Safety of Cimetidine in Obstetric Patients

Cimetidine has been suggested as a new approach to the prevention of the acid pulmonary aspiration syndrome in obstetric anaesthesia. In 20 patients in labour cimetidine 200 mg intravenously did not prolong labour or alter the pattern or strength of uterine contractions. The fetal heart rate did not show any alteration in rate or pattern and it was confirmed that cimetidine crosses the placenta. These findings suggest that further studies of cimetidine in obstetric patients would be safe.

Pharmacokinetic study of hydroxypropylmethylcellulose microparticles loaded with cimetidine

OBJECTIVES

The objective of this study was to assess the pharmacokinetic behavior of floating hydroxypropylmethylcellulose microparticles loaded with cimetidine (FMC) prepared using the non-solvent addition coacervation technique.

MATERIAL AND METHODS

Based on the physico-chemical characteristics of three formulations (FMC1, FMC2 and FMC3), FMC2 having a 1:3 ratio of cimetidine:HPMC was found optimum. For in vivo analysis, a new HPLC analytical method was developed and validated. The optimized formulations were subjected to in vivo studies to calculate the various pharmacokinetic parameters for developed optimized microparticulate formulation FMC3. The developed floating microparticles of cimetidine were further evaluated by in vivo experimentation.

RESULTS

The bioavailability parameters were found as: Cmax 1508.79 ± 37.95 ng/ml, Tmax 3.67 ± 0.17 h and AUC 14366.19 ± 377.64 ng h /mL.

CONCLUSIONS

For prolonged drug release in the stomach, developed floating microparticles of cimetidine (FMC3) may be used, thereby improving the bioavailability and patient compliance.

A study of the pharmacokinetic interactions of the direct renin inhibitor aliskiren with allopurinol, celecoxib and cimetidine in healthy subjects

OBJECTIVE

Aliskiren is the first in a new class of orally effective direct renin inhibitors approved for the treatment of hypertension. This multiple-dose study investigated the potential for pharmacokinetic interactions between aliskiren and three drugs, each predominantly eliminated by a different clearance/metabolic pathway: allopurinol (glomerular filtration), celecoxib (cytochrome P450 metabolism) and cimetidine (P-glycoprotein and organic anion/cation transporters).

RESEARCH DESIGN AND METHODS

Three open-label, multiple-dose studies in healthy subjects investigated possible pharmacokinetic interactions between aliskiren 300 mg od and allopurinol 300 mg od (n = 20), celecoxib 200 mg bid (n = 22), or cimetidine 800 mg od (n = 22). Subjects received aliskiren alone or co-administered with allopurinol, celecoxib or cimetidine. Allopurinol and celecoxib were also administered alone and in combination with aliskiren. Plasma drug concentrations were determined by LC/MS/MS.

RESULTS

Co-administration of aliskiren with allopurinol had no effect on allopurinol AUC(tau) (ratio of geometric means 0.93 [90% CI, 0.88, 0.98]) or oxypurinol AUC(tau) (mean ratio 1.12 [90% CI, 1.08, 1.16]) and C(max) (mean ratio 1.08 [90% CI, 1.04, 1.13]), with 90% CI within the bioequivalence range 0.80-1.25, and a minor effect on allopurinol C(max) (mean ratio 0.88 [90% CI, 0.78, 1.00]). Aliskiren co-administration had no effect on AUC(tau) or C(max) of celecoxib (mean ratios and 90% CI within range 0.80-1.25). Neither allopurinol nor celecoxib significantly altered aliskiren AUC(tau) or C(max) (geometric mean ratios 0.88-1.02 with 90% CI including 1.00, but with some 90% CI outside the 0.80-1.25 range due to high variability). Co-administration of aliskiren with cimetidine increased aliskiren AUC(tau) by 20% (mean ratio 1.20 [90% CI, 1.07, 1.34]) and C(max) by 25% (mean ratio 1.25 [90% CI, 0.98, 1.59]).

CONCLUSIONS

In this multiple-dose study, aliskiren showed no clinically relevant pharmacokinetic interactions when co-administered with allopurinol, celecoxib or cimetidine in healthy subjects.

Convenient and rapid determination of cimetidine in human plasma using perchloric acid-mediated plasma protein precipitation and high-performance liquid chromatography

This study demonstrates the analysis of cimetidine in human plasma with HPLC using a simplified sample preparation by protein precipitation with perchloric acid. Plasma cimetidine concentration was determined by plotting peak height ratio of cimetidine to ranitidine (internal standard, IS) against cimetidine concentrations in plasma. The cimetidine and ranitidine peaks were completely separated and no interference from plasma was observed. The lower limit of quantification (LLOQ) of the method was established at 0.1 microg/mL with a precision of 4.3% and a relative error of 1.9%. The average analytical recovery was >90% over the range of cimetidine concentrations (0.1-15.0 microg/mL). The linearity of calibration curve was excellent (r(2) > 0.999). The within- and between-day precision and accuracy, expressed as the coefficients of variation and relative error, were found to be less than 5%. Compared with previously reported methods, the analytical technique for cimetidine determination in human plasma presented here demonstrates comparable accuracy and precision, an acceptable analysis time, shorter and simpler sample preparation, and a reduced need for complicated equipment. The method presented here is simple and rapid, and the precision and sensitivity are appropriate for the determination of cimetidine in plasma in pharmacokinetic studies.

Pharmacokinetic interactions of the oral renin inhibitor aliskiren with lovastatin, atenolol, celecoxib and cimetidine

OBJECTIVE

Aliskiren is the first in a new class of orally effective renin inhibitors for the treatment of hypertension. This study investigated the interaction profile of aliskiren, which is of clinical importance because hypertensive patients often require concomitant drug therapy for associated comorbidities.

METHODS

Four separate studies investigated the pharmacokinetic interaction between single oral doses of aliskiren and lovastatin, atenolol, celecoxib or cimetidine, respectively. All studies involved healthy male volunteers aged 18-45 years. In 3 studies, subjects (n = 15 in each study) received single doses of aliskiren 150 mg alone, the test drug alone (lovastatin 40 mg, atenolol 100 mg or celecoxib 200 mg), or both drugs in combination, according to a 3-period crossover design. In the cimetidine study (n = 12), aliskiren 150 mg was administered alone or concomitantly with cimetidine 800 mg according to a two-period crossover design. Plasma concentrations of aliskiren and test drugs were determined by liquid chromatography and mass spectrometry methods. Pharmacokinetic parameters were derived from these data.

RESULTS

Mean AUC and t1/2 for aliskiren were not significantly changed by lovastatin, atenolol or celecoxib (< 10% difference between treatments). Aliskiren mean Cmax was not affected by either lovastatin or atenolol, although a non-significant 36% increase was observed with celecoxib. Modest, non-significant increases in aliskiren systemic availability followed coadministration with cimetidine (aliskiren mean AUC, Cmax and t1/2 increased by 17%, 19% and 15%, respectively). Aliskiren coadministration had no significant effect on the disposition of lovastatin, atenolol or celecoxib.

CONCLUSIONS

Overall, single doses of aliskiren showed no evidence of clinically important pharmacokinetic interactions with lovastatin, atenolol, celecoxib or cimetidine.

Feasibility of Biowaiver Extension to Biopharmaceutics Classification System Class III Drug Products

BACKGROUND

The extension of biowaivers (drug product approval without a pharmacokinetic bioequivalence study) to drugs belonging to Class III of the Biopharmaceutics Classification System (BCS) is currently a subject of much discussion.

OBJECTIVES

To assess the relationship between in vitro dissolution characteristics and in vivo absorption performance of immediate-release (IR) products containing cimetidine, a BCS Class III compound, in human subjects. To evaluate the feasibility and appropriateness of an extension of the biowaiver concept to BCS Class III compounds.

STUDY DESIGN AND PARTICIPANTS

BCS-conform dissolution tests were carried out on ten marketed cimetidine products from Thailand and Germany, as well as cimetidine tablet formulations containing cimetidine 400mg manufactured by direct compression using methacrylate copolymer (Eudragit) RS PO) as a release-retarding agent to yield three batches with significantly different release profiles. Twelve healthy male subjects were enrolled in a randomised, open-label, single-dose schedule based on a five-way Williams' design balanced for carryover effects. Subjects received the following treatments, with 1-week washout periods between: (i) Tagamet 400mg tablet; (ii) 7.5% methacrylate copolymer cimetidine tablet; (iii) 15% methacrylate copolymer cimetidine tablet; (iv) 26% methacrylate copolymer cimetidine tablet; and (v) Tagamet (300 mg/ 2 mL) intravenous injection. The area under the plasma concentration-time curve from 0 to 12 hours (AUC(12)) and AUC from time zero to infinity (AUC(infinity)), peak plasma concentration (C(max)), absolute bioavailability (F) and mean residence time (MRT) were evaluated and statistically compared among formulations. In vitro-in vivo correlation (IVIVC) analysis was then applied to elucidate the overall absorption characteristics of each tablet formulation.

RESULTS

The release properties of the ten marketed cimetidine products were shown to comply with current US FDA criteria for rapidly dissolving drug products. As expected, the in vitro dissolution profiles of the cimetidine tablets containing different percentages of methacrylate copolymer differed considerably from one another. However, in vivo results showed no significant difference in AUC(12), AUC(infinity), C(max) and F between the tablets manufactured with methacrylate copolymer and the innovator. The MRT values obtained from 26% methacrylate copolymer tablets were significantly longer than for the other two methacrylate copolymer formulations and the Tagamet tablets. Furthermore, IVIVC analysis showed that the 26% methacrylate copolymer tablets exhibited dissolution rate-limited absorption, whereas the other formulations showed permeability rate-limited absorption.

CONCLUSION

The results of the present study indicated that the absorption of cimetidine from IR tablets is, in general, limited by permeability rather than dissolution. IVIVC analysis demonstrated that only when the release was deliberately retarded (tablets containing 26% methacrylate copolymer), did the dissolution represent the rate-limiting step to drug absorption. On the in vitro side, it seems that 85% dissolution within 30 minutes, as currently required by the US FDA Guidance, is more than sufficient to guarantee bioequivalence of IR cimetidine products. For cimetidine and other BCS Class III drugs with a similar intestinal absorption pattern, application of the biowaiver concept seems to present little risk of an inappropriate bioequivalence decision.

Multiple dose pharmacokinetics and acute safety of piroxicam and cimetidine in the cat

The purpose of this study was to evaluate the multiple dose pharmacokinetics and acute safety of piroxicam and cimetidine alone and in combination in cats. Seven healthy cats were included in this randomized-crossover study. The cats were assigned to groups designated to receive cimetidine alone (15 mg/kg, p.o., q12 h), piroxicam alone (0.3 mg/kg, p.o., q24 h), and piroxicam combined with cimetidine (both at aforementioned doses). The cats were dosed for 10 days followed by at least a 2-week washout period between trials. Serial blood samples were collected following the first and last doses and analyzed utilizing a high-performance liquid chromatography with mass spectrometry detection (LC/MS) assay. Pharmacokinetic parameters were determined using noncompartmental analysis. Endoscopic evaluation of the gastric mucosa was performed and serum urea nitrogen (SUN), creatinine, alkaline phosphatase (ALP), and alanine transaminase (ALT) activities were evaluated. There were not a clinically relevant difference between the pharmacokinetic parameters of piroxicam administered alone or in combination with cimetidine after either the first or last dose. Gastric ulcers were not observed in any cats although gastric erosions were. The SUN, creatinine, ALP, and ALT activities remained within reference ranges for all cats. It appears that once daily, short-term use of piroxicam alone and in combination with cimetidine in cats is relatively safe based on the parameters evaluated in this study. However, further studies are necessary to determine the long-term gastrointestinal safety of piroxicam.

Renal interaction between itraconazole and cimetidine

Renal drug interactions can result from competitive inhibition between drugs that undergo extensive renal tubular secretion by transporters such as P-glycoprotein (P-gp). The purpose of this study was to evaluate the effect of itraconazole, a known P-gp inhibitor, on the renal tubular secretion of cimetidine in healthy volunteers who received intravenous cimetidine alone and following 3 days of oral itraconazole (400 mg/day) administration. Glomerular filtration rate (GFR) was measured continuously during each study visit using iothalamate clearance. Iothalamate, cimetidine, and itraconazole concentrations in plasma and urine were determined using high-performance liquid chromatography/ultraviolet (HPLC/UV) methods. Renal tubular secretion (CL(sec)) of cimetidine was calculated as the difference between renal clearance (CL(r)) and GFR (CL(ioth)) on days 1 and 5. Cimetidine pharmacokinetic estimates were obtained for total clearance (CL(T)), volume of distribution (Vd), elimination rate constant (K(el)), area under the plasma concentration-time curve (AUC(0-240 min)), and average plasma concentration (Cp(ave)) before and after itraconazole administration. Plasma itraconazole concentrations following oral dosing ranged from 0.41 to 0.92 microg/mL. The cimetidine AUC(0-240 min) increased by 25% (p < 0.01) following itraconazole administration. The GFR and Vd remained unchanged, but significant reductions in CL(T) (655 vs. 486 mL/min, p < 0.001) and CL(sec) (410 vs. 311 mL/min, p = 0.001) were observed. The increased systemic exposure of cimetidine during coadministration with itraconazole was likely due to inhibition of P-gp-mediated renal tubular secretion. Further evaluation of renal P-gp-modulating drugs such as itraconazole that may alter the renal excretion of coadministered drugs is warranted.

Liquid-phase microextraction based on carrier mediated transport combined with liquid chromatography–mass spectrometry

Recently, we demonstrated for the first time liquid-phase microextraction (LPME) of polar drugs based on carrier mediated transport. In this new extraction technique, selected analytes were extracted as ion-pairs from small volumes of biological samples, through a thin layer of a water immiscible organic solvent immobilised in the pores of a porous hollow fibre (liquid membrane), and into a microl volume of an acidic aqueous acceptor solution placed inside the lumen of the hollow fibre. In the current paper, this new extraction technique was combined with liquid chromatography-mass spectrometry (LC-MS) for the first time. Carrier mediated LPME was evaluated for several new model drugs (0.01 <log P< 1.76), the sample clean-up aspects were investigated in detail, and this new extraction technique was fully validated for the first time. Extractions were performed from 50 microl of human plasma samples, which provided sufficient material in combination with LC-MS. Sodium octanoate (50 mM) was added to the sample as carrier, 1-octanol (approximately 15 microl) was used as the liquid membrane in the wall of the hollow fibre, and 50 mM HCl was utilized as acceptor solution in the lumen of the hollow fibre. The addition of carrier to the samples was found to significantly improve extraction recoveries for the polar drugs tested, providing recoveries in the range 16-78%. Validation was accomplished for atenolol and cimetidine. Limits of quantification (S/N = 5) from 50 microl of plasma were 25 and 50 ng/ml for atenolol and cimetidine, respectively. The intra-day precision (R.S.D.) ranged from 7.8 to 17.2% and from 9.5 to 14.1% for atenolol and cimetidine, respectively, and corresponding inter-day precisions (R.S.D.) were within 6.7-1.4% and 7.7-20.3%. The method was linear in the range 25-1500 ng/ml for atenolol (r = 0.992), and 50-3500 ng/ml for cimetidine (r = 0.976). The accuracy of the method was found to be in range 89.1-99.6% and 83.4-86% for atenolol and cimetidine, respectively. The sample clean-up obtained by carrier mediated LPME was excellent, providing a significantly lower back-ground level in total ion current chromatograms by LC-MS as compared to protein precipitation.

Validation of a simplified method for determination of cimetidine in human plasma and urine by liquid chromatography with ultraviolet detection

A HPLC method was developed for determination of cimetidine in human plasma and urine. Plasma samples were alkalinized followed by liquid extraction with water-saturated ethyl acetate then evaporated under nitrogen. The extracts were reconstituted in mobile phase and injected onto a C(18) reversed-phase column; UV detection was set at 228 nm. Urine samples were diluted with an internal standard/mobile phase mixture (1:9) prior to injection. The lower limit of quantification in plasma and urine were 100 ng/ml and 10 microg/ml, respectively; intra- and inter-day coefficients of variation were </=4.2%. Advantages of this validated assay include a readily available internal standard, simplified plasma extraction and urine dilution methods, and applicability to clinical studies investigating the renal handling of cimetidine.

Development of an HPLC method for the determination of ranitidine and cimetidine in human plasma following SPE

A selective, sensitive and accurate high-performance liquid chromatographic method has been developed, validated and applied for the determination of ranitidine and cimetidine in plasma samples. The effects of mobile phase composition, buffer concentration, mobile phase pH and concentration of organic modifiers on retention of investigated drugs were investigated. Sample preparation was carried out by adding an internal standard, famotidine, and the clean-up procedure was accomplished using solid-phase extraction (SPE). This method uses ultraviolet detection, the separation used a Lichrocart Lichrospher 60 RP-select B column and the mobile phase consisted of 0.2% triethylamine (TEA), 0.04 mol l(-1) KH2PO4 at pH 6.8 and 14% acetonitrile. The recovery, selectivity, linearity, precision and accuracy of the method were evaluated from spiked human plasma. The method has been implemented to monitor ranitidine levels in clinical samples.

Glucocorticoid involvement in suppression of NK activity following surgery in rats

We studied plasma factors mediating suppression of NK activity (NKA) following surgery. Plasma from operated rats suppressed NKA of splenocytes, leukocytes, and purified natural killer (NK) cells, and charcoal stripping nullified suppression. The glucocorticoid antagonist mifepristone prevented suppression, whereas blockers of reactive oxygen metabolites, opioids, catecholamines, prostaglandin-E2, and histamine did not. NKA dropped as corticosterone levels peaked postoperatively, and administration of relevant doses of corticosterone suppressed NKA. Inhibition of glucocorticoid synthesis prevented plasma from suppressing NKA but merely attenuated NKA suppression in operated rats. Thus, postoperative concentrations of corticosterone can directly suppress NKA but additional factors probably act in vivo.

Pharmacokinetics of saquinavir co-administered with cimetidine

The present study evaluated the effect of cimetidine, a histamine H(2) receptor antagonist able to inhibit cytochrome P450 metabolism, on the steady-state pharmacokinetics of saquinavir soft gel. Twelve healthy volunteers (eight males and four females) participated in an open-label, double-phase pharmacokinetic study. Volunteers took saquinavir soft gel 1200 mg three times a day for 13 days and then saquinavir soft gel 1200 mg twice a day with cimetidine 400 mg twice a day from day 14 to 26. The pharmacokinetics of saquinavir on days 13 and 26 were compared. All 12 volunteers completed the study. The association of cimetidine with saquinavir soft gel 1200 mg twice a day resulted in a significant increase in saquinavir AUC(0-24) (120%; P = 0.023) and C(max) (179%; P = 0.019), whereas C(trough) did not differ significantly (32% increase; P = 0.272). Increased exposure to saquinavir was observed in healthy volunteers after co-administration with cimetidine. The most significant increase involved C(max). Further pharmacokinetic studies in HIV-infected subjects are warranted to confirm the boosting effect of cimetidine and to investigate any impact that the increase in saquinavir C(max) may have on intracellular accumulation of the drug.

Influence of lysine on cimetidine uptake and on excretion of cimetidine by the rat mammary gland

Cimetidine is actively transported into human and rat milk. However, the transporters involved have not been characterized. It is possible that xenobiotics may be actively transported into milk by an amino acid transport system. The objective of these studies was to determine the influence of lysine on the uptake of cimetidine into rat mammary explants (study 1), and on the excretion of cimetidine into rat milk (study 2). In study 1, excised lactating rat mammary epithelial tissue fragments were exposed to 3H-cimetidine and 14C-lysine in the presence of 10 microM, 1 mM, or 1 M cold lysine, and the uptake of 3H-cimetidine and 14C-lysine were measured by liquid scintillation counting after 5 or 20 minutes of incubation. After 5 minutes of incubation, 1 M lysine inhibited 3H-cimetidine uptake by 47.7% (SD +/- 6.5%), compared with 10 microM lysine (P < 0.05), and 14C-lysine uptake was also inhibited by 54.1% (SD +/- 6.4%) (P < 0.05). Similar results were seen after 20 minutes of incubation. In a randomized crossover study (study 2), 6 lactating female rats were infused to steady state with cimetidine (0.5mg/h) in the presence or absence of lysine (360mg/h). Cimetidine concentrations in serum and milk were determined by high-performance liquid chromatography. Cimetidine systemic clearance (28.6+/-15.0mL/kg/min vs. 38.9+/-3.9 mL/kg/min, mean +/- SD) and milk to serum cimetidine ratio (M/S) (28.0+/-16.1 vs. 28.9+/-6.7), respectively, were not significantly altered by the presence or absence of lysine. Although 1 M lysine inhibited uptake of cimetidine in rat mammary explants, the concentrations of lysine used in this study, which approached toxicity in vivo, produced no significant effects on cimetidine transport into milk or the systemic clearance of cimetidine.

Variability in cimetidine absorption and plasma double peaks following oral administration in the fasted state in humans: correlation with antral gastric motility

The role of gastrointestinal motility and pH in determining cimetidine bioavailability as well as double peaks in plasma profiles following oral administration, in the quiescent or active phase of antral motility, to humans in the fasted state was examined. Plasma cimetidine-time curves did not show the presence of double peaks in any subject following intravenous administration. The incidence of double peaks was 73% following oral administration and was independent of antral migrating motility complex phase. Further, it was found that oral administration of cimetidine in the quiescent phase resulted in significantly higher bioavailability and in other pharmacokinetic parameters compared to that obtained following administration in the active phase. Excellent linearity in plots of motility peaks vs. plasma peaks with slopes close to unity were evident for both quiescent (r(2)=0.93) and active phase (r(2)=0.97) administration. A total of 14 peaks out of 22 (10 subjects, 64%) and 20 out of 27 peaks (11 subjects, 74%), were accounted for in quiescent and active phase oral administration, respectively. The proximal occurrence of plasma peaks to antral motility peaks typical of phase III contractions strongly implies that motility patterns may be responsible for secondary maxima following oral cimetidine administration in the fasted state.

Head-column field-amplified sample stacking in capillary electrophoresis for the determination of cimetidine, famotidine, nizatidine, and ranitidine-HCl in plasma

In this study, low concentrations of histamine2-receptor (H2-)antagonists were effected across a water plug, with separation taking place in a binary buffer comprising ethylene glycol and NaH2PO4 (pH 5.0), and detection at 214 nm. Liquid-liquid extraction with ethyl acetate- isopropanol is shown to provide extracts that are sufficiently clean. The calibration curves were linear over a concentration range of 0.1-2.00 microg/mL cimetidine, 0.2-5.0 microg/mL ranitidine-HCl, 0.3-5.0 microg/mL nizatidine, and 0.1-3.0 microg/mL famotidine. Mean recoveries were > 82%, while the intra- and interday relative standard deviations (RSDs) and relative errors (REs) were all < 13%. The method is sensitive with a detection limit of 3 ng/mL cimetidine, 30 ng/mL ranitidine HCl, 50 ng/mL nizatidine and 10 ng/mL famotidine (S/N = 3, electric-driven injection 90 s). This newly developed capillary electrophoresis (CE) method was applied for the determination of analytes extracted from plasma taken from a volunteer dosing a cimetidine, ranitidine, and nizatidine tablet simultaneously. These three H2-antagonists can be detected in real samples by this method, excluding the low dosing of famotidine tablet.

Prediction of midazolam-CYP3A inhibitors interaction in the human liver from in vivo/in vitro absorption, distribution, and metabolism data

The extent of decreases in apparent hepatic clearance and intrinsic hepatic clearance of midazolam (MDZ) after intravenous administration of MDZ with concomitant oral administration of cimetidine (CIM), itraconazole (ITZ), or erythromycin (EM) was predicted using plasma unbound concentrations and liver unbound concentrations of inhibitors. When MDZ was concomitantly administered with CIM, the observed increase in MDZ concentration was successfully predicted using inhibition constants assessed by human liver microsome and liver-to-plasma unbound concentration ratios in rats. However, the extent of interaction with ITZ or EM was still underestimated even taking into account the concentrative uptake of inhibitors into liver. We could predict the degree of "mechanism-based" inhibition by EM on the hepatic metabolism of MDZ, after repeated administration of EM, by a physiological model incorporating the amount of active enzyme as well as the concentration of inhibitor. The maximum inactivation rate constant and the apparent inactivation constant of EM on MDZ metabolism were 0.0665 min(-1) and 81.8 microM, respectively. These kinetic parameters for the inactivation of the enzyme were applied to the physiological model with pharmacokinetic parameters of EM and MDZ obtained from published results. Consequently, we estimated that cytochrome P450 3A4 in the liver after repeated oral administration of EM was inactivated, resulting in 2.6-fold increase in the plasma concentration of MDZ. The estimated extent of increase in MDZ concentration in our study correlated well with the observed value based on metabolic inhibition by EM from published results.

Quantitative analysis of cimetidine in human plasma using LC/APCI/SRM/MS

A quantitative method was developed and validated for rapid and sensitive analysis of cimetidine in human plasma. The method involved the use of liquid chromatography (LC) coupled with atmospheric pressure chemical ionization (APCI) and selected reaction monitoring (SRM) mass spectrometry (MS). A cimetidine analog, SKF92374, was used as the internal standard. Separation of cimetidine and the internal standard was accomplished using a reverse-phase HPLC column (C18). The eluted components were ionized by the APCI source and subsequently detected by a highly selective triple quadrupole mass spectrometer in the SRM mode. Linear standard curves were obtained from 5 ng/mL (lower limit of quantitation) to 10,000 ng/mL. The results demonstrated excellent precision (%RSD 1. 1-8.9%) and accuracy (94.7-108.0%) over this range. In addition, the amount of plasma sample needed for analysis was small (50 muL), and the plasma pretreatment (analyte recovery >94%) was simple and time saving. This assay was used to evaluate cimetidine levels in premature infants following intravenous infusion of cimetidine.

High-throughput solid-phase extraction for the determination of cimetidine in human plasma

For the implementation and validation of an automated 'high-throughput' solid-phase extraction (SPE) system, using microtiter solid-phase technology and a pipetting robot, a SPE method previously validated manually for cimetidine in human plasma was adapted. Sample cleanup was performed by means of SPE using Microlute extraction plates in the 96-well format, each well filled with 50 mg of Varian C18 sorbent. Separation was performed by reversed-phase high-performance liquid chromatography (HPLC) with UV detection at 234 nm. The validated calibration range was from 0.100 to 5.00 mg/l, with an inaccuracy and imprecision below 20% at all concentration levels. Validation results on linearity, specificity, precision, accuracy and stability are shown and are found to be adequate. Cross-check analysis of samples from a clinical trial showed that there is a good correlation between results obtained by the automated method and results obtained by the manual method. The average sample preparation time for a technician decreased from approximately 4 min per sample to 0.6 min. A sample throughput of at least 160 samples per day can be achieved, the HPLC analysis time being the rate-limiting step.

Assessment of blood level with controlled-release dosage forms: effect of the rate constant of elimination of the drug

Prediction of the drug level in the volume of distribution was made using a numerical model taking into account the following facts: the kinetics of drug release out of the dosage form along the gastrointestinal tract, the kinetics of absorption in the blood compartment and the kinetics of elimination. Various parameters intervene significantly for a given dosage form. Some emphasis was placed upon the rate of elimination of the drug which appears to be the main characteristic for the drug, especially when it is delivered through controlled release dosage forms. The rate of release of the drug out of the dosage form, as well as the dose frequency and the gastrointestinal transit time were also considered. The drug level in the plasma was expressed by the amount of drug as a fraction of the amount of drug initially located in the dosage form.

An interaction study with cimetidine and the new angiotensin II antagonist valsartan

OBJECTIVE

This was a randomised, open, three-way crossover study in 12 healthy male volunteers to determine the effect of a single oral dose of cimetidine on the pharmacokinetics of a single oral dose of the angiotensin II receptor antagonist valsartan and vice versa. The volunteers received either valsartan alone (160 mg), or cimetidine alone (800 mg), or valsartan 1 h after cimetidine. The study was designed primarily to detect a possible influence of cimetidine on the rate and extent of absorption of valsartan.

METHODS

Plasma concentrations of valsartan and cimetidine, measured by means of high-performance liquid chromatography, were used to calculate pharmacokinetic parameters. The rate of absorption of valsartan and the fraction of the dose absorbed and systemically available after oral administration were calculated using data from an i.v. study with valsartan in healthy young volunteers.

RESULTS

The pharmacokinetics of cimetidine area under curve (AUC0-48 h), maximum concentration (Cmax), time to reach Cmax(tmax) and apparent terminal plasma half-life (t1/2) was not changed by co-administration of valsartan. For valsartan, the AUC0-48 h increased by 7% and the Cmax by 51% (ratio of geometric means) with co-administration of cimetidine. The higher value for Cmax was attributed to the initial increase in the rate of absorption of valsartan: ka was increased 2.7-fold and another indicator for the rate of absorption, Cmax/tmax, 2.2-fold. This effect was ascribed to inhibition of acid secretion by cimetidine, which leads to a higher gastric pH, thereby increasing the solubility of valsartan; the t1/2 of valsartan was not changed. After valsartan alone, 19% of the dose was absorbed, 23% with co-administration of cimetidine. It was estimated that only 2.2% of the possible change in AUC might be missed by giving a single high dose of cimetidine instead of multiple doses, with the aim to optimally inhibit formation of the inactive metabolite of valsartan. Cimetidine-related changes in the rate of elimination of valsartan were not anticipated, since the clearance from plasma occurs mainly by biliary excretion of unchanged valsartan; metabolism and renal excretion are only minor contributors. Therefore, even in the clinically relevant situation with multiple doses of valsartan and cimetidine, notable changes in the pharmacokinetics of valsartan, except for an increase in Cmax, are not to be expected. This increase in Cmax appears to be of no clinical significance. Valsartan alone and in combination with cimetidine was well tolerated by healthy subjects.

Influence of the antacid Maalox and the H2-antagonist cimetidine on the pharmacokinetics of cerivastatin

The possible influence of Maalox 70, an antacid based on magnesium-aluminum hydroxide, and the H2-antagonist cimetidine, both commonly prescribed in hypercholesterolemic patients, on the pharmacokinetics of the new HMG-CoA reductase inhibitor cerivastatin was investigated in 2 separate studies in 8 healthy young male subjects each. Cerivastatin plasma concentration/time profiles were assessed by a specific HPLC assay; in addition, total immunoreactive drug (cerivastatin plus metabolites) was determined by RIA. Single oral doses of 200 micrograms cerivastatin were administered under fasting conditions without or with 10 ml Maalox 70 suspension. The mean AUC and Cmax ratios (combined dosing/monodosing) including 90% confidence intervals were 0.92 (0.73-1.15) and 0.89 (0.72-1.10) for the HPLC data, and 0.99 (0.85-1.14) and 1.03 (0.82-1.30) for the RIA data, respectively. Thus, no interaction of the simultaneous administration of Maalox 70 on the pharmacokinetics of cerivastatin was observed. In a similar controlled, randomized nonblind 2-way crossover design the influence of the H2- antagonist and well-known cytochrome P450 enzyme inhibitor cimetidine was investigated. Eight healthy young male volunteers received single oral doses of 200 micrograms cerivastatin alone or on the fourth day of a 4-day cimetidine 400 mg b.i.d. pretreatment. The mean AUC and Cmax ratios (combined dosing/monodosing) including 90% confidence intervals were 0.98 (0.90-1.08) and 0.91 (0.78-1.07) for the RIA data, and 0.89 (0.82-0.96) and 0.93 (0.80-1.09) for the HPLC data, respectively, clearly indicating that cimetidine and cerivastatin did not interact pharmacokinetically. These results do not only reflect the apparent insensitivity of cerivastatin absorption to possible changes in gastric pH, but demonstrate that the metabolic pathways of cerivastatin, involved in its first-pass metabolism and elimination, are rather insensitive to cytochrome P450 enzyme inhibition induced by cimetidine.

Applications and simulations of a discontinuous oral absorption pharmacokinetic model

PURPOSE

To illustrate the application of a discontinuous oral absorption model to cimetidine and ranitidine plasma concentration versus time data to demonstrate the use of the model for drugs which display discontinuous oral absorption profiles, and to illustrate the effect of various model parameters on plasma drug concentration versus time profiles and bioavailability.

METHODS

A discontinuous oral absorption model was used to fit ranitidine and cimetidine serum concentrations following oral and intravenous administration. The model was also used to simulate bioavailability and plasma concentrations versus time profiles for various parameter values.

RESULTS

Serum concentrations following administration of ranitidine and cimetidine were well described by the model, and parameter estimates obtained were in agreement with literature values. Simulations demonstrate the effects of various absorption parameters and gastroin-testinal tract transit parameters on bioavailability and plasma concentration profiles.

CONCLUSIONS

This discontinuous oral absorption pharmacokinetic model can be a useful tool in characterizing absorption phases, disposition, and bioavailability of drugs exhibiting two absorption peaks following oral administration.

Lack of pharmacokinetic interaction between butorphanol nasal spray and cimetidine

The potential for a pharmacokinetic interaction between butorphanol nasal spray and cimetidine, under steady state conditions, was evaluated in 16 healthy male volunteers. Subjects received either a 1 mg butorphanol nasal spray every 6 h or a 300 mg cimetidine tablet every 6 h on days 1 4, the combination of two compounds every 6 h on days 5-8 and the original treatment as described in the first segment (days 1-4) on days 9-12. Serial blood and urine samples were collected on days 4, 8 and 12, and additional blood samples were taken immediately, prior to the morning dose on days 3, 7 and 11. Based on the analysis of the Cmin samples, the plasma concentrations of cimetidine and butorphanol achieved steady state by the third day of dosing. No statistically significant differences were found in the plasma concentrations of butorphanol or cimetidine (except for t1/2 and MRT) between any of the treatment phases. Butorphanol nasal spray and cimetidine can be co-administered without any adjustment of dosage for either drug.

Ferrous sulfate does not reduce serum levels of famotidine or cimetidine after concurrent ingestion

A series of randomized crossover studies were performed to determine whether there was a reduction in serum levels of cimetidine and famotidine when coingested with ferrous sulfate (300 mg). Coingestion of a ferrous sulfate tablet with cimetidine (300 mg) was associated with little reduction in serum cimetidine area under the curve (AUC) (mean versus mean, 20.8 versus 23.4 mumol.hr/L; mean percentage difference, -11%; 95% confidence interval [CI] of percentage difference, -26% to 4.2%) or peak concentration (Cmax) (mean versus mean, 5.1 versus 6.1 mumol/L; mean percentage difference, -16%; CI of percentage difference, -36% to 4%). Similarly, ferrous sulfate solution coingested with cimetidine caused little change in cimetidine AUC (mean versus mean, 19.9 versus 23.0 mumol.hr/L; mean percentage difference, -13%; CI of percentage difference, -34% to 7%) or Cmax (mean versus mean, 5.0 versus 5.0 mumol/L; mean percentage difference, 1%; CI of percentage difference, -18% to 20%). Concurrent ingestion of famotidine (40 mg) with a ferrous sulfate tablet did not result in significant reductions in serum famotidine AUC (mean versus mean, 1.78 versus 1.99 mumol.hr/L; mean percentage difference, -10%; CI of percentage difference, -34% to 13%) or Cmax (mean versus mean, 0.31 versus 0.32 mumol/L; mean percentage difference, -3%; CI of percentage difference, -27% to 22%). The formation of famotidine:iron(III) complexes was shown in methanol but was not observed in an aqueous buffer at pH 6.5. Ranitidine did not bind iron in an aqueous buffer and only weakly bound iron in methanol. Coingestion of ferrous sulfate with either cimetidine or famotidine does not cause a clinically relevant reduction in serum histamine H2-receptor blocker levels and, on the basis of in vitro binding experiments, iron is unlikely to interact with ranitidine.

Interaction of some drugs on the pharmacokinetics or pharmacodynamics of MPC-1304, a dihydropyridine Ca2+ antagonist

The aim of this study was to assess the pharmacokinetics and subsequent pharmacodynamic interaction of MPC-1304, a dihydropyridine Ca2+ antagonist, with other drugs in animal experiments. We measured the systolic blood pressure and heart rate of conscious spontaneously hypertensive rats implanted with battery-operated biotelemetry devices after combined administration of various drugs. Cimetidine (10 mg/kg) did not affect the reduction in systolic blood pressure and the increase in heart rate induced by MPC-1304, whereas it significantly increased the plasma concentration of a metabolite of MPC-1304 (M-1) compared to that detected when MPC-1304 was administered alone. When MPC-1304 was consecutively administered in combination with rifampicin (400 mg/kg) for 9 days, the plasma concentrations of MPC-1304 and of M-1 significantly decreased compared to those found when MPC-1304 alone was given. In spite of these reductions in plasma concentrations, rifampicin did not attenuate the hypotensive action induced by MPC-1304. When prazosin, reserpine, or methyldopa was administered in combination with MPC-1304, the hypotensive action was enhanced as compared to that by MPC-1304 alone or to that by the co-administered drug used alone (prazosin, reserpine, or methyldopa). Quinidine (10 mg/kg) affected neither the hypotensive action induced by MPC-1304 nor the plasma concentrations of MPC-1304 and M-1. These results indicate that cimetidine and rifampicin interact with MPC-1304 pharmacokinetically, without apparently changing the hypotensive action of MPC-1304, whereas quinidine does not affect the metabolism of MPC-1304, and that other hypotensive drugs, such as prazosin, reserpine, and methyldopa, potentiate the hypotensive action of MPC-1304.

Influence of cimetidine coadministration on the pharmacokinetics of sotalol enantiomers in an anaesthetized rat model: evidence supporting active renal excretion of sotalol

Sotalol (STL) is an amphoteric, chiral beta-adrenergic blocking drug useful in the treatment of both hypertension and ventricular arrhythmias. In the human and rat, STL enantiomers are predominantly cleared from the body by the kidney as intact drug. The renal clearance (Clr) of STL enantiomers substantially exceeds the glomerular filtration rate (GFR) in the human and rat. In this report, the hypothesis that STL enantiomers are excreted by an active renal transport system was investigated in the rat by coadministering racemic STL (10 mg kg-1) with cimetidine, an inhibitor of renal tubular secretion of organic cations. To compare the effects of short-term and sustained cimetidine exposure on STL enantiomer disposition, cimetidine was administered either as a single bolus (30 mg kg-1, n = 7) immediately prior to the STL dose, or as a 30 mg kg-1 bolus plus a 50 mg kg-1 infusion over the 6 h study period (n = 7). Blood and urine samples were collected over 6 h, during which time anaesthesia was maintained via intraperitoneal administration of pentobarbital. Cimetidine bolus and cimetidine infusion reduced STL enantiomer Clr by 43 and 59%, respectively, compared with respective saline controls. Significant stereoselectivity was observed in the cimetidine infusion group: systemic clearance, Clr (R > S), and AUC (S > R), although the magnitude of stereoselectivity was less than 5%. This study supports the hypothesis that STL enantiomers are predominantly cleared from the rat via a renal cationic transport mechanism and that this system can be competitively inhibited by the presence of cimetidine.

Effect of acute renal failure on neurotoxicity of cimetidine in rats

PURPOSE

We investigated the effect of acute renal failure on the neurotoxicity of cimetidine in rats.

METHODS

Experimental acute renal failure was produced by bilateral ureteral ligation. Cimetidine was intravenously infused to ureter ligated (UL) and control rats, and cimetidine concentration in plasma, brain and cerebrospinal fluid (CSF) were compared.

RESULTS

The cimetidine concentration in plasma was rapidly increased in UL rats as compared to control rats. The cimetidine concentration in CSF at the onset of convulsion did not depend on the infusion rate, suggesting that cimetidine in CSF equilibrates rapidly with the site of action for clonic convulsion. The cimetidine concentration in CSF of UL rats at the onset of clonic convulsion was lower than those of control rats.

CONCLUSIONS

Increased sensitivity to the drug on the central nervous system may contribute to increased toxicity of cimetidine with renal failure.

Active transport of cimetidine into human milk

Most xenobiotics are transferred from blood into breast milk by passive diffusion. However, an active transport mechanism has been speculated for cimetidine, and the purpose of this study was to characterize cimetidine transfer into human milk. Twelve healthy lactating volunteers received single oral doses of 100, 600, and 1200 mg cimetidine in a randomized, crossover design on 3 different days. Blood and milk specimens were collected and assayed for cimetidine. In vitro measurements, including skim to whole milk concentration ratio, milk pH, and free fractions in serum and milk were used for a diffusion model prediction of milk to serum concentration ratio of cimetidine; the mean milk/serum ratio (+/- SD) was 1.05 +/- 0.18. The observed milk/serum ratio (5.77 +/- 1.24) was 5.5 times higher than the milk/serum ratio predicted by diffusion. The observed milk/serum ratio for the three dosing regimens were not significantly different from one another. Time of peak concentration (tmax) in milk (3.3 +/- 0.7 hours) displayed a delay compared with serum tmax (1.7 +/- 0.6 hours). Oral clearance for 1200 mg cimetidine dose (0.47 +/- 0.11 L/hr/kg) was significantly lower compared with oral clearance values for 100 and 600 mg cimetidine doses (0.59 +/- 0.11 and 0.57 +/- 0.13 L/hr/kg, respectively). The maternal dose of cimetidine ingested by a suckling infant based on body weight was estimated to be 6.7%, which appears to be safe under normal conditions. This study provides the first definitive evidence of an active transport system for drug transfer into human milk, which may have broader consequences for the suckling infant.

Lack of interaction between nefazodone and cimetidine: a steady state pharmacokinetic study in humans

The steady-state pharmacokinetic interaction between nefazodone and cimetidine was evaluated in a three-period crossover study consisting of three treatments of 1 week duration with a 1 week washout between treatments. The 18 healthy, male study subjects received: nefazodone hydrochloride 200 mg twice daily (every 12 h) for 6 days; cimetidine 300 mg four times daily for 6 days; and 200 mg nefazodone hydrochloride twice daily + 300 mg cimetidine four times daily for 6 days. On day 7 of each treatment, only the morning dose was administered. Serial blood samples were collected for pharmacokinetic analysis after drug administration on day 7 of each treatment; blood samples for trough levels (Cmin) to assess attainment of steady state, were also collected just prior to the morning doses on days 2-7 of each study period. Plasma samples were assayed for cimetidine, and nefazodone and its metabolites hydroxynefazodone and m-chlorophenylpiperazine by specific, validated h.p.l.c. methods. Statistical analyses of Cmin data indicated that, regardless of treatment, steady state was achieved for cimetidine by day 2 and for nefazodone and its metabolites by day 3 of multiple dosing, and that there were no significant differences in Cmin levels between treatments. When nefazodone and cimetidine were co-administered for 1 week, no change in steady-state pharmacokinetic parameters for cimetidine, nefazodone or hydroxynefazodone was observed compared with each drug dosed alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of cimetidine in human plasma by high-performance liquid chromatography following liquid-liquid extraction

A new method is described for the determination of cimetidine in human plasma. The drug and internal standard (ranitidine) were separated on a Nucleosil C18 5 microns (25 x 4.6 mm I.D.) column using a mobile phase of acetonitrile-phosphate buffer, pH 6.2 (25:75, v/v) containing 2.5 g/l heptane sulphonic acid. The mobile phase was delivered at a flow-rate of 0.9 ml/min, detection was by ultraviolet absorption at 228 nm and concentrations were calculated on the basis of peak areas. The drugs were extracted from alkaline plasma into ethyl acetate using a salting out procedure which involved the addition of 100 ml of a saturated solution of K2CO3 to each 250-microliters plasma aliquot. The method was validated over the concentration ranges 50-3000 ng/ml and 100-7000 ng/ml for two separate studies. Mean coefficients of variation were less than 6% for both intra- and inter-assay in both studies and recoveries varied between 71 and 81%. The method was successfully applied to the determination of cimetidine in plasma for a pharmacokinetic study.

Determination of cimetidine in human plasma by free capillary zone electrophoresis

The separation of cimetidine from the metabolites cimetidine amide and cimetidine sulfoxide, endogenous creatinine and the internal standard ranitidine was achieved by capillary electrophoresis in less than 5 min. All compounds were well separated from cimetidine, including possible plasma ingredients, as the UV spectra of cimetidine standard and cimetidine from the plasma extract match. Plasma levels of cimetidine were determined in the range 250-3000 ng/ml in plasma and higher concentrations were determined by dilution of the sample with blank plasma.

Gastric pH influences the appearance of double peaks in the plasma concentration-time profiles of cimetidine after oral administration in dogs

The plasma concentration-time profiles of cimetidine often exhibit two peaks following oral administration of a single dose in the fasted state, while the concurrent administration of some antacids results in a lower extent as well as rate of absorption. In the present work, absorption of cimetidine after a single dose in the fasted state was studied as a function of gastric pH in male beagle dogs to determine whether gastric pH plays a role in the double peak phenomenon and/or can account for the decrease in bioavailability when antacids are coadministered. The extent of absorption of cimetidine was not influenced significantly by gastric pH, indicating that elevation of gastric pH is not the cause of decreases in the bioavailability of cimetidine when it is administered with antacids. Distinct double peaks or plateaux were noted in 8 of 10 plasma profiles when the gastric pH was 3 or below. Irregular absorption behavior was observed in 2 of 6 profiles in the pH range of 3 to 5, while single peaks were observed in all 10 profiles when the gastric pH was maintained at pH > or = 5. It was concluded that gastric pH is a major factor in the generation of cimetidine double peaks. Changes in gastric pH also resulted in changes in the apparent kinetics of absorption. Below pH 5, absorption mostly followed zero-order kinetics (9 of 16 profiles) or a more complex kinetic process involving at least two components to the absorption phase (5 of 16 profiles). At gastric pH > or = 5, however, absorption followed first order kinetics in 7 of 10 profiles. These differences in kinetics of absorption are postulated to arise from variations in gastric emptying as a function of pH and/or carryover effects of gastric pH into the upper intestine.

Ion-pair solid-phase extraction of cimetidine from plasma and subsequent analysis by high-performance liquid chromatography

An improved method is described for the solid-phase extraction of cimetidine from plasma or serum with subsequent analysis by HPLC. New aspects of the method include protein precipitation with metaphosphoric acid (5%, w/v), followed by selective adsorption of cimetidine and the internal standard ranitidine on the surface of a solid-phase phenyl (PH Bond Elut) column, using octanesulfonate as an ion-pairing agent. Separation was achieved on a LiChrosorb RP-18 column with a mobile phase consisting of acetonitrile-0.01 M phosphate buffer pH 3.0 containing 0.005 M octanesulfonate (22:78, v/v). The intra-assay coefficient of variation varied between 0.7 and 4.0%. The procedure provides cleaner and more stable samples and a better recovery (90 +/- 2.3%) and sensitivity (limit of detection 5 ng/ml and limit of quantitation 25 ng/ml) as compared with previous methods.

In vivo administration of H2 blockers, cimetidine and ranitidine, reduced the contents of the cytochrome P450IID (CYP2D) subfamily and their activities in rat liver microsomes

1. The effects of in vivo administration of H2 blockers, cimetidine and ranitidine (0.6 mmol/kg body weight/day, for 5 days), on several P450 isozymes, the P450IID (CYP2D) subfamily, and their monooxygenase activities in rat liver microsomes were investigated. 2. In vivo administration of cimetidine and ranitidine decreased the contents of P450 isozymes and the activities of P450-linked monooxygenase systems; i.e., benzphetamine N-demethylase, aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase. 3. The inhibitory effect on the enzymatic activities of the P450IID (CYP2D)-linked monooxygenase systems was studied by Western blot analysis with serum containing anti-CYP2D6 IgG, i.e., LKM1 autoantibody. The amount of P450IID (CYP2D) in liver microsomes decreased more remarkably in the group administered ranitidine or cimetidine in vivo than in controls. 4. The effects of cimetidine and ranitidine on the activities of the P450IID (CYP2D)-linked monooxygenase systems were investigated in vitro. The activities of debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase were inhibited in vitro by cimetidine or ranitidine at a higher concentration than that on in vivo administration of either H2 blocker. 5. The kinetic parameters for cimetidine or ranitidine as to the activities of debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase in liver microsomes were determined by means of Lineweaver-Burk plots. 6. The suppressive effects of cimetidine and ranitidine on the activities of P450IID (CYP2D)-linked monooxygenase systems in vivo were found to be due to a decrease of the content of the P450IID (CYP2D) protein.

Short report: a comparative study of the interaction between antacid and H2-receptor antagonists

The influence of concomitant antacid administration on the relative bioavailability of the H2-receptor antagonists cimetidine, famotidine, nizatidine and ranitidine, was investigated in a panel of 21 healthy, adult male volunteers in an eight-way crossover trial. Administration with antacid reduced the bioavailability of all agents tested. The reduction in area under the serum concentration-time curve (AUC) was greatest for cimetidine (23%) and ranitidine (26%) and least for nizatidine (12%) and famotidine (19%). Reductions in peak serum concentration (Cmax) followed a similar pattern. The times of peak serum concentrations were not affected by antacid. Comparison of the relative bioavailability among all drugs tested showed no statistically significant differences in the effect of antacid administration on these agents. However, a high degree of intersubject variability was observed.

Development of secondary sex characteristics in male rats after fetal and perinatal cimetidine exposure

Cimetidine has been reported to cause antiandrogenic effects in male pups of female rats receiving cimetidine during gestation. Because of conflicting reports of cimetidine causing permanent antiandrogenic effects in male rats, we studied the sexual development of male rats born to females receiving cimetidine. Water or water and cimetidine (194 mg/kg of body weight per day) were administered to pregnant rats from day 12 of gestation through weaning. A total of 130 male pups were studied. Testicular prostate gland/seminal vesicle weights, anogenital distance, serum testosterone and dihydrotestosterone levels, and seminiferous tubule areas were compared between the two groups. Transfer of cimetidine across the placenta and though breast milk was confirmed by HPLC analysis of serum from female littermates at 0, 10, and 20 days of age. With the exception of a smaller anogenital distance (p < 0.03) and a lower anogenital index (p < 0.05) in the cimetidine-exposed newborn rats, no statistically significant differences were observed in the measured parameters between the cimetidine-exposed and control groups. Cimetidine exposure during the fetal and perinatal periods did not alter the development of secondary sex characteristics in male rat pups.

Effects of exposure to standard- and nicotine-reduced-cigarette smoke on pharmacokinetics of theophylline and cimetidine in rats

Influences of exposure to standard- (containing nicotine and tar) and nicotine-reduced-cigarette smoke on the pharmacokinetics of theophylline (20 mg/kg, per os) and cimetidine (50 mg/kg, per os) were investigated in rats. Animals were exposed to standard- or nicotine-reduced-cigarette smoke for 8 min with a "smoking machine". In control rats, theophylline concentrations in plasma increased rapidly, peaked 2 h later, and then decreased gradually. Concentrations of theophylline in plasma of rats exposed to standard- and nicotine-reduced-cigarette smoke were suppressed in comparison with that of control rats, and the suppressive effect of nicotine-reduced-cigarette smoke was weaker than that of standard-cigarette smoke. The suppression of theophylline concentrations in plasma induced by exposure to cigarette smoke may be due to nicotine and other constituents of the cigarette smoke, even if the effects are slight. For cimetidine, no difference was found between drug concentration in plasma of rats exposed to nicotine-reduced-cigarette smoke and that of control rats; however, the drug concentration in plasma of rats exposed to standard-cigarette smoke was markedly suppressed. These results suggest that the suppression of cimetidine concentrations in plasma may be due solely to nicotine in cigarette smoke.

The effects of chronic oral diltiazem and cimetidine dosing on the pharmacokinetics and negative dromotropic action of intravenous and oral diltiazem in the dog

The kinetics and negative dromotropic action of intravenous (1 mg kg-1) and oral (5 mg kg-1) diltiazem were studied in dogs after acute doses, after treatment for 3 days with oral diltiazem (5 mg kg-1, t.i.d.), and after 3 days' treatment with oral diltiazem (5 mg kg-1 t.i.d.) and cimetidine (200 mg t.i.d.). Plasma concentrations of diltiazem and two of its metabolites, desacetyldiltiazem and desmethyldiltiazem were measured by HPLC. Chronic oral dosing significantly lowered both the systemic and oral clearance of diltiazem, with no changes in either the volume of distribution or blood binding of diltiazem. Cimetidine treatment resulted in a significant reduction in diltiazem oral clearance from chronic control with no effect on its systemic clearance. The AUCs of both metabolites increased by greater than threefold from acute to chronic oral dosing; however, the ratio of each metabolite's AUC to that of diltiazem AUC was not significantly altered. Cimetidine treatment significantly lowered these ratios. The negative dromotropic potency of diltiazem after the acute oral dose was three times greater than that after intravenous or chronic control dosing. Cimetidine treatment resulted in further lowering chronic oral diltiazem potency. These data indicate that the disposition and negative dromotropic action of diltiazem is dependent both on the route of administration and the duration of treatment, and can be altered by co-administration with cimetidine.

Pharmacokinetics of cimetidine during lactation: species differences in cimetidine transport into rat and rabbit milk

The disposition of cimetidine, including transfer into milk, was characterized in the rabbits and rats. Nursing rabbits and suckling offspring exhibited similar pharmacokinetics with a mean systemic disease (CL) in the adults and pups of 22.0 +/- 5.2 and 30.4 +/- 8.9 ml/min/kg, respectively. Cimetidine exhibited a distributional time lag and a prolonged T 1/2 in milk compared to serum (70 +/- 15 vs. 33 +/- 5 min), resulting in a time-dependent milk to serum (M/S) drug concentration ratio. The ratio of the area under the time curve of cimetidine in serum and milk was 1.49 +/- 0.37 and was comparable to a diffusional model predicted M/S ratio of 1.16 +/- 0.11. Unbound CL after a high cimetidine infusion regimen (16.6 +/- 7.3 ml/min/kg) was significantly less than that after two lower infusion rates (26.2 +/- 4.9 and 29.4 +/- 12.3 ml/min/kg, respectively). M/S determined at increasing steady-state serum concentrations were 1.03, 1.08 and 1.08, respectively, which agreed well with the corresponding predicted M/S (1.06, 1.14 and 1.13, respectively). Cimetidine was also administered to lactating rats and resulted in a concentration-dependent decrease in CL (11.2 +/- 1.5, 10.8 +/- 2.7 and 7.30 +/- 1.0 ml/min, respectively) after three increasing infusion rates. The steady-state M/S ratio decreased slightly from 31.9 +/- 9.0 to 26.5 +/- 9.5 and 24.6 +/- 6.4 with the increasing infusion rate. Steady-state M/S values were 6-fold higher than the predicted M/S value (4.19). Hence, cimetidine transport into rabbit milk appears to be governed by diffusion, whereas cimetidine transfer into rat milk may involve active transport.

Interaction between succinylcholine and cimetidine in rats

The hypothesis that histamine H2 receptor blockade adversely affects neuromuscular function was tested, in vivo, in rats anaesthetised with urethane during mechanical pulmonary ventilation. Succinylcholine was administered as a bolus and constant-rate infusion to maintain 49.2% (+/- 1.5 SEM) twitch suppression in 19 rats. Cimetidine iv, 3.2, 7.5, 10, 17.8, 23.7, 31.6, or 56.2 mg.kg-1 was then administered in groups of two to three rats. Cimetidine produced an immediate potentiation of twitch suppression followed by a transient reversal and then a continued potentiation. Peak potentiation occurred within 19.0 (+/- 2.7) sec and was maintained in 11 rats at steady-state. Reversal was evident 4.1 (+/- 0.4) min after cimetidine administration. There was a good relationship between peak potentiation and serum cimetidine concentration with 50% potentiation occurring at 46.5 (+/- 4.6) micrograms.ml-1. Potentiation at steady-state was not correlated to serum cimetidine concentration but there was a weak relationship between reversal and serum cimetidine concentration. These results support reports from patients of an interaction between cimetidine and succinylcholine.

Effect of Histamine H2Receptor Antagonists on the Secretion of Cerebrospinal Fluid in the Cat

Following a recent report that epithelial cells of the choroid plexus possess histamine H2 receptors, the effect of cimetidine and ranitidine, histamine H2 receptor antagonists, on the secretion and electrolyte content of CSF was examined. Fifty cats were divided into one control (n = 6) and six experimental groups. CSF was collected by puncture of the cisterna magna following pentobarbital anesthesia, and its volume, concentrations of Na+, K+, Cl-, and pH were determined. Cimetidine or ranitidine (50, 20, or 10 mg/kg) was injected intravenously 2 h after the start of the test, and their concentrations were measured in hourly blood samples and in 30-min aliquots of CSF in the 50 mg/kg experimental groups. Whereas the secretion of CSF did not change over 6 h in the control group, it decreased significantly by 30-60 min after injection of cimetidine or ranitidine and remained low for the following 6 1/2 h in all experimental groups except the 10-mg ranitidine group. Peak cimetidine and ranitidine concentrations in CSF in the 50-mg experimental groups were noted 60 and 90 min, respectively, after intravenous injection. CSF electrolyte concentrations and pH did not change during the test in any group. We conclude that intravenous cimetidine or ranitidine can significantly reduce CSF secretion in the cat, possibly by competitive inhibition of the histamine effect on H2 receptors located on the choroid plexus epithelial cell, or by a direct effect on the capillaries of the choroid plexus.

Use of a pharmacokinetic model incorporating discontinuous gastrointestinal absorption to examine the occurrence of double peaks in oral concentration-time profiles

Double peaks in the plasma concentration-time profile following oral administration have been reported for several compounds. A pharmacokinetic model incorporating discontinuous absorption was developed to simulate concentration-time profiles with double peaks. The gastrointestinal (GI) tract was divided into N compartments, with absorption occurring only from the second and Nth compartments. A two-compartment model was used to describe systemic drug disposition. The effect of gastric emptying and GI transit rate constants (Kl and K1, respectively), number of hypothetical gut compartments, and absorption rate constant at each site (Ka1, Ka2) on the time of occurrence of each peak (Tp1, Tp2), the theoretical fraction of the dose absorbed at each site (phi 1, phi 2), and the contribution of the second site to systemic drug exposure (expressed as phi 2rel) were examined. Simulated concentration-time profiles demonstrated that Tp2 was determined by Kt and N, while Tp1 was determined by K1 and Kt. Changes in Ka1 and Ka2 had no effect on Tp1 or Tp2. phi 1, phi 2, and phi 2rel were determined by Ka1, Ka2, and Kt, and simulations indicated that a secondary peak in the concentration-time profile will be evident only when phi 2rel is substantial. In addition, concentration-time data for ranitidine and cimetidine, which displayed double peaks, were fit with the model. The present model described both data sets well, and realistic pharmacokinetic and physiologic parameters (absorption rate constants, systemic bioavailabilities, GI residence times) were obtained.

Lack of effect of omeprazole, cimetidine, and ranitidine on the pharmacokinetics of ethanol in fasting male volunteers

The effects of three gastric antisecretory drugs on the pharmacokinetics of ethanol have been studied in a randomized crossover experiment. Male medical students (n = 12) took ethanol 0.8 g/kg body weight at 08.00 h after an overnight fast. On seven successive days before drinking ethanol they were given omeprazole 20 mg, cimetidine 800 mg, ranitidine 300 mg, or no drug, with a period of at least 7 days between treatments. The peak blood ethanol concentration of 21.9 to 22.8 mmol.l-1 occurred at 64 to 70 min after the end of drinking. The rate of disappearance of ethanol from the blood ranged from 3.0 to 3.3 mmol.l-1.h-1 and the rate of removal from the whole body ranged from 8.0 to 8.5 g.h-1. The apparent volume of distribution of ethanol was almost the same for all four treatments: mean 0.68 l.kg-1, corresponding to a mean total body water of 44 l (59% body weight). Mean areas under the concentration-time profiles of ethanol ranged from 83 to 87 mmol.l-1.h for the four treatments. It is concluded that omeprazole, cimetidine and ranitidine do not alter the kinetics of a moderate dose of ethanol.

Electrochromatographic solid-phase extraction for determination of cimetidine in serum by micellar electrokinetic capillary chromatography

A highly effective electrochromatographic solid-phase extraction and preconcentration method is reported for the determination of cimetidine in serum in the concentration range 0.233-11.4 micrograms/ml. Preconcentrated samples were determined by micellar electrokinetic capillary chromatography while ranitidine was used as an internal standard. Sample preparation included retention of the analyte on a C18 solid-phase cartridge, followed by elution assisted by an applied voltage of 150 V. From 0.5-ml serum samples, 20-50-microliters aliquots were collected for electrophoretic analysis. Within the studied concentration range, the method was linear and provided adequate precision.