Absorption Differences between Immediate-Release Dexmethylphenidate and dl-Methylphenidate

Quantitative Characterization of the Smoothness of Extended-release Methylphenidate Pharmacokinetic Profiles

OBJECTIVE

Extended-release methylphenidate (ER-MPH) formulations used to treat attention deficit hyperactivity disorder (ADHD) have complex pharmacokinetic (PK) profiles, resulting from differing ratios of immediate-release and extended-release components and/or their site of absorption. This study aimed to evaluate the smoothness of PK curves of ER-MPHs.

DESIGN

The integral of the second derivative squared was evaluated for modeled PK curves, with smaller values indicating a smoother curve. The calculated smoothness of each PK curve was normalized by dividing by Cmax 2 to derive a normalized smoothness parameter appropriate across the dose range of each formulation. Calculations used modeled PK curves from 100mg delayed-release and ER-MPH (DR/ER-MPH), 54mg osmotic release oral system MPH (OROS MPH), 60mg MPH controlled-release delivery (MPH CD), 60mg ER-MPH oral suspension (MEROS), 20mg ER dexmethylphenidate (d-MPH ER), and 60mg multilayer-release MPH (MLR-MPH).

RESULTS

The Cmax2-normalized smoothness value was consistent across DR/ER-MPH doses, allowing for relevant comparisons across formulations. Normalized smoothness values differed widely; the lowest normalized smoothness was 0.05 with DR/ER-MPH and ranged up to 9.56 with d-MPH ER.

CONCLUSION

DR/ER-MPH demonstrated a smoother PK profile compared to the highest dose of other ER-MPH formulations. While the benefits of a smooth PK profile remain to be tested clinically, having fewer peaks and troughs has been hypothesized to reduce waxing and waning of therapeutic effects throughout the day, and more gradual changes in MPH plasma levels have been hypothesized to lower the risk of likeability and potentially abate afternoon symptom rebound.

Potential for Underestimation of d-Methylphenidate Bioavailability Using Chiral Derivatization/Gas Chromatography

A tenable hypothesis is presented which explains disparities between older oral dl-MPH bioavailability data generated using chiral derivatization-gas chromatography versus more recent findings using chiral liquid chromatography. These disparities persist in current literature. The gas chromatographic methods found that the absolute bioavailability of d-MPH is 23% and that of l-MPH is 5% (i.e., 82% as the active d-isomer), while liquid chromatographic methods consistently report that approximately 99% of circulating MPH is d-MPH. Older methods used perfluoroacylated S-prolyl derivatizing agents which have a history of imprecision due to the susceptibility of the prolyl S-configuration to isomerize to the R-enantiomer. Accordingly, any R-prolyl impurity in the chiral derivatization reagent yields the (R,R,R)-MPH-prolyl diastereomer which, in being related as the opposite enantiomer of (S,S,S)-prolyl-MPH, co-elutes with l-(S,S)-MPH. This results in overestimation of the percent l-MPH at the expense of underestimating d-MPH. Unless compelling reasons exist to justify use of any chiral discriminators, less complex and less costly achiral analysis of plasma MPH appears appropriate for d-MPH quantitation since 99% exists as d-MPH. However, simultaneous plasma monitoring of d-MPH and l-MPH may be warranted when alterations in first-pass hepatic metabolism by carboxylesterase 1 (CES1) occurs. For example, (a) with transdermal dl-MPH delivery; (b) in cases of concomitant dl-MPH and a CES1 inhibitor, e.g., ethanol, which elevates l-MPH and d-MPH concentrations; (d) in forensic studies of intravenous or intranasal dl-MPH abuse; (e) were dl-MPH to be formulated as a free base sublingual product; or (f) as emerging advances in dl-MPH gene-dose effects warrant isomer correlations.

Subterminal hydroxyeicosatetraenoic acids: Crucial lipid mediators in normal physiology and disease states

Cytochrome P450 (P450) enzymes are superfamily of monooxygenases that hold the utmost diversity of substrate structures and catalytic reaction forms amongst all other enzymes. P450 enzymes metabolize arachidonic acid (AA) to a wide array of biologically active lipid mediators. P450-mediated AA metabolites have a significant role in normal physiological and pathophysiological conditions, hence they could be promising therapeutic targets in different disease states. P450 monooxygenases mediate the (ω-n)-hydroxylation reactions, which involve the introduction of a hydroxyl group to the carbon skeleton of AA, forming subterminal hydroxyeicosatetraenoic acids (HETEs). In the current review, we specified different P450 isozymes implicated in the formation of subterminal HETEs in varied tissues. In addition, we focused on the role of subterminal HETEs namely 19-HETE, 16-HETE, 17-HETE and 18-HETE in different organs, importantly the kidneys, heart, liver and brain. Furthermore, we highlighted their role in hypertension, acute coronary syndrome, diabetic retinopathy, non-alcoholic fatty liver disease, ischemic stroke as well as inflammatory diseases. Since each member of subterminal HETEs exist as R and S enantiomer, we addressed the issue of stereoselectivity related to the formation and differential effects of these enantiomers. In conclusion, elucidation of different roles of subterminal HETEs in normal and disease states leads to identification of novel therapeutic targets and development of new therapeutic modalities in different disease states.

The impact of methylphenidate and its enantiomers on dopamine synthesis and metabolism in vitro

Methylphenidate (MPH), a psychostimulant, is an effective first-line treatment for the symptoms associated with Attention-Deficit/Hyperactivity Disorder (ADHD). Although most MPH formulations are composed of the racemic 1:1 mixture of the two enantiomers (d- and l-threo), converging lines of evidence indicate that d-threo MPH seems to be superior to the l-isomer. We aimed to investigate whether MPH racemic mixture or pure enantiomers influence the enzyme activity of tyrosine hydroxylase (TH), monoamine oxidase B (MAO-B), catechol-O-methyltransferase (COMT), and aldehyde dehydrogenase (ALDH) in vitro in homogenates of rat PC12 cells incubated with racemic, d- and l-threo MPH (1nM up to 100μM), or a vehicle for control. We could observe dose dependent enhancement of TH activity with d-threo MPH, probably due to its higher affinity to the enzyme, which we could confirm for d-threo versus l-threo MPH via docking and molecular dynamic simulations analysis. MAO-B enzyme activity was found to be enhanced when incubated with both d- and l-isomers but not with the racemic mixture. This conflicting result was hypothesized to be due to possible aggregation of the two enantiomers or other molecular conformations. Such a possible interaction was observed indirectly, when TH was incubated with constant d-threo MPH while increasing l-isomer (increasing total MPH concentrations). Hence, TH activity was slightly decreased with increased l-isomer. In conclusion, the current in vitro investigation points to the stereoselectivity of the investigated enzymes and pharmacological effects of MPH enantiomers.

The Clinical Pharmacokinetics of Amphetamines Utilized in the Treatment of Attention-Deficit/Hyperactivity Disorder

Amphetamine (AMP), an indirectly acting psychostimulant approved for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children, adolescents, and adults, is among the most long-standing therapeutic agents in all of clinical psychopharmacology. This review focuses on AMP absorption, metabolism, and elimination brought to bear on comparative pharmacokinetics in its various formulations. A comprehensive search of the published literature was conducted using MEDLINE (PubMed) and Google Scholar databases through April 2017 to retrieve all pertinent in vitro and human studies for review and synthesis. Additionally, Food and Drug Administration (FDA) databases were accessed for otherwise unavailable data when possible. Initially available as racemic (dl)-AMP, this drug was later supplanted by enantiopure (d)-AMPH or enantioenriched (75:25 dl)-AMP formulations; although racemic AMP returned as an approved drug to treat ADHD in 2014. Presently, there are several immediate-release (IR) formulations available, including d-AMP, dl-AMP, and mixed amphetamine salts, which are neither racemic nor the pure d-enantiomer (i.e., a 3:1 mixture of d-AMP and l-AMP). Furthermore, new modified-release AMP formulations, including an oral suspension and an orally disintegrating tablet, are now available. A lysine-bonded prodrug form of d-AMP also serves as a treatment option. Oral AMP is rapidly absorbed, with high absolute bioavailability, followed by extensive metabolism involving multiple enzymes. Some metabolic pathways exhibit stereoselective biotransformations favoring the l-isomer substrate. Drug exposure exhibits dose-proportional pharmacokinetics. Body weight is a fundamental determinant of differences in observed AMP plasma concentrations. IR formulations typically provide a T_max from 2 to 3 hours. In replicated studies, children exhibit a shorter plasma T_1/2 (∼7 hours) relative to adults (∼10 to 12 hours). There are few documented pharmacokinetic drug interactions of clinical significance beyond influences of drug-induced alteration of urinary pH. The array of AMP formulations addressed in this review offer flexibility in dosing, drug onset, and offset to assist in individualized pharmacotherapy of ADHD.

Ethanol Interactions With Dexmethylphenidate and dl-Methylphenidate Spheroidal Oral Drug Absorption Systems in Healthy Volunteers

BACKGROUND/PURPOSE

Ethanol coadministered with immediate-release dl-methylphenidate (dl-MPH) or dexmethylphenidate (d-MPH) significantly increases the geomean maximum plasma concentration (Cmax) of d-MPH 22% and 15%, respectively, and elevates overall drug exposure and psychostimulant effects. We asked the question: Are these ethanol-MPH interactions based more fundamentally on (1) inhibition of postabsorption d-MPH metabolism or (2) acceleration of MPH formulation gastric dissolution by ethanol in the stomach? This was investigated using the pulsatile, distinctly biphasic, spheroidal oral drug absorption systems of dl-MPH and d-MPH.

METHODS

In a randomized, 4-way crossover study, 14 healthy subjects received pulsatile dl-MPH (40 mg) or d-MPH (20 mg), with or without ethanol (0.6 g/kg), dosed 4 hours later. These 4 hours allowed the delayed-release second MPH pulse to reach a more distal region of the gut to preclude gastric biopharmaceutical influences. Plasma was analyzed using a highly sensitive chiral method. Subjective/physiological effects were recorded.

FINDINGS/RESULTS

Ethanol increased the second pulse of d-MPH Cmax for dl-MPH by 35% (P < 0.01) and the partial area under the plasma concentration curve from 4 to 8 hours by 25% (P < 0.05). The respective values for enantiopure d-MPH were 27% (P = 0.001) and 20% (P < 0.01). The carboxylesterase 1-mediated transesterification metabolite ethylphenidate served as a biomarker for coexposure. Ethanol significantly potentiated stimulant responses to either formulation.

IMPLICATIONS/CONCLUSIONS

These findings support drug dispositional interactions between ethanol and MPH as dominant over potential biopharmaceutical considerations. Understanding the pharmacology underlying the frequent coabuse of MPH-ethanol provides rational guidance in the selection of first-line pharmacotherapy for comorbid attention-deficit/hyperactivity disorder-alcohol use disorder.