Plasma dopa concentrations after different preparations of levodopa in normal subjects

Effects of medicines used to treat gastrointestinal diseases on the pharmacokinetics of coadministered drugs: a PEARRL Review

Objectives

Drugs used to treat gastrointestinal diseases (GI drugs) are widely used either as prescription or over-the-counter (OTC) medications and belong to both the 10 most prescribed and 10 most sold OTC medications worldwide. The objective of this review article is to discuss the most frequent interactions between GI and other drugs, including identification of the mechanisms behind these interactions, where possible.

Key findings

Current clinical practice shows that in many cases, these drugs are administered concomitantly with other drug products. Due to their metabolic properties and mechanisms of action, the drugs used to treat gastrointestinal diseases can change the pharmacokinetics of some coadministered drugs. In certain cases, these interactions can lead to failure of treatment or to the occurrence of serious adverse events. The mechanism of interaction depends highly on drug properties and differs among therapeutic categories. Understanding these interactions is essential to providing recommendations for optimal drug therapy.

Summary

Interactions with GI drugs are numerous and can be highly significant clinically in some cases. While alterations in bioavailability due to changes in solubility, dissolution rate, GI transit and metabolic interactions can be (for the most part) easily identified, interactions that are mediated through other mechanisms, such as permeability or microbiota, are less well-understood. Future work should focus on characterising these aspects.

Drug-drug interactions in pharmacologic management of gastroparesis

BACKGROUND

Gastroparesis is a disorder characterized by delayed gastric emptying due to chronic abnormal gastric motility. The treatment of the disease often entails the co-administration of several classes of pharmacological agents. These agents may be metabolized via the same pathway. Inhibition or induction of a shared metabolic pathway leads to change in the systemic levels of prescribed drugs, possibly leading to undesired clinical outcomes.

PURPOSE

This review discusses different pharmacological treatment for gastroparesis patients and describes the potential for drug-drug interactions (DDIs) in some of the combinations that are currently used. Prokinetic agents such as metoclopramide and domperidone are the cornerstone in treatment of gastroparesis. Antiemetic agents such as promethazine and ondansetron are frequently administered to gastroparesis patients to reduce nausea and vomiting. Gastroparesis is prevalent in diabetic patients and therefore antidiabetic agents are also prescribed. Many of these co-administered drugs are metabolized via common drug metabolizing enzymes and this can trigger potential DDIs. The scientific literature was reviewed from the years 1975-2014 for original research articles and reviews that evaluated DDIs in gastroparesis. Many commonly prescribed combinations were predicted to cause potential DDIs in gastroparesis patients. This review will help inform about potential hazardous combinations. This information will hopefully lead to less adverse effects and more successful gastroparesis management.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

18F-dopa PET evidence that tolcapone acts as a central COMT inhibitor in Parkinson's disease

Tolcapone is a potent, selective, and reversible inhibitor of cathecol-O-methyl-transferase (COMT). This enzyme plays a crucial role in the extraneural inactivation of catecholamine neurotransmitters. Tolcapone's ability to inhibit central COMT in humans at therapeutic concentrations is not yet clear. The aim was to determine the effect of tolcapone on central COMT activity in Parkinson's disease (PD) using (18)F-dopa positron emission tomography (PET). The study was a randomized two-way crossover study. Twelve PD patients were recruited. On the treatment days patients were given either tolcapone (200 mg) or placebo together with levodopa/carbidopa (100/125 mg) 1 h before the injection of (18)F-dopa. Data were acquired in 25 frames over 94 min for the first PET scan period. At the end of this period the patients were removed from the scanner for 90 min and subsequently repositioned and data acquired in six 10-min time frames over 60 min. Influx constants (Ki) were computed using a graphical approach with a plasma input function. Mean (18)F-dopa putamen Ki's for the first 30-90 min, primarily reflecting central dopa decarboxylase (DDC) activity, were similar in PD patients whether tolcapone was present (0.0078 +/- 0.0031 min(-1)) or absent (0.0078 +/- 0.0030 min(-1)). Mean putamen Ki values calculated 180-240 min after injection of (18)F-dopa, reflecting both central DDC and COMT activity, were unchanged from 30-90' values in the presence of tolcapone (0.0079 +/- 0.0030), implying blockade of central COMT, but were significantly reduced (0.0059 +/- 0.0028) in the absence of this drug. These findings are compatible with clinical doses of tolcapone having a significant blocking effect on peripheral and central COMT but not DDC activity in PD.

New, selective catechol-O-methyltransferase inhibitors as therapeutic agents in Parkinson's disease

Levodopa remains the most effective drug for Parkinson's disease (PD). However, its benefits are limited owing to extensive metabolism by catechol-O-methyltransferase (COMT), especially if levodopa is used in combination with peripheral dopa-decarboxylase inhibitors. A new generation of potent, orally active, selective, and reversible COMT inhibitors has become available recently. Among these, tolcapone and entacapone have been best characterised. Preclinical and clinical studies have shown that COMT inhibitors markedly enhance levodopa availability and prolong its plasma half-life. In recent large clinical trials they proved to be able to ameliorate motor fluctuations, reduce disability, and decrease levodopa requirements in PD patients. The tolerability profiles of entacapone and tolcapone are good. COMT inhibition promises to become an important means of extending the benefits of levodopa therapy in PD.

Pharmacokinetic drug interactions with gastrointestinal motility modifying agents

Drugs may affect gastrointestinal motility and, therefore, absorption of other concomitantly administered drugs. Gastrointestinal prokinetic agents increase the rate of gastric emptying and also upper intestinal motility. These effects would be expected to increase the initial rate of absorption of orally administered drugs, but reduce total bioavailability of the agents. Metoclopramide has been shown to increase the rate of absorption of several classes of drug, reflected by reduced time taken to achieve maximal plasma concentration (tmax) and increased maximal plasma concentration (Cmax). However, the effect of these agents on the area under the plasma concentration-time curve from zero to infinity (AUC0-infinity), when measured, is not consistent. Cisapride and domperidone appear to have similar effects, but there are relatively less data available regarding these products. Opioids may delay gastric emptying considerably, an effect which will often have significant clinical and therapeutic implications. Most of the data confirming this observation concern oral analgesics, but the effect should be considered when prescribing any oral medication. Drugs with anticholinergic or sympathomimetic activity are likely to have a similar effect and this is confirmed, in the main, by the limited data available. Although many effects reported in the literature are of limited clinical importance, they may be significant when prescribing a drug with a narrow therapeutic index, especially if it is absorbed poorly.

A comparison of standard Madopar and controlled release Madopar in Parkinson's disease

In a randomised, double-blind cross-over study, objective measures were used to compare the effect of a single oral dose of a standard preparation of levodopa-benserazide (Madopar M) with a sustained-release preparation (Madopar HBS) in 9 Parkinsonian patients with 'end of dose deterioration'. The response of patients to an optimised regimen of each preparation was also assessed using a patient diary. In all patients the onset of effect of Madopar HBS following a single dose was delayed compared with Madopar M. The duration of effect of a single dose of Madopar HBS was substantially (38-120%) longer than Madopar M in five patients and the same or shorter in four patients. According to the patients' diaries, six patients noted an increase in 'on' hours while on Madopar HBS. The duration and severity of dyskinesia was similar for the two preparations. Madopar HBS is likely to be useful in some Parkinsonian patients with 'end of dose deterioration'. As the effect of each dose is delayed it is probably best given in combination with standard Madopar.

Co-administered oral metoclopramide does not enhance the rate of absorption of oral diazepam

Intravenous metoclopramide is known to increase the rate of absorption of oral diazepam if administered at the same time. It has been suggested that oral metoclopramide has the same effect. In this study, six healthy volunteers received oral diazepam (0.2 mg/kg) on two separate occasions, either alone or with oral metoclopramide (10 mg), given simultaneously. In contrast to the effects of intravenous metoclopramide, oral metoclopramide did not increase the rate of absorption of oral diazepam.

Clinical pharmacokinetics of anti-parkinsonian drugs

Of the neurological disorders, none can claim a battery of therapeutic agents based upon as rational a pharmacology as can Parkinson's disease. In this review, the clinical pharmacokinetics of the major classes of anti-Parkinsonian drugs is discussed. Although they are the oldest drugs in the anti-Parkinsonian armamentarium, little pharmacokinetic data are available regarding the anticholinergic and antihistaminic agents. Based on elimination half-lives of 10 to 18 hours, most could probably be effectively given on a twice-daily schedule. Amantadine is unique among anti-Parkinsonian agents both in lacking a clearly defined mechanism of action and in being eliminated from the body exclusively by renal excretion of unchanged drug. Thus the normal decline of renal function in the elderly Parkinsonian population becomes an important factor in avoiding potential drug toxicity. The pharmacokinetics and pharmacodynamics of levodopa are complex. Since it is an amino acid, it follows metabolic pathways and must compete for absorption and brain uptake with a number of large neutral amino acids. It has a short elimination half-life and, as Parkinson's disease progresses, the brain loses its capacity to store the drug and becomes dependent in a moment-to-moment fashion on plasma levodopa concentrations, creating therapeutic response fluctuations in over 50% of patients. Pharmacokinetic considerations in the management of these response fluctuations are discussed. The newest class of anti-Parkinsonian agents are the direct acting dopamine receptor agonists. These drugs, all derivatives of ergot, have more prolonged durations of anti-Parkinsonian action than levodopa. However, other than bromocriptine, clinical experience with members of this class of drugs is still limited.

The pharmacokinetics of intravenous and oral levodopa in patients with Parkinson's disease who exhibit on-off fluctuations

We have studied the clinical effects and pharmacokinetics of levodopa infusions and oral therapy in seven patients with Parkinson's disease. They all showed on-off fluctuations whilst receiving long-term treatment with levodopa in combination with a peripheral decarboxylase inhibitor. Intravenous infusion at a constant rate for up to 16 h resulted in a smoother clinical response, and maintained plasma levodopa concentrations within narrower limits compared with conventional oral therapy. Following infusion rates of 32-80 mg h-1 (0.5-1.3 mg kg-1 h-1) the plasma concentration associated with optimum therapeutic response lay between 0.3 and 1.6 mg l-1. There was considerable variation in the oral absorption and elimination of levodopa, both within and between subjects. The concentration of 3-OMe dopa in plasma hardly increased during each day's levodopa therapy. In all cases levels were greater than the maximum concentrations of levodopa, sometimes by as much as a factor of 10. In contrast to most previous reports on the pharmacokinetics of levodopa, the data presented here are consistent with a two-compartment kinetic model. It is not known whether the difference in pharmacokinetics is due to chronic therapy or whether it is specific to those patients who show on-off phenomena, but such changes might be related in some way to the development of fluctuations in clinical response.

The effects of domperidone on the absorption of levodopa in normal subjects

The effect of simultaneous oral administration of 20, 40, or 80 mg domperidone on the pharmacokinetics of an oral 500 mg dose of levodopa was studied in eight normal women. No significant differences in maximum plasma levodopa concentration, the time of its attainment, or the area under the plasma levodopa concentration versus time profile occurred. Domperidone significantly reduced the incidence of adverse gastrointestinal effects due to levodopa administration.

The effect of carbidopa on the pharmacokinetics of intravenously administered levodopa: the mechanism of action in the treatment of parkinsonism

The pharmacokinetics of short and long intravenous infusions of levodopa with and without concurrent oral administration of carbidopa was studied in 9 parkinsonian patients. Carbidopa reduced by 50% both the infusion rate required to produce a clinical response and the time required for plasma clearance of levodopa. Using this value for clearance, it is estimated that carbidopa doubles the bioavailability of orally administered levodopa. Carbidopa did not alter the therapeutically effective plasma concentration of levodopa, suggesting that carbidopa does not modify the so-called enzymatic blood-brain barrier. The decline of the plasma levodopa concentration was biphasic; carbidopa modestly increased half-lives of both phases. The apparent volume of distribution was not significantly altered. Carbidopa did not change the duration of the clinical response after the discontinuation of short infusions. From these observations we infer that the therapeutic effects of carbidopa can be attributed to doubling the bioavailability of orally administered levodopa and halving its plasma clearance.

The treatment of Parkinson's disease

In the management of Parkinson's disease physicians now have at their disposal a number of useful therapeutic tools (Figure 3). (Formula: see text) Anticholinergic drugs and amantadine suffice in the early stages of the disease when little disability is present. Levodopa combined with a peripheral decarboxylase inhibitor is the treatment of choice thereafter, and with the appearance of fluctuations it is necessary to increase the frequency of dosage of levodopa and to consider adding bromocriptine. Stereotactic surgery is useful in patients with severe tremor which is unresponsive to drug therapy.

Anti-parkinsonian drugs today

The drug treatment of Parkinson's disease has progressed through 3 main stages: firstly, the use of anticholinergic drugs and amantadine; then the introduction of levodopa and its association with peripheral decarboxylase inhibitors; and finally the use of direct acting dopamine agonist drugs. Levodopa, however, remains the most effective single drug in Parkinson's disease. Unfortunately, the side effects associated with long term levodopa treatment today constitute an important cause of functional disability. 'Positive' side effects such as involuntary movements and psychiatric disorder remain difficult to manage without causing an increase in Parkinsonian immobility; conversely, management of the 'negative' phenomenon of the 'off' period is limited by these same side effects. This has generated renewed interest in the highly complex pharmacokinetic and pharmacodynamic properties of levodopa, with the aim of reducing fluctuations in plasma levodopa (and hence brain dopamine) concentrations by sustained release oral or continuous parenteral administration of the drug.

Interactions affecting drug absorption

The influence of drug-drug and drug-food interactions affecting the absorption of orally administered medication is reviewed. Drug-drug interactions can be classified in terms of indirect effects by one drug on gastrointestinal tract physiology influencing the absorption of other drugs, or direct interactions involving altered pH, adsorption, absorption, or chelation. Most, but not all, drug-drug interactions result in reduced or delayed systemic drug availability. Drug-food interactions may result in reduced, delayed, or increased systemic drug availability. The absorption of only a small number of drugs is unaffected by concomitant food intake. The degree of interaction and whether it positively or negatively affects drug absorption depends on a number of factors including the physical and chemical nature of the drug, the formulation, the type of meal, and the time interval between eating and dosing. Mechanisms of drug-food interactions are not well characterised. They clearly involve both direct and indirect factors in a similar fashion to drug-drug interactions, but indirect factors probably predominate. Reduced or delayed drug absorption is generally attributed, at least in part, to delayed stomach-emptying due to food. Increased absorption may also result from delayed stomach-emptying facilitating greater drug dissolution before it passes from the stomach into the small intestine. Increased bioavailability of some drugs, e.g. propranolol, metoprolol and labetalol, may be related to reduced presystemic clearance. The potential clinical implications of drug-drug and drug-food interactions must be taken into account with oral medications in order to minimise variations in systemic drug availability and hence in clinical efficacy.

Factors contributing to variability in drug pharmacokinetics. I. Absorption

Apart from the physical and chemical properties of a drug, and also the dosage form in which it is presented, many other factors may affect the absorption of orally administered compounds and give rise to variable systemic availability. Three such factors, gastrointestinal (GI) disease, drug-drug interactions, and drug-food interactions are considered here. Although information regarding these factors, particularly GI disease, is scarce and sometimes conflicting, it is clear that they may give rise to variable drug absorption. The wide spectrum of effects on different drugs precludes the application of general rules and guidelines in drug therapy in most cases. Drugs, dosage forms, and various interactions should be considered individually. Also the types of interactions described in this review must be combined with other factors to be discussed later in this series when considering the influence of variable absorption, distribution, metabolism, and excretion on drug pharmacokinetics and clinical efficacy.

The management of Parkinson's disease

It is useful to divide Parkinsonian patients into those whose signs are confined to tremor, rigidity and akinesia, and those with evidence of a more diffuse disturbance. The treatment of choice in the former is levodopa combined with a peripheral decarboxylase inhibitor. At the onset of the disease, when disability is minimal, amantadine or anticholinergic drugs may suffice. Bromocriptine is useful in some patients who derive only short-lived benefit from each dose of levodopa. The role of stereotactic surgery is now confined to patients with an incapacitating unilateral tremor which has not improved with drug therapy. In elderly patients with evidence of diffuse cerebral dysfunction such as dementia, grasp reflex, hyper-reflexia or severe postural hypotension, the beneficial effect of these drugs is often outweighed by the side effects. Small doses of levodopa alone may be tried. Anticholinergic drugs and amantadine should be avoided in such patients.

Dosage form design for improvement of bioavailability of levodopa II: bioavailability of marketed levodopa preparations in dogs and parkinsonian patients

To estimate the absolute bioavailability of oral levodopa, plasma concentrations and urinary excretion of levodopa and its metabolites were determined in beagle dogs and in parkinsonian patients after intravenous and oral drug administration. The absolute bioavailability of orally administered levodopa was estimated to be about 35% in both dogs and patients; however, the total amount absorbed of intact drug and levodopa metabolites was estimated to be 80--90% of the administered dose. Due to the similarities of the pharmacokinetic characteristics of levodopa found in beagle dogs and in humans, beagle dogs can serve as a model to study bioavailability, absorption, and metabolic mechanisms.

Drug absorption in gastrointestinal disease with particular reference to malabsorption syndromes

There is a considerable range in the dose of many drugs that is required to produce a given pharmacological effect in an individual patient. This individual variation in dose requirement is sometimes reflected in the wide scatter in the steady state plasma concentration that follows the same oral dose of a drug given to any group of subjects. Such individual differences are largely due to variation in the rate of elimination of drugs. Gastrointestinal disease may also alter oral dose requirements by producing variation in both the amount and rate of drug absorption. These changes may be reflected in the plasma concentration/time curve that follows an oral dose. The amount of drug abosorbed is simultaneously affected by many factors. These include the physicochemical properties of the drug and the physiological factors that operate within the gut, as well as the presence of other substances such as food, or interaction with other drugs in the gut. The availability of the drug within the intestinal lumen is largely governed by its dissolution characteristics, particularly factors which can interfere with dissolution of the drug product in the gut. Physiological factors within the gut that affect oral drug absorption include gastric emptying rate and intestinal motility, the pH of the gastrointestinal fluids, the activity of gastrointestinal drug metabolising enzymes (e.g. monoamine oxidase and dopa decarboxylase) or drug metabolising bacteria and the surface area of the gut. Many factors affect gastric emptying. These include disease, surgery and other drugs. A change in the rate of gastric emptying alters the rate of drug delivery from the stomach to the duodenum and upper small intestine. This may profoundly alter the plasma concentration/time curve that follows oral administration of many drugs. For some drugs, proximal jejunal disease may reduce, delay or increase the apparent amount of drug absorbed. Reduced absorption of an antibiotic leads to a fall in the peak plasma concentration. If the peak falls below the minimum inhibitory concentration for a particular organism then therapeutic failure may occur, if it is assumed that the peak plasma concentration is all important for antimicrobial activity. Excessive drug absorption may lead to drug toxicity. Abnormal drug absorption is a feature of lower small intestinal conditions such as Crohn's disease. This suggests that drug absorption is not confined to the jejunum but continues throughout the small intestine. It is not always possible to predict the pattern of drug malabsorption from a knowledge of the physicochemical and pharmacokinetic properties of the drug and the pathophysiology of the disease. The rate and amount of drug absorbed be one patient may differ from that in another patient with the same condtion. Although these differences reflect normal individual variation, they are also related to the extent and activity of disease at the time of study...