Cimetidine dynamics after repeated intravenous injection

Pharmacokinetic and pharmacodynamic studies in man simulating acute and chronic treatment with oral pirenzepine

Nine healthy, male subjects received controlled-rate i.v. infusions of a new formulation of pirenzepine to produce constant plasma levels of 40 ng/ml and 105 ng/ml. They also received stepped infusions resulting in plasma levels of 20, 40, 80 and 40 ng/ml for defined periods. Peptone-stimulated gastric acid and volume secretion and near point vision decreased dose dependently, whereas gastric acidity was unchanged. There was a significant correlation between inhibition of gastric acid secretion and the pirenzepine concentration in plasma and in gastric juice. During the stepped i.v. infusion, changes in near point vision were closely related to the plasma drug concentration. Antimuscarinic side-effects occurred more frequently when the plasma drug level was high. Overall, there was a close relationship between the plasma concentrations and the effects and side-effects of pirenzepine. Its gastric inhibitory action was characterized only by a reduction in gastric volume secretion. Increasing plasma concentrations during the first days of treatment may be essential for its efficacy as an antiulcer drug.

Effect of a 2 g cimetidine infusion on twenty-four-hour intragastric pH in critically ill patients

We evaluated the effect of a cimetidine continuous infusion (2 g in 24 h) on the intragastric pH of 16 critically ill patients (11 men, 5 women, mean age 45 years). During the 24 h pre-trial period and the subsequent 24 h cimetidine infusion, an intragastric combined electrode was placed in the fundus and the pH recorded with a portable pH module and data collection unit. In each patient, the cimetidine infusion induced a prolonged rise of intragastric pH. For all patients the mean percentage of readings above pH 4.0 was 11% pre-trial and 75% during the cimetidine 24 h infusion (p less than 0.001). The percentages of readings above 5.0, 6.0, 7.0 were also significantly higher during infusion than pre-trial in the 16 patients. After starting the cimetidine infusion, there was a concomitant rise of median plasma cimetidine and median intragastric pH in the 7 patients studied. After 6 h, median plasma cimetidine remained above 1 mcg/ml. These results and recent data from the literature suggest that in critically ill patients a continuous infusion of cimetidine might prevent stress ulcerations better than bolus injections by maintaining intragastric pH above 4.0 during longer time intervals.

Pharmacodynamics and pharmacokinetics of parenteral histamine (H2)-receptor antagonists

Parenteral histamine (H2)-receptor antagonists are frequently used to prevent upper gastrointestinal bleeding caused by stress-induced gastric mucosal damage in critically ill patients. It is generally agreed that the goal of therapy in this syndrome is the consistent elevation of gastric pH levels above a certain value, often set at 4, in order to prevent the underlying mucosal damage from progressing to bleeding. The three H2-receptor antagonists currently available in a parenteral form and suitable for this mode of prophylaxis are cimetidine, ranitidine, and famotidine. The pharmacodynamic and pharmacokinetic properties of these agents, as they relate to their use in prevention of stress ulceration bleeding, are discussed here. These agents are more noted for their pharmacodynamic and pharmacokinetic similarities in acid suppression, elimination, and metabolism than for their differences. Ranitidine and famotidine are more potent than cimetidine, and famotidine has a slightly longer half-life than do cimetidine and ranitidine, but current dosing recommendations take these differences into account so that the agents have equivalent efficacy. Cimetidine and ranitidine have been widely used in this application. Less experience has been obtained, to date, with famotidine. Recent studies with primed, continuous infusions of cimetidine indicate that dosing schedule may be the key to obtaining better efficacy in prophylaxis of stress-related mucosal damage. Similar studies with ranitidine have not yielded results as promising as those with cimetidine, however, and few data are available on famotidine.

Dose-dependent pharmacokinetics of cimetidine in the rat

The pharmacokinetics of cimetidine were studied in the rat after intraperitoneal administration of 10, 40 and 100 mg/kg. The area under the plasma concentration-time curve increased more than proportionately with dose. The total plasma clearance of cimetidine decreased as the dose increased (4.11 to 2.21 l/h per kg) with a consequent increase in half-life (24.0 to 37.9 min) but no change in volume of distribution (mean 2.31 l/kg). The fraction of dose excreted unchanged increased slightly with dose (0.37 to 0.45), whereas the fraction excreted as the sulphoxide metabolite decreased significantly with increased dose (0.35 to 0.14). Both the renal clearance (1.52 to 0.99 l/h per kg) and the formation clearance of the sulphoxide metabolite (1.45 to 0.30 l/h per kg) decreased with increasing dose. Residual clearance, calculated as the difference between total clearance and the sum of renal and metabolic clearance, did not change with dose (mean 1.08 l/h per kg). The formation clearance of the sulphoxide metabolite became saturated at a lower cimetidine concentration than the renal clearance.

Inhibition of pentagastrin-stimulated gastric acid secretion and plasma levels of the new histamine H2-receptor antagonist ramixotidine dihydrochloride (CM 57755) in human volunteers

The new competitive histamine H2-receptor antagonist, ramixotidine 2 HCl (CM 57755), has been tested in healthy male volunteers for its ability to inhibit pentagastrin-stimulated gastric acid secretion. In the first study, in 8 subjects, pentagastrin 6 micrograms.kg-1 was injected s.c., 90 min after the following 4 oral treatments given in random order at weekly intervals: placebo, 100, 200 and 400 mg CM 57755. Gastric contents were collected over 15-min periods during the 2 h after pentagastrin stimulation. In a second, similar study, 8 subjects received placebo, 0.5 and 1.0 g CM 57755 and 800 mg cimetidine, 120 min before a 2 h i.v. infusion of 6 micrograms.kg-1.h-1 pentagastrin. Cumulative gastric secretion in placebo-treated subjects was 46 +/- 14 and 62 +/- 11 mmol H+.2 h-1 (mean +/- SD), respectively, in the first and second studies. It was significantly reduced only after 400 mg CM 57755 in the first study. In the second study either dose of CM 57755 and cimetidine caused a significant reduction in gastric acid secretion. Average plasma levels of ramixotidine were dose-related after 0.2 and 1.0 g and ranged from 0.3 and 1.6 micrograms/ml, respectively, at 60 min to 0.5 and 3.7 micrograms/ml at 180 min. The peak cimetidine level averaged 3.6 micrograms/ml at 150 min. Individual CM 57755 plasma levels throughout the test period were fairly consistent with the inhibition of cumulative gastric acid secretion scored concurrently in each subject. No subjective side-effects attributable to the treatments were reported, and no abnormal findings were seen in the ECG or in laboratory tests.

The comparative efficacy of cimetidine and ranitidine in controlling gastric pH in critically ill patients

The comparative efficacy of intravenous cimetidine and ranitidine in controlling gastric pH in 100 intensive care unit patients was assessed in a double blind, prospective, randomized study. The total number of gastric pH determinations and the number of pH determinations with pH less than five were recorded. Patients received either cimetidine or ranitidine via continuous infusion, with dosage adjustments for patients with renal insufficiency. Antacids were administered each time the gastric pH was less than five. There was no difference overall in the number of patients who had at least one gastric pH determination less than pH 5. There was however, a larger proportion of patients with greater than or equal to 10, greater than or equal to 15, greater than or equal to 20 and greater than or equal to 25 per cent of gastric pH determination less than pH 5 in the cimetidine group than in the ranitidine group. This difference was statistically significant for greater than or equal to 25 per cent. The drugs were well tolerated. Ranitidine was as effective as cimetidine and possibly more so in controlling gastric pH.

Pharmacokinetics and pharmacodynamics of cimetidine and metabolites in critically ill children

Cimetidine and antacids are the mainstays of therapy for the prophylaxis of stress-induced ulceration in critically ill children. Previous cimetidine dosing recommendations have been empiric because of a lack of knowledge about cimetidine disposition kinetics in children. Thirty children, mean age 9 +/- 3.2 years, were admitted to the study with the following primary diagnoses: closed head injury (23 patients), sepsis (four), gunshot wound (two), and bleeding gastric ulceration (one). The mean dose of cimetidine was 26 mg/kg/day, administered intravenously over 15 minutes in four divided doses. Cimetidine disposition was best described by a biphasic elimination curve with t1/2 values for cimetidine, cimetidine sulfoxide, and hydroxymethyl cimetidine of 1.39, 2.6, and 4.7 hours, respectively. Cimetidine plasma concentrations were maintained at greater than or equal to 0.5 microgram/ml for a significantly longer time in patients who received greater than or equal to 20 mg/kg/day. Most patients had a plasma cimetidine concentration below 0.5 to 1.0 microgram/ml 4 hours after infusion. The mean apparent volume of distribution and total body clearance for cimetidine were 1.23 L/kg and 10.4 ml/min/kg, respectively. A significant correlation was found between age and either apparent volume of distribution (r = 0.76, P less than 0.001) or total body clearance (r = 0.75, P less than 0.001). No significant correlation between cimetidine concentrations in either plasma or gastric juice and gastric pH could be determined. However, seven of nine patients who received only cimetidine had a gastric pH of greater than or equal to 4 at 2 hours after infusion when the plasma cimetidine concentration was greater than or equal to 1.0 or the gastric juice concentration was greater than or equal to 2.0 microgram/ml. The mean gastric pH was 2.2 at 6 hours, when plasma and gastric juice concentrations of cimetidine were greater than or equal to 1.0 microgram/ml. On the basis of our data, a cimetidine dosage of 20 to 30 mg/kg/day administered in six divided doses should provide for average steady-state plasma cimetidine concentrations of 1.3 to 2.0 micrograms/ml.

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.