The influence of gastrointestinal transit on drug absorption in healthy volunteers

Formulation strategies to improve the efficacy of intestinal permeation enhancers. Adv Drug Deliv Rev 2021;177:113925. [PMID: 34418495 DOI: 10.1016/j.addr.2021.113925]

The use of chemical permeation enhancers (PEs) is the most widely tested approach to improve oral absorption of low permeability active agents, as represented by peptides. Several hundred PEs increase intestinal permeability in preclinical bioassays, yet few have progressed to clinical testing and, of those, only incremental increases in oral bioavailability (BA) have been observed. Still, average BA values of ~1% were sufficient for two recent FDA approvals of semaglutide and octreotide oral formulations. PEs are typically screened in static in vitro and ex-vivo models where co-presentation of active agent and PE in high concentrations allows the PE to alter barrier integrity with sufficient contact time to promote flux across the intestinal epithelium. The capacity to maintain high concentrations of co-presented agents at the epithelium is not reached by standard oral dosage forms in the upper GI tract in vivo due to dilution, interference from luminal components, fast intestinal transit, and possible absorption of the PE per se. The PE-based formulations that have been assessed in clinical trials in either immediate-release or enteric-coated solid dosage forms produce low and variable oral BA due to these uncontrollable physiological factors. For PEs to appreciably increase intestinal permeability from oral dosage forms in vivo, strategies must facilitate co-presentation of PE and active agent at the epithelium for a sustained period at the required concentrations. Focusing on peptides as examples of a macromolecule class, we review physiological impediments to optimal luminal presentation, discuss the efficacy of current PE-based oral dosage forms, and suggest strategies that might be used to improve them.

Impact of gastrointestinal tract variability on oral drug absorption and pharmacokinetics: An UNGAP review

The absorption of oral drugs is frequently plagued by significant variability with potentially serious therapeutic consequences. The source of variability can be traced back to interindividual variability in physiology, differences in special populations (age- and disease-dependent), drug and formulation properties, or food-drug interactions. Clinical evidence for the impact of some of these factors on drug pharmacokinetic variability is mounting: e.g. gastric pH and emptying time, small intestinal fluid properties, differences in pediatrics and the elderly, and surgical changes in gastrointestinal anatomy. However, the link of colonic factors variability (transit time, fluid composition, microbiome), sex differences (male vs. female) and gut-related diseases (chronic constipation, anorexia and cachexia) to drug absorption variability has not been firmly established yet. At the same time, a way to decrease oral drug pharmacokinetic variability is provided by the pharmaceutical industry: clinical evidence suggests that formulation approaches employed during drug development can decrease the variability in oral exposure. This review outlines the main drivers of oral drug exposure variability and potential approaches to overcome them, while highlighting existing knowledge gaps and guiding future studies in this area.

Individual differences in timing of peak positive subjective responses to d-amphetamine: Relationship to pharmacokinetics and physiology

Rate of delivery of psychostimulants has been associated with their positive euphoric effects and potential addiction liability. However, information on individual differences in onset of d-amphetamine's effects remains scarce. We examined individual differences in the time to peak subjective and physiological effects and the pharmacokinetics/pharmacodynamics of oral d-amphetamine. We considered two independent studies that used different dosing regimens where subjects completed the drug effects questionnaire at multiple time points post d-amphetamine. Based on the observation of distinct individual differences in time course of drug effects questionnaire "feel", "high", and "like" ratings (DEQH+L+F) in Study 1, subjects in both studies were categorized as early peak responders (peak within 60 minutes), late peak responders (peak > 60 minutes) or nonresponders; 20-25% of participants were categorized as early peak responders, 50-55% as late peak responders and 20-30% as nonresponders. Physiological (both studies) and plasma d-amphetamine (Study 1) were compared among these groups. Early peak responders exhibited an earlier rise in plasma d-amphetamine levels and more sustained elevation in heart rate compared to late peak responders. The present data illustrate the presence of significant individual differences in the temporal pattern of responses to oral d-amphetamine, which may contribute to heightened abuse potential.

Pharmacokinetics of an Oral Drug (Acetaminophen) Administered at Various Times in Relation to Subcutaneous Injection of Exenatide (Exendin-4) in Healthy Subjects

Exenatide is an incretin mimetic with potential glucoregulatory activity in type 2 diabetes. This randomized, single-blind, placebo-controlled 6-way crossover study assessed exenatide's effect on acetaminophen pharmacokinetics. Of 40 randomized healthy subjects, 39 completed the study. On the placebo day, acetaminophen (1000 mg) was ingested and placebo injected subcutaneously at 0 hours. On exenatide days, acetaminophen was ingested at -1, 0, +1, +2, and +4 hours, relative to the 10 mug exenatide injected subcutaneously at 0 hours. With exenatide injection, mean plasma acetaminophen AUC(0-12 h) values were reduced by 11% to 24% (vs placebo). Peak plasma acetaminophen concentrations were similar for the -1-hour and placebo groups and reduced by 37% to 56% at other times. The most frequent adverse events were generally mild to moderate nausea and vomiting. Exenatide treatment concurrent with or preceding acetaminophen ingestion slowed acetaminophen absorption but had minimal effect on the extent of absorption.

Toward an increased understanding of the barriers to colonic drug absorption in humans: implications for early controlled release candidate assessment

The purpose of this study was to increase the understanding of in vivo colonic drug absorption in humans by summarizing and evaluating all regional in vivo human absorption data with focus on the interpretation of the colonic absorption data in relation to intestinal permeability and solubility. In addition, the usefulness of the Biopharmaceutics Classification System (BCS) in early assessment of the in vivo colonic absorption potential of controlled release drug candidates was investigated. Clinical regional absorption data (Cmax, Tmax, and AUC) of 42 drugs were collected from journal articles, abstracts, and internal reports, and the relative bioavailability in the colon (Frel(colon)) was obtained directly or calculated. Bioavailability, fraction dose absorbed, and information if the compounds were substrates for P-glycoprotein (P-gp) or cytochrome P450 3A (CYP3A) were also obtained. The BCS I drugs were well absorbed in the colon (Frel(colon) > 70%), although some drugs had lower values due to bacterial degradation in the colon. The low permeability drugs (BCS III/IV) had a lower degree of absorption in the colon (Frel(colon) < 50%). There was a clear correlation between in vitro Caco-2 permeability and Frel(colon), and atenolol and metoprolol may function as permeability markers for low and high colonic absorption, respectively. No obvious effect of P-gp on the colonic absorption of the drugs in this study was detected. There was insufficient data available to fully assess the impact of low solubility and slow dissolution rate. The estimated in vivo fractions dissolved of the only two compounds administered to the colon as both a solution and as solid particles were 55% and 92%, respectively. In conclusion, permeability and solubility are important barriers to colonic absorption in humans, and in vitro testing of these properties is recommended in early assessment of colonic absorption potential.

Comparison of metoclopramide oral tablets and solution in treatment of dysmotility-like dyspepsia

PURPOSE

The clinical effects of metoclopramide oral solution and tablets in patients with dysmotility-like dyspepsia (DLD) were compared.

METHODS

In a prospective study, 63 patients with DLD, selected according to Rome II criteria and randomly divided into three groups, received metoclopramide tablets 5 mg t.i.d., metoclopramide oral solution 2 mg t.i.d., or placebo for one week. Assessment was based on symptom score determined by interviews and on gastric emptying time measured by ultrasonography before and after drug consumption. Two antral and stomach body biopsies were taken to evaluate infection with Helicobacter pylori and the intensity of gastritis.

RESULTS

Symptom score and gastric emptying time were significantly reduced in both metoclopramide groups (p < 0.05) but not in the control group. Patients receiving the oral solution had a greater reduction in the symptom score than those receiving tablets (p < 0.05). The absence or presence of and intensity of gastritis or H. pylori infection had no correlation with the symptom scores or gastric emptying times.

CONCLUSION

Metoclopramide oral solution 2 mg t.i.d. was at least as effective as metoclopramide tablets 5 mg t.i.d. in reducing symptoms of DLD and gastric emptying time.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

Pharmacokinetics and pharmacodynamics of oral testosterone enanthate plus dutasteride for 4 weeks in normal men: implications for male hormonal contraception

Oral administration of testosterone enanthate (TE) and dutasteride increases serum testosterone and might be useful for male hormonal contraception. To ascertain the contraceptive potential of oral TE and dutasteride by determining the degree of gonadotropin suppression mediated by 4 weeks of oral TE plus dutasteride, 20 healthy young men were randomly assigned to 4 weeks of either 400 mg oral TE twice daily or 800 mg oral TE once daily in a double-blinded, controlled fashion at a single site. All men received 0.5 mg dutasteride daily. Blood for measurement of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone, dihydrotesterone (DHT), and estradiol was obtained prior to treatment, weekly during treatment, and 1, 2, 4, 8, 12, 13, 14, 16, 20, and 24 hours after the morning dose on the last day of treatment. FSH was significantly suppressed throughout treatment with 800 mg TE once daily and after 4 weeks of treatment with 400 mg TE twice daily. LH was significantly suppressed after 2 weeks of treatment with 800 mg TE, but not with 400 mg TE. Serum DHT was suppressed and serum estradiol increased during treatment in both groups. High-density lipoprotein cholesterol was suppresed during treatment, but liver function tests, hematocrit, creatinine, mood, and sexual function were unaffected. The administration of 800 mg oral TE daily combined with dutasteride for 28 days significantly suppresses gonadotropins without untoward side effects and might have utility as part of a male hormonal contraceptive regimen.

Gastric emptying in patients with insulin dependent diabetes mellitus and bioavailability of thioctic acid-enantiomers

The objective of this study was to determine the impact of prolonged gastric emptying in patients with insulin dependent diabetes mellitus (IDDM) on the bioavailability of the R(+)- and S(-)-thioctic acid (TA) enantiomers. Gastric emptying time (GET) was assessed in 30 healthy volunteers and 22 patients with IDDM using sequential ultrasonography after a standardized solid-liquid test meal. Pharmacokinetics and absolute bioavailability (F) of the TA-enantiomers were studied using a randomized, open two-way crossover design with administrations of oral and intravenous single doses of 200 mg rac-TA. GET in healthy subjects was 134.7+/-21.6 min, the normal range was calculated from 88.3 to 181.1 min. The mean GET in all IDDM patients was significantly prolonged (178.2+/-28.1 min; P<0.001). Only 50% of the patients (n=11) were found to have normal GET (group A), the other half of the population (n=11) were considered to have delayed GET (group B). Mean GET values were 156.9+/-21.5 in group A (P=0.028) and 199.4+/-13.9 min in group B, respectively, suggesting that gastric motility is significantly different from non-diabetic controls even in patients with apparently normal gastric emptying. Times to peak plasma concentrations (t(max)) of both TA-enantiomers were similar in both groups and thus, unrelated to measures of gastric emptying. In contrast, maximum concentrations (C(max)) and area-under-the-curve values (AUC) of both enantiomers were reduced by about 30% in patients with delayed GET. Although these differences resulted in statistical significance for the AUC of both enantiomers (P<0.05), linear regression analysis showed only modest correlation between GET and the extent of TA-enantiomer absorption (r2=0.31 and 0.22 for R(+)-/S(-)-TA, respectively). The study suggests that prolonged gastric emptying is frequently present in IDDM. Delayed gastric emptying, however, does not substantially affect the rate and extent of absorption of both TA-enantiomers.

XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl] aminomethyl)-1-cyclohexane acetic acid], a novel gabapentin prodrug: II. Improved oral bioavailability, dose proportionality, and colonic absorption compared with gabapentin in rats and monkeys

The absorption of gabapentin (Neurontin) is dose-dependent and variable between patients. Rapid clearance of the drug necessitates dosing three or more times per day to maintain therapeutic levels. These deficiencies appear to result from the low capacity, limited intestinal distribution, and variable expression of the solute transporter responsible for gabapentin absorption. Saturation of this transporter at doses used clinically leads to unpredictable drug exposure and potentially ineffective therapy in some patients. XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl]aminomethyl)-1-cyclohexane acetic acid] is a novel prodrug of gabapentin designed to be absorbed by high-capacity nutrient transporters located throughout the intestine. XP13512 was efficiently absorbed and rapidly converted to gabapentin after oral dosing in rats and monkeys. Exposure to gabapentin was proportional to prodrug dose, whereas exposure to intact XP13512 was low. In rats, >95% of an oral dose of (14)C-XP13512 was excreted in urine in 24 h as gabapentin. In monkeys, oral bioavailability of gabapentin from XP13512 capsules was 84.2% compared with 25.4% after a similar oral Neurontin dose. Compared with intracolonic gabapentin, intracolonic XP13512 gave a 17-fold higher gabapentin exposure in rats and 34-fold higher in monkeys. XP13512 may therefore be incorporated into a sustained release formulation to provide extended gabapentin exposure. XP13512 demonstrated improved gabapentin bioavailability, increased dose proportionality, and enhanced colonic absorption. In clinical use, XP13512 may improve the treatment of neuropathic pain, epilepsy, and numerous other conditions by increasing efficacy, reducing interpatient variability, and decreasing frequency of dosing.

Gastrointestinal transit of liquids in unfed cynomolgus monkeys

In this study, the gastric emptying rate and oro-caecal transit time of two liquid volumes (20 and 60 ml) were compared in unfed cynomolgus monkeys. First, the acetaminophen method was used to determine the gastric half-emptying time (t(50%)). The mean t(50%) from seven monkeys was 21.2 min for the 20-ml volume and 27.8 min for the 60-ml volume. This mean t(50%) in monkeys is similar to that reported previously in fasting humans. Next, the sulfasalazine-sulfapyridine method was used to determine median oro-caecal transit times for the 2 liquid volumes; these times were 2.5 h for the 20-ml volume and 2.3 h for the 60-ml volume, which are about 1.5 h shorter than previously reported transit times in humans. An increase in volume administered did not significantly change either the t(50%) or oro-caecal transit time. The data also show that variability in both t(50%) and oro-caecal transit time within each monkey is not as great as the large variability between monkeys. Consequently, cynomolgus monkeys are good model animals to use for studies on the gastric emptying of drug-containing liquids after fasting; however, analysis of results from bioavailability studies must compensate for differences in the oro-caecal transit time between monkeys and humans.

Is there an interaction between H2-antagonists and alcohol?

H2-antagonists are commonly prescribed drugs and alcohol use is widespread in the community. Any possible interaction may be important because of the frequent co-administration of both drugs and the potential for unexpected impairment of pyschomotor function, in particular, driving skills. Hepatic ADH is the major site of alcohol metabolism. ADH is also found in the stomach, but it is uncertain whether gastric ADH is able to metabolise a significant amount of alcohol in vivo. Significant first-pass metabolism can be demonstrated at lower doses of alcohol, and if alcohol is given after meals. Varying degrees of extraction of alcohol from the portal circulation probably explains the data regarding first pass metabolism rather than gastric metabolism by gastric ADH. H2-receptor antagonists inhibit gastric ADH activity to a variable extent. If gastric metabolism of alcohol is negligible then this inhibition has no relevance. Given the uncertainty regarding a mechanism of interaction, only carefully conducted studies in controlled environments will answer the question. The large inter-subject variability of alcohol absorption means that any study which seeks to determine the effect of an H2-receptor antagonist on ethanol metabolism must have sufficient numbers. A cross-over design, with each subject acting as his own control, is preferable to avoid ascribing an effect to treatment rather than to chance. The alcohol dosing studies are reviewed and the results summarised according to dose of alcohol given. At a dose of 0.15 g/kg of alcohol, four commonly used H2-antagonists may cause a small increase in blood alcohol concentrations in certain conditions. This absolute increase is very small. The magnitude of effect is far less than the effect of taking a meal before alcohol. At doses of 0.3 g/kg and above the majority of evidence favours no interaction between H2-antagonists and alcohol. There is no interaction at doses that would be expected to impair psychomotor skills (above 25 mg/dl). There remains a question regarding the cumulative effect of repeated small doses of alcohol and further studies are required. The relationship between ethanol absorption and gastric emptying raises the possibility that the effects of H2-receptor antagonists observed at very low doses of alcohol may be due to the acceleration of gastric emptying by these drugs. This is an attractive hypothesis that explains many aspects of the debate, but studies of the effect of H2-antagonists on gastric emptying have been conflicting.

Effects of surgery on the pharmacokinetic parameters of drugs

As with other forms of stress, surgery is a time of fluctuating haemodynamics, physiological shifts, intense metabolic changes, and protein catabolism and anabolism. At times, these changes can occur within hours. There is a reasonable body of literature concerning the adverse effects of surgery on gastric emptying, but not on the effects of surgery on the distribution, metabolism and excretion of drugs. Gastric emptying is diminished or absent following major procedures, which prevent the delivery of orally administered drugs to their major site of absorption, the small bowel. Changes in the paracellular absorption of drugs may occur postoperatively, although transcellular absorption appears to be unimpaired. Distribution is affected by changes in blood volume, alterations in circulation, increases in the extracellular fluid and changes in the circulating plasma protein levels, such as albumin and alpha 1-acid glycoprotein. Little is known about alterations in drug metabolism following surgery. However, it has been shown that systemic hypoxia alters the function of some of the cytochrome P450 (CYP) system and increased levels of cytokines have an effect on the metabolism of at least 1 drug. In addition, the renal elimination of drugs is affected in patients postoperatively, although the effects of biliary clearance in this period are difficult to determine. Despite the lack of research into pharmacokinetics during the postoperative period, and given the immense and often sudden changes observed in patients post-surgery, it is reasonable to recommend vigilance with respect to drug therapy during this period.

Comparative pharmacokinetic study of a floating multiple-unit capsule, a high-density multiple-unit capsule and an immediate-release tablet containing 25 mg atenolol

The purpose of this study was to evaluate the possible advantages of floating and high-density dosage forms and their influence on pharmacokinetic parameters. Atenolol was chosen as a model drug because of its poor absorption in the lower gastrointestinal tract. Three formulations containing 25 mg atenolol, a floating multiple-unit capsule, a high-density multiple-unit capsule, and an immediate-release tablet were compared with respect to estimated pharmacokinetic parameters. The two multiple-unit dosage forms were composed of compressed minitablets and had sustained release properties. The bioavailability of the two gastroretentive preparations with sustained release characteristics was significantly decreased when compared to the immediate-release tablet. The floating minitablets seemed to be retained longer in the stomach than the high-density dosage form. The first atenolol concentration detectable in the plasma and the time to peak Tmax were delayed for the floating dosage form. For the parameters Cmax and AUC 0-infinity, the lower limit of the 90% confidence interval was outside the bioequivalence range (0.80-1.25). This study showed that it was not possible to increase the bioavailability of a poorly absorbed drug such as atenolol using gastroretentive formulations. Atenolol absorption was delayed and the maximum plasma concentration was diminished.

Pharmacokinetic interactions between alcohol and other drugs

The frequent use of alcohol (ethanol) together with prescription drugs gives any described pharmacokinetic interaction significant clinical implications. The issue is both the effect of alcohol on the pharmacokinetics of various drugs and also the effect of those drugs on the pharmacokinetics of alcohol. This review discusses these pharmacokinetic interactions but also briefly describes some other effects of alcohol that are clinically relevant to drug prescribing. The use of several different study designs may be required before we can confidently state the presence or absence of any alcohol-drug interaction. Short term administration of alcohol in volunteers is the most common study design but studies of social drinking and prolonged moderate alcohol intake can be important in some situations. Community-based studies may illustrate the clinical relevance of any interaction. Alcohol can affect the pharmacokinetics of drugs by altering gastric emptying or liver metabolism (by inducing cytochrome P450 2E1). Drugs may affect the pharmacokinetics of alcohol by altering gastric emptying and inhibiting gastric alcohol dehydrogenase. The role of gastric alcohol dehydrogenase in the first-pass metabolism of alcohol is reviewed in this article and the arguments for and against any potential interaction between alcohol and H2 receptor antagonists are also discussed. The inhibition of the metabolism of acetaldehyde may cause disulfiram-like reactions. Pharmacodynamic interactions between alcohol and prescription drugs are common, particularly the additive sedative effects with benzodiazepines and also with some of the antihistamine drugs; other interactions may occur with tricyclic antidepressants. Alcohol intake may be a contributing factor to the disease state which is being treated and may complicate treatment because of various pathophysiological effects (e.g. impairment of gluconeogenesis and the risk of hypoglycaemia with oral hypoglycaemic agents). The combination of nonsteroidal anti-inflammatory drugs and alcohol intake increases the risk of gastrointestinal haemorrhage.

Inter-individual and intra-individual variability of ethanol concentration-time profiles: comparison of ethanol ingestion before or after an evening meal

1. The magnitude of the variability of ethanol absorption is an important factor for studies that seek to determine the significance of potential interactions between ethanol and drugs. The aim of this study was to determine the extent of inter- and intra-individual variability of ethanol concentration-time profiles in fasted and fed subjects. 2. Twenty-four healthy male subjects were randomized to receive ethanol 0.3 g kg-1 before an evening meal on two study days and ethanol 0.3 g kg-1 after an evening meal on two study days. Plasma ethanol concentrations were measured at intervals from 0-240 min. 3. There were significant differences in the mean area under the ethanol concentration-time curve (AUC), the mean peak ethanol concentration (Cmax), the mean ethanol elimination slope and the time to peak ethanol concentration between the fed and fasted subjects. There were no significant differences between the first and second study days for either fed or fasting subjects for all parameters. 4. There was no statistically significant difference in inter- or intra-subject variance between fed and fasted studies although the coefficients of variation (standard deviation expressed as a percentage of the mean) for the differences between the first and second study day were higher for fed studies. 5. The large inter- and intra-individual variability of alcohol absorption for both fasted and fed subjects must be considered in the design of alcohol-drug interaction studies.

Pharmacokinetic considerations in gastrointestinal motor disorders

Although it has been recognised that alterations in gastrointestinal motility, whether induced by physiological or pathological processes, have significant effects on the pharmacokinetics of orally administered drugs, this subject has received inappropriately little attention. Studies relating to this topic have focused on healthy volunteers and animals and have largely been confined to the effects of single drug doses. There is limited information about the effects of disease on pharmacokinetics under steady-state conditions. Changes in gastrointestinal motility may affect the pharmacokinetics of orally administered drugs by altering the rate of delivery, bioavailability or mucosal absorption of the drug. In general the rate of absorption and time taken to achieve maximal plasma concentrations for well absorbed drugs may be modified by changes in gastrointestinal motility, but overall bioavailability is not usually affected. In these cases the therapeutic and clinical effects of the alteration in pharmacokinetics will, therefore, depend on which parameters are important for the action of the drug. For poorly absorbed drugs both the rate of absorption and bioavailability are likely to be altered by changes in gastrointestinal motility. However, the complex effects of food and disease, as well as the properties and formulation of any drug (solubility, ease of dispersion, delayed release formulation) often make the prediction of the magnitude, or even the direction, of any effect difficult to predict. Drugs with direct effects on gastrointestinal motility may influence their own patterns of absorption. In patients with gastrointestinal motility disorders, drugs administered in a controlled release formulation, or those with poor bioavailability, are most likely to have a poorly predictable therapeutic effect. Care should be taken to ensure that the formulation of the drug, its timing of administration in relation to meals and the use of coadministered drugs optimise, or at least ensure consistent absorption.

Drug interactions with antacids. Mechanisms and clinical significance

Concomitant use of antacid preparations with other medications is common. The potential for antacid-drug interactions is dependent upon the chemistry and physical properties of the antacid preparation. The intragastric release of free aluminum and magnesium ions has potent effects on gastrointestinal function and on drug pharmacokinetics. Antacid-drug interactions may occur secondary to changes in gastrointestinal motility or alterations in gastric and urinary pH. Direct adsorption also results in decreased drug bioavailability. Human drug interaction studies are usually performed with healthy volunteers; extrapolation of these results to clinical situations may not always be valid. However, the current literature would suggest that significant interactions with antacids do occur with certain members of the quinolone, nonsteroidal anti-inflammatory drug (NSAID) and cephalosporin classes of drugs. Notable interactions also occur with tetracycline, quinidine, ketoconazole and oral glucocorticoids. These interactions are particularly relevant in the patient with sepsis, cardiac disease or inflammatory syndromes.

Absorption of polar drugs following caecal instillation in healthy volunteers

We have investigated colonic drug absorption in man by the caecal instillation of a multi-component solution of atenolol, cimetidine, frusemide, hydrochlorothiazide and salicylic acid. We found that salicylic acid absorption from this solution was delayed but complete whereas the absorption of atenolol, cimetidine, frusemide and hydrochlorothiazide was four- to five-fold lower than expected from oral bioavailability studies.

Effects of a non-absorbable osmotic load on drug absorption in healthy volunteers

1. We have studied the effects of a non-absorbable osmotic load on the absorption of a multicomponent solution of frusemide, atenolol, hydrochlorothiazide and salicylic acid in six healthy volunteers. 2. Each subject was studied on up to four separate occasions. The drugs were administered in one of four solutions: a) a mannitol/electrolyte solution, b) a double-strength mannitol/electrolyte solution, c) a glucose/electrolyte solution and d) water. Lactulose or sulphasalazine were added as oro-caecal transit markers. Lactulose was included in the mannitol- and glucose-based solutions, adding a further non-absorbable osmotic load, and sulphasalazine was added to the water, adding little osmotic load. 3. The absorption of atenolol and hydrochlorothiazide was two- to three-times less from all lactulose-containing solutions than from the sulphasalazine-containing solution. The absorption of frusemide and salicylic acid was similar from all four solutions. 4. The largest non-absorbable osmotic load impaired the absorption of atenolol and hydrochlorothiazide most and the incorporation of glucose only partly restored absorption. 5. These results suggest that transmucosal water movement is an important determinant of atenolol and hydrochlorothiazide absorption but is less relevant for the absorption of frusemide and salicylic acid. Furthermore, these data demonstrate a previously unrecognised interaction between a commonly prescribed laxative--lactulose, and atenolol and hydrochlorothiazide.