography with fluorescence detection. The faecal nematode egg count reduction test (FECRT) (n=39) and evaluation of the clinical efficacy were performed at day 14 post-treatment (n=6), where a predominance of IVM highly resistant nematodes was observed.

Results and conclusions

Neither the overall kinetic behaviour nor the IVM systemic exposure differed among all the tested oral formulations. Equivalent efficacy results were obtained for the different preparations, with an evident therapeutic failure to control Haemonchus spp. and Teladorsagia circumcincta, which correlates with a high degree of nematode resistance to IVM.

Impact of food administration on lopinavir-ritonavir bioequivalence studies

A bioequivalence study in 16 Caucasian healthy volunteers (eight male, eight female), comparing plasma drug concentrations after a single oral dose of lopinavir and ritonavir (400 and 100mg, respectively), was carried out following a two-period, two-sequence, two-treatment, randomized crossover design. Formulations were given 15 min after a moderate-fat breakfast in order to diminish both the intrinsic highly-variable performance and the sex differences observed in bioequivalence trials under fasting conditions. Ninety percent confidence intervals for the Test/Reference (T/R) ratio of geometric means for area under concentration-time curve (AUC) and maximum concentration (C(MAX)), either for lopinavir or ritonavir, were within the range of 0.80-1.25. Coprandial administration of formulations not only reduced the number of subjects required for bioequivalence assessment, reducing both ethical and economic cost of the trial, but also the sex differences in the T/R ratio of means.

Influence of efflux transporters on drug metabolism: theoretical approach for bioavailability and clearance prediction

Cytochrome P450 enzymes and efflux transporters, expressed in the intestine and/or in the liver, play important roles in drug clearance and oral bioavailability. The relative contribution of transporters and enzymes in drug metabolism is still controversial. Some antiepileptic drugs, such as carbamazepine, phenytoin and phenobarbital (phenobarbitone), show time-dependent and dose-dependent pharmacokinetics due to their inductive effect on both efflux transporters and enzymes. However, steady-state plasma drug concentrations for each antiepileptic drug do not relate to oral daily dose in the same way, with decreased or increased apparent clearance according to the drug. A multicompartment pharmacokinetic model was developed in order to explain these different behaviours using a single mechanism of inductive action. The key for solving these apparent dissimilarities was to consider in the model the unique physiological connection that intestine, liver and bloodstream have. Efflux transporters not only enhance enzymatic competition in relation to first-order processes, but also change the predominance of some elimination routes. For instance, the carbamazepine-10,11-epoxide formation increases at the expense of other carbamazepine metabolites, enhancing both the systemic and presystemic elimination of parent drug. Conversely, the major hepatic metabolism of phenytoin diminishes in favour of its minor intestinal elimination, decreasing the total drug clearance.

Total, unbound plasma and salivary phenytoin levels in critically ill patients

OBJECTIVE: To assess the reliability of salivary phenytoin (PHT) concentrations and predicted free PHT levels by Sheiner-Tozer equation in order to substitute measured free PHT concentrations in critically ill patients. METHODOLOGY: Twenty-four neurocritically ill adult patients receiving intravenous PHT were included in the study. Analyses of total, free plasma and saliva PHT concentrations were performed by fluorescence polarization immunoassay. Plasma albumin levels were also determined. RESULTS: Free PHT concentrations as well as salivary levels better correlate to clinical effect than total drug concentrations. Linear regression analysis showed a strong correlation between estimated free PHT concentrations by Sheiner-Tozer and measured free PHT levels (r=0.835; p<0.001) and salivary PHT concentrations and measured free PHT concentrations (r=0.964; p<0.001). Sheiner-Tozer equation could be misleading in the presence of displacing drugs. CONCLUSIONS: Saliva may serve as a feasible fluid to plasma in order to be used as a surrogate for free concentration monitoring of PHT in this population.

Systemic and presystemic conversion of carbamazepine to carbamazepine-10,11-epoxide during long term treatment

INTRODUCTION: Carbamazepine (CBZ) undergoes biotransformation, being CYP3A4 the major cytocrome P450 (CYP) enzyme catalyzing the carbamazepine-10,11-epoxide (EPOX) formation, which is quantitatively the most important pathway in CBZ metabolism. There is evidence of dose-dependent elimination of this drug due to its autoinduction capacity. Moreover, published data showed an incomplete bioavailability of CBZ since its absorption increases when grapefruit juice was administered. Both CYP3A4 and MRP2 (located in the enterocyte) are autoinduced during long term use of CBZ. As the other enzymes involved in CBZ metabolism are negligible in the gut, presystemic biotransformation through CYP3A4 could be responsible for the bioavailability of the drug as well as EPOX formation. OBJECTIVE: The purpose of our study was to assess the importance of presystemic formation of EPOX during the autoinduction of CBZ versus the daily administered dose. PATIENTS AND METHODS: 40 adults (average age: 28 years) and 29 children (average age: 9 years) receiving CBZ as monotherapy were included in the study. CBZ and EPOX plasma concentrations were analyzed by a previous validated HPLC method. RESULTS AND CONCLUSION: The results obtained confirmed the metabolic induction after chronic administration and provided new elements to suggest a strong contribution of dose-dependent bioavailability in the non linear kinetics of CBZ.

The Influence of Cardiovascular Physiology on Dose/Pharmacokinetic and Pharmacokinetic/Pharmacodynamic Relationships

Inter- and intraindividual variability in the relationship between dose and clinical--or pharmacodynamic--response of a drug can be analysed in two steps: firstly, by considering the plasma pharmacokinetic response to a given dose and, secondly, by the connection between both pharmacokinetic and pharmacodynamic responses. As the cardiovascular system is the means of transport of endogenous and exogenous substances, blood flow fraction destined to each organ determines the relative mass of solute in plasma, which is constantly in contact with the tissue. Hence, not only the rate but also the extent of drug transfer would be increased when tissues are irrigated by a higher fraction of cardiac output. Aging and circadian rhythms present similar cardiac output distribution patterns when moving from young to aged adult and from nocturnal to diurnal hours. These two changes lead to an increased blood flow delivery to the extra-splanchnic-renal region in the elderly and in the morning, but with a decreased cardiac output in aged individuals and an increased one during the day. This scenario allows us to forecast substance concentrations outside the blood vessels, which are responsible for the extent of drug elimination and the intensity of drug effect. So available data on disposition and pharmacodynamics of drugs might be explained from another point of view that challenges current knowledge. Furthermore, the administration of cardiovascular active drugs might reverse the chronological sequence between pharmacokinetic and pharmacodynamic responses, since they could modify blood flow distribution.

Multiplicative dependence of the first order rate constant and its impact on clinical pharmacokinetics and bioequivalence

The purpose of the study was to investigate the factors upon which the first order rate constant depends in order to assess the impact on the body-drug concentration when it changes throughout a treatment. A reasoning for linking the first order rate constants with several factors in a multiplicative way was proposed out taking into account kinetic and thermodynamic aspects. A multi-compartment model for drug disposition was analyzed and compartment mean drug concentrations at steady state were obtained as a function of the model kinetic constants. At the moment, only four factors were identified as responsible for the actual rate constant value. Apart from an intrinsic kinetic constant, they are the inverse of compartment volume, the transfer surface area, the fraction of mass able to be transferred, and the fraction of mass that effectively can be transferred. In clinical practice some of these factors might change throughout time (because of chronophysiological rhythms or drug availability at the action sites) and consequently, an inconstant extra-plasma/plasma drug concentration ratio could be obtained. In conclusion, therapeutic response prediction for a treatment does not upon plasma drug concentration monitoring. Equivalence in plasma drug exposure should not mean therapeutic equivalence, because differences in the rate and in the extent between test and reference products may induce a change in the action site/plasma drug concentration ratio.

In vitro approach to study the influence of the cardiac output distribution on drug concentration

Blood flow is not constant during the day, not only due to cardiac output variation but to the variable blood flow fraction supplied to the organs. To what extent these variations could affect the relative drug concentration between two different tissues, is the purpose of this work. In order to study that, a device was designed which took into account different fluid flows towards two flasks. Connections between flasks and pump are shown in text (figure 1). At the bottom of each flask a non-miscible liquid (dichloromethane: CH2Cl2, 100 mL, places 2 and 3) with the circulating fluid (water, 1350 mL, places 1, A, B) was placed. Malachite Green (MG) was introduced as solute into the system (place 1, 5 mL, 700 mg/L aqueous solution). The pump output was set at 1080 mL/min, serving more fluid (five times) to flask A than to flask B. Samples were drawn from compartments 1, A and B, and from organic compartments, during experiences taking several hours. MG concentrations were colorimetrically measured at 619 nm. Room temperature was between 15-25 degrees C. CH2Cl2 MG concentration ratio([2]/[3]) was similar to fluid flow ratio (sigma/(1-sigma)) all over the experience time. Other experiences changing the stop time, room temperature, CH2Cl2 volumes and places, pump output distribution, led us to different evidences that supported a theoretical model. As a conclusion, the most important feature was that whereas MG concentrations in water (1, A, B) were close, in CH2Cl2 these were very different. So, is drug effect prediction reliable by monitoring free drug concentrations in blood?

The influence of cardiac output distribution on the tissue/plasma drug concentration ratio

In pharmacokinetics, it is currently assumed that blood and interstitial spaces belong to the central compartment, when the solute is quickly equilibrated between both fluids. Taking into account that the same extracellular fluid dissolves the drug either in the plasma or in the interstitium, both drug concentrations (intra and intravascular) should be identical. However, this equality may not exist when there is a dissimilar distribution of blood flow among the organs. A closed, two-zone, three-compartment model was mathematically investigated. Compartment 1 was intravascular, and compartments 2 and 3 were extravascular. The fluid within the compartment 1 was supposed to be pushed by a pump, and to be distributed towards two different zones. One of these zones was in contact with the compartment 2, and the other with compartment 3. The drug could be exchanged between the compartments 1 and 2 or 1 and 3, by mean of first-order kinetics (k12, k21, k13, k31). It was assumed a very fast flow that assured instantaneous homogeinity of drug concentration in compartment 1. Pressure was kept constant, so an increase in the pump output distribution towards one zone is compensated by a decrease towards the other zone. At time infinite the drug concentration (C) ratio between compartments yielded: C2/C1 = k12.sigma.V1/(k21.V2) and C3/C1 = k13.(1-sigma).V1/(k31.V3), being sigma: the pump output fraction served to the zone where compartment 2 was located, and Vi: the volumes of compartments. So, at the equilibrium the concentrations are not necessarily identical between the extravascular and intravascular sites. In conclusion, as the cardiac output distribution changes due to circadian rythms and cardiovascular active drug administration, current therapeutic drug monitoring and bioequivalence studies using plasma as biologic fluid would be controversial issues.

Lipidic matrix of albendazole sulphoxide: is it an alternative for systemic infections?

As albendazole sulphoxide (ABZS) shows better dissolution properties than albendazole (ABZ), a lipidic matrix with this drug was formulated in order to evaluate if its absorption and so systemic infection chemotherapy could be improved. A cross-over, randomised study in 8 healthy volunteers was carried out, after single administration of 1 g of albendazole or albendazole sulphoxide in lipidic matrix of Gelucire 44/14 (ABZLM and ABZSLM). Absorption was followed performing albendazole sulphoxide dosage in urine samples by high pressure liquid chromatography analysis, during 48 hours. Significant differences were found (p = 0.02) between the urinary recoveries (% E48), being 1.74% and 0.19% the percentage of dose recovered when ABZSLM or of ABZLM were respectively administered. In a previous study of our group similar values were obtained of urinary recovery percentages after albendazole sulphoxide powder administered to another group of healthy volunteers. Lipidic matrix does not improve the physicochemical properties of albendazole sulphoxide powder.

Lipidic matrix of albendazole: an alternative for systemic infections

Albendazole is a poorly water soluble drug, with low oral bioavailability, used in pharmacological treatment of a systemic disease as hydatid parasitosis. Lipidic matrices of Gelucires (44/14 and 35/02) were developed. After "in vitro" studies, one formulation was chosen for a single dose study in 8 healthy volunteers, with a cross-over and randomised design, taking a commercially available tablet as reference. Drug absorption was followed by albendazole sulphoxide dosage in urine by high pressure liquid chromatography. Neither albendazole nor albendazole sulphoxide were recovered in urine after tablet administration while 0.18% (+/- 0.06) of dose was recovered after lipidic matrix administration in the first 24 hours. Besides ageing control were performed up to 18 months post-elaboration. Lipidic matrix with Gelucire 44/14 was revealed as a promising attempt for oral pharmaceutical form in albendazole systemic treatment.

Average parameters as a trend to reduce the residual variability in bioequivalence trials

Bioavailability and bioequivalence evaluations of drug products carried out using the experimental maximum concentration (Cmax) and the experimental time to reach Cmax (Tmax), are not advisable for slow-release formulations and for trials performed with saliva as biologic fluid. When slow-release curves are considered the drug concentration profiles usually show multiple peaks, making it difficult to compute a Cmax,Tmax value. The saliva profile throughout time shows a high variability observed as more than one peak in the saliva concentration versus time curves. In both cases, even if there is a major peak, when the statistical analysis of the data is performed, an important variability in Cmax results in high values in the residual variance of the ANOVA test. Consequently, the power of the bioequivalence test decreases and sometimes it is not possible to conclude on bioequivalence. The average concentration (Cav), the average maximum concentration (Cmax,av) and Cmax,av/Cav x 100 (%Cmax,av) are proposed in this paper as possible parameters in order to evaluate the profile of the concentration-time curves, as they reduce the residual variability in bioequivalence studies.

Average parameters in bioavailability studies: an application to slow-release amitriptyline formulation

In order to assess the extent and the rate of absorption in bioavailability studies, area under the curve (AUC), experimental maximum concentration (Cmax) and experimental time to reach Cmax (Tmax), are used. But when slow-release formulations are considered, the drug concentration-time curves usually show multiple peaks, and it is difficult to compute a Cmax and Tmax value. In case a Cmax value is computed, important variability in this parameter results in high values in the residual variance of the ANOVA test. So in order to decrease the high variability, average parameters: average concentration (Cav), average maximum concentration (Cmax,av) and Cmax,av x 100/Cav (%Cmax,av), are proposed. These new parameters were applied in a bioavailability study of slow-release amitriptyline formulation.

Bioavailability comparison between albendazole and albendazole sulphoxide in rats and man

Albendazole (ABZ) is a broad spectrum anthelmintic benzimidazole with very low bioavailability, and its activity is due to its main metabolite, Albendazole sulphoxide (ABZS). This work demonstrates the improvement of bioavailability when the ABZS is directly administered, compared with the ABZ administration, both orally given. This observation may be used as an interesting target in the design of new drugs with antihelmintic activity in systemic diseases, using ABZS as a parent drug.

Influence of the emulsion sign in phenytoin bioavailability

To study the influence of the emulsion sign in phenytoin bioavailability, two emulsions (w/o and o/w) were prepared. Bioavailability studies were carried out with both emulsions in Wistar male rats. The study was of a randomized two-way cross over design. A radiotracer technique was chosen as analytical procedure, due to the small blood sample collected. 14C-phenytoin was synthesized with a high yield and suitable radiochemical purity. It is concluded, from a biopharmaceutic point of view, that the emulsion sign does not seem to affect the amount of phenytoin absorbed, as it is shown through the comparison of the areas under curve up to 24 h. An emulsion with oil as an external phase is responsible of a longer absorption, taking into account the apparent elimination constant rates.

Bioavailability study of furosemide prodrugs in rats

According to the hydrolysis performance "in vitro" and lipophylicity, two Furosemide (F) prodrugs were chosen from a series of acyloxymethylesters of F synthesized previously: P1 (acetyloxymethyl-4-chloro-N-furfuryl-5-sulfamoylanthranilate ) and P4 (pivaloyloxymethyl-4-chloro-N-furfuryl-5-sulfamoylanthranilate+ ++). The bioavailability studies were assayed over two groups of eight male Wistar rats as a randomized two-way crossover and balanced design: group 1) a solution of P1 in propylenglycol/ethyl acetate vs an aqueous solution of F, and group 2) P1 vs. P4 solutions in oleic acid (P1#, P4#). These assays showed a better absorption performance of P1 and P4 than F, while the two prodrugs showed a similar bioavailability. The oleic acid seems to be responsible for the delay in the recovery of 50% of the total amount of F excreted in urine (T50%). When the monitoring is done in plasma after the administration of P1#, P1 was not detected as circulating prodrug. The analytical determinations of F in urine and plasma were done by high performance liquid chromatography (HPLC). From the urinary excretion data, a slope that indicates a slow elimination was found with a half-life of 12 hours approximately.

Development of absorption furosemide prodrugs: synthesis, in vitro and in vivo evaluation

Six acyloxymethyl esters of Furosemide were synthesized and the structures were determined by chemical and spectroscopic methods. Lipophilicity parameters were analysed by high performance liquid chromatography (HPLC). Hydrolysis performances in human plasma and intestinal fluids anticipate their properties as absorption prodrugs of Furosemide. A bioavailability study carried out with 8 male Wistar rats with one of the synthesized prodrug (acetyloxymethyl 4-chloro-N-furfuryl-5-sulfamoylanthranilate) showed a greater absorption in relation to Furosemide. The percentages of mean urinary recovery of Furosemide for the prodrug and for the standard solution of the drug were 20.84 and 14.36 respectively. The doses were 10 mg/Kg in Furosemide. The analytical determinations of Furosemide in biological fluids were done by HPLC.

Alternative approach to relative bioavailability and bioequivalence evaluation, with drugs following Michaelis-Menten elimination kinetics

An alternative approach to bioavailability and bioequivalence assessment is presented. By a modified Wagner-Nelson procedure, the parameters of a monocompartmental model are calculated, after single oral dose administration trials. The usefulness of the procedure described here is that it permits comparison between two different brands of drug in multiple doses, without the need to administer repeated doses. Only one dose is necessary in order to calculate model parameters and infer steady-state levels.