The Antinociceptive Effects of Tramadol and/or Gabapentin on Rat Neuropathic Pain Induced by a Chronic Constriction Injury.

Preclinical Research The current work evaluates the interaction between two commonly used drugs, tramadol (Tra) and gabapentin (Gbp). Dose-response curves (DRC) and isobolographic analysis were used to confirm their synergistic antihyperalgesic and anti-allodynic responses in a rat neuropathic pain model involving chronic constriction injury of the sciatic nerve and in von Frey and acetone tests. Tra and Gbp produced dose-dependent antihyperalgesic and anti-allodynic effects. Dose-response studies of combinations of Tra and Gbp in combination showed the DRC was leftward-shifted compared to the DRCs for each compound alone. One combination demonstrated both antihyperalgesic and anti-allodynic effects greater than those observed after individual administration. The remaining combinations demonstrated an additive effect. The Tra+Gbp combination demonstrated a potentiative effect with smaller doses of Tra. Additionally, it was determined lethal dose 50 (LD50 ) of Tra alone and tramadol + Gbp 10 using mice to 48 h post administration. The DRC (death) were similar for Tra alone and in Tra in combination, despite the improved effectiveness of Tra in the presence of GBP, 10 mg/kg. A combination of these drugs could be effective in neuropathic pain therapy because they can produce potentiative (at a low dose) or additive effects. Drug Dev Res 77 : 217-226, 2016. © 2016 Wiley Periodicals, Inc.

Effect of tramadol on metamizol pharmacokinetics and pharmacodynamics after single and repeated administrations in arthritic rats.

Combined administration of certain doses of opioid compounds with a non-steroidal anti-inflammatory drug can produce additive or supra-additive effects while reducing unwanted effects. We have recently reported that co-administration of metamizol with tramadol produces antinociceptive effect potentiation, after acute treatment. However, none information about the effect produced by the combination after chronic or repeated dose administration exists. The aims of this study were to investigate whether the antinociceptive synergism produced by the combination of metamizol and tramadol (177.8 + 17.8 mg/kg, s.c. respectively) is maintained after repeated treatment and whether the effects observed are primarily due to pharmacodynamic interactions or may be related to pharmacokinetics changes. Administration of metamizol plus tramadol acute treatment significantly enhanced the antinociceptive effect of the drugs given alone (P < 0.05). Nevertheless, this effect decreased about 53% after the chronic treatment (3 doses per day, for 4 days). No pharmacokinetic interaction between metamizol and tramadol was found under acute treatment (P > 0.05). The mechanism involved in the synergism of the antinociceptive effect observed with the combination of metamizol and tramadol in single dose cannot be attributed to a pharmacokinetic interaction, and other pharmacodynamic interactions have to be considered. On the other hand, when metamizol and tramadol were co-administered under repeated administrations, a pharmacokinetic interaction and tolerance development occurred. Differences found in metamizol active metabolites' pharmacokinetics (P < 0.05) were related to the development of tolerance produced by the combination after repeated doses. This work shows an additional preclinical support for the combination therapy. The clinical utility of this combination in a suitable dose range should be evaluated in future studies.

Comparative in vitro dissolution study of carbamazepine immediate-release products using the USP paddles method and the flow-through cell system.

Dissolution profiles of four carbamazepine immediate-release generic products (200 mg tablets) and the reference product Tegretol® were evaluated using the USP paddles method and an alternative method with the flow-through cell system, USP Apparatus 4. Under official conditions all products met the Q specification, dissolution profiles of generic products were similar to the dissolution profile of the reference product (f 2 > 50) and model-independent parameters showed non significant differences to the reference product except mean dissolution time for product A (p < 0.05). On the other hand, when the flow-through cell system was used, none of the products met the pharmacopeial specification at 15 min and product A did not reach dissolution criteria at 60 min, dissolution profiles of all generic products were not similar to the reference product profile (f 2 < 50) and all model-independent parameters showed significant differences compared to the reference product (p < 0.05). Weibull's model was more useful for adjusting the dissolution data of all products in both USP apparatuses and Td values showed significant differences compared to the reference product (p < 0.05) when USP Apparatus 4 was used. These results indicate that the proposed method, using the flow-through cell system, is more discriminative in evaluating both, rate and extent of carbamazepine dissolution process from immediate-release generic products.

In vitro release of ketoprofen suppositories using the USP basket and the flow-through cell dissolution methods.

In order to study the release characteristics of ketoprofen suppositories under the hydrodynamic environment generated by USP Apparatus 1 and 4, the dissolution profiles of the Mexican reference product (100 mg) were determined. Phosphate buffer pH 8 and 1% sodium lauryl sulfate (SLS) aqueous solutions were proved as dissolution mediums. Baskets were rotated at 100 rpm with USP Apparatus 1 and different flow rates from 16-32 mL/min with USP Apparatus 4 were used. Drug samples were taken and quantified during 60 min by UV analysis at 260 nm. Mean dissolution time (MDT) and dissolution efficiency (DE) were calculated by model-independent methods. Data were also fitted to several kinetic models. Poor dissolution was found in both dissolution mediums when USP basket method was used (< 10% dissolved) while better results were obtained with USP Apparatus 4 when 1% SLS at 24 mL/min was used (43.6% dissolved, MDT of 25.5 min and DE of 25.0%). Kinetics showed a great variability when the USP Apparatus 1 was used, and Gompertz fitted well for data of 1% SLS at 24 mL/min (R(2)(adjusted) > 0.99). The results suggest the need to establish an adequate dissolution method to evaluate the release kinetics of ketoprofen from suppositories.

Effect of antioxidants on captopril floating matrices.

Stability of captopril in a controlled release formulation has been a challenge for some time. The sustained release of captopril from floating matrices has been studied varying the antioxidant load, the sodium bicarbonate proportion and the compaction pressure. Although in many cases the effect of compaction pressure remains hidden, actual results show that matrices compacted at 55 MPa have smaller density and float in the dissolution medium while those compacted at 165 MPa float only adding sodium bicarbonate. The increase of compaction pressure reduces the hydration volume and increases the time necessary to attain its maximum. These changes are attributed to lower matrix porosity and to the consequent diminution of water and drug transport. Increasing ascorbic acid proportions increase the matrix hydration volume and the drug released. The use of sodium ascorbate and the substitution of 15% polymer with sodium bicarbonate reduce the matrix hydration volume, shorten the matrix hydration process and increase the drug released. This is attributed to carbon dioxide bubbles that decrease the matrix coherence and expand the matrix volume, facilitating drug dissolution and only a limited further matrix expansion. The antioxidant protection provided by sodium ascorbate was lesser of that of ascorbic acid because of greater molecular mass and lesser release rate.

Effect of antioxidants on captopril floating matrices.

Stability of captopril in a controlled release formulation has been a challenge for some time. The sustained release of captopril from floating matrices has been studied varying the antioxidant load, the sodium bicarbonate proportion and the compaction pressure. Although in many cases the effect of compaction pressure remains hidden, actual results show that matrices compacted at 55 MPa have smaller density and float in the dissolution medium while those compacted at 165 MPa float only adding sodium bicarbonate. The increase of compaction pressure reduces the hydration volume and increases the time necessary to attain its maximum. These changes are attributed to lower matrix porosity and to the consequent diminution of water and drug transport. Increasing ascorbic acid proportions increase the matrix hydration volume and the drug released. The use of sodium ascorbate and the substitution of 15% polymer with sodium bicarbonate reduce the matrix hydration volume, shorten the matrix hydration process and increase the drug released. This is attributed to carbon dioxide bubbles that decrease the matrix coherence and expand the matrix volume, facilitating drug dissolution and only a limited further matrix expansion. The antioxidant protection provided by sodium ascorbate was lesser of that of ascorbic acid because of greater molecular mass and lesser release rate.

The antinociceptive efficacy of morphine, metamizol, or their combination in an experimental rat model with different levels of inflammatory pain.

The purpose of this work was to evaluate the antinociceptive efficacy of an optimal morphine and metamizol combination on different levels of nociception (levels I, II, and III) using the "Pain-induced functional impairment model in the rat". The effect of acetylsalicylic acid was examined as a reference drug at the same levels of nociception. The antinociceptive effects produced by morphine (3.2 mg/kg s.c.) and metamizol (177.8 mg/kg s.c.) were studied either individually or in combination. The antinociceptive efficacies were expressed as either areas under the curve (AUCs), maximum effects as functionality index in percent of the time course, or the antinociceptive effects produced at 2 h after administration. Unlike morphine, the antinociceptive effects of acetylsalicylic acid decreased with increasing intensity of nociception. In summary, the analysis of antinociceptive efficacies produced by the co-administration of these drugs for different levels of nociception revealed that co-administration provided potentiated and better antinociceptive coverage throughout our observation time than did the individual drugs or the expected theoretical sum (using AUC or effects after 2 h). This is the first study to demonstrate that an optimal morphine and metamizol combination is able to produce potentiation of antinociceptive effects during intense pain.

Pharmacokinetics and pharmacodynamics of metamizol in co-administration with morphine under acute and chronic treatments in arthritic rats.

OBJECTIVE: To investigate the relationship between metamizol pharmacokinetics and the antinociceptive effect produced after subcutaneous administration of metamizol (177.8 mg/kg) alone or in combination with morphine (3.2 mg/kg), under acute and chronic treatments. METHODS: Antinociception was assessed using the pain-induced functional impairment model in rat (PIFIR). Serial blood samples were collected from the same animals to study the pharmacokinetics of metamizol. KEY FINDINGS: The co-administration of the drugs in single dose, confirmed the potentiation of their individual antinociceptive effects. When the drugs were administered alone following the chronic schedule, a pronounced tolerance development to their antinociceptive effects was found, whereas it was significantly attenuated when they were administered together. Metamizol pharmacokinetics was unaltered by the presence of morphine. Plasma concentrations of 4-methylaminoantipyrine, an active metabolite markedly decreased under chronic administration. CONCLUSIONS: The mechanism involved in the potentiation of the antinociceptive effect produced by the combination, cannot be explained by the interaction of morphine on metamizol pharmacokinetics. Other pharmacokinetic interactions along with known pharmacodynamic interactions in which metamizol active metabolites contribute, should be considered. The frequency of administration enhances tolerance development and induces metamizol elimination process.

Enhancement of antinociception by co-administration of ibuprofen and caffeine in arthritic rats.

It has been observed that caffeine improves antinociceptive efficacy of some non-steroidal antiinflammatory drugs (NSAIDs) in several experimental models, however, these effects have been questioned in humans. Controversy in clinical studies may be due to the use of different protocols as well as to high interindividual variability in patient response. In addition, the antinociceptive interaction of ibuprofen+caffeine has not been studied. To assess a possible synergistic antinociceptive interaction, the antinociceptive effects of ibuprofen, and caffeine administered either separately or in combinations were determined in a model of arthritic pain: "Pain-induced functional impairment in the rat (PIFIR model). The antinociceptive efficacies were evaluated using several dose-response curves and time courses. The antinociceptive effects from the combination that produced the greater effect were compared with the maximal antinociceptive effect of either morphine or acetylsalicylic acid alone. The animals were administered with 0.05 ml intra-articular (i.a.) of uric acid to induce nociception. Groups of six rats received either acetylsalicylic acid, morphine, ibuprofen or caffeine, or a combination ibuprofen+caffeine (18 combinations). We report here that caffeine (17.8 and 31.6 mg/kg) is able to potentiate the antinociceptive effect of ibuprofen. This investigation showed that six combinations presented effects of potentiation and twelve combinations only showed antinociceptive effects not different from that of ibuprofen alone. The maximum antinociceptive effect was 270.7+/-12.7 area units (au), produced by ibuprofen 100 mg/kg+caffeine 17.8 mg/kg; this effect was greater than the maximum produced by morphine 17.8 mg/kg (244.7+/-22.9 au) in these experimental conditions. The maximum potentiation was 197 % produced with the combination of ibuprofen 17.8 mg/kg+caffeine 17.8 mg/kg. These results suggest that the antinociceptive effect of ibuprofen was significantly potentiated by doses of caffeine that by themselves are ineffective in this model.

The antispasmodic activity of Buddleja scordioides and Buddleja perfoliata on isolated intestinal preparations.

The antispasmodic activity of extracts from the aerial parts of Buddleja scordioides and Buddleja perfoliata (family: Scrophulariaceae) was studied on isolated tissue preparations from rabbit and guinea pig intestine. The chloroformic extract from the plants exhibited a significant relaxation on the spontaneous contraction of isolated rabbit jejunum at concentrations ranging from 1 to 400 microg/ml, and also caused an inhibitory effect on both K+ and Ca2+ induced contractions in the same tissue. The extracts at moderate doses (50 microg/ml) reduced 5-hydroxytryptamine (5-HT), acetylcholine and histamine induced contractions on isolated guinea pig ileum. Therefore, B. scordioides and B. perfoliata possess similar relaxant mechanism of action, in view of the fact that both inhibit K+ induce contraction and act through serotoninic, muscarinic and histaminic receptors. So, these data support the idea that the extracts may interfere either with calcium mobilization from intracellular stores, or with calcium interaction with regulatory proteins (e.g., calmodulin), or in other steps in the calcium signaling pathway. This leads us to suggest that the spasmolytic effect of both Buddleja species on smooth muscular contractility are due to the same or similar compounds occurring in these two species, which might be present in similar quantities.

High-performance liquid chromatographic assay for morphine in small plasma samples: application to pharmacokinetic studies in rats.

In order to perform a reliable pharmacokinetic study of morphine during subchronic treatment in rats, an easy, rapid, sensitive and selective analytical method was proposed and validated. The analyte and internal standard (naloxone) were extracted from plasma samples (100 microL) by a single solid-phase extraction method prior to reverse-phase high performance liquid chromatography (HPLC) along with electrochemical detection (ED). Standard calibration graphs were linear within a range of 3.5-1,000 ng/mL (r=0.999). The intra-day coefficients of variation (CV) were in the range of 5.8-8.5% at eight concentration levels (7-1,000 ng/mL) and the inter-day coefficient of variation ranged from 7.4 to 8.8%. The intra-day assay accuracy was in the range of -5-10% and the inter-day assay accuracy ranged from 3.0 to 9.3% of relative error (RE). The limit of quantification was 3.5 ng/mL using a plasma sample of 100 microL (15.8% of CV and 12.8% of RE). Plasma samples were stable for 2 months at -20 degrees C. This method was found to be suitable for pharmacokinetic studies in rats, after subcutaneous administration of morphine (5.6 mg/kg per day) in subchronic treatment for 6 and 12 days.

Antinociceptive effects of a new sigma-1 receptor antagonist (N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamide) in two types of nociception.

Pain has become an active clinical challenge due its etiological heterogeneity, symptoms and mechanisms of action. In the search for new pharmacological therapeutic alternatives, sigma receptors have been proposed as drug targets. This family consists of sigma-1 and sigma-2 receptors. The sigma-1 system is involved in nociception through its chaperone activity. Additionally, it has been shown that agonist to these receptors promote related sensitisation and pain hypersensitisation, suggesting the possible use of antagonists for sigma-1 receptors as an alternative therapy. The aim of this study was to evaluate the antinociceptive effect of a new sigma-1 receptor antagonist N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamida (NMIN) in two types of pain (arthritic and neuropathic) and to compare its efficacy and potency with reference drugs. The antinociceptive effects of NMIN were quantitatively evaluated using the pain-induced functional impairment model in the rat and the acetone test in a rat model of neuropathic pain. NMIN (sigma-1 receptor affinity of 324nM) did not show any antinociceptive activity in the arthritic pain model but showed a dose-dependent anti-allodynic effect in neuropathic pain. NMIN showed a similar efficacy compared to the effects obtained with morphine and the sigma-1 antagonist BD-1063. However, these reference drugs showed increased potency compared with NMIN. Our results suggest that sigma-1 receptors may play an important direct role in neuropathic pain but not in arthritic pain, supporting the hypothesis that NMIN may be useful for the treatment of neuropathic pain.

HPLC method with solid-phase extraction for determination of (R)- and (S)-ketoprofen in plasma without caffeine interference: application to pharmacokinetic studies in rats.

A fast and reproducible high-performance liquid chromatography method has been developed for the determination of (R)- and (S)-ketoprofen. Ketoprofen enantiomers were determined in plasma samples (50 µL), after solid-phase extraction, using diclofenac as internal standard. Analyses were performed on a (S, S)-Whelk-O 1 stainless steel column (5 µm, 250 × 4.6 mm) using hexane-ethanol-acetic acid (93:7:0.5, v/v/v) as the mobile phase and detection at 254 nm. The method was selective for ketoprofen enantiomers in the presence of caffeine and endogenous plasma compounds. Standard curves were linear (R(2) > 0.999) over the concentration range of 0.25-12.50 and 0.25 µg/mL was taken as the limit of quantification. The intra- and interday precision (relative standard deviation) values were <15.0% and the accuracy (relative error) was within ±12.0% at 1.0, 5.0 and 10.0 µg/mL. Enantiomer recoveries yielded 100.0 ± 15%. No significant differences were determined in plasma samples stored at room temperature for 24.0 h, after two freeze-thaw cycles, and between 0 and 4 weeks at -20°C (P > 0.05). The validated method was successfully applied in determination of (S)-ketoprofen in Wistar rats after oral administration of 3.2 mg/kg of (S)-ketoprofen alone or 3.2 mg/kg of (S)-ketoprofen + 17.8 mg/kg of caffeine.

Enhancement of antinociception but not constipation by combinations containing tramadol and metamizole in arthritic rats.

BACKGROUND AND AIMS: The use of a combination of analgesics could provide an optimal pain treatment with minimal side effects. Combinations of tramadol and some nonsteroidal anti-inflammatory drugs have demonstrated synergistic antinociceptive effects as well as a significantly reduced occurrence of adverse effects. The purpose of this study was to investigate the antinociceptive and constipation effects of tramadol and metamizole alone or in combination in rats and to discern among the types of drug interactions that exist using dose-response curves and an isobolographic analysis. METHODS: The antinociceptive effects of tramadol and metamizole, alone or in various combination ratios, were quantitatively evaluated using the "pain-induced functional impairment model in the rat." Additionally, the constipation effect was evaluated using the charcoal meal test. RESULTS: Tramadol (3.2-56.2 mg/kg) and metamizole (56.2-562.3 mg/kg) demonstrated a dose-dependent response with tramadol being more efficacious and potent than metamizole. Twenty-five different combinations of tramadol with metamizole were analyzed, and the evaluated combinations exhibited antinociceptive effects that were either additive or potentiative. An optimal combination was established with 3.2 mg/kg of tramadol and 316.2 mg/kg of metamizole. However, the constipation observed with this combination was more severe than that observed with the administration of tramadol alone. Our results reveal a possible interaction between the two drugs, which may be pharmacokinetic and/or pharmacodynamic in nature. CONCLUSIONS: The preclinical antinociceptive interaction and adverse effects produced by the combination of tramadol and metamizole suggests that caution should be exercised when using this combination in the clinical therapy of pain.

Antinociceptive effects of tramadol in co-administration with metamizol after single and repeated administrations in rats.

Combinations of two analgesic drugs of the same or different class are widely used in clinical therapy to enhance its antinociceptive effects and reduce the side effects. In order to evaluate a possible antinociceptive synergistic interaction of metamizol s.c., a nonsteroidal antiinflammatory drug (NSAID), and tramadol s.c., an atypical opioid (opioid receptor agonist), were administered alone or in combination. In the present study, the antinociceptive efficacy and the possible development of pharmacological tolerance produced by the combination tramadol plus metamizol during a 4-day treatment in rats using the plantar test was evaluated. Male Wistar rats were s.c. injected with tramadol (17.8 mg/kg), metamizol (177.8 mg/kg) or the combination tramadol plus metamizol three times a day for 4 days. Both metamizol and tramadol produced antinociceptive effects with a low rate trend towards tolerance development at the end of the treatment. The antinociceptive efficacy of tramadol and metamizol co-administration gradually decreased after the second injection. These data suggest that when the combination is given in a unique administration it results in an important potentiation of their individual antinociceptive effects. But, the repeated coadministration of tramadol plus metamizol results in a development of tolerance.

High-performance liquid chromatographic assay for metamizol metabolites in rat plasma: application to pharmacokinetic studies.

In order to evaluate the pharmacokinetics of metamizol in the presence of morphine in arthritic rats, after subcutaneous administration of the drugs, an easy, rapid, sensitive and selective analytical method was proposed and validated. The four main metamizol metabolites (4-methylaminoantipyrine, 4-aminoantipyrine, 4-acetylaminoantipyrine and 4-formylaminoantipyrine) were extracted from plasma samples (50-100µl) by a single solid-phase extraction method prior to reverse-phase high performance liquid chromatography with diode-array detection. Standard calibration graphs for all metabolites were linear within a range of 1-100µg/ml (r(2)≥0.99). The intra-day coefficients of variation (CV) were in the range of 1.3-8.4% and the inter-day CV ranged from 1.5 to 8.4%. The intra-day assay accuracy was in the range of 0.6-9.6% and the inter-day assay accuracy ranged from 0.9 to 7.5% of relative error. The lower limit of quantification was 1µg/ml for all metabolites using a plasma sample of 100µl. Plasma samples were stable at least for 4 weeks at -20°C. This method was found to be suitable for studying metamizol metabolites pharmacokinetics in arthritic rats, after simultaneous administration of metamizol and morphine, in single dose.

Effect of metamizol on morphine pharmacokinetics and pharmacodynamics after acute and subchronic administration in arthritic rats.

The aim of the present study was to investigate whether metamizol alters the pharmacokinetics of morphine and to determine the relationship between morphine plasma levels and antinociceptive effect produced after co-administration of drugs under acute and subchronic treatments using the pain-induced functional impairment model in rat (PIFIR model). Administration of morphine+metamizol under acute treatment produced a significantly higher antinociceptive effect than that obtained with morphine alone (P<0.05). This effect remained unaltered after subchronic treatments for 6 and 12 days. In addition, after the simultaneous administration of the drugs in a single dose, a pharmacokinetic interaction was found, which significantly (P<0.001) increased maximum plasma concentration (C(max)), concentration at 4h (C(4h)), partial areas under the plasma concentration-time curve from zero to 4h (AUC(0-4)) and from zero to 24h (AUC(0-24)). Moreover, whereas plasma concentration of morphine markedly decreased up to 4h (C(4h)) after subchronic administration of the opioid, multiple dosing of the morphine+metamizol combination produced an accumulation of the drug in plasma (P<0.001). The increase observed in morphine plasma levels after co-administration of metamizol may be explained by a possible enzymatic inhibition of the glucuronosyl-transferase system involved in the metabolism of morphine. This study reveals both a pharmacodynamic and a pharmacokinetic interaction between morphine and metamizol, leading to an increased antinociceptive effect and a delay in tolerance development.

Anti-nociceptive synergism of morphine and gabapentin in neuropathic pain induced by chronic constriction injury.

In order to detect an anti-nociceptive interaction between morphine and gabapentin, the anti-allodynic and anti-hyperalgesic effects of these drugs, administered either separately or in combination, were determined using the von Frey and acetone tests in a rat model of neuropathic pain (Bennett model). Morphine and gabapentin individually induced moderate attenuation of mechanical hyperalgesia, whereas the morphine and gabapentin combination completely decreased hyperalgesia. Morphine showed its maximal effect at 30 min post-injection in the acetone test; however, this effect gradually returned to the baseline value. Gabapentin did not produce an anti-allodynic effect, whereas the morphine and gabapentin combination completely decreased allodynia behavior at 30 min post-injection, an effect that persisted until 120 min. The area under the curve (AUC) of the anti-allodynic or anti-hyperalgesic effects produced by the combinations were significantly greater than the theoretical sum of effects produced by each drug alone or similar to the theoretical sum. The analysis of the effect, expressed as the AUC of the time course, supports the hypothesis that the combination of these drugs is useful in neuropathic pain therapy.

Comparison of dissolution profiles for albendazole tablets using USP apparatus 2 and 4.

The in vitro dissolution of albendazole from three different commercially available products (200 mg tablets) was studied using U.S. Pharmacopeia (USP) Apparatus 2 and USP Apparatus 4 in order to compare the release performance of the drug in two essentially different dissolution systems. For both cases, 0.1 N HCl was used as dissolution medium. Only the reference product and one of the generic products studied met the 80% USP 24 specification for albendazole dissolved at 30 min, using USP Apparatus 2. Although the reference product reached 80% of albendazole dissolved at 30 min when Apparatus 4 was used, the generic products' dissolution performance was markedly reduced in this system. Though dissolution rate was slower using Apparatus 4, the total quantity of albendazole dissolved from the reference product, represented by area under the dissolution profile, was practically the same regardless of the system used. Dissolution kinetics of albendazole was adequately described by Weibull's function for all the products. The dissolution time (t(d)) derived from data fitting to this function showed significant differences among the products studied. Data analysis based on analysis of variance (ANOVA) showed nonequivalence among the dissolution profiles of generic products compared with the reference product either with the dissolution vessel system or the flow-through cell, as well as nonequivalence among the dissolution profiles using both apparatuses with the same product. Though differences in the dissolution profiles for generic products against the reference product in both systems were found, USP Apparatus 4 showed higher discriminative capacity in differentiating the release characteristics of the products tested.