Inhibition constants, inhibitor concentrations and the prediction of inhibitory drug drug interactions: pitfalls, progress and promise.

Strategies and standards for predicting the likelihood of pharmacokinetically significant inhibitory drug-drug interactions for drug development purposes which rely primarily on projected in vivo concentrations of cytochrome P450 (CYP) or transporter inhibitors, [I], and in vitro estimates of their inhibitory constants, K(i), were specified in several commentaries based upon a conference held by the European Federation of Pharmaceutical Sciences (EUFEPS) several years ago. Since then the application of those strategies and standards has met with varying degrees of success. Many of the vexing issues that were identified in the EUFEPS Conference Report remain, while other issues are systematically being resolved. This article briefly reviews the underlying strategy in the prediction of the significance of inhibitory DDIs using [I]/K(i) ratios; some of the difficulties or pitfalls associated with the predictive application of [I]/K(i) ratios; and some of the recent refinements of the general strategy.

Cyclooxygenase-2 inhibition by rofecoxib reverses naturally occurring fever in humans.

Cyclooxygenase (COX) exists as constitutive (COX-1) and inducible (COX-2) isoforms. Nonsteroidal antiinflammatory drugs (NSAIDs) such as ibuprofen and diclofenac inhibit both COX-1 and COX-2. The role of COX-2 in the genesis of fever in monkeys and humans was examined with use of the specific COX-2 inhibitor rofecoxib. Rofecoxib was administered to monkeys made febrile by 6 microg/kg intravenous lipopolysaccharide. Induced pyrexia was followed by oral rofecoxib (1 or 3 mg/kg), diclofenac (3 mg/kg), or vehicle. Rofecoxib and diclofenac rapidly reversed the elevated temperature (P < .05 versus vehicle for 3 mg/kg rofecoxib and diclofenac at 70 to 90 minutes after dosing). A single-dose, parallel-group, double-blind randomized trial was conducted in 94 patients with fever caused by a viral-type illness. Mean baseline temperature was similar for all groups (-38.5 degrees C). Patients received oral doses of 12.5 mg rofecoxib, 25 mg rofecoxib, 400 mg ibuprofen, or placebo and the mean +/- SE change in oral temperature at 4 hours after dosing was -0.97 degrees C +/- 0.11 degrees C, -1.19 degrees C +/- 0.09 degrees C, -1.20 degrees C +/- 0.11 degrees C, and 0.01 C +/- 0.17 C, respectively (P < .001 for active treatments versus placebo). Specific inhibition of COX-2 by rofecoxib results in antipyretic activity in monkeys and humans comparable to dual COX-1/COX-2 inhibitors such as diclofenac or ibuprofen. The data support the hypothesis that it is the COX-2 isoform that is primarily involved in the genesis of fever in humans.

The use of in vitro methods to predict in vivo pharmacokinetics and drug interactions.

With the dramatic change underway in the process of drug discovery and development it has become increasingly important to define, both qualitatively and quantitatively, the dispositional features of new chemical entities (NCEs) as early in the process as possible. To that end strategies have emerged that are designed to enable reasonable predictions about a NCE's absorption from the gastrointestinal tract, systemic bioavailability and likelihood for significant pre-systemic clearance, character of metabolic processing both within the gastrointestinal tract and the liver, in vivo pharmacokinetics (PK), and likelihood for clinically significant interactions with other drugs. To some extent these strategies have embraced interspecies allometric scaling in which findings in animals are extrapolated to predict outcomes in humans. However, a greater emphasis in recent years has been placed on predicting human PK and the likelihood of clinically significant drug-drug interactions for NCEs solely from in vitro experiments. These general strategies have been methodologically streamlined so that hundreds or even thousands of experiments on a given NCE can be conducted within several days. Dispositional data from these pre-clinical experiments is useful for rapidly identifying potential marketing advantages for NCEs, and for screening out those substances that should not be placed into more expensive and labor-intensive animal experiments or brought to clinical trial. The key issue in these strategies is the accuracy with which pre-clinical findings predict clinical outcomes. Based largely on retrospective analyses the current state of the art exhibits a high percentage of useful predictions. However, there are many examples in which the prediction of either human PK or clinical drug-drug interactions from pre-clinical data has failed. The reasons for inaccurate predictions are manifold, and may include the actual in vitro methodology used, inappropriate model selection, and errant scale-up factors. Additionally, in vitro methods may fail to account for complex hepatobiliary processing including transport phenomena and Phase II metabolism. Progress has been made in establishing humanized methodologies that accurately describe these processes, with a view toward reconstituting the contributions of each into a more complex and accurate depiction and prediction of in vivo PK and drug-interaction potential.

Influence of some novel N-substituted azoles and pyridines on rat hepatic CYP3A activity.

A series of N-substituted heteroaromatic compounds structurally related to clotrimazole was synthesized, and the effects of these compounds on ethosuximide clearance in rats were determined as a measure of their abilities to induce cytochrome P4503A (CYP3A) activity. Ethosuximide clearance and in vitro erythromycin N-demethylase activity were shown to correlate. In this series, imidazole or other related heteroaromatic "head groups" were linked to triphenylmethane or other phenylmethane derivatives. Within the series, it was found that 1-triphenylmethane-substituted imidazoles elicited the greatest increase in CYP3A activity, and that among the triphenylmethyl-substituted imidazoles, the highest activities were achieved by the substitution of F- or Cl- in either the meta or para position of one of the phenyl rings. Diphenylmethyl-substituted pyridine was effectively devoid of activity. Compounds eliciting the largest increase in CYP3A activity (viz. 1-[(3-fluorophenyl)diphenylmethyl]imidazole, 1-[(4-fluorophenyl)diphenylmethyl]imidazole, and 1-[tri-(4-fluorophenyl)methyl]imidazole) produced little or no increase in ethoxyresorufin O-dealkylase (EROD) activity (i.e. CYP1A), whereas benzylimidazole, which elicited only a small increase in CYP3A activity, produced an almost 9-fold increase in CYP1A activity. For a series of eleven compounds exhibiting a wide range of influence on CYP3A activity, a positive correlation was found between ethosuximide clearance and hepatic CYP3A mRNA levels.

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.

Norastemizole Sepracor.

Sepracor and Janssen are developing the histamine H1 antagonist, norastemizole (an active metabolite of Johnson & Johnson's Hismanal), for the potential, non-sedating treatment of allergy. Sepracor expects to file an NDA with the FDA by the fourth quarter of 2000 [337315,358429]. As of September 1999, Sepracor was conducting two large-scale phase III seasonal allergic rhinitis studies [340260]. Sepracor expects norastemizole to be the most potent non-sedating histamine, with equal or more rapid onset of action than other therapies [229516]. Norastemizole is 13- to 16-fold more potent as an H1 antagonist than astemizole and 20- to 40-fold more potent in inhibiting histamine-induced bronchoconstriction. Following a single dose of norastemizole (25 mg p.o.), there is significant attenuation of histamine-induced wheal and flare responses within 30 min. The drug's major advantage is its lack of cardiotoxicity or interactions with other drugs that increase the risk of developing serious arrhythmias [301469]. In July 2000, Morgan Stanley Dean Witter predicted filing for FDA approval for allergic rhinitis during the first half of 2001, and a partnership announcement around the time of this NDA filing. The analysts also forecast European sales of $8.3 million in 2002, rising to $16.7 m by 2005 [384868].

Lack of pharmacokinetic interaction between rofecoxib and methotrexate in rheumatoid arthritis patients.

Rofecoxib is a highly selective and potent inhibitor of cyclooxgenase-2 (COX-2). Methotrexate is a disease-modifying agent with a narrow therapeutic index frequently prescribed for the management of rheumatoid arthritis. The objective of this study was to investigate the influence of clinical doses of rofecoxib on the pharmacokinetics of methotrexate in patients with rheumatoid arthritis. This was a randomized, double-blind, placebo-controlled study in 25 rheumatoid arthritis patients on stable doses of methotrexate. Patients received oral methotrexate (7.5 to 20 mg) on days -1, 7, 14, and 21. Nineteen patients received rofecoxib 12.5, 25, and 50 mg once daily on days 1 to 7, 8 to 14, and 15 to 21, respectively. Six patients received placebo on days 1 to 21 only to maintain a double-blinded design for assessment of adverse experiences. Plasma and urine samples were analyzed for methotrexate and its major although inactive metabolite, 7-hydroxymethotrexate. The AUC(0-infinity) geometric mean ratios (GMR) and their 90% confidence intervals (90% CI) (rofecoxib + methotrexate/methotrexate alone) for day 7/day -1, day 14/day -1, and day 21/day -1, for rofecoxib 12.5, 25, and 50 mg, were 1.03 (0.93, 1.14), 1.02 (0.92, 1.12), and 1.06 (0.96, 1.17), respectively (p > 0.2 for all comparisons to day -1). All AUC(0-infinity), GMR and Cmax GMR 90% CIs fell within the predefined comparability limits of (0.80, 1.25). Similar results were observed for renal clearance of methotrexate and 7-hydroxymethotrexate at the highest dose of rofecoxib tested (50 mg). It was concluded that rofecoxib at doses of 12.5, 25, and 50 mg once daily has no effect on the plasma concentrations or renal clearance (tested at the highest dose of rofecoxib) of methotrexate in rheumatoid arthritis patients.

Differential kinetics of phenytoin in elderly patients.

The elderly have a relatively high risk of developing adverse drug reactions. Phenytoin continues to be a preferred drug for treating generalised tonic-clonic seizures in the elderly and simple partial seizures that generalise. Phenytoin is eliminated almost entirely by hepatic oxidation. The principle enzymes responsible are cytochrome P450 (CYP)2C9 and CYP2C19. CYP2C9 is saturated by therapeutic doses of phenytoin, and at steady state both enzymes are probably operant in most people. The nonlinear pharmacokinetics of phenytoin make it a difficult drug for which to establish safe and effective administration regimens. An important area of inquiry is whether the differential disposition kinetics of phenytoin in the elderly render its administration an even more difficult challenge. Moreover, since the elderly are generally subject to more polypharmacy than younger adults, are they, as a result, subject to either more frequent or more severe drug interactions with phenytoin than younger adults? In order to examine these issues we were interested in learning the extent to which old age might affect the plasma protein binding of phenytoin, its hepatic metabolism and, ultimately, its pharmacokinetic profile. With regard to the latter we looked carefully at the methods that have been used to characterise the disposition kinetics of phenytoin in general, and in the elderly, in particular. There are many conflicting findings with regard to the effect of age on the disposition kinetics of phenytoin. However, the strategies used for estimating kinetic parameters for phenytoin [viz the maximum rate of metabolism/elimination (Vmax) and the Michaelis-Menton constant (Km)] exhibit deficiencies that could account for some of the disparate findings. Certainly, more careful prospective studies focusing on the effects of age on phenytoin disposition kinetics are warranted. However, in light of the information currently available, no special attention need be paid to the initiation of phenytoin administration in elderly patients who are taking multiple anticonvulsants. On the other hand, for the elderly receiving phenytoin monotherapy, the initiation of phenytoin administration should occur at lower doses than would be customary for younger adults, and phenytoin blood concentrations should be appropriately monitored in order to evaluate individual Vmax and Km values for informed dosage adjustments.

Exercise training does not alter cytochrome P-450 content and microsomal metabolism.

The purpose of this investigation was to determine whether increased endurance exercise capacity alters total hepatic cytochrome P-450 content and cytochrome P-450 (CYP1A and CYP2B) mediated hepatic microsomal mixed-function oxidase drug metabolism. Twenty adult male Sprague-Dawley rats were randomly assigned to either a control (C) or an endurance trained group (ET). ET rats were progressively trained 5 d.wk-1 for 11 wk. Both C and ET rats were administered in random order single posttraining doses of probe drugs theophylline (probe for CYP1A) and antipyrine (probe for CYP2B). Soleus muscle citrate synthase activity of ET rats was significantly greater (P < 0.01) than for C rats (mean +/- SD; C, 26.4 +/- 1.3 mumol.g-1.min-1; ET, 46.1 +/- 2.7). In contrast, total liver cytochrome P-450 content was not significantly different (P > 0.01) among C and ET rats (mean +/- SD; C, 0.554 +/- 0.055 nmol.mg-1 liver protein; ET, 0.604 +/- 0.080). Likewise, the posttraining C and ET single-sample plasma clearances of theophylline (mean +/- SD; C, 1.89 +/- 0.360 1.h-1.kg-1 total liver weight; ET, 2.08 +/- 0.49) and antipyrine (mean +/- SD; C, 6.44 +/- 1.56 1.h-1.kg-1 total liver weight; ET, 6.51 +/- 1.02) were not significantly different (P > 0.01). Therefore, it was concluded that strenuous endurance training of 11 wk duration did not alter total hepatic cytochrome P-450 content or CYP1A or CYP2B activity.

Convenient synthesis of 6-nor-9,10-dihydrolysergic acid methyl ester.

6-Nor-9,10-dihydrolysergic acid methyl ester (IV) was prepared by demethylation of 9,10-dihydrolysergic acid methyl ester (II) with 2,2,2-trichloroethyl chloroformate, followed by reduction of the intermediate carbamate (III) with zinc in acetic acid. The 6-ethyl-V and 6-n-propyl-VI derivatives were prepared by alkylation of IV with the appropriate halide. All of the ergoline derivatives were evaluated for stereotyped behavior in rats, with 6-nor-6-ethyl-9,10-dihydrolysergic acid methyl ester (V) being active but much less potent than apomorphine. Compound VI was evaluated for its effect on blood pressure; at a dose of 30 mg/kg ip, it significantly lowered, diastolic pressure in normotensive rats.

Use of limited laboratory-acquired therapeutic drug monitoring data in determining individualized drug requirements.

Routinely acquired therapeutic drug monitoring (TDM) data from 220 patients were used to estimate patient-specific pharmacokinetic parameters for the following drugs: aminoglycoside antibiotics (gentamicin and tobramycin), digoxin, theophylline, carbamazepine, procainamide, phenobarbital, and quinidine. A microcomputer based set of algorithms operating on two relatively unconstrained TDM values estimated pharmacokinetic parameters with which future TDM levels were forecast. Mean prediction errors (mpe) and root mean squared errors (rmse) were used as measures of predictive performance. Values of mpe deviated from zero by less than one microgram per l for digoxin and by less than one microgram per l for all other drugs. Values of rmse were also small when viewed in the context of the respective therapeutic plasma concentration ranges.

Phenylbutazone-warfarin interaction in the dog.

The administration of phenylbutazone together with warfarin to dogs resulted in an elevation of the free fraction of warfarin in the plasma from 2-6 to 8-0% thus providing direct support for the notion that phenylbutazone induced inhibition of warfarin binding to plasma proteins. This inhibition as evaluated by a kinetic method was accompanied by a two-fold decrease in the plasma half-life of warfarin from 18-4 h in control animals to 9-6 h in phenylbutazone-treated animals. Marked increases in warfarin-induced hypoprothrombinaemia were observed when at doses up to 8 mg kg-1 (orally) it was given with phenylbutazone (50 mg kg-1, orally). The unbound fraction of warfarin in canine plasma ranged from 1-7 to 4-3% indicating individual differences in the extent of the plasma binding of warfarin in the dog.

Topical application of a chlordane-containing ectoparasiticide: effect on the plasma half-life of warfarin in dogs.

Brief exposure of dogs to topical chlordane solutions resulted in a significant and long-lasting decrease in the biological half-life of orally administered warfarin. The effect is presumed to be an expression of chlordane's well-documented inductive effect on hepatic microsomal drug metabolizing enzymes and its long-term storage in fat depots. The facility with which chlordane is absorbed through the intact skin of dogs may render casually-treated animals unsuitable for subsequent pharmacologic study for long periods of time.

Drug interactions of H2-receptor antagonists.

Three drug interactions of nizatidine and of other antisecretory agents were studied comparatively. First, the effects of nizatidine, cimetidine and ranitidine on the dispositional kinetics of theophylline were evaluated in chronic obstructive pulmonary disease (COPD) patients. Second, the effect of magnesium/aluminium hydroxide on the relative bioavailability of nizatidine, famotidine, cimetidine and ranitidine was evaluated in healthy volunteers. Finally, the effects of nizatidine and omeprazole on the dispositional kinetics of phenytoin were evaluated in healthy volunteers. Only cimetidine altered the steady-state kinetics of oral theophylline, slowing theophylline clearance by 25%. Each of the H2-receptor antagonists exhibited a modest decline in relative bioavailability when ingested with antacid. Antacid ingestion decreased the bioavailability of famotidine, ranitidine and cimetidine by 20-25%, and the bioavailability of nizatidine by 12%. Each of these effects was statistically significant. Finally, it was found that neither omeprazole nor nizatidine affected the single dose kinetics of phenytoin.

[New guidelines on chronic pancreatitis : interdisciplinary treatment strategies]. / Neue Leitlinie zur chronischen Pankreatitis : Interdisziplinäre Behandlungsstrategien.

Chronic pancreatitis is a common disorder associated with significant morbidity and mortality. Interdisciplinary consensus guidelines have recently updated the definitions and diagnostic criteria for chronic pancreatitis and provide a critical assessment of therapeutic procedures. Diagnostic imaging relies on endoscopic ultrasound (EUS) as the most sensitive technique, whereas computed tomography (CT) and magnetic resonance imaging (MRI)/magnetic resonance cholangiopancreatography (MRCP) remain a frequent preoperative requirement. Endoscopic retrograde cholangiopancreatography (ERCP) is now used mostly as a therapeutic procedure except for the differential diagnosis of autoimmune pancreatitis. Complications of chronic pancreatitis, such as pseudocysts, duct stricture and intractable pain can be treated with endoscopic interventions as well as open surgery. In the treatment of pseudocysts endoscopic drainage procedures now prevail while pain treatment has greater long-term effectiveness following surgical procedures. Currently, endocopic as well as surgical treatment of chronic pancreatitis require an ever increasing degree of technical and medical expertise and are provided increasingly more often by interdisciplinary centres. Surgical treatment is superior to interventional therapy regarding the outcome of pain control and duodenum-preserving pancreatic head resection is presently the surgical procedure of choice.

The use of single-sample clearance estimates to probe hepatic drug metabolism in rats. IV. A model for possible application to phenotyping xenobiotic influences on human drug metabolism.

1. Conditions were examined under which estimates of drug clearance made from a single measurement of plasma concentration effectively represented multi-sample estimates of clearance for carbamazepine, quinidine, and paracetamol. When plasma concentrations were measured at various post-dose times, both individual and mean values of single-sample clearance estimates (CL) corresponded closely to multi-sample clearance estimates. Best post-dose sampling times were: carbamazepine, 3 h; quinidine, 10 h; and paracetamol, 6 h. 2. Single sample clearance estimates, CL, were calculated for seven drugs employed as probes of hepatic drug-metabolizing activity in rats. Valproic acid was investigated as a probe of microsomal and peroxisomal oxidases; antipyrine, theophylline, ethosuximide, carbamazepine and quinidine as probes of hepatic mixed-function oxidases (MFO), and paracetamol as a probe for UDP-glucuronosyltransferase activity. 3. A clearance index (CI, namely, probe CL after xenobiotic pretreatment divided by control probe CL) was calculated for each probe. Eight pretreatments were used: phenobarbital (PB), beta-naphthoflavone (beta NF), polychlorinated biphenyls (PCB), rifampin, pregnenolone-16 alpha-carbonitrile (PCN), clofibric acid, cimetidine, and piperonyl butoxide. The effect of each xenobiotic pretreatment on all probe CL values was consolidated and plotted as the logarithm of the CI, and a distinct pattern or handprint evolved for each pretreatment. 4. We conclude that the use of multiple single-sample probes of hepatic MFO activity can be useful in structuring handprints to characterize xenobiotic-mediated effects on hepatic MFO. This minimally invasive in vivo approach may have application in investigating and possibly phenotyping MFO activity in human subpopulations that are subject to sustained exposure to particular xenobiotics.

Use of single sample clearance estimates of cytochrome P450 substrates to characterize human hepatic CYP status in vivo.

1. Single sample clearance estimates (CL/F) of orally administered ethosuximide were obtained in four groups of healthy adult subjects. One group was treated with phenobarbital to induce CYP2B/2C and CYP3A activity; one group was treated with rifampin to induce CYP3A activity; one group consisted of cigarette smokers (increased CYP1A activity), and one group was untreated (controls). Ethosuximide CL/F values were slightly, though not significantly, increased among cigarette smokers (12.5% increase) and the phenobarbital group (25% increase), but rifampin treatment resulted in a significant increase (65%). 2. The influence of rifampin treatment on the single sample oral clearances of antipyrine, theophylline, phenytoin, carbamazepine, ethosuximide, quinidine, valproic acid, and lorazepam was investigated to determine whether rifampin induces only CYP3A. Rifampin treatment significantly increased the oral clearance of each drug from 1.4-fold (valproic acid) to 3.4-fold (quinidine). These findings indicate that the inductive effect of rifampin extends well beyond CYP3A.

Dispositional and pharmacodynamic characteristics of brodifacoum in warfarin-sensitive rats.

A dose-response curve for the hypoprothrombinemic effect of brodifacoum 3-[-3(4'-bromobiphenyl-4-yl) 1,2,3,4-tetrahydronaphth-1-yl] -4-hydroxycoumarin, was constructed using doses ranging from 0.1 to 0.33 mg/kg. Brodifacoum exhibited a remarkably steep dose-response curve. Brodifacoum failed to exhibit a dose-dependent effect on the degradation rate constant (kdeg) for prothrombin complex activity (PCA) after a PCA-synthesis-blocking dose of warfarin. Both phenobarbital pretreatment and SKF525A treatment altered to anticoagulant response to brodifacoum. Phenobarbital decreased the anticoagulant effect, whereas SKF525A increased it, suggesting that a substantial portion of brodifacoum-induced hypoprothrombinemia is mediated by brodifacoum itself rather than by metabolites. Finally, rats dosed orally with brodifacoum (0.2 mg/kg p.o.) were sacrificed in groups of 3-5 at various times up to 120 h after the dose. Brodifacoum was assayed in serum, small intestine, and liver by an HPLC method. Brodifacoum disappeared slowly from serum with a half-life of 156 h. Disappearance from small intestine was rapid, for 24 h, but intestinal levels began increasing from 24 to 72 h after the dose. Concentrations in liver were rapidly established, and exceeded serum concentrations by 20-fold. Brodifacoum levels in liver remained relatively constant for 96 h. Sustained liver concentrations after a single dose may partially account for brodifacoum's apparent potency relative to warfarin.

An investigation of the relationship between estrogen, estrogen metabolites and blood cholesterol levels in ovariectomized rats.

17 beta-Estradiol (E2) has long been known for protecting against coronary heart disease by lowering cholesterol levels in premenopausal women. A recent study in our laboratory suggested that two hydroxylated metabolites of E2 possess similar hypocholesterolemic effects in male rats. This effect has been further investigated with additional estrogen metabolites in ovariectomized rats with a view toward mimicking the true postmenopausal situation in humans. Their effects in reproductive tissues were also evaluated histologically. Fundamentally, the following issues were addressed: (1) Do oxidized metabolites of estradiol lower total cholesterol levels? (2) Can a hypocholesterolemic effect be achieved without eliciting estrogenic activities on reproductive tissues? The results of this investigation showed that a number of oxygenated metabolites of estradiol can lower cholesterol levels. Among them, 4-hydroxyestradiol (4-OHE2) produced a striking hypocholesterolemic effect and a substantial uterotropic effect. 2-Hydroxyestradiol (2-OHE2), 2-methoxyestradiol (2-meoE2) and 2-methoxyestrone (2-meoE1) produced a significant decrease in cholesterol levels at doses that did not produce significant uterotropic effects.