Comparative bioavailability of two oral formulations of levofloxacin in healthy Mexican volunteers.

OBJECTIVE: To assess the bioequivalence of two levofloxacin 500 mg tablets marketed in Mexico. MATERIAL AND METHODS: The clinical investigation was designed as a randomized, open-labeled, two-part, two-treatment, two-period crossover study, in 27 healthy male volunteers. 1 tablet of each formulation was administered with 200 ml of water after 10 h overnight fast. After dosing, serial blood samples were collected for a period of 24 h. Plasma concentrations were determined by a validated high-performance liquid chromatographic method and pharmacokinetic parameters were obtained by non-compartmental approach. Analysis of variance (ANOVA) was carried out using log-transformed AUClast, AUC yen and Cmax and untransformed tmax, and 90% confidence intervals for AUClast, AUC yen and Cmax were calculated. If the 90% confidence intervals (CI) for AUClast, AUC yen and Cmax fell fully within the interval 80 - 125%, the bioequivalence of the two formulations was established. RESULTS: The means (test and reference) for AUClast were 58.869 and 56.297 microg x h/ml, for AUC yen were 63.456 and 60.748 microg x h/ml and for Cmax were 8.691 and 8.445 microg/ml. The geometric mean ratios of the test formulation to reference formulation for AUClast, AUC yen and Cmax (CI) were 104.53% (102.73 - 106.36%), 104.37% (102.04 - 106.75%) and 103.45% (95.57 - 111.97%), respectively. All 90% CI for AUClast, AUC yen and Cmax fell within the Mexican Federal Commission for Prevention of Sanitary Risks (COFEPRIS) accepted bioequivalence range of 80 - 125%. CONCLUSIONS: Based on the results, the formulations tested are bioequivalent.

Oral pharmacokinetics of meloxicam in the rat using a high-performance liquid chromatography method in micro-whole-blood samples.

The pharmacokinetics of meloxicam, a potent analgesic and antiinflammatory drug used in several rheumatic diseases, has been studied in rats that received oral doses of 3.2, 5.6 or 10 mg/kg of meloxicam. Blood samples were obtained at selected times during 24 h after administration, and meloxicam concentrations were determined by a validated high-performance liquid chromatography (HPLC) method, using micro-whole-blood samples, developed in our laboratory. After administration of meloxicam, blood concentrations increased reaching a dose-dependent maximal concentration in about 2 h. Then, concentrations decayed with a half-life of 9 h. An increase in C(max) and AUC as a function of the dose was observed, and no statistically significant difference was observed in AUC/dose or C(max)/dose between doses. However, linearity could not be concluded because of the wide variability observed.

Use of cyclosporin-A in experimental spinal cord injury: design of a dosing strategy to maintain therapeutic levels.

Cyclosporin-A (CsA) is frequently used as an immunosuppressive agent in experimental transplantations. CsA has been used in nervous tissue transplants in spinal cord injury (SCI). However, optimal results have not been obtained. This is likely due to the fact that SCI alters CsA pharmacokinetics and hence fixed dose regimens are not adequate. In this study, several CsA dosing regimens were evaluated in Long-Evans female rats subjected to a severe low thoracic (T8) SCI by the contusion method. Serum CsA concentrations were measured to determine which dosing regimen allowed CsA levels to be maintained within the therapeutic window. It was found that administration of 2.5 mg/kg/12 h intraperitoneally during the first 2 days after SCI (acute phase) followed by 5 mg/kg/12 h orally thereafter (subacute and chronic phases) yields CsA circulating levels within the therapeutic window, i.e., 0.120-0.275 microgram/mL. This dosing regimen represents a suitable alternative to fixed dosing to achieve an optimal CsA-induced immunosuppression in experimental models of SCI.

Alteration of cyclosporin-A pharmacokinetics after experimental spinal cord injury.

The pharmacokinetics of the immunosuppressive agent cyclosporin-A (CsA) were studied in rats submitted to spinal cord (SC) injury. A single CsA 10 mg/kg dose was given either intraperitoneally (i.p.) or orally to rats submitted to experimental SC injury at the T8 level. Twenty four hours after lesion (acute stage of SC injury) i.p. CsA bioavailability was increased, while t1/2 was prolonged. However, oral bioavailability was reduced. Seven weeks after lesion (chronic stage of SC injury) CsA bioavailability, by either route, was not significantly different from control values. Results indicate that parenteral CsA bioavailability is increased during the acute stage of SC lesion, probably due to an impaired elimination. Oral bioavailability, however, is decreased, since there is also an important reduction in gastrointestinal CsA absorption that overrides the effect of impaired elimination. Alterations in CsA pharmacokinetics appear to revert during the chronic stage of SC injury. Changes in CsA bioavailability, depending on the route of administration and on time, must be considered to design an adequate immunosuppressive treatment in SC injury.

Pharmacokinetics of oral nifedipine in different populations.

Nifedipine disposition varies among populations. Reports on oral nifedipine pharmacokinetics show that peak plasma levels and AUC values are higher in Mexican and Japanese than in European and North American subjects. Increased nifedipine bioavailability in the nonwhite populations is likely due to nutritional habits. Certain flavonoids that inhibit the first-pass metabolism of dihydropyridines are present in the diets of both Mexican and Japanese. Differences in phenotypes may play a role in interethnic variability.

Bioavailability of oral cyclosporine in healthy Mexican volunteers: evidence for interethnic variability.

The existence of population variations in cyclosporine pharmacokinetics could be expected, as this drug, similar to nifedipine, is biotransformed by cytochrome P-450 subfamily 3A4, and the existence of interethnic variability in nifedipine disposition has been demonstrated previously. The bioavailability of oral cyclosporine was studied in 23 healthy Mexican volunteers receiving 7.5-mg/kg doses of cyclosporine. Blood samples were drawn over 24 hours, and concentration of cyclosporine in whole blood was determined by a radioimmunoassay using monoclonal antibodies specific for the unchanged drug. The bioavailability of cyclosporine exhibited wide interindividual variability. Maximum concentration (Cmax) ranged from 528 ng/mL to 2,689 ng/mL, area under the concentration-time curve (AUC) ranged from 6,550 ng.hr/mL to 18,562 ng.hr/mL, and time to reach Cmax (tmax) ranged from 1 to 8 hours. Half-life (t1/2) exhibited less important variations, ranging from 4.4 to 9.1 hours. The bioavailability of oral cyclosporine in Mexicans was higher than that reported for white populations under similar conditions. The present results suggest the existence of interethnic variability in the pharmacokinetics of cyclosporine, as is the case with nifedipine.

Evidence for the involvement of nitric oxide in the antinociceptive effect of ketorolac.

The involvement of nitric oxide in the antinociception produced by ketorolac was assessed using the pain-induced functional impairment model in the rat: 800 micrograms of NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthesis, or saline was injected intra-articularly in a hind limb joint previously injured with uric acid. Animals then received ketorolac, dipyrone or no drug. Ketorolac and dipyrone produced a significant antinociceptive effect which was reduced by pretreatment with NG-nitro-L-arginine methyl ester, but not with saline. It is concluded that the antinociceptive effect of both drugs involves the local participation of nitric oxide.

Effect of caffeine coadministration and of nitric oxide synthesis inhibition on the antinociceptive action of ketorolac.

The effects of caffeine and nitric oxide synthesis inhibition on the antinociceptive action of ketorolac were assessed using the pain-induced functional impairment model in the rat. Nociception was induced by the intra-articular injection of uric acid. Ketorolac, but not caffeine, produced an antinociceptive effect which was reduced by NG nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis. Caffeine coadministration potentiated the ketorolac effect. L-NAME induced a dose-dependent reduction of this potentiation. The results suggest the participation of the L-arginine-nitric oxide-cyclic GMP pathway in the caffeine potentiation of ketorolac-induced antinociception.

Influence of sex on the pharmacokinetics of tolmetin in the rat.

The purpose of this study was to investigate sex-related differences in the pharmacokinetics of tolmetin, a potent nonsteroidal anti-inflammatory drug, in the rat. Male and female Wistar rats received oral tolmetin at two dose levels, 3.2 and 10 mg/kg. Blood samples were drawn at selected times after drug administration, and tolmetin concentration in whole blood was determined. Tolmetin was rapidly absorbed in all cases. C(max) increased with the dose, but was similar in both sexes. Notwithstanding, tolmetin half-life was significantly prolonged in females compared with males. As a result of the prolonged half-life, area under the curve values were significantly higher in females than in males. Tolmetin clearance was significantly reduced in females. The present results strongly suggest sex-related differences in the pharmacokinetics of tolmetin in the rat. Tolmetin elimination appears to be impaired in females, compared with males. The existence of sex-related differences in tolmetin pharmacokinetics in other species, including humans, requires further investigation.

Antagonism by potassium of the inhibition of (Na(+)+K+) ATPase produced by an endogenous digitalis-like compound extracted from mammalian heart.

The antagonism by potassium of (Na(+)+K+)ATPase inhibition was analyzed in order to further study the presence of endogenous digitalis-like activity in mammalian heart. Digoxin and a partially purified extract prepared from guinea pig cardiac tissue inhibited (Na(+)+K+)ATPase, also obtained from guinea pig heart, in a dose-dependent manner in agreement with Michaelis-Menten kinetics. The effects of both inhibitors were antagonized by increasing the KCl concentration from 0.5 to 5 mM. Hunter-Downs plots were linear, suggesting that the interaction between (Na(+)+K+)ATPase and either digoxin or the extract could be considered as competitive. Ks estimated for KCl from these plots was similar to the K0.5 value obtained by activating the enzyme with KCl, and thus suggesting that both inhibitors, digoxin and the extract, were acting on (Na(+)+K+)ATPase. Digoxin and the extract were able to inhibit [3H]ouabain specific binding to the enzyme in a dose-dependent manner in agreement with a competitive interaction. Ki values estimated from binding experiments were similar to those calculated from Hunter-Downs plots. Data strongly suggest the presence in guinea pig heart of an endogenous compound which is able to inhibit (Na(+)+K+)ATPase solely by binding to the digitalis receptors.

Pharmacokinetic-pharmacodynamic modeling: why?

At present, pharmacokinetic-pharmacodynamic (PK-PD) modeling has emerged as a major tool in clinical pharmacology to optimize drug use by designing rational dosage forms and dosage regimes. Quantitative representation of the dose-concentration-response relationship should provide information for prediction of the level of response to a certain level of drug dose. Several mathematical approaches can be used to describe such relationships, depending on the single dose or the steady-state measurements carried out. With concentration and response data on-phase, basic models such as fixed-effect, linear, log-linear, E(MAX), and sigmoid E(MAX) can be sufficient. However, time-variant pharmacodynamic models (effect compartment, acute tolerance, sensitization, and indirect responses) can be required when kinetics and response are out-of-phase. To date, methodologies available for PK-PD analysis barely suppose the use of powerful computing resources. Some of these algorithms are able to generate individual estimates of parameters based on population analysis and Bayesian forecasting. Notwithstanding, attention must be paid to avoid overinterpreted data from mathematical models, so that reliability and clinical significance of estimated parameters will be valuable when underlying physiologic processes (disease, age, gender, etc.) are considered.

Determination of oral rifampin pharmacokinetic parameters in Mexicans and comparison with other populations: absence of evidence for interethnic variability.

Oral pharmacokinetics of rifampin were studied in eight Mexican young healthy male volunteers after administration of a 600 mg oral dose. After an overnight fast, subjects received medication and blood samples were drawn at selected times over a 24-h period. Rifampin plasma levels were determined by HPLC. Pharmacokinetic parameters (mean +/- SEM) were: Cmax 13.514 +/- 1.775 micrograms/ml, tmax 1.88 +/- 0.30 h, AUC 73.61 +/- 9.48 micrograms.h/ml and half-life 2.98 +/- 0.29 h. Results were compared with those obtained for other populations under similar conditions in order to explore the possibility of interethnic variability, since it has been reported that rifampin pharmacokinetics in Indonesian subjects differ from those found in Europeans. Pharmacokinetic data found in Mexicans were comparable with those observed in British, Indian, Japanese and Italian individuals. As the pharmacokinetics of rifampin seem to be similar in different populations, it is concluded that ethnic origin does not appear to play an important role. Therefore, dosing regimens designed for Caucasians can be extrapolated for other populations.

Pharmacokinetics of oral ranitidine in Mexicans.

The pharmacokinetics of oral ranitidine were studied in 24 Mexican male healthy volunteers. Subjects received a tablet containing 150 mg of ranitidine (Azantac, Glaxo de México, Mexico City) after an overnight fast and blood samples were drawn at several times for a period of 24 h. Ranitidine concentration in plasma was measured by high performance liquid chromatography and pharmacokinetic parameters were determined by non-compartmental analysis. Ranitidine plasma concentration increased with time, reaching a maximum of (mean +/- SEM) 484 +/- 34 ng/ml in 2.7 +/- 0.2 h. Plasma levels then decayed with a terminal half-life of 4.8 +/- 0.3 h. The area under the plasma concentration against time curve was 2440 +/- 126 ngh/ml. Oral ranitidine pharmacokinetic parameters in Mexicans appeared to be similar to those previously reported for Caucasians.

Comparison between Sprague-Dawley and Wistar rats as an experimental model of pharmacokinetic alterations induced by spinal cord injury.

Two strains of rats, Sprague-Dawley and Wistar, were assayed in order to determine which strain is the more suitable experimental model for the study of pharmacokinetic alterations induced by spinal cord injury. Animals were submitted to spinal cord contusion at the T8-T9 level by the weight drop method. A single acetaminophen oral dose (100 mg/ kg) was administered 24 h after injury and blood samples were drawn for a period of 4 h. Acetaminophen concentration in whole blood was determined by high performance liquid chromatography and pharmacokinetic parameters were estimated. For both strains, Cmax and AUC were significantly lower, whereas tmax remained unchanged, in injured animals compared to sham-injured controls. Circulating acetaminophen concentrations were higher; therefore, pharmacokinetic alterations were more easily discerned, in Sprague-Dawley than in Wistar rats. It is concluded that the Sprague-Dawley strain is a more suitable model for the study of pharmacokinetic alterations induced by spinal cord injury.

Analgesic efficacy and bioavailability of ketorolac in postoperative pain: a probability analysis.

BACKGROUND: The analgesic efficacy and bioavailability of 30 mg intramuscular ketorolac was studied in 24 patients with severe or very severe postoperative pain. METHODS: Pain and pain relief were determined by a five-point verbal rating scale and data were submitted to a probability analysis. Ketorolac plasma levels were determined by high-performance liquid chromatography. RESULTS: Two patients chose not to finish the study; 22 patients completed the study achieving at least good pain relief. Of these 22 patients, 13 reached complete pain relief. Ketorolac was rapidly absorbed. Notwithstanding, pain relief increased gradually, showing considerable delay with regard to plasma concentrations. Analysis of the probability-time curves revealed that 25% of the patients obtained moderate pain relief at 7 min after ketorolac administration, 50% at 11 min, 75% at 29 min, and 95% at 60 min. Good pain relief was achieved in 25, 50, and 75% of the patients at 1.1, 1.8, and 2.7 h, respectively. Complete pain relief was achieved in 25% and 50% of the patients at 2.6 h and 3.7 h, respectively. The probability of exhibiting an acceptable pain relief in responsive patients for more than 5 h was 0.97. No serious side effects were detected. CONCLUSIONS: Results show that 30 mg intramuscular ketorolac is an adequate treatment for postoperative pain in the Mexican population. Therefore, the use of higher doses is not justified. Due to gradual installation of analgesia, administration of additional analgesic medication before 1 h is not recommended.

Comparative bioavailability evaluation of two cyclosporine oral formulations in healthy Mexican volunteers.

BACKGROUND: The use of conventional cyclosporine (Sandimmune) requires great care, as this drug exhibits a narrow therapeutic index and wide interindividual variability in its pharmacokinetics. Recently, a new microemulsion formulation (Neoral) was developed. With this formulation, cyclosporine is absorbed at the small intestine without the presence of bile. Therefore, the objective of this study was to compare the bioavailability of cyclosporine after the administration of conventional and microemulsion formulations in healthy Mexican volunteers in order to approach the optimal dosage regimen of microemulsion in the Mexican population. METHODS: The trial was conducted using 23 healthy volunteers according to a randomized crossover design. Volunteers received one 7.5-mg/kg dose as each formulation, with a 1-week washout period between treatments. Blood samples of 0.5 mL were obtained according to the following schedule: 0, 0.5, 1, 2, 3, 4, 5, 6, 8, 12, and 24 h after medication. RESULTS: These indicated that Cmax and AUC0-24 values were higher with the microemulsion than with the conventional formulation. CONCLUSIONS: The microemulsion had a better absorption profile than the conventional formulation, because plasma levels with the conventional formulation demonstrated oscillations rather than reflecting an erratic absorption. Lower doses of the microemulsion are required to obtain Cmax values similar to those obtained with conventional cyclosporine.

Determination of Three Acetylator Phenotypes in a Mexican Population Using Sulfamethazine Metabolic Ratio.

Eighty-six Mexican young healthy volunteers received a 500-mg oral sulfamethazine dose. Urine was collected 0--6 h after medication and acetylated, and unchanged sulfamethazine were determined by the Bratton--Marshall method. The frequency distribution of acetylated/unchanged sulfamethazine, determined by probit analysis, appeared to be trimodal, allowing the discrimination of three acetylator phenotypes. The frequencies of rapid, intermediate, and slow acetylators were 28%, 42%, and 30% respectively, being consistent with the Hardy-Weinberg law.

Discrepancy between Bioavailability and Hypotensive Effect of Oral and Sublingual Nifedipine.

Nifedipine, 10-mg capsules, were given orally and sublingually to six healthy volunteers according to a randomized crossover design. Nifedipine plasma levels, blood pressure, and heart rate were determined at several times after medication. C(max) was higher (134 plus minus 17 vs. 93 plus minus 2 ng ml(minus sign1), mean plus minus SD, P < 0.01) and occurred earlier (0.5 vs. 1 h) with oral than with sublingual nifedipine. However, there was no significant difference in AUC (268 plus minus 56 vs. 288 plus minus 35 ng h ml(minus sign1)) nor in t(1/2) (1.8 plus minus 0.2 vs. 1.9 plus minus 0.3 h), indicating that sublingual administration decreased the rate but not the extent of nifedipine absorption. Notwithstanding the difference in C(max), both routes yielded a similar reduction in diastolic blood pressure of 13 plus minus 1 mm Hg. Heart rate increase, which reflects the activation of homeostatic mechanisms, was greater with oral than with sublingual nifedipine, that is, 18 plus minus 1 vs. 13 plus minus 1 beats min(minus sign1), P < 0.01. It is concluded that slower absorption after sublingual administration increases nifedipine hypotensive efficiency by producing less counteracting homeostatic responses than the more rapidly absorbed oral nifedipine.

Evidence against the participation of mu- and kappa-opioid receptors in the analgesic activity of ketorolac in rats.

The possibility that activation of opioid receptors is involved in the analgesic activity of ketorolac was explored. The analgesic effects of ketorolac, of ketocyclazocine, the prototype kappa-agonist, and of morphine, the prototype mu-agonist, were assayed in the pain-induced functional impairment model in the rat. All three drugs induced a significant analgesic effect in this model. Naloxone was able to antagonize the effects of ketocyclazocine and morphine. However, the effect of ketorolac was not blocked by naloxone, although a high dose, 3.2 mg kg-1, capable of blocking kappa-receptors was used. It is concluded that activation of mu- or kappa-opioid receptors, by either a direct or an indirect mechanism, does not play a role in the analgesic activity of ketorolac.

Characterization of the analgesic effect of paracetamol and caffeine combinations in the pain-induced functional impairment model in the rat.

The analgesic activities of paracetamol (100, 178, 316 and 562 mg kg-1), caffeine (10, 18, 32 and 56 mg kg-1) and combinations of these doses were tested on a pain-induced functional impairment model in the rat. Dysfunction of the right hind limb was induced by an intra-articular injection of 30% uric acid in the knee. Drugs were given orally and the recovery of functionality over time was considered as an expression of analgesia. Paracetamol alone induced a dose-dependent analgesic effect whereas caffeine alone did not show any activity at the assayed doses. Combinations of 316 mg kg-1 paracetamol with either 10, 18, 32 or 56 mg kg-1 caffeine yielded analgesic effects significantly greater than that of paracetamol alone. The highest potentiation was observed with a paracetamol-caffeine mixture of 316-32 mg kg-1. Caffeine coadministration, however, did not significantly change paracetamol plasma levels. No potentiation was obtained with other combinations. Paracetamol plasma levels and analgesic effect observed with administration of 316 mg kg-1 paracetamol alone or 316-32 mg kg-1 of paracetamol-caffeine were fitted to the sigmoidal Emax model according to the Hill equation. The curves obtained were parallel, but that of the combination was shifted to the left. It is concluded that caffeine is able to potentiate the analgesic effect of paracetamol by a pharmacodynamic mechanism, but this only occurs at certain dose combinations.