Dissolution and Absorption of Nipedipine from Nifedipine-Polyvinylpyrrolidone Coprecipitate

Moisture sorption by polymeric excipients commonly used in amorphous solid dispersion and its effect on glass transition temperature: I. Polyvinylpyrrolidone and related copolymers

Moisture plays a critical role in the stability of amorphous solid dispersions (ASD) as it can lower the glass transition temperature (T_g) and thereby increase molecular mobility resulting in drug crystallization. A systematic study on moisture sorption by four polyvinylpyrrolidone (PVP) having different molecular weights (Kollidon® 12, 17, 30, and 90) and two related copolymers (Kollidon® VA64; Soluplus®) was conducted at 25 and 40 °C as a function of relative humidity to determine effects of absorbed moisture on T_g and potential stability of ASDs. A VTI dynamic moisture sorption analyzer was used, where experimental conditions were first established such that equilibrium was reached and there was no significant hysteresis loop between sorption and desorption isotherms. The PVPs had identical moisture sorption profiles and were highly hygroscopic, reaching 22-24% and 41-42% w/w moisture at 25 °C/60% RH and 25 °C/80% RH, respectively. Kollidon® VA64 and Soluplus® were relatively less hygroscopic, reaching, respectively, about half and one-fourth the moisture content of PVPs at 25 °C/60% RH. Moisture sorption at 40 °C was relatively lower than that at 25 °C. The high moisture sorption drastically decreased T_g of polymers, which roughly agreed with theoretical calculations based on the Gordon-Taylor/Kelley-Bueche equation, although deviation occurred, possibly due to hydrogen bonding between polymer and moisture.

Evaluation of the inhibitory potential of HPMC, PVP and HPC polymers on nucleation and crystal growth

Supersaturated drug delivery systems are generally developed for improving solubility and dissolution of poorly soluble drugs.

Modification of physicochemical characteristics of active pharmaceutical ingredients and application of supersaturatable dosage forms for improving bioavailability of poorly absorbed drugs

New chemical entities are required to possess physicochemical characteristics that result in acceptable oral absorption. However, many promising candidates need physicochemical modification or application of special formulation technology. This review discusses strategies for overcoming physicochemical problems during the development at the preformulation and formulation stages with emphasis on overcoming the most typical problem, low solubility. Solubility of active pharmaceutical ingredients can be improved by employing metastable states, salt forms, or cocrystals. Since the usefulness of salt forms is well recognized, it is the normal strategy to select the most suitable salt form through extensive screening in the current developmental study. Promising formulation technologies used to overcome the low solubility problem include liquid-filled capsules, self-emulsifying formulations, solid dispersions, and nanosuspensions. Current knowledge for each formulation is discussed from both theoretical and practical viewpoints, and their advantages and disadvantages are presented.

Development of coated nifedipine dry elixir as a long acting oral delivery with bioavailability enhancement

To develop the long acting nifedipine oral delivery with bioavailability enhancement, a nifedipine dry elixir (NDE) containing nifedipine ethanol solution in dextrin shell was prepared using a spray-dryer, and then coated nifedipine dry elixir (CNDE) was prepared by coating NDE with Eudragit acrylic resin. The physical characteristics and bioavailability of NDE and CNDE were evaluated, and then compared to those of nifedipine powder. NDE and CNDE, which were spherical in shape, had about 6.64 and 8.68-8.75 μm of geometric mean diameters, respectively. The amount of nifedipine dissolved from NDE for 60 min increased about 7- and 40-fold compared to nifedipine powder in pH 1.2 simulated gastric fluid and pH 6.8 simulated intestinal fluid, respectively. Nifedipine released from CNDE was retarded in both dissolution media compared with that from NDE. After oral administration of NDE, the C(max) and AUC(0→8h) of nifedipine in rat increased about 13- and 7-fold, respectively, and the Tmax of nifedipine was reduced significantly compared with those after oral administration of nifedipine powder alone. The AUC(0→8h) and T(max) of nifedipine in CNDE increased markedly and the C(max) of nifedipine in CNDE was significantly reduced compared to those in NDE. It is concluded that CNDE, which could lower the initial burst-out plasma concentration and maintain the plasma level of nifedipine over a longer period with bioavailability enhancement, might be one of potential alternatives to the marketed long acting oral delivery system for nifedipine.

Development and evaluation of controlled porosity osmotic pump for nifedipine and metoprolol combination

Background

A system that can deliver multi-drug at a prolonged rate is very important for the treatment of various chronic diseases such as diabetes, asthma and heart disease. Controlled porosity osmotic pump tablet (CPOP) system was designed to deliver Nifedipine (NP) and Metoprolol (MP) in a controlled manner up to 12 h. It was prepared by incorporating drugs in the core and coated with various types (PVP, PEG-400 and HPMC) and levels (30, 40 and 50% w/w of polymer) of pore former at a weight gain of 8, 12 & 15%.

Results

Formulation variables like type and level of pore former and percent weight gain of membrane was found to affect the drug release from the developed formulations. Drug release was inversely proportional to the membrane weight but directly related to the level of pore former. Burst strength of the exhausted shell was inversely proportional to the level of pore former, but directly affected by the membrane weight. Results of scanning electron microscopy (SEM) studies showed the formation of pores in the membrane from where the drug release occurred. Dissolution models were applied to drug release data in order to establish the mechanism of drug release kinetics. In vitro release kinetics was subjected to superposition method to predict in vivo performance of the developed formulation.

Conclusion

The developed osmotic system is effective in the multi-drug therapy of hypertension by delivering both drugs in a controlled manner.

A nanofibrous sheet-based system for linear delivery of nifedipine

We developed a nanofibrous sheet-based system to achieve linear release of nifedipine for oral delivery. The nanofibrous sheets of micro-porosity were fabricated by the electrospinning method, using a biocompatible polymer, poly (lactic-co-glycolic acid). The sheets were then used as a drug diffusion barrier by capping and sealing a compressed tablet, composed of nifedipine and a solubility enhancer, polyvinylpyrrolidone. The nanofibrous sheets of different thicknesses were prepared to vary the rate of drug diffusion in this work. In vitro drug release study revealed that as the sheet thickness increased, drug release was more retarded, where a lag phase of drug release became more evident. We were to realize linear drug release by combining two distinctly capped tablets, each showing a different drug release, which exhibited an almost linear release of nifedipine during 24h (R(2)>0.986). The drug release profile was not influenced by the pH of the release media as the morphological structure of nanofibrous sheets was seen to be not very different at both pHs 1.2 and 6.8. Therefore, we conclude that a combination of two tablets, each capped with nanofibrous sheets of different thickness, is a promising system for linear delivery of oral drug.

Drug–polymer interaction and its significance on the physical stability of nifedipine amorphous dispersion in microparticles of an ammonio methacrylate copolymer and ethylcellulose binary blend

Using spectroscopic and thermal analysis, this study investigated drug-polymer interaction and its significance on the physical stability of drug amorphous dispersion in microparticles of an ammonio polymethacrylate copolymer (Eudragit RL) (RL) and ethylcellulose (EC) binary blend (RL/EC = 2:1 w/w) prepared for use in controlled release of poorly water-soluble drugs. Solid dispersion of the model drug, nifedipine in the microparticles could be described as an ideal amorphous mixture for drug loadings up to 11% w/w. The antiplasticizing effect of the polymer blend was indicated by a significant increase in the glass transition point from approximately 50 degrees C for the amorphous nifedipine to approximately 115 degrees C for its solid solution. Moreover, shifts in infrared vibration wavenumber of nifedipine carbonyl and amine groups suggested that the hydrogen bonds (H-bonds) originally formed among nifedipine molecules were broken and replaced by those formed between nifedipine and polymers in the microparticles. Further infrared analysis on nifedipine amorphous dispersions with a single polymer, namely RL or EC, confirmed the proposed hydrogen-bonding interactions; and their stability study results suggested that both antiplasticizing effects and hydrogen bonding of EC and RL with nifedipine might be responsible for the physical stability of the microparticles of nifedipine amorphous dispersion with a RL/EC binary blend.

One-step preparation of drug-containing microparticles to enhance the dissolution and absorption of poorly water-soluble drugs using a 4-fluid nozzle spray drier

We studied the use of a 4-fluid nozzle spray drier as a new one-step method for preparing drug-containing microparticles to enhance the dissolution and absorption of poorly water-soluble drugs. We employed ethenzamide (EZ) and flurbiprofen (FP) as poorly water-soluble drugs and lactose (LAC) and mannitol (MAN) as water-soluble carriers for microparticles. EZ-ethanol or FP-acetone/methanol (2:1) solutions and aqueous solutions of LAC or MAN were simultaneously supplied through different liquid passages of a 4-fluid nozzle spray drier and then dried to obtain LAC or MAN microparticles containing EZ or FP. The dissolution of EZ from the EZ/LAC and EZ/MAN microparticles was much faster than that from EZ powder. The dissolution of EZ was more rapid from the EZ/MAN microparticles than the EZ/LAC microparticles. The dissolution of FP from the FP/MAN microparticles was greatly enhanced because of large effective surface area of FP dispersed in microparticles following rapid dissolution of MAN. The absorption of FP after oral administration of the FP/MAN microparticles to rats was markedly increased. The results demonstrate that the 4-fluid nozzle spray drier can be used for the one-step preparation of drug-containing microparticles that enhance the dissolution and absorption of poorly water-soluble drugs and that overcome the problem of finding a common solvent for drugs and carriers.

Controlled release of drug via methylcellulose-carboxyvinylpolymer interpolymer complex solid dispersion

The purpose of this research was to examine the controlled release of phenacetin (PHE) from solid dispersion by the formation of an interpolymer complex between methylcellulose (MC) and carboxyvinylpolymer (CP). The PHE/polymer composition ratio was fixed at 20:80 (w/w) in the solid dispersion. The effect of the MC/CP ratio and molecular weight of MC on the PHE release was studied. The release of PHE from the solid-dispersion granules depended on the MC/CP ratio, with a ratio of 50:50 giving the lowest rate of release. In aqueous solution, this MC/CP ratio resulted in the lowest transmittance, suggesting a maximal extent of interpolymer complex formation between MC and CP. Furthermore, at a MC/CP ratio of 50:50, the release of PHE from the solid dispersion granules decreased as the molecular weight of the MC increased, reaching a plateau at molecular weights >or=180,000. The contributions of diffusion and polymer relaxation to PHE release increased as the molecular weight of the MC increased. This study shows that it is feasible to control the release of PHE from MC-CP solid dispersion granules by modulating complex formation between MC and CP, which can be accomplished by altering the MC/CP ratio and the molecular weight of MC.

Establishment of new preparation method for solid dispersion formulation of tacrolimus

The aim of this study was to establish a new preparation method for solid dispersion formulation (SDF) of tacrolimus, a poorly water-soluble drug, without dichloromethane, because no use of dichloromethane is recommended by ICH harmonized tripartite guideline. To select the appropriate carrier, three different SDFs with polyethylene glycol 6000 (PEG 6000), polyvinylpyrrolidone (PVP) and hydroxypropylmethylcellulose (HPMC) were prepared by the conventional solvent method, in which tacrolimus and the carrier were completely dissolved in the mixture of dichloromethane and ethanol. Powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) patterns indicated that tacrolimus exists in an amorphous state in all three SDFs. The supersaturated dissolution profiles of tacrolimus were observed in all SDFs, and the highest level of supersaturation for tacrolimus was obtained and maintained for 24h from SDF with HPMC. On the other hand, the supersaturated level from SDF with PEG 6000 or PVP decreased rapidly. The in vivo oral absorption study in dogs showed that bioavailability of tacrolimus from SDF with HPMC was remarkably improved compared with the crystalline powder. It was clarified that HPMC is the most appropriate carrier for SDF of tacrolimus. Then, SDF of tacrolimus was prepared by the new method, which allows us to make SDF of tacrolimus by swelling HPMC with ethanol, in which tacrolimus was completely dissolved. This new method does not need dichloromethane. The physicochemical properties of SDF with HPMC prepared by the new method were the same as those of SDF prepared by the conventional solvent method. Furthermore, SDF with HPMC prepared by the new method was still stable after stored at 40 degrees C for 3 months. The pharmacokinetic parameters after oral administration in monkeys showed no significant difference (P>0.01) between SDFs with HPMC prepared by the two methods. In conclusion, we have established the new preparation method for SDF of tacrolimus with HPMC and the new method makes it possible to prepare SDF of tacroliumus without dichloromethane.

Evaluation of Hypromellose Acetate Succinate (HPMCAS) as a Carrier in Solid Dispersions

The utility of hypromellose acetate succinate (HPMCAS), a cellulosic enteric coating agent, as a carrier in a solid dispersion of nifedipine (NP) was evaluated in comparison with other polymers, including hypromellose (HPMC), hypromellose phthalate (HPMCP), methacrylic acid ethyl acrylate copolymer (MAEA), and povidone (PVP). An X-ray diffraction study showed that the minimum amount of HPMCAS required to make the drug completely amorphous was the same as that of other cellulosic polymers, and less than that in dispersions using non-cellulosic polymers. Hypromellose acetate succinate showed the highest drug dissolution level from its solid dispersion in a dissolution study using a buffer of pH 6.8. This characteristic was unchanged after a storage test at high temperature and high humidity. The inhibitory effect of HPMCAS on recrystallization of NP from a supersaturated solution was the greatest among all the polymers examined. Further, the drug release pattern could be modulated by altering the ratio of succinoyl and acetyl moieties in the polymer chain. Our results indicate that HPMCAS is an attractive candidate for use as a carrier in solid dispersions.

Physical stability of solid dispersions of the antiviral agent UC-781 with PEG 6000, Gelucire 44/14 and PVP K30

This paper describes the physical stability of solid dispersions of UC-781 with PEG 6000, Gelucire 44/14 and PVP K30 prepared by the solvent and melting methods. The concentration of the drug in the solid dispersions ranged from 5 to 80% w/w. The solid dispersions were stored at 4-8 and 25 degrees C (25% RH), then their physicochemical properties were analysed by differential scanning calorimetry (DSC), X-ray powder diffraction and dissolution studies as a function of storage time. The DSC curves of solid dispersions of UC-781 with PVP K30 did not show any melting peaks corresponding to UC-781 after storage, indicating no recrystallization of the drug. The DSC data obtained from PEG 6000 and Gelucire 44/14 showed some variations in melting peak temperatures and enthalpy of fusion of the carriers. It was shown that the enthalpy of fusion of PEG 6000 in the dispersions increased after storage; it was more pronounced for samples stored at 25 degrees C compared to those at 4-8 degrees C indicating the reorganization of the crystalline domains of the polymer. Similarly, the enthalpy of fusion of Gelucire 44/14 in the solid dispersions increased as a function of time. Dissolution of UC-781 from all solid dispersions decreased as a function of storage time. While these observations concurred with the DSC data for all solid dispersions, they were not reflected by X-ray powder diffraction data. It was concluded that it is the change of the physical state of the carriers and not that of the drug, which is responsible for the decreased dissolution properties of the solid dispersions investigated.

Improving the dissolution and bioavailability of nifedipine using solid dispersions and solubilizers

Nifedipine (NF) is a poorly water-soluble drug, of low and irregular bioavailability after oral administration. Although some reports have attempted to improve the dissolution of NF using solid dispersions and solubilizers, little literature information is available on the in vivo performance of such preparations. The aim of the present work was to improve the therapeutic efficacy of NF via incorporation into different types of carriers, and to investigate their in vitro dissolution and bioavailability in rabbits. Nifedipine solid dispersions were prepared by fusion, solvent, and freeze-drying methods with polyethylene glycol (PEG) 6000 and PEG monomethylether 5000 (PEG MME 5000). Complexation of NF with beta-cyclodextrin (beta-CyD) and solubilization by sodium lauryl sulfate (SLS) have also been studied. The dissolution was determined by the flow-through cell in order to maintain perfect sink conditions. The solid dispersions resulted in a significant increase in the dissolution rate as compare to pure drug. The highest NF dissolution rate was obtained from solid dispersions containing 95% PEG 6000 prepared by the solvent method. While, unexpectedly, the highest absorption in rabbits was obtained from 95% PEG 6000 prepared by the fusion method. Compared to SLS, beta-CyD gave higher in vitro and in vivo values. Differential scanning calorimetry (DSC) and powder x-ray diffractometry indicated that NF in solid dispersions is homogeneously distributed, and no drug crystallized out of the system. The DSC thermograms of NF-beta-CyD complex and NF/SLS solid mixture showed a decrease in the NF endothermic peak. The x-rays showed different diffraction patterns of pure NF and pure carrier, suggesting the formation of a new solid form.

The role of the kneading paddle and the effects of screw revolution speed and water content on the preparation of solid dispersions using a twin-screw extruder

The twin-screw hot-melt extrusion process is useful for preparing solid dispersions which can improve the dissolution and absorption of drugs. The kneading paddle elements of the screws play an important role in changing the crystallinity and dissolution properties of a solid dispersion of kneaded nifedipine-hydroxypropylmethylcellulose phthalate (NP-HPMCP). After operating the machine, a small amount of kneaded material adhering to the screws was collected and its physicochemical properties examined. Samples from the kneading paddle with a twist angle of 60 degrees were transparent and exhibited super-saturation on dissolution testing. When the kneading paddle elements were detached from the screws and only the feed screw elements were operated, the physicochemical properties of the extruded material were significantly influenced by the operating conditions of the machine e.g. revolution rate of screws, and the amount of water added to the feed materials. Slow revolution of the screws and the addition of a suitable amount of water to the mixture increased the rate of drug dissolution, although no super-saturation occurred. As the kneading paddle elements can retain the mixture in the machine for a longer period under intense shear, desired solid dispersions can be prepared routinely irrespective of the operating conditions. Moreover, a capillary rheometer can be useful to predetermine the amount of water added and the temperature for the preparation of solid dispersions using a twin-screw extruder.

Solid-state characterization of nifedipine solid dispersions

The purpose of this study is to characterize the nature and solid-state properties of a solid dispersion system of nifedipine (33.3% w/w) in a polymer matrix consisting of Pluronic F68 (33.3% w/w) and Gelucire 50/13 (33.3% w/w). The nature of nifedipine dispersed in the matrix was studied by powder X-ray diffractometry (PXRD), differential scanning calorimetry (DSC) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The rate and extent of water uptake of the solid dispersion were determined by weight gain. The dissolution rate of nifedipine solid dispersion was determined using Apparatus 2 of USP XXIII (1995). Quantitative PXRD showed that the saturation solubility of nifedipine in the polymer matrix is 2.1-3.0% w/w and indicated an excess of crystalline nifedipine in the solid dispersion. The maximum water uptake by the solid dispersion exposed to 75% RH at 45 degrees C was 3.3 times higher than for the dispersion exposed to 65% RH at 25 degrees C. Over 8 weeks, PXRD and DRIFTS of the nifedipine matrix stored at 25 or 4 degrees C were unchanged, showing constancy of crystallinity and intermolecular interactions. For a given mass of nifedipine (20 mg) and for a given particle size of nifedipine (<850 microm), the initial release rate of nifedipine from the solid dispersion was faster (46.2% of the nifedipine dissolved in 20 min) than that of the pure drug (1.2% of the nifedipine dissolved in 20 min). The results indicate that the nifedipine solid dispersion is physically stable over 8 weeks. Nifedipine is released faster from the solid dispersion than from the pure crystalline drug of the same particle size.

A nifedipine coground mixture with sodium deoxycholate. II. Dissolution characteristics and stability

Nifedipine is a poorly water soluble drug that demonstrates low bioavailability. In a previous study, a coground mixture of nifedipine with sodium deoxycholate (DCNa), a bile salt, immediately produced colloidal particles when dispersed in water. In this study, the effect of the weight fraction of DCNa, grinding time, dissolution media, and storage conditions on colloidal particle formation in solution was investigated. The coground mixture was prepared with a vibration rod mill, and its solid state was characterized using powder X-ray diffraction. A laser diffraction particle size analyzer was used to determine the particle size distribution curve in water. The size of particles formed in solution decreased with an increase in the weight fraction of DCNa and grinding time. A nifedipine-DCNa (1:2 w/w) mixture coground for 30 min was used in the experiments. Colloidal particle formation from the coground mixture was also observed in dissolution media of water and a pH 6.8 buffer solution at 37 degrees C. Most precipitates passed through a filter with a pore size of 0.8 microm, but the particle size distribution in water was different from that in the pH 6.8 buffer solution. DCNa exhibited not only micellar solubilization for drug crystals, but also a retarding effect on drug crystal growth in a supersaturated solution. The latter effect could serve to form colloidal particles in solution. When stored under 75% relative humidity at 40 degrees C for 1 month, the amorphous coground mixture crystallized, and the particle size in water markedly increased. Therefore, the weight fraction of DCNa, grinding time, dissolution media, and humidity during storage influence the dissolution characteristics of nifedipine from a coground mixture.

Physical properties of solid dispersion of a nonsteroidal anti-inflammatory drug (M-5011) with Eudragit E

Some acidic nonsteroidal anti-inflammatory drugs (NSAIDs) are poorly soluble in the stomach. In this study, M-5011, d-2-[4-(3-methyl-2-thienyl) phenyl] propionic acid, was used as a model substance. To increase the dissolution rate of M-5011, a solid dispersion of M-5011 was prepared by the powder mixing method using Eudragit E-100 (aminoacryl methacrylate copolymer) as a carrier. Evaluation by X-ray diffraction and differential scanning calorimetry (DSC) revealed that M-5011 easily formed a solid dispersion with E-100. The dissolution behavior of a physical mixture prepared immediately after mixing and the mixture stored for 14 days at 40 degrees C were examined. It was observed that the former, containing a great deal of E-100, showed a fairly good dissolution behavior, and the latter had a better dissolution rate. The mechanism of the interaction of M-5011 and E-100 was investigated by infrared (IR) spectroscopy and nuclear magnetic resonance (NMR). The interaction was simulated by NMR using a monomer of Eudragit E-100.

Effect of water-soluble carriers on dissolution characteristics of nifedipine solid dispersions

Solid dispersions of nifedipine (NP) with polyethylene glycols (PEG4000 and PEG6000), hydroxypropyl-beta-cyclodextrin (HP beta CD), and poloxamer 407 (PXM 407) in four mixing ratios were prepared by melting, solvent, and kneading methods in order to improve the dissolution of NP. The enhancement of the dissolution rate and the time for 80% NP dissolution T80% depended on the mixing ratio and the preparation method. The highest dissolution rate and the T80% as short as 15 min were obtained from PXM 407 solid dispersion prepared by the melting method at the mixing ratio of 1:10. The X-ray diffraction (XRD) patterns of solid dispersions at higher proportions of carriers demonstrated consistent with the results from differential scanning calorimetric (DSC) thermograms that NP existed in the amorphous state. The wettability and solubility were markedly improved in the PXM 407 system. The presence of intermolecular hydrogen bonding between NP and PEGs and between HP beta CD and PXM 407 was shown by infrared (IR) spectroscopy.

Control of medicine release from solid dispersion composed of the poly(ethylene oxide)-carboxyvinylpolymer interpolymer complex by varying molecular weight of poly(ethylene oxide)

Solid dispersion composed of the poly(ethylene oxide) (PEO)-carboxyvinylpolymer (CP) interpolymer complex containing phenacetin (PHE) was prepared by using nine grades of PEO having different molecular weights from 2000 to 4500000. We attempted to control the medicine release from the PEO-CP solid dispersion by varying the molecular weight of PEO. The physicochemical properties of the solid dispersion were analyzed by powder X-ray diffractometry and thermal analysis. The interaction between PEO and CP was analyzed by IR spectroscopy. Transmittance of the polymer solution was measured to study the complexation between PEO and CP. The release profile of PHE varied depending on the molecular weight of PEO. The minimum release rate was observed at the PEO molecular weight of 35000. It was found that the amount of the PEO-CP complex formation by hydrogen bonding changed depending on the molecular weight of PEO. These results indicate that it is feasible to control the medicine release from the PEO-CP solid dispersion by varying the molecular weight of PEO.

Design of toxicokinetic studies

1. Toxicokinetics is defined as pharmacokinetic studies in animals during actual toxicity studies or under conditions mimicking them (species, duration, dose level, etc.). 2. Toxicology studies require toxicokinetics to check whether systemic exposure reflects administered dose. In particular, it is important to know whether the absence of toxicity at a given dose is due to the innocuousness of the compound or to its poor bioavailability. 3. Pivotal toxicology studies may require different toxicokinetic support than that of early studies, as more is learned of the compound and its metabolites. Considerations need to be placed on such factors as the choice of biological matrix for drug assay, the relevance of metabolites, and which dose levels require the most pharmacokinetic investigation.

Metabolism and kinetics of amlodipine in man

1. The disposition of amlodipine, R,S,2-[(2-aminoethoxy)methyl]-4-(2-chlorophenyl)-3-ethoxycarbonyl- 5- methoxycarbonyl-6-methyl-1,4-dihydropyridine has been studied in two human volunteers using single oral and intravenous doses of 14C-amlodipine. The drug was well absorbed by the oral route while the mean oral bioavailability for unchanged drug was 62.5%. 2. Renal elimination was the major route of excretion with about 60% of the dosed radioactivity recovered in urine. Mean total recovered radioactivity in urine and faeces amounted to 84% for both the oral and intravenous routes. 3. Apart from a small amount of unchanged amlodipine (10% of urine 14C), only pyridine metabolites of amlodipine were excreted in urine. The majority (greater than 95%) of the metabolites excreted in the 0-72 h post-dose period were identified; the major metabolite was 2-([4-(2-chlorophenyl)-3-ethoxycarbonyl-5-methoxycarbonyl-6-methyl- 2-pyridyl]methoxy) acetic acid and this represented 33% of urinary radioactivity. The data indicate that oxidation of amlodipine to its pyridine analogue is the principal route of metabolism with subsequent metabolism by oxidative deamination, de-esterification and aliphatic hydroxylation. 4. For the two volunteers, amlodipine concentrations in plasma declined with a mean half-life of 33 h, while slower elimination of total drug-related material from plasma was observed, consistent with prolonged excretion (up to 12 days) of metabolites in urine and faeces. Only amlodipine and pyridine metabolites were found in the circulation. As these pyridine derivatives have minimal calcium antagonist activity the efficacy of amlodipine in man can most probably be attributed to the parent drug.