Pharmacokinetics and metabolism of diltiazem in healthy males and females following a single oral dose

Metabolic N-Dealkylation and N-Oxidation as Elucidators of the Role of Alkylamino Moieties in Drugs Acting at Various Receptors

Metabolic reactions that occur at alkylamino moieties may provide insight into the roles of these moieties when they are parts of drug molecules that act at different receptors. N-dealkylation of N,N-dialkylamino moieties has been associated with retaining, attenuation or loss of pharmacologic activities of metabolites compared to their parent drugs. Further, N-dealkylation has resulted in clinically used drugs, activation of prodrugs, change of receptor selectivity, and providing potential for developing fully-fledged drugs. While both secondary and tertiary alkylamino moieties (open chain aliphatic or heterocyclic) are metabolized by CYP450 isozymes oxidative N-dealkylation, only tertiary alkylamino moieties are subject to metabolic N-oxidation by Flavin-containing monooxygenase (FMO) to give N-oxide products. In this review, two aspects will be examined after surveying the metabolism of representative alkylamino-moieties-containing drugs that act at various receptors (i) the pharmacologic activities and relevant physicochemical properties (basicity and polarity) of the metabolites with respect to their parent drugs and (ii) the role of alkylamino moieties on the molecular docking of drugs in receptors. Such information is illuminative in structure-based drug design considering that fully-fledged metabolite drugs and metabolite prodrugs have been, respectively, developed from N-desalkyl and N-oxide metabolites.

Mapping human microbiome drug metabolism by gut bacteria and their genes

Individuals vary widely in their responses to medicinal drugs, which can be dangerous and expensive owing to treatment delays and adverse effects. Although increasing evidence implicates the gut microbiome in this variability, the molecular mechanisms involved remain largely unknown. Here we show, by measuring the ability of 76 human gut bacteria from diverse clades to metabolize 271 orally administered drugs, that many drugs are chemically modified by microorganisms. We combined high-throughput genetic analyses with mass spectrometry to systematically identify microbial gene products that metabolize drugs. These microbiome-encoded enzymes can directly and substantially affect intestinal and systemic drug metabolism in mice, and can explain the drug-metabolizing activities of human gut bacteria and communities on the basis of their genomic contents. These causal links between the gene content and metabolic activities of the microbiota connect interpersonal variability in microbiomes to interpersonal differences in drug metabolism, which has implications for medical therapy and drug development across multiple disease indications.

Pharmacogenomics, pharmacokinetics and pharmacodynamics: interaction with biological differences between men and women

Pharmacological response depends on multiple factors and one of them is sex-gender. Data on the specific effects of sex-gender on pharmacokinetics, as well as the safety and efficacy of numerous medications, are beginning to emerge. Nevertheless, the recruitment of women for clinical research is inadequate, especially during the first phases. In general, pharmacokinetic differences between males and females are more numerous and consistent than disparities in pharmacodynamics. However, sex-gender pharmacodynamic differences are now increasingly being identified at the molecular level. It is now even becoming apparent that sex-gender influences pharmacogenomics and pharmacogenetics. Sex-related differences have been reported for several parameters, and it is consistently shown that women have a worse safety profile, with drug adverse reactions being more frequent and severe in women than in men. Overall, the pharmacological status of women is less well studied than that of men and deserves much more attention. The design of clinical and preclinical studies should have a sex-gender-based approach with the aim of tailoring therapies to an individual's needs and concerns.

Repeated dosing of piperine induced gene expression of P-glycoprotein via stimulated pregnane-X-receptor activity and altered pharmacokinetics of diltiazem in rats

This study investigated the effect of piperine on the gene expression of P-glycoprotein (P-gp) as well as pregnane-X-receptor (PXR) activity and also its implication on the bioavailability of diltiazem, a P-gp substrate. The effect of piperine on the systemic exposure of diltiazem was examined in rats after the intravenous and oral administration of diltiazem with/without 2 week pretreatment with piperine. Compared with the control group given diltiazem (20 mg/kg) alone, the pretreatment with piperine (10 or 20 mg/kg, once daily for 2 weeks) decreased the oral exposure of diltiazem by 36-48% in rats. Consequently, the bioavailability of oral diltiazem was significantly lower (p < 0.05) after the 2 week pretreatment with piperine. The pretreatment with piperine for 2 weeks also reduced the systemic exposure of desacetyldiltiazem, a major active metabolite of diltiazem by approximately 73%, accompanied by a significant decrease in the metabolite-parent ratio. In contrast to the oral pharmacokinetics, piperine did not affect the intravenous pharmacokinetics of diltiazem in rats. Immunoblot analysis indicated that the protein expression level of intestinal P-gp was significantly enhanced after the 2 week pretreatment with piperine in rats. In addition, piperine increased the PXR reporter activity in human hepatoma cells. Taken together, the 2 week pretreatment with piperine significantly induced intestinal P-gp expression in conjunction with stimulated PXR activity and decreased the oral exposure of diltiazem and desacetyldiltiazem in rats.

Effects of lovastatin on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats: possible role of cytochrome P450 3A4 and P-glycoprotein inhibition by lovastatin

OBJECTIVES

The purpose of this study was to examine the effects of lovastatin on cytochrome P450 (CYP) 3A4 and P-glycoprotein (P-gp) in vitro and then to determine the effects of lovastatin on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats.

METHODS

The pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined after orally administering diltiazem (12 mg/kg) to rats in the presence and absence of lovastatin (0.3 and 1.0 mg/kg). The effect of lovastatin on P-gp as well as CYP3A4 activity was also evaluated.

KEY FINDINGS

Lovastatin inhibited CYP3A4 enzyme activity with a 50% inhibition concentration of 6.06 µM. In addition, lovastatin significantly enhanced the cellular accumulation of rhodamine-123 in MCF-7/ADR cells overexpressing P-gp. Compared with the control (given diltiazem alone), the presence of lovastatin significantly altered the pharmacokinetic parameters of diltiazem. The areas under the plasma concentration-time curve (AUC) and the peak concentration of diltiazem were significantly increased (P < 0.05, 1.0 mg/kg) in the presence of lovastatin. Consequently, the absolute bioavailability values of diltiazem in the presence of lovastatin (11.1% at 1.0 mg/kg) were significantly higher (P < 0.05) than that of the control group (7.6%). The metabolite-parent AUC ratio in the presence of lovastatin (1.0 mg/kg) was significantly (P < 0.05) decreased compared with the control group.

CONCLUSIONS

It might be considered that lovastatin resulted in reducing the first-pass metabolism in the intestine and/or in the liver via inhibition of CYP3A4 and increasing the absorption of diltiazem in the intestine via inhibition of P-gp by lovastatin.

Effect of hesperidin on the oral pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats

Objectives

This study was to investigate the effect of hesperidin, an antioxidant, on the bioavailability and pharmacokinetics of diltiazem and its active major metabolite, desacetyldiltiazem, in rats.

Methods

A single dose of diltiazem was administered orally (15 mg/kg) in the presence or absence of hesperidin (1, 5 or 15 mg/kg), which was administered 30 min before diltiazem.

Key findings

Compared with the control group (given diltiazem alone), hesperidin (5 or 15 mg/kg) significantly altered the pharmacokinetic parameters of diltiazem, except for 1 mg/kg hesperidin. The area under the plasma concentration-time curve from time 0 h to infinity (AUC0-∞) was significantly (5 mg/kg, P &lt; 0.05; 15 mg/kg, P &lt; 0.01) increased by 48.9–65.3% and the peak plasma concentration (Cmax) was significantly (P &lt; 0.05) increased by 46.7–62.4% in the presence of hesperidin (5 or 15 mg/kg). Consequently, the absolute bioavailability (F) of diltiazem with hesperidin was significantly (5 mg/kg, P &lt; 0.05; 15 mg/kg, P &lt; 0.01) higher than that in the control group. Hesperidin (5 or 15 mg/kg) significantly (P &lt; 0.05) increased the AUC0-∞ and 15 mg/kg of hesperidin significantly (P &lt; 0.05) increased the Cmax of desacetyldiltiazem. However, the metabolite-parent ratio (MR) of desacetyldiltiazem was not significantly changed in the presence of hesperidin.

Conclusions

Hesperidin significantly enhanced the oral bioavailability of diltiazem in rats. It might be considered that hesperidin increased the intestinal absorption and reduced the first-pass metabolism of diltiazem in the intestine and in the liver via an inhibition of cytochrome P450 3A or P-glycoprotein.

Semiphysiologically based pharmacokinetic models for the inhibition of midazolam clearance by diltiazem and its major metabolite

Prediction of the extent and time course of drug-drug interactions (DDIs) between the mechanism-based inhibitor diltiazem (DTZ) and the CYP3A4 substrate midazolam (MDZ) is confounded by time- and concentration-dependent clearance of the inhibitor. Semiphysiologically based pharmacokinetic (PBPK) models were developed for DTZ and MDZ with the major metabolite of DTZ, N-desmethyldiltiazem (nd-DTZ), incorporated in the DTZ model. Enzyme kinetic parameters (k(inact) and K(I)) for DTZ and nd-DTZ were estimated in vitro and used to model the time course of changes in the amount of CYP3A4 in the liver and gut wall, which in turn, determined the nonlinear elimination of MDZ and DTZ, and the corresponding DDI. The robustness of the model prediction was assessed by comparing the results of the prediction to published DTZ pharmacokinetic and DTZ/MDZ interaction data. A clinical study was conducted to further validate the predicted increase of MDZ exposure after DTZ treatment. The model predicted the nonlinear disposition of DTZ after single and multiple oral doses. The clinical study showed that DTZ treatment resulted in 4.1- and 1.6-fold increases in MDZ exposure after oral and intravenous MDZ administration, respectively, suggesting that the DDI in the gut wall plays an important role in the DTZ/MDZ interaction. The semi-PBPK model incorporating the DDI at the gut wall, and the effect of nd-DTZ successfully predicted the nonlinear disposition of DTZ and its interaction with MDZ. Moreover, model simulation suggested that both DTZ and nd-DTZ contributed to the overall inhibitory effect after DTZ administration, and the values of the in vitro estimated inhibition parameters and CYP3A4 turnover rate are critical for the prediction.

effect of atorvastatin on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats

The purpose of this study was to investigate the effect of atorvastatin, HMG-CoA reductase inhibitor, on the pharmacokinetics of diltiazem and its active metabolite, desacetyldiltiazem, in rats. Pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined in rats after oral administration of diltiazem (15 mg x kg(-1)) to rats pretreated with atorvastatin (0.5 or 2.0 mg x kg(-1)). Compared with the control (given diltiazem alone), the pretreatment of atorvastatin significantly altered the pharmacokinetic parameters of diltiazem. The peak concentration (Cmax) and the areas under the plasma concentration-time curve (AUC) of diltiazem were significantly (p < 0.05, 0.5 mg x kg(-1); p < 0.01, 2.0 mg x kg(-1)) increased in the presence of atorvastatin. The AUC of diltiazem was increased by 1.40-fold in rats pretreated with 0.5 mg x kg(-1) atorvastatin, and 1.77-fold in rats pretreated with 2.0 mg x kg(-1) atorvastatin. Consequently, absolute bioavailability values of diltiazem pretreated with atorvastatin (8.4-10.6%)were significantly higher (p < 0.05) than that in the control group (6.6%). Although the pretreatment of atorvastatin significantly (p < 0.05) increased the AUC of desacetyldiltiazem, metabolite-parent AUC ratio (M.R.) in the presence of atorvastatin (0.5 or 2.0 mg x kg(-1)) was significantly decreased compared to the control group, implying that atorvastatin could be effective to inhibit the metabolism of diltiazem. In conclusion, the concomitant use of atorvastatin significantly enhanced the oral exposure of diltiazem in rats.

High-performance liquid chromatography-mass spectrometry analysis of diltiazem and 11 of its phase I metabolites in human plasma

The aim of the present work was to develop a high-performance liquid chromatography-mass spectrometry method for analysis of diltiazem (DTZ) and metabolites in human plasma after single dose administration (120 mg). Human plasma samples (1 ml) were cleaned up by a solid phase extraction procedure (C18 cartridges) using codeine as an internal standard. Reconstituted extracts were separated on a reversed-phase C8 column with a linear gradient mobile phase system. The run time per sample analysis was 11 min. Detection was performed using selected ion monitoring following atmospheric pressure chemical ionization. The lower limit of quantification was estimated to be 1 microg/l (in spiked plasma) for all available reference compounds (i.e. DTZ and five metabolites). Validation of the method showed good linearity, precision and accuracy for quantification of these six reference compounds. In addition, tandem MS analyses of human plasma sampled from healthy individuals after peroral intake of 120 mg DTZ revealed that the method enabled detection of six additional metabolites for which reference compounds were not available.

Pharmacokinetic interaction between tacrolimus and diltiazem: dose-response relationship in kidney and liver transplant recipients

OBJECTIVE

To study the dose-response relationship of the pharmacokinetic interaction between diltiazem and tacrolimus in kidney and liver transplant recipients.

DESIGN

Nonrandomised seven-period stepwise pharmacokinetic study.

PATIENTS

Stable kidney (n = 2) and liver (n = 2) transplant recipients maintained on oral tacrolimus twice daily but not taking diltiazem.

METHODS

Patients were treated with seven incremental dosages of diltiazem (0 to 180 mg/day) at > or = 2-weekly intervals. At the end of each interval, 13 blood samples were taken over a 24-hour period to allow determination of morning (AUC(12)), evening (AUC(12-24)) and 24-hour (AUC(24)) areas under the concentration-time curve for tacrolimus, as well as AUC(24) for diltiazem and three of its metabolites.

RESULTS

There was considerable interpatient variability in tacrolimus-sparing effect. In the two kidney transplant recipients, an increase in tacrolimus AUC(24) occurred following the 20 mg/day dosage of diltiazem (26 and 67%). The maximum increase in tacrolimus AUC(24) occurred at the maximum diltiazem dosage used (180 mg/day), when the increase was 48 and 177%. In the two liver transplant recipients, an increase in tacrolimus AUC(24) did not occur until a higher diltiazem dosage (60 to 120 mg/day) was given. The increase at the maximum diltiazem dosages used (120 mg/day in one and 180 mg/day in the other) was also lower (18 and 22%) than that exhibited by the kidney transplant recipients. The increase in tacrolimus AUC(12) was similar to the increase in AUC(12-24) when diltiazem was given in the morning only (dosages < or = 60 mg/day). Hence, diltiazem affects blood tacrolimus concentrations for longer than would be predicted from the half-life of diltiazem in plasma.

CONCLUSIONS

The mean tacrolimus-sparing effect of diltiazem was similar in magnitude to the cyclosporin-sparing effect previously reported. Whether the lesser tacrolimus-sparing effect with diltiazem seen in the liver transplant recipients was due to functional differences in the transplanted liver is not known, but it was not due to lower plasma diltiazem concentrations. Diltiazem makes a logical tacrolimus-sparing agent because of the potential financial savings and therapeutic benefits. Because of interpatient variability, the sparing effect should be demonstrated in each patient and not merely assumed.

Effect of administration route and length of exposure on pharmacokinetics and metabolism of diltiazem in dogs

The objective of this study was to systematically determine the pharmacokinetics and metabolism of diltiazem (DTZ) after a single i.v. dose, and after single and multiple oral (p.o.) doses. Four mongrel dogs (3 M, 1 F), aged 1-3 years, body weight 19-25 kg, were each given a single 30 mg dose of DTZ as a solution by i.v injection, the same dose orally from an immediate release tablet (Cardizem, Aventis Pharma, Canada, QC), and also t.i.d. for 10 doses. A 3-4 week washout period was allowed between each treatment. Blood samples (4 ml each) were obtained after each treatment from each animal via a cephalic vein at 0 (just before dosing), 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 6.0, 8.0, and 12.0 h post dose. Urine samples were collected for 24 h. The plasma samples were immediately separated by centrifugation and stored at -20 degrees C until analysis. The results showed that the bioavailability after a single p.o. dose of DTZ was 26+/-24%. Following a single i.v. dose, DTZ declined bi-exponentially with a terminal half-life (t1/2) of 4.2+/-1.7 h. N-Monodesmethyl DTZ (M(A)), deacetyl DTZ (M1), and deacetyl N-monodesmethyl DTZ (M2) were the major metabolites. Contrary to the results observed in clinical studies, there were no increase of plasma concentrations of DTZ after repeated doses (accumulation factor R = 0.94+/-0.51). Plasma concentrations of M1 decreased following repeated oral doses, accompanying by an increase of plasma concentrations of M2, although these changes were not statistically significant (p >0.05). This study cautions the use of mongrel dogs for direct extrapolation to humans, particularly for chronic pharmacokinetics studies of DTZ.

A simple and sensitive high-performance liquid chromatography assay of diltiazem and main metabolites in renal transplanted patients

A sensitive and specific reversed-phase high-performance liquid chromatographic method was developed for determination of the benzothiazepine calcium channel blocker diltiazem and three of its main metabolites in human plasma. A solid-phase extraction method (C18) is described for isolating diltiazem and the metabolites N-demethyl-diltiazem (M(A)), deacetyldiltiazem (M1) and N-demethyl-deacetyl-diltiazem (M2) from human plasma spiked with trans-diltiazem as internal standard. Chromatographic analysis of the eluate was accomplished using a reversed-phase column (C8) and a mobile phase consisting of acetonitrile, potassium dihydrogen phosphate-buffer (pH 2.9), triethylamine and methanol (280:598:2:90 v/v/v/v). The limit of detection was 5 ng/ml for diltiazem and 2.5 ng/ml for the metabolites (UV detection). Recoveries were > 75% for all substances. Precision analysis indicated a coefficient of variation below 5% for both diltiazem and metabolites at plasma concentrations ranging from 30 to 120 ng/ml for diltiazem and from 15 to 60 ng/ml for the metabolites. The accuracy was < 3% for diltiazem at all concentrations investigated. Metabolites showed an accuracy of < 7% at higher concentrations, however at low concentration the accuracy was < 16%. A pilot study has been performed in order to study whether the method was applicable to patients and volunteers. Plasma samples from renal transplanted patients treated with cyclosporin A and healthy volunteers did not show any interference by cyclosporin A with the determination of diltiazem and metabolites.

Diltiazem disposition and metabolism in recipients of renal transplants

Diltiazem is widely prescribed in Australasia as a cyclosporine-sparing agent. On seven separate occasions at 2-week intervals, the authors studied eight patients who had undergone renal transplantation, were treated with cyclosporin A, and were in stable condition. The patients were administered escalating doses of conventional-release diltiazem, and (in an extension study) controlled-diffusion diltiazem, to consider the disposition and metabolism of diltiazem. Blood samples were drawn during a 24-hour period, and the AUC(0)(24) of diltiazem and three major metabolites was determined. Results demonstrated that seven patients had comparable diltiazem metabolite AUC(0)(24) profiles, despite considerable variability in parent diltiazem areas under the curve (AUCs), with DM-DTZ > DA-DTZ > DMDA-DTZ. The eighth patient displayed a different metabolite profile. The controlled-diffusion formulation reduced the interpatient variability in diltiazem AUC(0)(24) from 46-fold to <3-fold, but it did not appear to have release characteristics consistent with the manufacturer's specifications. The apparent bioavailability of the conventional-release diltiazem formulation appeared to be highly variable, and this has implications for its use in recipients of organ transplants. The dosage escalation demonstrated a linear relationship between dose and AUC for diltiazem and for each of the metabolites. There may be a subset of patients who display a different diltiazem metabolite profile.

Pharmacokinetics and haemodynamic effect of diltiazem in rats: effect of route of administration

Diltiazem is a calcium antagonist widely used for the treatment of angina and hypertension. Previous studies in patients have shown that the haemodynamic effects of diltiazem are greater after parenteral rather than oral administration. The rat has been used as an animal model to determine the effect of the route of administration on the pharmacokinetic and haemodynamic effects of diltiazem. The results showed that plasma concentrations of diltiazem were more than 10 times higher after the intra-arterial dose. The plasma concentrations of the major metabolites were also higher after intra-arterial administration, although only for deacetyl diltiazem (M1) did the difference reach statistical significance (P < 0.05). The haemodynamic effects (on blood pressure and heart rate) of diltiazem were considerably greater after intra-arterial administration; this was attributed mainly to the much higher plasma concentrations of diltiazem. The hypotensive and chronotropic effects of diltiazem were similar; Emax and EC50 for diastolic blood pressure were 72+/-19% and 4.4+/-5.9 microg mL(-1); for heart rate they were 77+/-32% and 10.0+/-11.7 microg mL(-1), respectively. The haemodynamic effects of diltiazem are much greater after intra-arterial administration, mainly because of the much higher plasma concentrations of the drug. The contribution by the metabolites would be minimal after this route of administration.

Effects of age, gender, and diurnal variation on the steady-state pharmacokinetics of BMS-181101, an antidepressant, in healthy subjects

OBJECTIVES

To investigate the effects of age, gender, and diurnal variation on the safety, tolerability, and steady-state pharmacokinetics of BMS-181101, an antidepressant, in humans.

METHODS

This was a multiple-dose parallel-design study in 51 healthy subjects (12 young and 12 elderly men and 12 young and 15 elderly women). Each subject received a 15 mg oral dose of BMS-181101 every 12 hours on days 1 through 6 and one dose on day 7. After the evening dose on day 6 and morning dose on day 7, serial blood samples were collected at specified times after administration. Plasma was analyzed for BMS-181101 with use of an HPLC method.

RESULTS

Male subjects tolerated BMS-181101 better than female subjects. The mean values for area under the plasma concentration-time curve over the dosing interval tau (AUC tau; 58.8 to 102.4 ng.hr/ml) and elimination half-life t1/2; 5.7 to 10.4 hours) for the elderly subjects were significantly greater than those for the young subjects (39.0 to 64.3 ng.hr/ml and 3.2 to 4.5 hours). The mean values for peak plasma concentration (Cmax; 14.7 to 25.2 ng/ml) and AUC tau (52.4 to 102.4 ng.hr/ml) for the women were significantly greater than those for the men (9.08 to 15.3 ng/ml and 39.0 to 73.6 ng.hr/ml). The mean values for Cmax (14.7 to 25.2 ng/ml) and AUC tau (54.8 to 102.4 ng.hr/ml) on the morning of day 7 were significantly greater than those after the evening dose on day 6 (9.08 to 17.3 ng/ml; 39.0 to 83.4 ng.hr/ml).

CONCLUSIONS

An initial lower dose or appropriate titration of daily doses of BMS-181101 may be necessary for the treatment of elderly and female subjects, and the pharmacokinetics of BMS-181101 exhibited significant diurnal effects.

Steady-state plasma concentrations of diltiazem and its metabolites in patients and healthy volunteers

Diltiazem (DTZ) is a calcium antagonist widely used in the treatment of angina and hypertension. It is extensively metabolized in humans via N-demethylation, O-demethylation, deacetylation, and oxidative deamination, yielding a host of metabolites, some of which have potent pharmacological properties. After our initial identification of O-desmethyl DTZ (Mx) and N,O-didesmethyl DTZ (MB) as major metabolites of DTZ and our subsequent of identification of their chemical synthesis, an improved high-performance liquid chromatography assay was developed to determine the plasma concentrations of DTZ and seven of its major basic metabolites, including the previously unquantitated Mx and MB. The system consisted of a C18 analytical column protected by a C18 cartridge guard column and a variable wavelength ultraviolet detector set at 237 nm. The mobile phase was a mixture of methanol, 0.04 M ammonium acetate, and acetonitrile (38:36:26) containing 0.08% triethylamine, with final pH of the mobile phase adjusted to 7.5. The system was operated at room temperature isocratically at a flow rate of 1.2 ml/min. Using verapamil as an internal standard, DTZ and the basic metabolites in plasma were determined in young healthy volunteers (n = 21) and in patients with ischemic heart disease (n = 19) at steady state after repeated oral doses of 60 mg DTZ four times daily. Preliminary results show that steady-state plasma concentrations of DTZ and its metabolites were higher in the older patients than in young healthy subjects (p < 0.05).

Effect of phenobarbital pretreatment on the pharmacokinetics and metabolism of diltiazem in rats

In order to study the effect of cytochrome P-450 isozyme induction on the pharmacokinetics and metabolism of diltiazem (DTZ), male Sprague-Dawley rats weighing 300-600 g were randomly assigned to two groups. The enzyme induction group (n = 4) received phenobarbital 60 mg/kg i.p. once daily for 4 days, whereas the control group (n = 6) received normal saline for the same duration. Each rat then received a single oral dose of DTZ in solution (20 mg/kg). Blood samples (0.5 ml) were collected from each rat via an implanted polyethylene catheter (0.040" i.d.) in the right carotid artery at 0 (just before dosing), 0.25, 0.5, 1,2,3,4,6,8 and 10 h post-dose. Arterial plasma concentrations of DTZ and its metabolites M(A), M1, M2, M4 and M6 were determined by HPLC. Pharmacokinetics parameters were calculated using non-linear regression. The results showed that both mean Cmax and AUC of DTZ were lower (871.6 vs 79.8 ng/ml; 1171 vs 101.9 ng-h/ml), but the mean Cmax of the primary metabolites M1 and M(A) was higher after phenobarbital (M1 413.0 vs 648.9 ng/ml; M(A) 683.0 vs 814.8 ng/ml). The highest increase was seen in the mean Cmax and AUC of the secondary metabolite M2 (837.5 vs 2585.7 ng/ml; 3312.1 vs 13156.5 ng-h/ml). In contrast, plasma concentrations of the O-desmethylated metabolites M4 and M6 did not increase after phenobarbital. These results suggest that both deacetylation and N-demethylation of DTZ in rats are catalyzed by drug metabolizing enzymes inducible by phenobarbital.

Gender differences in exercise and recovery blood pressure responses in normal volunteers given diltiazem

This preliminary phase I study was conducted in healthy volunteers to determine whether gender differences exist in the hemodynamic effects of diltiazem at rest, during exercise, and after exercise. At comparable serum concentrations of the drug, women demonstrated lower systolic and diastolic pressure during exercise and after exercise. ST slope after diltiazem administration in women became less positive during exercise and was gender specific. Heart rate and P-R interval changes were not gender dependent. Results of this study demonstrate that some hemodynamic responses to diltiazem are gender specific while others are not. It indicates that direct comparison studies may be required to detect such differences. In healthy women, hypotension after exercise and the effects of diltiazem are more synergistic than in men. Such a gender difference in response may be an important consideration in determining the correct dosages of this drug for treatment of hypertension.

Endogenous natriuretic factors 3: isolation and characterization of human natriuretic factors LLU-alpha, LLU-beta 1, and LLU-gamma

A low molecular weight endogenous substance believed to be responsible for extracellular fluid homeostasis in mammals has been sought for many years. Our goal is to isolate and structurally characterize this putative "natriuretic hormone." We have developed an assay using the conscious rat to measure prolonged natriuresis (Benaksas et al (1993) Life Sciences, 52, 1045-1054), the activity originally described for this putative substance. Using this assay we have identified a number of natriuretic compounds isolated from human uremic urine. The collected urine is processed by ultrafiltration (< or = 3 kDa), gel filtration chromatography (G-25) and extraction with isopropanol and diethyl ether. The organic soluble material is then subjected to sequential high-performance liquid chromatography. We report here the initial characterization of two pure isolates (LLU-alpha and LLU-gamma) obtained by this method, and the structural elucidation of a third pure compound, LLU-beta 1, a natriuretic and previously unreported metabolite of the drug diltiazem.

Pharmacokinetics and metabolism of diltiazem in rats following a single intra-arterial or single oral dose

Diltiazem (DTZ) 20 mg/kg was given to male Sprague-Dawley rats either orally (p.o.) or intra-arterially (i.a.) over a 5 min period (n = 6 for each group). Plasma concentrations of DTZ and its major basic metabolites were determined by high performance liquid chromatography assay (HPLC) as previously described over a 10 h period. The major metabolites found in the rat plasma were M2, followed by M6, MA, M1, and then M4. The metabolite Mx was measurable only in some of the plasma samples, and MB was not detected in this species. The mean apparent half-life (t1/2) of the measurable metabolites were longer than the parent DTZ. The metabolism profiles were qualitatively similar between the two routes of administration. Quantitatively, however, the plasma concentrations of the metabolites were higher after the i.a. route. These results are in agreement with a previous study reported in rabbits, and suggest that deacetylation of DTZ and MA in the blood is extremely important in this species.

The effect of multiple doses of ranitidine on the pharmacokinetics and metabolism of diltiazem in dogs

In order to determine the potential pharmacokinetic drug interaction between ranitidine and diltiazem (DTZ), each of ten male beagle dogs, age 2.7-4.0 years, weight 13-16 kg, received a single oral dose of sustained release DTZ with and without previous multiple oral doses of ranitidine (150 mg bid for five doses). The dog was selected as the animal model because the pharmacokinetics and metabolism profiles of DTZ are similar to those in humans and because sustained release DTZ capsules can be administered with ease to this species. Following the oral dose of DTZ, blood samples (5 ml each) were obtained via a cephalic vein at 0 (just before dosing), 1, 2, 3, 4, 5, 6, 8, 12, 18, 24, 30, 36, and 48 h after the dose. Urine samples were collected for 48 h post dose. Plasma and urine concentrations of DTZ and its major metabolites N-monodesmethyl DTZ (MA), deacetyl DTZ (M1), and deacetyl N-monodesmethyl DTZ (M2) were determined by HPLC. Pharmacokinetic parameters were calculated by non-linear curve fitting, and the effect of ranitidine was evaluated by two-factor analysis of variance (ANOVA). Pre-treatment of the animals did not significantly alter the disposition of DTZ (p > 0.05). Similar to the results reported in clinical studies, there were large variations in the plasma and urine concentrations of DTZ and its major metabolites among the beagle dogs. The effect of ranitidine on the disposition of DTZ was highly variable.