On the intestinal transit of a single non-disintegrating object

Hierarchical Mass Transfer Analysis of Drug Particle Dissolution, Highlighting the Hydrodynamics, pH, Particle Size, and Buffer Effects for the Dissolution of Ionizable and Nonionizable Drugs in a Compendial Dissolution Vessel

Dissolution is a crucial process for the oral delivery of drug products. Before being absorbed through epithelial cell membranes to reach the systemic circulation, drugs must first dissolve in the human gastrointestinal (GI) tract. In vivo and in vitro dissolutions are complex because of their dependency upon the drug physicochemical properties, drug product, and GI physiological properties. However, an understanding of this process is critical for the development of robust drug products. To enhance our understanding of in vivo and in vitro dissolutions, a hierarchical mass transfer (HMT) model was developed that considers the drug properties, GI fluid properties, and fluid hydrodynamics. The key drug properties include intrinsic solubility, acid/base character, pK_a, particle size, and particle polydispersity. The GI fluid properties include bulk pH, buffer species concentration, fluid shear rate, and fluid convection. To corroborate the model, in vitro dissolution experiments were conducted in the United States Pharmacopeia (USP) 2 dissolution apparatus. A weakly acidic (ibuprofen), a weakly basic (haloperidol), and a nonionizable (felodipine) drug were used to study the effects of the acid/base character, pK_a, and intrinsic solubility on dissolution. 900 mL of 5 mM bicarbonate and phosphate buffers at pH 6.5 and 37 °C was used to study the impact of the buffer species on drug dissolution. To investigate the impacts of fluid shear rate and convection, the apparatus was operated at different impeller rotational speeds. Moreover, presieved ibuprofen particles with different average diameters were used to investigate the effect of particle size on drug dissolution. In vitro experiments demonstrate that the dissolution rates of both the ionizable compounds used in this study were slower in bicarbonate buffer than in phosphate buffer, with the same buffer concentration, because of the lower interfacial buffer capacity, a unique behavior of bicarbonate buffer. Therefore, using surrogates (i.e., 50 mM phosphate) for bicarbonate buffer for biorelevant in vitro dissolution testing may overestimate the in vivo dissolution rate for ionizable drugs. Model simulations demonstrated that, assuming a monodisperse particle size when modeling, dissolution may overestimate the dissolution rate for polydisperse particle size distributions. The hydrodynamic parameters (maximum shear rate and fluid velocity) under in vitro conditions in the USP 2 apparatus under different rotational speeds are orders of magnitude higher compared to the in vivo situation. The inconsistencies between the in vivo and in vitro drug dissolution hydrodynamic conditions may cause an overestimation of the dissolution rate under in vitro conditions. The in vitro dissolution data supported the accuracy of the HMT for drug dissolution. This is the first drug dissolution model that incorporates the effect of the bulk pH and buffer concentration on the interfacial drug particle solubility of ionizable compounds, combined with the medium hydrodynamics effect (diffusion, convection, shear, and confinement components), and drug particle size distribution.

Evolution of a detailed physiological model to simulate the gastrointestinal transit and absorption process in humans, part II: extension to describe performance of solid dosage forms

The physiological absorption model presented in part I of this work is now extended to account for dosage-form-dependent gastrointestinal (GI) transit as well as disintegration and dissolution processes of various immediate-release and modified-release dosage forms. Empirical functions of the Weibull type were fitted to experimental in vitro dissolution profiles of solid dosage forms for eight test compounds (aciclovir, caffeine, cimetidine, diclofenac, furosemide, paracetamol, phenobarbital, and theophylline). The Weibull functions were then implemented into the model to predict mean plasma concentration-time profiles of the various dosage forms. On the basis of these dissolution functions, pharmacokinetics (PK) of six model drugs was predicted well. In the case of diclofenac, deviations between predicted and observed plasma concentrations were attributable to the large variability in gastric emptying time of the enteric-coated tablets. Likewise, oral PK of furosemide was found to be predominantly governed by the gastric emptying patterns. It is concluded that the revised model for GI transit and absorption was successfully integrated with dissolution functions of the Weibull type, enabling prediction of in vivo PK profiles from in vitro dissolution data. It facilitates a comparative analysis of the parameters contributing to oral drug absorption and is thus a powerful tool for formulation design.

Fast-dissolving microparticles fail to show improved oral bioavailability

Oral dosage forms are the preferred means of delivering drugs for systemic absorption. However, development problems occur for drugs with poor water solubility and/or gastrointestinal permeability. It is generally believed that the in-vivo bioavailability of poorly water-soluble drugs from Class II of the Biopharmaceutics Classification System can be improved by increasing the dissolution rate. We have attempted to increase the in-vivo oral bioavailability of a model Class II drug (griseofulvin) by preparing rapidly-dissolving particles. The solvent-diffusion method was used to prepare particles with hydrophilic surfactants (Brij 76/Tween 80 surfactant blend) and in-vivo studies were conducted in rats. The griseofulvin particles produced were bipyramidal in habit with a particle size of 2.18 ± 0.12 mm; they contained crystalline drug and a relatively large proportion (12% w/w) of hydrophilic surfactant. The latter and the small particle size ensured rapid particle dispersion and dissolution in-vitro. Thus, within 30 min of the in-vitro dissolution test, the bipyramidal particles had released ∼70% of drug compared with ∼10% from the starting material (particle size 12.61 ± 1.11 μm). However, the rapid and increased drug dissolution in-vitro was not translated to rapid and enhanced absorption in-vivo, and the oral bioavailability of the model drug was found to be the same from the control and from the bipyramidal particles. The poor in-vivo performance of the bipyramidal particles showed that although the dissolution rate of a Class II drug is thought to be a good indicator of its in-vivo bioavailability, this is not always the case.

Investigation of human pharmacoscintigraphic behavior of two tablets and a capsule formulation of a high dose, poorly water soluble/highly permeable drug (efavirenz)

Human pharmacoscintigraphic behavior of two tablets and a capsule formulation of a high dose, poorly water soluble, highly permeable, micronized drug (efavirenz) was investigated. The tablets and capsule, prepared with samarium oxide and neutron activated to produce radioactive samarium-153, were evaluated for their in vivo disintegration and gastrointestinal (GI) transit in healthy subjects under fasted condition. Scintigraphic images were acquired to coincide with blood sampling times to assess the plasma concentration-time profile in relation to in vivo disintegration and GI transit. The mean gastric emptying times were approximately the same for all three formulations. Although in vivo dosage form disintegration was faster for Tablet A as compared to Tablet B and was similar between Tablet A and the capsule, Tablet A showed a slower rate and extent of drug absorption than Tablet B and the capsule. The results of this study eliminated the initial hypothesis that the difference in in vivo performance between the two tablet formulations is due to a different rate of in vivo disintegration and suggest that for this drug the in vivo dissolution rate of the drug from its disintegrated dosage form was a more important factor affecting the rate and extent of drug absorption.

The role of gamma-scintigraphy in oral drug delivery

The gastrointestinal tract is usually the preferred site of absorption for most therapeutic agents, as seen from the standpoints of convenience of administration, patient compliance and cost. In recent years there has been a tendency to employ sophisticated systems that enable controlled or timed release of a drug, thereby providing a better dosing pattern and greater convenience to the patient. Although much about the performance of a system can be learned from in vitro release studies using conventional and modified dissolution methods, evaluation in vivo is essential in product development. The non-invasive technique of gamma-scintigraphy has been used to follow the gastrointestinal transit and release characteristics of a variety of pharmaceutical dosage forms. Such studies provide an insight into the fate of the delivery system and its integrity and enable the relationship between in vivo performance and resultant pharmacokinetics to be examined (pharmacoscintigraphy).

Use of hydroxypropyl methylcellulose acetate succinate in an enteric polymer matrix to design controlled-release tablets of amoxicillin trihydrate

A controlled-release table of amoxicillin trihydrate was developed by use of a matrix formulation based on the enteric polymer hydroxypropyl methylcellulose acetate succinate (HPMCAS). Sustained drug release was shown by in vitro dissolution testing; the polymer could suppress drug release in the presence of gastric pH but could enhance drug release in the presence of small intestinal pH, compared with compacts of pure drug. Grinding or physical mixing of the drug with the polymer, an alteration in normal compaction pressure, or a substitution of other enteric polymers did not markedly affect drug release from compacts. Physicochemical testing of samples confirmed that the method of mixing did not alter powder morphology. An ethanolic granulation procedure was used in the production of final tablets (21 x 10 mm) containing amoxicillin (750 mg), HPMCAS, anhydrous directly compressible lactose, and lubricants. These large tablets showed a promising sustained-release effect in vitro when a variable-pH-shift dissolution procedure was used. However, single-dose studies with a panel of fasting subjects showed that the tablets had a relative bioavailability of only 64.4%. Other pharmacokinetic parameters confirmed a lack of therapeutic advantage of these tablets over an equivalent dose of conventional capsules.

The influence of density on the gastrointestinal transit of pellets

The gastric emptying, intestinal transit and caecum arrival times of 1 mm pellets of density 1.5 and 2.8 g cm-3 have been assessed in fed and fasted volunteers by means of gamma-scintigraphy. The pellets were prepared by extrusion/spheronisation, coated with ethylcellulose and labelled with technetium-99m. The position of the pellets in the gastro-intestinal tract was followed by a double-headed gamma camera to allow detailed information over a period of up to 10 h. Analysis of variance established that there was a highly significant difference in the time for 50% of the pellets to empty from the stomach both in fed and fasted states. The 2.8 g cm-3 pellets had an extended resident time in both the fed and fasted states. The gastric emptying time was prolonged in the fed state. There was no significant difference in intestinal transit time between the two formulations nor whether the volunteers were fed or fasted. The caecum arrival time was therefore modified only by the gastric emptying time.

The dissolution rate and bioavailability of hydrochlorothiazide in pellet formulations

The influence of non-active ingredients in the manufacture of pellets on in-vitro dissolution rate and on bioavailability of hydrochlorothiazide has been studied. Pellets were formulated using either microcrystalline cellulose or microcrystalline cellulose-carboxymethylcellulose sodium blends as matrix, and hydrochlorothiazide as the active ingredient. In-vitro drug release from the different pellet formulations was retarded in comparison to a conventional tablet formulation and was dependent on the nature of the non-active ingredient and, for the microcrystalline cellulose-carboxymethylcellulose sodium blend, of the dissolution medium. In-vivo bioavailability of both pellet formulations was low compared with that of the conventional tablet and the plasma concentration-time profiles did not suggest slow release.

Effect of particle size and food on gastric residence time of non-disintegrating solids in beagle dogs

The gastric residence times of various sizes of radio-opaque particles and tablets were measured in beagle dogs by X-ray, both in the fasted state and after a single meal. During the course of the studies, changes in intragastric pH were also monitored with a radiotelemetric pH sensor, the Heidelberg capsule. The gastric residence time increased with increasing particle size and with particles greater than or equal to 5 mm in diameter approached a plateau value both in the fasted state and after feeding. This value was about 7.5 h after feeding and about 1.5 h in the fasted state, and probably corresponded to the occurrence of the interdigestive migratory myoelectric complex (IMMC wave). The pH in the stomach was variable in the fasted state, but an abrupt pH increase (up to pH 6-7) was observed during the emptying of larger tablets. In some instances this high pH in the stomach was maintained until the next IMMC wave occurred. The gastric emptying of larger tablets administered with food was also associated with an abrupt pH increase.

Drug delivery to the proximal colon

The transits of a capsule and a multiparticulate pellet system have been monitored through the gastrointestinal tract in six healthy volunteers. Both preparations moved together through the stomach and small intestine, reaching the colon, on average, 4 h after dosing. Within the colon the pellets dispersed and moved at a slower rate than the capsule. There was considerable intersubject variability in the large bowel transit times. The findings are discussed in terms of drug delivery to the colon.

Gastrointestinal transit of pellet systems in ileostomy subjects and the effect of density

The transit of pellets of different densities (0.94 and 1.96 g cm-3) through the gastrointestinal tract of ileostomy subjects has been followed using the technique of gamma scintigraphy. In contrast with earlier reported results, the transit of the pellets was not affected by particle density.

Gastrointestinal transit of an osmotic tablet drug delivery system

The gastrointestinal transit of a radiolabelled osmotic tablet drug delivery system has been monitored in groups of young and elderly healthy subjects, using gamma scintigraphy. The gastric emptying and small intestinal transit times were similar for both groups of subjects. The units were observed to move through the gastrointestinal tract at about the same rate as the released contents, arriving at the caecum on average 7 h after dosing. The data suggest that tablet adhesion to the mucosal surface is unlikely to be the mechanism responsible for the side effects reported for the indomethacin formulation Osmosin.