Time-dependent disposition of beta-naphthoflavone in the rat

The pharmacokinetics of beta-naphthoflavone (BNF) have been investigated in rats following various modes of intravenous administration. From intravenous bolus studies it was established that BNF showed a high blood clearance (130 ml/min/kg) and no detectable excretion of unchanged compound in the urine. The volume of distribution for BNF was large (6 L/kg), and binding to plasma proteins extensive (96%). Intravenous infusion studies where the length of infusion was increased from 1 to 8 hr showed marked signs of time-dependent pharmacokinetics. During continuous infusions the plasma concentrations accrued for approximately 1 hr, after which plasma concentrations declined in an apparent exponential fashion to a plateau value. In the short infusion studies the postinfusion half-life (27 min) was significantly shorter than the terminal half-life after bolus administration (40 min). Time-dependent clearance of BNF resulting from enhancement/induction of P450IA enzymes is proposed as the mechanism for these unusual pharmacokinetic features. The use of antipyrine as an independent probe for P450 activity gave similar trends in antipyrine clearance for various modes of BNF administration. Computer simulations based on an autoinduction model for time-dependent clearance were consistent with the observations on BNF in the rat.

Clinical prosthodontic management of a new implant system

Just prior to the advent of the branemark implant, the Harvard consensus on implantology condemned all maxillary implants. While implant utilization has skyrocketed in the last few years integration of implants in the maxilla is a persistent problem and even the branemark implant enjoys a lower success rate in this bone. These observations underscore the higher failure rate of the maxillary implant and suggest the need for development of a new implant design configuration that would favour long term successful integration despite the attenuated maxillary attributes. Such is the intent of the ostial implant. The basic considerations concerning this device, its bioengineering, experimental performance and clinical trials are discussed elsewhere. The purpose of this paper is to describe clinical prosthodontic applications of this implant system.

Nonlinear first-pass metabolism of nifedipine in healthy subjects

1. A dose-range study was carried out with nifedipine (NF) in 11 healthy subjects, who received orally 5, 10 and 20 mg in capsules (cap) and 20 mg as slow-release tablet. 2. The dose-normalized area under the curve (AUC) of NF showed significant nonlinearity: AUC of 5 mg (cap) was 312 +/- 179, of 10 mg (cap) 357 +/- 186 and of 20 mg (cap) 424 +/- 174 (ng.h/ml). The AUC of the tablet was significantly lower than that of 10 and 20 mg (cap), but not different from 5 mg (cap). 3. The dose-normalized AUC of the pyridine metabolite (M-0) significantly decreased with increasing dose: AUC of 5 mg (cap) was 244 +/- 88, of 10 mg (cap) 194 +/- 96 and of 20 mg (cap) 120 +/- 37 (ng.h/ml). 4. The excretion of M-I (which is ester-hydrolysed M-0) into urine was not different for any of the doses. 5. It is concluded that NF exhibits nonlinear first-pass metabolism, which concomitantly affects the formation of M-0, but not that of M-I.

Drug metabolizing capacity in vitro and in vivo--II. Correlations between hepatic microsomal monooxygenase markers in phenobarbital-induced rats

Pretreatment with various doses of phenobarbital (PB) has been used to create a pool of rats with a wide range of hepatic microsomal monooxygenase activity to systematically examine relationships between and within in vivo and in vitro markers. The in vivo clearance of tolbutamide (TOL), theophylline (TH), antipyrine (AP) and its metabolites were determined in the same rats used for hepatic microsome preparation and assessment of P450 content and activities (via 7-ethoxycoumarin O-deethylase (ECOD), 7 ethoxyresorufin O-deethylase, 7-methoxycoumarin O-demethylase (MCOD) and aldrin epoxidase determinations). A graded dose-response relationship was found between PB treatment and most but not all parameters. The need for careful selection of in vivo and as well as in vitro markers is apparent from these studies. The most responsive parameters--TOL and AP clearances, MCOD and ECOD activities--were also those producing the strongest in vivo-in vitro correlations. Despite the diffuse nature of the PB induced response in P450 complement, good predictive relationships were apparent between ECOD and TOL clearance (r2 = 0.88).

Drug metabolizing capacity in vitro and in vivo--I. Correlations between hepatic microsomal monooxygenase markers in beta-naphthoflavone-induced rats

Relationships between and within in vivo and in vitro markers of drug oxidative metabolism have been investigated in rats displaying a wide range of hepatic microsomal monooxygenase activity due to prior treatment with various doses of the inducing agent beta-naphthoflavone (BNF). BNF induction produced large dose-related changes in the in vivo clearance (CL) of theophylline (TH), antipyrine (AP) and the individual AP metabolite formation clearances, 4-hydroxyantipyrine (4H) and norantipyrine, and the in vitro parameters, 7-ethoxycoumarin O-deethylase, 7-ethoxyresorufin and P450. No trends were observed with the formation clearance of 3-hydroxymethylantipyrine and 7-methoxycoumarin O-demethylase whilst a negative response was observed with aldrin epoxidase. The selectivity of the markers towards BNF induction was coincident with the degree of covariance observed between these parameters. Strong correlations were observed in particular between CL(TH) and CL(4H) and ECOD and EROD indicating the high predictive value of these parameters. These studies demonstrate that under the well controlled conditions which may be imposed in animal environments predictively useful relationships (r2 greater than 0.8) can be established between in vitro and in vivo markers of hepatic microsomal monooxygenase activity.

Factors contributing to the accelerated clearance of theophylline and antipyrine in adults with exocrine pancreatic disease

1. Oxidative drug metabolizing capacity has been assessed by oral antipyrine and/or theophylline tests in consecutive patients with chronic pancreatitis (CP; alcoholic 24, idiopathic 47), acute pancreatitis (AP; 28) and pancreatic cancer (CA; 11). Most of the patients had drastically reduced their alcohol consumption and dietary fat intake for variable periods before the tests. Excellent bioavailability of theophylline was confirmed from paired oral and intravenous tests in seven subjects, including two with exocrine pancreatic failure. 2. The clearance of theophylline in the patients was faster than in 15 controls with a 'healthy lifestyle' [median 104 (range 18-320) ml h-1 kg-1 vs median 68 (range 50-97) ml h-1 kg-1, P less than 0.01]. The difference was especially apparent in the groups with alcoholic CP (P less than 0.001 and idiopathic CP (P less than 0.01), but not in the AP and CA group as a whole, although drug clearance in some 50% of those cases exceeded the reference range. 3. There was good correlation between theophylline and antipyrine clearance in a subset of 91 subjects who had both tests (15 controls, 76 patients), but antipyrine was much less sensitive as a marker of enzyme induction. This suggests that enzyme induction in pancreatic disease preferentially involves the polycyclic aromatic hydrocarbon-inducible subfamily of cytochrome P-450. 4. The lack of correlation between pancreatic secretory capacity in 56 cases, judged by a secretin-pancreozymin test, and theophylline clearance suggests that enzyme induction is not secondary to pancreatic dysfunction. 5. Multivariate regression analysis identified approximately 50% of variability in clearance of each probe.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of two azole antifungal drugs, ketoconazole, and fluconazole, as modifiers of rat hepatic monooxygenase activity

The mechanism of action of azole antifungal agents is believed to involve inhibition of fungal cytochrome P-450, and, therefore, an investigation of the interaction of these drugs with mammalian cytochrome P-450 systems should provide some indication of their selectivity as antifungal agents. The ability of ketoconazole and fluconazole, the latter representing a new generation of triazole antifungal agents, to modify rat mixed function oxidase activity has been investigated in vitro with hepatic microsomes and in vivo using a N-methyl-[14C] antipyrine breath test. As a measure of selectivity the results have been compared with antifungal potency. Ketoconazole is more potent than fluconazole by an order of magnitude in inhibiting metabolism by O-dealkylation of ethoxycoumarin, methoxycoumarin and ethoxyresorufin (IC50 values of 6, 5 and 130 microM for ketoconazole respectively). The effects on the regio- and stereospecific hydroxylation of [14C] testosterone were also measured; the IC50 values for inhibition of total testosterone metabolism were 0.1 mM and greater than 3 mM for ketoconazole and fluconazole respectively. Marked selectivity differences were observed for the two drugs as indicated by ketoconazole being a potent inhibitor of 7 alpha-hydroxylation of testosterone (IC50 20 microM) while fluconazole did not inhibit this activity at 3 mM. In vivo investigations using a range of doses confirmed their ranking for inhibitory potency; the ED50 values for maximum demethylation rate were 17 mumol/kg and greater than 60 mumol/kg for ketoconazole and fluconazole respectively. Thus fluconazole has a lower propensity to interact with rat hepatic cytochrome P-450 and can be considered a more selective antifungal agent as its in vivo antifungal potency is an order of magnitude greater than ketoconazole.

Theophylline and antipyrine disposition in smoking and non-smoking epileptic subjects

Theophylline and antipyrine disposition has been compared in smoking epileptic patients, non-smoking epileptic patients and non-smoking healthy volunteers. Although clear differences in drug clearance and half-life were evident as a result of anticonvulsant drug therapy, no effect of smoking was discernible. Thus, additive effects from induction of the hepatic microsomal monooxygenase system in man by anticonvulsant drugs and polycyclic aromatic hydrocarbons (in cigarette smoke) were not evident.

Kinetics of drug metabolism inhibition: use of metabolite concentration-time profiles

A simple model simulating the kinetics of drug metabolism inhibition interaction is investigated. The relative sensitivity of drug and metabolite concentration-time profiles as indices of inhibition is assessed. Under steady-state conditions where inhibitor concentrations are maintained constant, the determination of metabolite in addition to drug kinetics provides little additional information when inhibition is nonselective in nature. However metabolite profiles do offer increased sensitivity when parallel routes of metabolism exist and there is selectivity of inhibitory action. Non-steady-state conditions are also investigated as they often apply in inhibition studies; the inhibitor is often administered as a single dose or as a multiple dosing regimen rather than by a constant rate intravenous infusion. Under the former conditions, when inhibitor concentrations in the body fluctuate during the study, metabolite kinetics can be more useful than drug kinetics. The changes evident in the metabolite concentration-time profiles are substantial due to both the inhibition per se and the nonlinearity in the system arising from inhibitor elimination. It is concluded that metabolite kinetics provide a useful aid in the detection of drug metabolism inhibition interactions.

Glucuronidation in vitro and in vivo. Comparison of intestinal and hepatic conjugation of morphine, naloxone, and buprenorphine

The glucuronidation of morphine, naloxone, and buprenorphine by the liver and intestine has been assessed both in vitro and in vivo in the rat. Using microsomes, Vmax/Km ratios were estimated as measures of intrinsic enzyme activity, and a wide range of ratios were obtained (500-fold). The ratio for the intestine was consistently less than for the liver, and the rank order of activities within each tissue was morphine less than naloxone less than buprenorphine. Using various routes of administration, plasma concentration-time profiles for each compound were determined and used to estimate hepatic and intestinal extraction ratios. These extraction ratios were dose-independent and more substantial for the liver than the intestine. For each tissue, buprenorphine and naloxone showed similar extraction, whereas morphine was less. Plasma binding and blood/plasma concentration ratios were determined, and perfusion models were used to calculate intrinsic clearance. This in vivo parameter of enzyme activity showed a wide range (200-fold), comparable to the spread of microsomal Vmax/Km ratios. Although the in vivo parameters consistently gave measures 10- to 30-fold higher than the in vitro parameters, the rank order was identical for the two sets. These data suggest that a comparative approach to correlate in vitro and in vivo data has advantages over the use of absolute drug characteristics.

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.

Plasma concentration-response relationship for cimetidine inhibition of drug metabolism in the rat

Cimetidine inhibition of antipyrine elimination has been studied in the rat over a range of steady state cimetidine concentrations. Cimetidine is a potent inhibitor of antipyrine metabolism with a concentration of about 1.25 mg/liter causing a 50% decrease in the total plasma clearance of antipyrine. The degree of inhibition of antipyrine clearance caused by cimetidine is dependent upon its plasma concentration, but the relationship is not linear. The formation clearances of all antipyrine metabolites measured--3-hydroxymethylantipyrine, 4-hydroxyantipyrine, and norantipyrine--are inhibited by cimetidine at all concentrations used. The susceptibility of the different metabolites to cimetidine inhibition does vary. The renal clearance of antipyrine was also decreased by cimetidine by an unknown mechanism. Analysis of the individual formation clearances suggests that the inhibition of oxidative metabolism due to cimetidine is caused by its binding to two classes of enzyme sites--a high affinity, low capacity and a low affinity, high capacity site. These two different sites would appear to be responsible for the production of different metabolites of antipyrine. The true nature of the selectivity of cimetidine inhibition of antipyrine metabolism is apparent from the formation clearances but not from the urinary metabolite patterns.

Effect of pregnenolone carbonitrile, promethazine and antipyrine pretreatment on antipyrine metabolite formation in rats

The effect of three inducers of cytochrome P-450-mediated drug oxidations (Pregnenolone carbonitrile, promethazine and antipyrine) on antipyrine metabolite kinetics has been investigated using the urinary metabolite pattern and 14CO2 exhalation rate (CER)-time profile following [N-methyl-14C]antipyrine administration. The CER-time profiles showed the characteristic changes associated with induction, namely, increased maximum CER and decreased half-life, previously observed in phenobarbitone and beta-naphthaflavone-induced rats. Calculation of formation rate constants based on urinary recovery of 3-hydroxymethyl-, 4-hydroxy- and nor-antipyrine indicated no clear selectivity of induction by any pretreatment. However, the percentage increase of the latter two metabolites was two- to four-fold greater than for the former metabolite. The use of the metabolite ratio (3-hydroxymethylantipyrine/norantipyrine) is proposed to assess the qualitative nature of induction of antipyrine metabolism.

Temporal effects on antipyrine metabolite kinetics in Aroclor 1254-treated rats

The in vivo consequences of a single dose of Aroclor 1254 (50 mg/kg) on the drug metabolizing capacity of rats were investigated. A noninvasive method, employing [N-methyl-14C]-antipyrine where both 14CO2 exhalation and urinary excretion of 4-hydroxy-, 3-hydroxymethyl-, and norantipyrine were monitored, was used. A group of rats were sequentially tested over a 3-week period to characterize temporal patterns. The antipyrine metabolite kinetic approach demonstrated that induction of hepatic cytochrome P-450 is maximal 3-6 days after Aroclor 1254 administration and the effects were apparent for at least a further 14-17 days. Evidence is presented to suggest selective effects of Aroclor 1254 on different cytochromes P-450 are apparent in vivo.

Disposition of metronidazole and its effects on sulphasalazine metabolism in patients with inflammatory bowel disease

The pharmacokinetics of metronidazole were studied after oral and intravenous administration to seven patients with Crohn's disease and five patients with ulcerative colitis. The oral/intravenous availability ratio was 0.97 +/- 0.2 in the Crohn's patients and 0.90 +/- 0.1 in the colitics (mean +/- s.e. mean). Plasma clearance was 3.24 +/- 0.2 l h-1 and 4.1 +/- 0.5 l h-1, respectively. These differences were not statistically significant. Ten of the patients on long term sulphasalazine were studied to observe the effect of 2 weeks metronidazole therapy on plasma sulphapyridine concentration. The sulphapyridine concentration changed from 22.1 +/- 2.0 micrograms ml-1 to 15.95 +/- 4.5 micrograms ml-1 in the Crohn's patients and 26.0 +/- 6.0 micrograms ml-1 to 36.4 +/- 8.5 micrograms ml-1 in the colitis group, pre- and post-metronidazole. These differences were not statistically significant. These results suggest that metronidazole does not interfere with diazo-link splitting of sulphasalazine and that patients with Crohn's disease handle metronidazole in a similar manner to patients with colitis.

Cervical spine movement during orotracheal intubation

We measured cervical spine movement during orotracheal intubation in 16 anesthetized patients without neck injury. Comparisons were made using straight and curved laryngoscope blades without stabilization, Philadelphia collar stabilization, and in-line stabilization by an assistant. There was cervical spine movement in all cases. There was no significant difference in movement without stabilization when comparing straight and curved laryngoscope blades (P = .8536). There was no significant decrease in movement when a Philadelphia collar was applied (P = 1.000). There was a significant decrease in movement when in-line stabilization was applied (P = 0.0056). Although none of the methods tested totally prevented cervical spine movement during orotracheal intubation, the least movement was obtained by the use of in-line stabilization by an assistant. The type of laryngoscope blade used or application of a Philadelphia collar did not reduce movement significantly.

Inhibition of antipyrine metabolite formation. Steady state studies with cimetidine and metyrapone in rats

Antipyrine metabolite kinetics have been characterized in rats receiving an intravenous infusion of either saline, cimetidine, or metyrapone. 14CO2 exhalation rate-time profiles following [N-methyl-14C] antipyrine administration demonstrate that, when either cimetidine or metyrapone is maintained at a steady state plasma concentration, a constant degree of inhibition is evident. Under the conditions imposed in this in vivo study, the inhibitory potency of metyrapone is approximately 6 times that observed with cimetidine. Rate constants for the formation of 4-hydroxy-, 3-hydroxymethyl-, and norantipyrine have been calculated using breath and urinary metabolite data under the steady state inhibitory states. Metyrapone nonselectively inhibits the formation of all three oxidative metabolites by approximately one-third. Cimetidine inhibition is selective where rates of 3-methyl-hydroxylation and N-demethylation are reduced by 50% yet 4-hydroxylation is unaffected. The value of assessing inhibitory responses under steady state conditions is discussed and the nonlinear nature of the kinetics of inhibition, when a single bolus dose of inhibitor is used, is illustrated by means of computer simulation.

Induction of cytochromes P-450 in pancreatic disease: consequence, coincidence or cause?

We have examined the pharmacokinetics of antipyrine and of theophylline--validated probes for cytochromes P-450 activities--in a series of patients with pancreatic disease. The half-life of each drug was significantly lower, and its clearance faster, in patients than in controls and this pattern was detected in the subgroups with acute pancreatitis (6), chronic pancreatitis (22), or pancreatic cancer (4). These data suggest induction of cytochromes P-450 in all forms of exocrine pancreatic disease. Enzyme induction is unlikely to be secondary to pancreatic malfunction since there was no correlation between prevailing exocrine status, as assessed by secretin-pancreozymin tests, and the half-life or clearance of either drug. The corollary is that induction of the mono-oxygenases by environmental agents, both recognised and unidentified, is a primary event in pancreatic disease. The possible relevance of this finding is discussed.

Antipyrine metabolite kinetics in healthy human volunteers during multiple dosing of phenytoin and carbamazepine

Antipyrine total clearance and the formation clearance of its major metabolites were studied in normal, healthy male volunteers before and after multiple dosing for approximately three weeks with phenytoin (six subjects) and carbamazepine (six subjects). Total antipyrine clearance increased on average by 91% after phenytoin dosing and by 61% after carbamazepine and individual increases correlated well with mean plasma concentrations of the anti-epileptic drug. The increase in total clearance resulted largely from increased formation clearances of the 4-hydroxy and 3-hydroxymethylantipyrine metabolites with minimal effect on the norantipyrine pathway, following treatment with both enzyme-inducing drugs. It is concluded that both phenytoin and carbamazepine have similar effects on antipyrine metabolism and that these effects are mediated by induction of specific forms of cytochrome P450.

Quantitation of extrahepatic metabolism. Pulmonary and intestinal conjugation of naphthol

There is much evidence in the literature to indicate extrahepatic conjugation of naphthol by various isolated tissue preparations. Using in vivo methodology, where multiple sites of input are employed, it has been demonstrated that both lung and intestinal mucosa make a significant contribution in the conjugation of naphthol in the rat. Under the conditions studied, both of the above organs (pulmonary extraction ratio 0.44, intestinal extraction ratio 0.46) are more than twice as effective as the liver in eliminating naphthol (hepatic extraction ratio 0.18). This multiple sites of input method has been compared with a multiple sites of sampling method. Similar results have been obtained in quantifying pulmonary conjugation of naphthol. The relevant merits of the two approaches are discussed.

Changes in the disposition of promethazine during multiple dosing in the rabbit

During a multiple dosing regimen, the area under the promethazine blood concentration-time profile progressively decreased indicating auto-induction of metabolism. The increase in promethazine clearance (mean 35%) was not reflected in changes in either elimination half-life or minimum blood concentrations during the dosing interval. This was attributed to a deep compartment in the disposition of promethazine in the rabbit. The changes in promethazine clearance were accompanied by proportionally larger changes in the clearance of the monodesmethylpromethazine metabolite. The effect of promethazine pretreatment on the clearance of antipyrine was also studied and was found to be significantly increased by a mean of 17% following pretreatment with promethazine. However, the changes in the clearance of antipyrine did not highly correlate with those of promethazine and monodesmethylpromethazine. This may indicate that promethazine induces metabolic systems in the rabbit for which antipyrine is not a good substrate.

Effect of promethazine and isosafrole on rat-hepatic microsomal mono-oxygenase activity: comparison with classic inducers phenobarbitone and beta-naphthoflavone

The characteristics of induction of rat-hepatic microsomal mono-oxygenase activity by promethazine and isosafrole have been investigated and compared with the classic inducers phenobarbitone and beta-naphthoflavone. Both promethazine and isosafrole pretreatments result in increased cytochromes P-450, and enhanced NADPH-cytochrome c reductase, aminopyrine N-demethylase, dichloronitroanisole O-demethylase, ethoxycoumarin O-deethylase and ethoxyresorufin O-deethylase activity. Isosafrole but not promethazine increased the liver to body weight ratio. It is concluded that promethazine and isosafrole pretreatment produces an induction of the rat-hepatic microsomal mono-oxygenase system which shows both phenobarbitone- and polycyclic aromatic hydrocarbon-type characteristics.

Inhibition of antipyrine metabolite formation in rats in vivo

The effect of four inhibitors of cytochrome P-450-mediated drug oxidations (SKF 525A, cimetidine, metyrapone and alpha-naphthoflavone) on the urinary metabolite pattern and 14CO2 exhalation rate (CER)-time profile following [N-methyl-14C]antipyrine administration has been investigated. The CER-time profiles indicated that inhibition of antipyrine metabolism was in the rank order SKF 525A greater than cimetidine greater than metyrapone greater than ANF. The urinary metabolite patterns showed selectively in action towards particular pathways, 3-hydroxylation being primarily decreased by SKF 525A and cimetidine, and N-demethylation by ANF. The results provide further evidence for involvement of multiple forms of cytochrome P-450 in antipyrine metabolism.

In vivo capacity of hepatic and extrahepatic enzymes to conjugate phenol

Phenol has a high metabolic clearance in the rat and shows extensive first pass metabolism. The ability of the intestinal mucosa, liver, and lung to conjugate phenol is investigated over a 35-fold dose range by employing a judicious choice of route of administration. Comparison of blood phenol concentration-time profiles, following intravenous administration into the jugular and the hepatic portal veins, indicates extensive hepatic conjugation of phenol at low doses. However, the capacity of the hepatic enzymes is readily saturated. This observation is confirmed in isolated perfused livers. Comparison of blood phenol concentration time profiles, following vascular administration into the carotid artery and the jugular vein, indicates substantial pulmonary conjugation of phenol. Although the extent of pulmonary conjugation is less than the hepatic contribution, it is evident over a wider dose range than the latter organ. Intestinal conjugation of phenol is assessed by comparison of data from intraduodenal and hepatic portal venous administration. At low doses of phenol (less than 1 mg/kg), intestinal and hepatic conjugation are comparable. The capacity of the intestinal conjugating enzymes is remarkably high; at large doses (greater than 5 mg/kg), intestinal conjugation far exceeds the contribution of the hepatic and pulmonary enzymes.

First pass conjugation of phenol following rectal administration in rats

The systemic availability of phenol following rectal administration to rats has been compared to intraduodenal and intravenous administration via the hepatic portal and jugular veins. Phenol blood concentration-time curves after the rectal route of dosing indicate that first pass conjugation by the intestinal mucosa and liver is drastically reduced when compared to the other routes of administration.

Drug metabolite concentration-time profiles: influence of route of drug administration

In order to assess the contribution of an active metabolite to the overall pharmacological response following drug administration it is necessary to characterise the metabolite concentration-time profile. The influence of route of drug administration on metabolite kinetics has been investigated by computer simulation. Comparisons between simulated profiles and published concentration-time data have been carried out. A route dependence in metabolite concentration-time curves is readily apparent provided the metabolite kinetics are formation rate limited and the hepatic clearance of drug is greater than 25 l/h (medium to highly cleared). Oral drug administration produces a triphasic metabolite concentration-time profile whereas only two phases are discernable after intravenous drug administration. The magnitude of the difference in maximum metabolite concentration is directly proportional to the hepatic clearance of drug due to first-pass metabolite production. The route dependence in the shape of the metabolite concentration-time curves is most dramatic when the absorption and distribution of drug and the elimination of metabolite is rapid. A reduction in the rate of either of these processes alters the shape of the metabolite concentration-time profile such that the consequence of first-pass metabolite formation may be reduced.

Determinants of systemic availability of promethazine in rabbits

Promethazine blood concentration-time curves have been determined in 7 rabbits following intravenous, oral and hepatic portal vein administration of promethazine. This phenothiazine has a large volume of distribution and a high metabolic clearance resulting in low blood concentrations particularly when the oral route is used. Analysis of the areas under the blood concentration-time curves indicates that hepatic first-pass metabolism is the major determinant of promethazine's low oral availability. Absorption from the gastrointestinal tract is essentially complete in most rabbits and the contribution of metabolism by the intestinal mucosa is minimal. The present findings are compared with the literature on other phenothiazines.

Antipyrine metabolite kinetics in rats: studies on dose and time dependence

Antipyrine metabolite kinetics have been investigated in the rat with respect to dose and time dependence. The metabolic pathways, 4-hydroxylation, benzylic oxidation, and N-demethylation, are of equal quantitative importance (approximately 20 per cent of dose) and show no dose dependence over the range 25-500 mg kg-1. By using [N-methyl-14C]-antipyrine, the single carbon fragment lost by N-demethylation may be monitored as 14CO2. Serial sampling of 14CO2 exhalation rate provides a half-life estimate which, according to theoretical principles, reflects the antipyrine plasma half-life. When both half-lives were measured in the same animals a statistically significant correlation was evident. At doses of 250 mg kg-1 and 500 mg kg-1 there is an increase in CER half-life (218 and 303 min respectively) when compared to a dose below 100 mg kg-1 (152 min). The metabolite formation rate constants are decreased accordingly at the high doses but are invariant over the dose range 25-100 mg kg-1. Although inter-rat variation in antipyrine metabolite kinetics was substantial, intra-rat variability was small. The noninvasive nature of determining antipyrine metabolite kinetics via breath and urine analysis provides a potentially useful animal model system to investigate the factors influencing hepatic mixed function oxidase activity in vivo.

Absorption of prednisolone in patients with Crohn's disease

The absorption of prednisolone in patients with Crohn's disease was investigated. Seven patients with Crohn's disease and eight normal control subjects were given a tracer dose of tritiated prednisolone with 20 mg cold prednisolone by mouth. On a separate occasion they were given an intravenous injection of radiolabelled prednisolone. After oral ingestion only 53.4 +/- 11.7% of labelled material was excreted in the urine of Crohn's patients compared with 82.5 +/- 3.6% in the normal subjects. The oral/intravenous availability ratio was 0.61 +/- 0.14 in Crohn's patients and 0.89 +/- 0.07 in the normal group. Areas under plasma concentration-time curves were lower in patients than normal subjects and the oral/intravenous ratios were 0.6 +/- 0.2 and 0.86 +/- 0.09 respectively. Faecal excretion of radioactivity after oral ingestion was greater in Crohn's patients (19.3 +/- 2.5%, n = 3) than in normal subjects (7 +/- 2.8%, n = 4). The range for each type of measurement was much wider in the patient group than in the normal subjects. These data suggest that patients with Crohn's disease do not absorb prednisolone normally and that absorption varies between patients.

Pharmacokinetics of promethazine and its sulphoxide metabolite after intravenous and oral administration to man

Blood concentrations of promethazine and promethazine sulphoxide have been measured following oral and intravenous administration of promethazine to seven healthy male volunteers. Promethazine disposition is characterised by a large volume of distribution (1970 1) and a high blood clearance (1.141 min-1). Less than 1% of the dose is excreted unchanged in the urine, therefore total body clearance is essentially metabolic clearance. In accord with this high clearance the oral availability of promethazine is only 25%. The absorption of promethazine from the gastrointestinal tract exceeds 80% in most subjects. Minimal metabolism by the gastrointestinal mucosa is implicated. Promethazine sulphoxide pharmacokinetics are consistent with a pronounced first pass effect. Although the area under the curve for this metabolite is not route dependent, there is a marked alteration in the shape of the metabolite curve when oral and intravenous data are compared. Evidence is presented to support the hypothesis that S-oxidation of promethazine is predominantly an hepatic event. The conclusions of previous investigators with regard to the role of the gut mucosa in S-oxidation of phenothiazines is critically assessed.

Interpretation of CO2 exhalation rate data from demethylation of aminopyrine and its metabolite monomethylaminoantipyrine

1 Aminopyrine breath tests make use of the commercially available (N-dimethyl-[14C])-aminopyrine. A pharmacokinetic model has been proposed to relate 14CO2 exhalation rates (CER) to the demethylation of ([14C]-methyl)-aminopyrine (AP) and -monomethylaminoantipyrine (MAP). 2 computer simulations based on the model show that the shape of the CER-time profile is largely dependent on the ratio of the MAP to AP elimination rate constants. If this ratio equals 0.5 then the CERs decline in the monoexponential fashion. Ratios less than 0.5 result in concave biexponential curves whereas ratios greater than 0.5 result in convex curves. When demethylation is not complete for both compounds the transfer from biexponential to monoexponential behaviour will only occur at ratios greater than 0.5. 3 The resolution of concave biexponential CER-time profiles to give accurate estimates of AP and MAP elimination rate constants can only be achieved when the length of the experiment is adequate. The commonly employed 2 microCi tracer dose of aminopyrine is insufficient to monitor CER over the necessary time period to detect the proposed biexponential decline.

Azo reduction of sulphasalazine in healthy volunteers

1 A comparison of total sulphapyridine saliva concentrations resulting from administration of sulphapyridine and sulphasalazine at equimolar doses has been carried out. It is concluded that the extent of azo reduction of sulphasalazine to release sulphapyridine is complete in healthy volunteers. 2 An ampicillin course of treatment alters the azo reduction of sulphasalazine. The extent of cleavage is reduced by one third on average. There is no change in the rate of absorption of the release sulphapyridine. 3 An ampicillin course of treatment does not alter the disposition of sulphapyridine. The changes noted in acetyl conjugate plasma concentrations are entirely consistent with reduced availability of sulphapyridine.

Aminopyrine demethylation kinetics. Use of metabolite exhalation rates as an index of enhanced mixed-function oxidase activity in vivo

The usefulness of determining aminopyrine demethylation kinetics via monitoring of metabolite exhalation rats had been assessed. In rats receiving [N-dimethyl-14C]aminopyrine, a biexponential decline in the 14CO2 exhalation rate is apparent when monitoring is continued over a 5 to 6-hr period. Both the fast phase (representing the first demethylation) and the slower phase (representing the second demethylation) are markedly influenced by phenobarbital and promethazine pretreatment. These changes are consistent with enhanced mixed-function oxidase activity. Examination of the urinary excretion products of aminopyrine obtained in these studies support this claim. The sensitivity of the above index of mixed-function oxidase activity is increased considerably by the use of crossover experimental designs. Considerable interanimal variation is observed in the metabolite production in both exhaled air and urine from rats administered aminopyrine.

Quantification of naphthyl conjugates. Comparison of high-performance liquid chromatography and selective enzyme hydrolysis methods

1. An h.p.l.c. method for the simultaneous determination of naphthol, naphthyl glucuronide and naphthyl sulphate in urine is described. This procedure is based on the selective formation of an ion pair between the sulphate conjugate and tetrabutyl lammonium, which allows its resolution from the glucuronide on a reverse phase column. 2. The h.p.l.c. method is sed to assess the selective enzyme hydrolysis method which is frequently employed in drug conjugate assays. Results suggest that the use of sulphatase and beta-glucuronidase may give data which are quantitatively and qualitatively erroneous.

Influence of phenobarbitone pretreatment on disposition of amidopyrine and its metabolites in rat

Plasma concentration-time profiles for amidopyrine and its primary desmethyl metabolite have been determined in control rats and in rats pretreated with phenobarbitone. The kinetics of both compounds indicate enhanced clearance following pretreatment. Metabolite excretion in urine, bile and breath support the plasma data. It would appear that pheno barbitone pretreatment results in quantitative but not qualitative changes in the metabolism of amidopyrine in rat.