Cimetidine pharmacokinetics and dosage requirements in children

The pharmacokinetics of cimetidine (10 mg/kg) were investigated in 11 children following an oral dose and in 9 children following an intravenous dose. The children ranged in age from 4-13 years and were undergoing radiology for upper gastrointestinal tract pain. Compared with a group of adults, the children had a higher total body clearance (11.6 +/- 3.4 versus 7.0 +/- 2.5 ml/min per kg; P less than 0.005), a larger apparent volume of distribution (1.24 +/- 0.40 versus 0.80 +/- 0.24 l/kg; P less than 0.005) and a shorter elimination half-life (83 +/- 26 versus 122 +/- 16 min; P less than 0.001) of cimetidine. Renal clearance in children comprised 70% of total body clearance, more than double that of adults (9.0 +/- 1.9 versus 4.2 +/- 2.1 ml/min per kg; P less than 0.001). The area under the cimetidine plasma concentration: time curve after the oral dose was on average 42% in children compared with adults. The mechanism for the increased elimination of cimetidine in children is suggested to be an increase in the renal tubular secretory transport of cimetidine in the kidney. A statistically significant negative correlation was observed between age and cimetidine renal clearance. A cimetidine dosage regimen of approximately 30 mg/kg per day in three to four divided doses would be an appropriate dose in children.

The use of an in vitro binding assay to predict histamine H2-antagonist activity

Thirty-seven compounds were evaluated for their ability to inhibit histamine stimulated adenylate cyclase and to inhibit 3H-tiotidine binding. The compounds examined included a number of known H2-antagonists and a number of potential H2-antagonists of diverse chemical structure. The correlation between the Ki values from the binding assay and from the inhibition of adenylate cyclase was calculated to be r = 0.99, p less than 0.001. Thus 3H-tiotidine binding in guinea-pig cerebral cortex can be used to give a valid assessment of histamine H2-receptor activity.

Cimetidine clearance in the obese

Six subjects with normal weight (mean weight = 62 kg) and six obese subjects (mean weight = 140 kg) were given a single intravenous cimetidine infusion of 600 mg over 10 to 15 minutes. Both groups of subjects had normal serum creatinine levels and were matched with respect to age, desirable body weight, height, renal function, and sex. Compared with subjects of normal weight, obese subjects had higher cimetidine systemic (1147 and 637 ml/min) and renal (808 and 318 ml/min) clearances. Volume of distribution at steady state was of the same order for the two groups (82 and 84 L), but the t 1/2 was shorter in the obese group (1.2 and 1.9 hr). Obese subjects had lower cimetidine sulfoxide serum concentrations and greater cimetidine sulfoxide renal clearance (856 and 509 ml/min). Cimetidine systemic clearance and cimetidine sulfoxide renal clearance values were of the same order in the two groups when normalized by the value of weight raised to the 0.76 and 0.5 powers. Under the assumptions of an average weight of 70 kg and that average serum concentrations produced by cimetidine, 300 mg iv every 6 hours, are appropriate, people with normal renal function and body weight usually receive 48 mg/day/weight0.76. This same dosage in obese individuals with normal serum creatinine values should result in the same average steady-state serum concentrations. In our obese subjects, the mean cimetidine dose would have been approximately 500 mg iv every 6 hours.

Increased cimetidine clearance in burn patients

To determine the etiology of the decreased efficacy of cimetidine in burned and critically ill surgical patients, we studied the kinetics and dynamics of the drug in eight burned patients at a mean of 12 days after the burn. The kinetics were compared with those of nine healthy controls. The elimination half-life of cimetidine in burned patients was significantly reduced (2.2 v 1.5 hours), and total clearance significantly increased (8.2 v 13.3 mL/min/kg). Creatinine and total cimetidine clearance were highly correlated with size of the burn. In the patients studied, 63% of the dose was excreted in eight hours, compared with 45% after 24 hours in controls. Gastric pH was maintained at 4 or higher as long as plasma levels of cimetidine were held above 0.5 microgram/mL. Thus, the increased clearance of cimetidine might explain the decreased effectiveness of this drug in burned and possibly other surgical patients. Dose schedules may need to be altered to compensate for the enhanced clearance of this drug.

Augmentin bioavailability following cimetidine, aluminum hydroxide and milk

Previous studies [Jackson et al. 1980] have shown that the bioavailability of Augmentin is not affected by food. The present work has shown that aluminum hydroxide, milk and cimetidine do have some influence on the bioavailability of a single dose of oral Augmentin, but the small differences observed are unlikely to be of therapeutic importance. It is concluded that Augmentin may be administered in clinical practice with any of these substances.

The pharmacokinetics of cimetidine and metabolites in a neonate

Limited pharmacokinetic data have been reported concerning the use of cimetidine in the neonate for the management of gastrointestinal hemorrhage. After receiving cimetidine at a dose of 10 mg/kg/d in a full-term infant, the steady-state peak and four-hour concentrations were 3.5 and 1.8 micrograms/ml, respectively. A mean half-life of 3.6 h for cimetidine and 2.2 h for cimetidine sulfoxide were determined. The cimetidine half-life was prolonged, and the total body clearance decreased as compared with values reported in critically ill children and adults.

Specific 3H-cimetidine binding to receptors in the submandibular gland of the rat

Recent studies indicate that the secretagogue action of histamine on the salivary gland is mediated via H2-receptors. In this study we attempt to characterize the histamine H2-receptors in the submandibular gland of the rat. The results show the presence of a specific 3H-cimetidine binding site in the gland. However, the binding constants of this site are not fully characteristic of the specific 3H-cimetidine-H2-receptor binding. They tend to indicate that the binding of 3H-cimetidine to the gland membrane is similar to the well-characterized 3H-cimetidine-imidazole-recognition-receptor binding found in the membrane of the brain, gastric mucosae and atrium of various mammalian species.

Famotidine, a new H2-receptor antagonist, does not affect hepatic elimination of diazepam or tubular secretion of procainamide

In 8 healthy male volunteers the pharmacodynamic responses to a single dose of diazepam and a single dose of procainamide were assessed before and after pre-treatment with the H2-receptor antagonist famotidine in a randomized crossover study. The pharmacokinetics of diazepam and procainamide were also studied, and the binding of famotidine to human liver microsomes was also measured. Cimetidine induced binding changes with a spectral dissociation constant (Ks) of 0.87 mM, whereas famotidine produced no measurable spectral alteration in concentrations up to 4 mM. The elimination half-life (t1/2: 45.6 h) and total plasma clearance (CL: 0.28 ml/min/kg) of diazepam were not significantly altered by famotidine (t1/2 = 39.0 +/- 11.4 h; CL = 0.31 +/- 0.08 ml/min/kg). Similarly, there was no enhancement of the sedative effect of diazepam by famotidine. The pharmacodynamics and pharmacokinetics of procainamide and N-acetylprocainamide (NAPA), too, were not significantly changed by famotidine: procainamide t1/2 2.9 vs 3.0 h under famotidine and renal clearance (CLR) 436 vs 443 ml/min; and NAPA CLR 195 vs 212 ml/min under famotidine. The data suggest that famotidine, in contrast to cimetidine, does not affect the pharmacokinetics of diazepam (hepatic elimination) or procainamide (tubular secretion). This new H2-receptor antagonist appears to be devoid of an interaction potential for either type of drug elimination.

Saturable transport of cimetidine from cerebrospinal fluid to blood in rats

Cimetidine, a histamine H2 receptor antagonist, is known to have some effects on central nervous system, and is reported to emerge in the cerebrospinal fluid (CSF). We evaluated the elimination of cimetidine from CSF by the method of ventriculo-cisternal perfusion in rats. The extraction ratio of cimetidine at the low perfusate concentration (14 nM) was 0.190 +/- 0.023 (n = 3), while that significantly (p less than 0.02) decreased to 0.0916 +/- 0.081 (n = 3) at the high concentration (4 mM). No significant difference was observed in the production rate of CSF between these two concentrations. These findings suggest that cimetidine is excreted from CSF to blood by a saturable transport system.

Toxicology of cimetidine

Cimetidine has been shown to have low acute toxicity in dogs and rodents. Repeated-dose studies of up to 24 months' duration in rodents at doses up to 950 mg day-1 kg-1 showed few adverse effects. Liver weight was consistently increased at the highest dose and testis, prostate and seminal vesicle weights were reduced in a dose- and time-related fashion. Cimetidine was not carcinogenic in the rat. In tests of up to 1 year's duration in dogs two animals receiving 504 mg day-1 kg-1 had to be killed before the end of the study. They had degenerative changes in the liver and renal tubular nephrosis. These and other dogs at 504 mg day-1 kg-1 had elevated serum transaminases. No such changes were seen at 366 mg day-1 kg-1 or less. Prostate weights were reduced in a dose- and time-related fashion. In a 7-year study in dogs, specifically designed for the purpose, no changes of the stomach mucosa were seen during regular biopsy. Although shown to be a mild anti-androgen, cimetidine produced no significant adverse effects in reproductive studies. The large body of evidence that cimetidine is not a risk for gastric cancer is reviewed. Over 30 million patients have so far been treated with cimetidine and the prediction from the animal studies that it would be an extremely safe therapeutic agent has been borne out in practice.

Cimetidine interaction with amitriptyline

The interaction of cimetidine with amitriptyline was assessed by means of amitriptyline and nortriptyline plasma concentration measurements, standing blood pressure and pulse rate, digit symbol substitution, and visual analogue scales. Cimetidine increased plasma amitriptyline concentrations and decreased plasma nortriptyline concentrations, apparently by inhibiting presystemic metabolism. The changes in blood pressure, pulse rate and digit symbol substitution correlated with changes in concentrations of amitriptyline in plasma and expected changes based on a dose ranging preliminary experiment. Changes in subjective ratings of effects correlated with changes in nortriptyline concentrations in plasma.

Pharmacokinetic aspects during continuous ambulatory peritoneal dialysis: a literature review

Since its introduction some years ago continuous ambulatory peritoneal dialysis (CAPD) has proved to be a valuable alternative to haemodialysis in the treatment of uraemia. Factors contributing to the transport of solutes through the peritoneal membrane are discussed and the literature concerning the pharmacokinetic aspects of CAPD is reviewed.

An assessment of the physiological significance of cimetidine interactions with copper and zinc in biofluids as based on the computer-simulated distribution of the involved complexes at therapeutic levels of the drug

The hypothesis was formerly put forward that the main therapeutic action of cimetidine (the histamine H2-receptor antagonist marketed as Tagamet) as well as some of its side effects might be mediated by its interactions with essential metal ions. The present paper reports the potentiometric study of the coordination of the drug with copper(II) and zinc(II) in NaCl 0.15 mol dm-3 at 37 degrees C. Special attention was paid to copper complexes, due to (i) the involvement of cimetidine in rheumatoid arthritis evolution which could be related to the well-established role of copper against this disease, (ii) the anti-ulcer and anti-inflammatory properties of copper. In particular, the copper-cimetidine-histamine and copper-cimetidine-histidine ternary systems were investigated. Computer simulations of the distribution of cimetidine, zinc and copper in blood plasma were performed at therapeutic levels of the drug. No influence can be expected from cimetidine on the bioavailability of these metal ions, the opposite being also true. The mediation of copper in the action of cimetidine on rheumatoid arthritis should thus be ruled out, the influence of the drug being rather interpretable in terms of reduction of histamine release. Similarly, the sexual dysfunctions due to cimetidine administration are unlikely to arise from the interactions of drug with zinc in blood plasma. The possible involvement of copper and zinc in cimetidine gastrointestinal absorption is also discussed.

Histamine H2-antagonist drug interactions in perspective: mechanistic concepts and clinical implications

Histamine H2-antagonists have the ability to produce fundamental changes in the absorption and disposition of other drugs. However, there are similarities and differences between the H2-antagonists in this respect, depending on the process involved. By increasing the intragastric pH any H2-antagonist has the potential of altering the absorption of weak acids or weak bases. However, since the rise in intragastric pH is not immediate, as with antacids, this type of interaction might be avoided for concomitantly administered, rapidly absorbed drugs. Whereas cimetidine inhibits hepatic mixed-function oxidase drug metabolism, ranitidine does not have this characteristic. Clinical studies have found that cimetidine produces a 20 to 60 percent decrease in the clearance of 23 drugs (such as warfarin, theophylline, quinidine, phenytoin, imipramine, propranolol, nifedipine). Marketed and investigational H2-antagonist drugs differ in their ability to inhibit drug metabolism due to the combined characteristics of cytochrome P-450 binding affinity and therapeutic dosage. Cimetidine also inhibits the renal-tubular secretion of other weak bases (such as procainamide). Management suggestions are presented to help clinicans predict and avoid failure in drug therapy as a result of these drug interactions.

The influence of orally administered cimetidine and theophylline on the elimination of each drug in patients with chronic airways obstruction

We studied the effect of cimetidine and theophylline on the metabolism of each drug in 19 elderly patients with stable chronic obstructive pulmonary disease. Each patient received either regular release or sustained release oxtriphylline. Cimetidine was given to some patients in both groups after steady state for theophylline was achieved. Multiple serum theophylline and cimetidine measurements were performed. Cimetidine decreased theophylline clearance by an average of 37.8% (26.2 to 46.7) for regular release and 33.4% (22.2 to 45.0) for sustained release oxtriphylline. The apparent volume of distribution was not changed; however, the half-life for theophylline was increased. As there is a linear relationship between the theophylline clearance before and after the addition of cimetidine and a linear relationship between trough and peak values before and after cimetidine, these 2 relationships may be used to predict clearance values and serum levels after cimetidine treatment when the initial values are known.

Pharmacokinetic–pharmacodynamic modelling and simulation using the electrical circuit simulation program spice2

The use of the electrical circuit simulation program SPICE2 for performing digital computer simulations of linear and non-linear pharmacokinetic-pharmacodynamic models is described. SPICE2 utilizes the principles of network thermodynamics (thermodynamics of electrical circuits). These principles dictate analogy between the conservation laws of chemical reactions and mass transport and Kirchhoff's laws of current and voltage balance, and also prove that Fick's law of diffusion is isomorphous with the conductance form of Ohm's law. Detailed descriptions of program inputs, formats, and options for simulation of linear and non-linear pharmacokinetic-pharmacodynamic systems are provided, with appropriate examples. Single as well as multiple dose simulations (accumulation kinetics and dynamics) are discussed. The advantages of SPICE2 over other available simulation packages, including user-friendliness, ease of operation, versatility, power, and the economy of time and effort afforded, are emphasized. The educational value of SPICE2 as a highly versatile tool for teaching both fundamental and complex pharmacokinetic-pharmacodynamic concepts, as well as its routine usage in elucidating complex research problems, are also discussed.

Histamine H2 receptor desensitization in HL-60 human promyelocytic leukemia cells

Recent studies have suggested that cyclic AMP (cAMP) may be involved in regulation of cell growth and differentiation of cancer cells. Incubating HL-60 cells in the presence of the specific H2 agonist dimaprit resulted in 30-fold increases in cAMP levels (EC50 = 5.7 X 10(-6) M) and morphological changes suggestive of cell maturation along the granulocyte pathway. However, cells cultured with 10(-5) M dimaprit showed more than an 80% decrease in their cAMP response to subsequent addition of H2 agonists, whereas the cAMP response to prostaglandin E2 was unaltered. Desensitization was time-dependent (halftime approximately 2.5 hr with 10(-5) M dimaprit), dose-dependent (dimaprit EC50 = 1.4 X 10(-6) M) and completely prevented by 10(-3) M cimetidine. Desensitization of HL-60 cells for 4 hr with 10(-5) M dimaprit followed by the addition of 10(-3) M cimetidine resulted in total recovery of the cAMP response in less than 24 hr. The pharmacologically inactive analog N-methyldimaprit (SK&F 92054) did not increase cAMP production or cause desensitization to H2 stimulation. Desensitization was observed in the presence or absence of a phosphodiesterase inhibitor, indicating that induction of cAMP-phosphodiesterase was not involved in this process. No difference in the number of [3H]tiotidine binding sites was observed between control and dimaprit-desensitized HL-60 cells. Based on these results, we suggest that H2 receptor agonists caused an agonist-dependent desensitization, presumably due to an uncoupling of receptors from adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cimetidine on the hepatic extraction of indocyanine green, the portal pressure and the systemic circulation in patients with cirrhosis of the liver

The effect of cimetidine on hepatic and systemic haemodynamic parameters was studied in seven patients with portal hypertension due to alcohol-induced cirrhosis of the liver and in one patient with peliosis hepatis following oral contraceptive steroids. The intravenous administration of cimetidine (350 mg as bolus, followed by 2 mg/min over 60 min) reduced the hepatic extraction of continuously infused indocyanine green (ICG) by 27%; this was statistically significant (P less than 0.01). Since the ICG clearance, calculated independently of hepatic perfusion, was lowered by 19%, this effect seems to be mainly due to a reduced capacity of the liver to remove the dye from the blood, rather than due to changes in perfusion. Cimetidine did not influence the elevated portal pressure in the patients with cirrhosis, or the normal pressure in the patient with peliosis hepatis. No significant effect was observed on heart rate, mean arterial pressure, pulmonary artery pressure, pulmonary capillary pressure and cardiac output. These studies indicate that the reduction of the hepatic ICG extraction following cimetidine is more the result of an inhibited capacity of the liver to remove the dye than of changes in the hepatic perfusion or in the systemic circulation.