Radioimmunoassay for cortisol using antibodies against prednisolone conjugated at the 3-position.

A radioimmunoassay for the measurement of cortisol in plasma was developed. The procedure requires a one-step precipitation of the proteins in plasma or a one-step extraction with methylene chloride in urine. The method is precise, accurate, sensitive, and very specific. The primary antibody was prepared against prednisolone-3- carboxymethoxime-bovine serum albumin. The method is sensitive to 12.5 pg/ml of plasma. The antiserum has little interference from other endogenous steroids and metabolites of cortisol. 0830 hour plasma levels of cortisol in twenty normal human adults averaged 150 +/- 33 ng/ml.

Isotretinoin kinetics after 80 to 320 mg oral doses.

Twelve healthy male subjects received 80, 160, 240, and 320 mg doses of oral isotretinoin as multiples of 40 mg capsules separated by 2-week washout periods in a randomized, crossover design. Blood samples were drawn at specific times over a 72-hour period after dosing. Blood concentrations of isotretinoin as well as its major metabolite, 4-oxo-isotretinoin, were determined by a specific HPLC method. In addition to the normal laboratory battery of tests, serum triglyceride levels were determined before the first dose and again 72 hours after each of the four doses. Mean (+/- SD) maximum concentrations after 80 to 320 mg doses were 366 +/- 159, 820 +/- 474, 1056 +/- 547, and 981 +/- 381 ng/ml, whereas the respective AUC0-infinity values were 3690 +/- 1280, 7030 +/- 4140, 9780 +/- 6080, and 9040 +/- 2900 ng X hr/ml. The observed apparent elimination t1/2 remained approximately the same (14.7 hours) for each dose. The maximum concentration and AUC values for isotretinoin appear to be dose proportional from 80 to 240 mg but plateau at the 320 mg dose level. Therefore, because isotretinoin blood concentrations may not increase with higher doses in the fasting state, single, oral doses in excess of 240 mg should be used with caution. The data also suggest that elevated triglyceride levels are not a simple function of isotretinoin blood concentrations across the entire study population and dose range studied, but that in subjects with triglyceride levels in excess of the normal range triglyceride levels were positively related to isotretinoin blood concentrations.

Dietary saccharin kinetics.

Six adult female subjects who use saccharin-containing products in their diet were asked to take divided equal doses of saccharin every 6 hr to maintain their average daily intake for 3 days. At the end of this period, each subject took a single dose that was equal to one divided dose. Saccharin concentrations in plasma and urine samples were used to assess the kinetic profile. Saccharin absorption was rapid with maximum concentrations in plasma in 0.5 to 1.0 hr. Maximum plasma concentrations and areas under the plasma concentration-time curves were proportional to dose. Renal clearance exceeded glomerular filtration rate in all cases and approximated renal plasma flow when corrected for the saccharin free fraction in plasma. Mean elimination half-life was 7.5 hr and mean apparent volume of distribution was 264 l. The kinetic parameters indicate that saccharin is distributed as a function of lean rather than total body mass (suggesting that saccharin does not distribute into body fat). This observation, together with data from studies in animals, suggests that there may be one or more high-retention compartments for saccharin.

Interaction of chloramphenicol with phenytoin and phenobarbital. Case report.

The effect of chloramphenicol therapy (48 mg/kg/day) on the serum concentrations of phenytoin and phenobarbital was studied in a patient previously stabilized on anticonvulsant medications. Phenytoin, 12 mg/kg/day, and phenobarbital, 5 mg/kg/day resulted in serum concentrations averaging 10.8 microgram/ml before and 30.5 microgram/ml, after chloramphenicol therapy. A reduction in dose of both phenytoin and phenobarbital was required to minimize adverse effects during the course of chloramphenicol therapy. An average daily dose of phenytoin of 9.1 mg/kg resulted in an average serum concentration of 17.8 microgram/ml. A daily dose of phenobarbital of 4.0 mg/kg resulted in an average serum concentration of 37.1 microgram/ml. These changes indicate 50.5% and 40.4% decreases in clearance of phenytoin and phenobarbital. Multiple-dose nonlinear regression analysis of phenytoin and phenobarbital serum concentration data obtained during chloramphenicol therapy indicated a 62.5% and a 29.5% decrease in clearance. Subsequent serum concentration monitoring demonstrated a similar reduction in phenobarbital clearance when chloramphenicol was added to phenobarbital alone.

Pharmacokinetics and pharmacodynamics of methadone in patients with chronic pain.

Concentrations of methadone in plasma, estimates of pain relief, and pupillary size were determined after a single intravenous dose (10 to 30 mg) of methadone hydrochloride to eight patients with chronic pain, five of whom had cancer. The pharmacokinetic parameter estimates reveal rapid and extensive distribution (Varea) and a slow apparent elimination half-life (t1/2) (mean Varea = 3.59 L/kg and harmonic mean t1/2 = 23 hours). The harmonic mean blood clearance is 106 ml/min, the harmonic mean renal clearance is 3.9 ml/min, the mean hepatic extraction ratio is 0.089, and plasma protein binding is 86% to 89%. These results suggest that only the free (unbound) fraction of methadone present in blood is extracted by the liver and that methadone can be classified as a low (hepatic)-extraction drug. The data were fit to a pharmacokinetic-pharmacodynamic model to obtain estimates of the steady-state plasma methadone concentration required to produce 50% of the maximum pain relief. This value varied from 0.04 to 1.13 micrograms/ml (mean = 0.29 micrograms/ml). These results indicate substantial interindividual variation in the relationship between changes in plasma methadone concentration and analgesia in patients with chronic pain receiving opioids. A pharmacokinetic-pharmacodynamic model may be useful for the individualization of analgesic dosage and therefore the optimization of pain management in patients with chronic pain.

Pharmacokinetic-pharmacodynamic relationships of methadone infusions in patients with cancer pain.

To determine the relationship between changes in plasma methadone concentration and pharmacodynamic effects, plasma methadone profiles and pharmacodynamics (analgesia and sedation) were measured during and after the continuous infusion of methadone for 180 to 270 minutes in 15 patients with pain caused by cancer. An increase in plasma methadone concentration resulted in a rapid increase in pain relief or sedation. The estimates of values of 50% of maximum effect (Css50) for pain relief and sedation obtained with a pharmacokinetic-pharmacodynamic model varied tenfold to twentyfold among patients; the mean Css50 value for pain relief (0.359 +/- 0.158 [SD] micrograms/ml) was virtually the same as the mean Css50 value for sedation (0.336 +/- 0.205 [SD] micrograms/ml). Similarly, the mean gamma (slope function) for pain relief (4.4 +/- 3.8 [SD]) and sedation (5.8 +/- 5.4 [SD]) did not differ. Examination of hysteresis plots of data obtained during the infusion and for 4 to 5 hours after cessation of the infusion revealed a very rapid equilibration between plasma methadone values and the sites mediating pain relief. There was no indication of the development of tolerance to the pharmacodynamic effects of methadone during the study. This report describes a method for quantitating the pharmacokinetic-pharmacodynamic relationships of the desirable and undesirable effects of opioid analgesics.

Trimoprostil plasma concentration--gastric acid inhibition relationships: potentiation by food.

A single, oral, 1.5-mg dose of trimoprostil was taken before a standard meal and a matching placebo was taken after a standard meal by 10 subjects (group A). A second group of 10 subjects took placebo before a meal and trimoprostil after the meal (group B), while a third group took placebo both before and after the standard meal (group C). Food-stimulated gastric acid production was measured by intragastric titration for 6.5 hr after dosing. Trimoprostil taken after the meal had a greater effect on gastric acid secretion than when taken before the meal: Duration of effect was 5 to 5.5 hr in group B and 2 to 2.5 hr in group A. Blood samples were drawn and assayed for trimoprostil by gas chromatography-mass spectrometry. Mean trimoprostil plasma concentration and mean inhibition of gastric acid secretion data were fit to two models by the Hill equation. The mean plasma concentration associated with 50% inhibition of gastric acid secretion was 1.25 ng/ml. Trimoprostil plasma concentrations between 3 and 4 ng/ml were associated with 70% to 80% gastric acid inhibition. Overall, there appears to be a pharmacokinetic-pharmacologic correlation between trimoprostil plasma concentrations and inhibition of gastric acid secretion. Trimoprostil (1.5 mg) in the presence of food appears to have a therapeutic advantage, in that it decreases acid secretion longer than when taken without food and suffers no loss of bioavailability.

Propoxyphene and norpropoxyphene kinetics after single and repeated doses of propoxyphene.

Plasma concentrations of propoxyphene (P) and its pharmacologically active metabolite norpropoxyphene (NP) were determined in normal subjects after single 130-mg oral doses and during and after 13 consecutive oral doses of 130 mg P, and in former heroin addicts who were maintained on 900 to 1200 mg of P per day. The data were analyzed using a first-pass elimination pharmacokinetic model. Both P and NP cumulated during repeated dosing to levels 5 to 7 times those after the first dose. In contrast, "maintenance" patients exhibited steady-state trough plasma NP cumulation that exceeded that of P by a factor of 13. Several changes in P and NP kinetics occurred during repeated dosing with P to the normal subjects: P clearance decreased from 994 to 508 ml/min, NP clearance decreased from 454 to 2210 ml/min, P half-life (t 1/2) increased from 3.3 to 11.8 hr, NP t 1/2 increased from 6.1 to 39.2 hr, and area under the concentration time curves for P and NP were doubled. These changes in kinetics during repeated dosing resulted in more extensive cumulation of P and NP than would be predicted from the single-dose kinetic profile. Changes in the extent of first-pass elimination of P result in variability in plasma P and NP that may contribute to P-induced toxicity.

Influence of phenytoin and phenobarbital on the disposition of a single oral dose of clonazepam.

Clonazepam (CZP) was measured in the plasma of eight subjects for 48 hr after a 0.03-mg/kg oral dose. After pretreatment for 19 days with phenytoin (DPH, 4.3 mg/kg/day), plasma CZP concentrations were determined in the same subjects after another 0.03 mg/kg oral dose of CZP. The same protocol was followed in eight additional subjects using phenobarbital (PB, 1.4 mg/kg/day) instead of DPH. DPH pretreatment lowered mean plasma CZP concentration in 8 of the 12 time points. DPH pretreatment increased CZP clearance by 46% to 58% and decreased CZP half-life (t1/2) by 31%. Both changes were statistically significant. After PB pretreatment the mean plasma CZP concentration was lowered by an average of 11%, but the decrease was statistically significant for only 1 of the 12 time points. PB decreased mean CZP t1/2 by 11% and increased CZP clearance by 19% to 24%, but only the increase in clearance was statistically significant. Both DPH and PB increased CZP clearances and decreased the areas under the plasma concentration-time curves without altering the volumes of distribution. This observation is consistent with induction of CZP metabolism. The overall effect of DPH (4.3 mg/kg/day) was greater than the effect of PB (1.4 mg/kg/day). Neither the DPH or PB had a significant effect on the extent of CZP protein binding.

Relationships between CSF drug concentrations, receptor binding characteristics, and pharmacokinetic and pharmacodynamic properties of selected 1,4-substituted benzodiazepines.

Pharmacokinetic profiles of the 1,4-substituted benzodiazepines are defined by their absorption, distribution, metabolism, and excretion characteristics. An ability to cross the blood-brain barrier and the onset of pharmacological activity have been associated with the physiochemical properties of the benzodiazepines. In addition, drug concentrations in the CSF correlate with the unbound drug concentrations in blood or plasma. Duration of pharmacological activity of the benzodiazepines in humans is associated with the affinity of these compounds for the benzodiazepine receptors in human brain. Therefore, benzodiazepines with high affinity for the benzodiazepine receptor sites in human brain tend to exhibit prolonged half-lives of elimination from the CSF which correlate with the prolonged duration of clinical and pharmacological effects and lower therapeutic doses of these drugs in vivo.

Clinical pharmacokinetics of the retinoids.

Etretinate, isotretinoin (13-cis-retinoic acid), and tretinoin (all-trans-retinoic acid) are retinoic acid analogues comprising a group of compounds known as the retinoids. However, they exhibit distinct and important differences with regard to their therapeutic and toxicological profiles. Tretinoin, due to a low oral therapeutic index, is limited almost exclusively to topical application, whereas etretinate and isotretinoin are therapeutically effective when given systemically by the oral route. Clinical doses of isotretinoin range from 0.5 to 8 mg/kg/day, with acute side effects appearing following doses of 1 mg/kg/day or greater. Plasma concentrations of isotretinoin following single and multiple doses peak between 2 to 4 hours and exhibit elimination half-lives of 10 to 20 hours. Isotretinoin blood concentration-time curves following a single- or multiple-dose regimen are well described by a linear model with biphasic disposition characteristics. Etretinate, which possesses a narrower therapeutic concentration range than isotretinoin, is used clinically at doses between 0.5 to 1.5 mg/kg/day; acute side effects appear following doses of 0.5 mg/kg/day or more. In most conditions, the retinoids produce a maximal effect in about 8 weeks (at the highest tolerated dose), with a slow recurrence of symptoms usually occurring within several weeks following cessation of treatment - except in the treatment of cystic acne with isotretinoin. Maintenance or intermittent dosing usually results in a prolongation of remission. Pharmacokinetically, the major difference between isotretinoin and etretinate is the much longer elimination half-life (120 days) of etretinate following long term administration. Recently, however, blood concentration versus time curves from day 1 to day 180 of etretinate therapy have been fitted by a single polyexponential pharmacokinetic equation without the need to invoke non-linearity in the kinetics. The observed lengthening of the elimination half-life with multiple dosing may thus be due to a lack of assay sensitivity at drug concentrations seen after single-dose administration, rather than to time-related alterations in the pharmacokinetics of etretinate.

Pharmacokinetics of oral cibenzoline in arrhythmia patients.

The pharmacokinetics of oral cibenzoline were studied in 30 arrhythmia patients as part of an ascending multiple-dose efficacy study. The elimination half-life of the drug following repetitive dosing ranged from 7.6 to 22.3 hours, with a harmonic mean of 12.3 hours (n = 24), and increased with age and decreasing renal function. The drug exhibited apparent dose proportional and linear pharmacokinetics over the range of doses studied. Multivariate analysis revealed that the patients' age and serum creatinine concentration accounted for 71% of the variability in the range of beta values (terminal elimination rate constant), and that 69.5% of the intersubject variability in the steady-state trough plasma concentrations could be accounted for by the patients' age, weight and serum creatinine concentration. These data suggest that, although there is some intersubject variability in the elimination and accumulation of cibenzoline, much of the variability can be explained by the patients' age, weight and renal function.

Drugs and endogenous ligands compete for receptor occupancy.

Dietary and endogenous ligands compete with drugs for receptor occupancy and therefore should be considered during therapeutic interventions and during pharmacokinetic/pharmacodynamic modeling. When disease is the result of an overabundance of these natural ligands, antibodies and/or their Fab fragments may be useful as therapeutic agents to reverse the effects of the natural ligands.

Optimizing the use of biomarkers, surrogate endpoints, and clinical endpoints for more efficient drug development.

Biomarkers and surrogate endpoints are critical to the future of efficient drug development. Definitions, a conceptual model, and a conceptual framework for validating and bridging biomarkers to clinical endpoints are provided in this presentation. In addition, a few examples are provided to support the development concept. Poor correlation between a biomarker and its clinical endpoint may result from (1) poor measurement of one or both, (2) selection of an inappropriate biomarker, or, more important, (3) use of an inappropriate clinical endpoint. Pharmacokinetic/pharmacodynamic (PK/PD) modeling output can be no better than biomarkers or surrogate endpoints used for the modeling. As we increase our understanding of biomarkers, surrogate markers, and the mechanistic basis for the processes of interest, biomarker and surrogate endpoint predictive power will no longer be an issue and PK/PD modeling inputs and outputs will improve.

Selecting and validating biologic markers for drug development.

Biochemical and clinical markers are critical for efficient development of new molecular entities. Biologic markers of drug effect, sometimes referred to as "surrogate" markers, are used when such clinical outcome measures as survival are substantially delayed relative to predictive biochemical changes or clinical effects of the new molecular entity. Biologic markers have generally been used for early-phase decision-making studies and accelerated regulatory approvals for much-needed drugs to treat cancer and acquired immune deficiency syndrome (AIDS). The rationale for these two uses of biologic markers is different and therefore the foundation required for establishing and validating each may be different. The theoretical foundation required for a marker that will be used to justify the regulatory approval of a treatment for a life-threatening disease should be greater than the required for an early decision-making study with an angiotensin II antagonist that will be used to treat mild to moderate hypertension. Use of CD4 counts as "surrogate markers" for prolonged survival was inappropriate. In contrast, changes in angiotensin-II concentrations and other renin-angiotensin system biochemical markers, observed for the first time in a study in humans, with a purported angiotensin-II receptor antagonist indicate that the new molecular entity is working as hoped. This is a good decision-making tool, because theory indicates that these changes should lead to reduced blood pressure, which is a predictive "surrogate" for reduction in subsequent cardiovascular events. Surrogate, biologic markers should be used only if they have a rational theoretical basis, are proven in preclinical or clinical experience, and are measured with validated methods. Different validation-acceptance criteria for decision-making markers compared with markers used for regulatory approval must be prospectively acknowledged and delineated.

Combined pharmacokinetic/pharmacodynamic (PK/PD) modeling.

Several compartmental and noncompartmental approaches have been successfully applied to PK/PD modeling. Although great opportunities exist to expand on these techniques, application of these methods to relate concentrations of all drugs to their effects is possible.

Physiologic pharmacokinetic modeling.

Although physiologic modeling has not gained the widespread acceptance that was originally projected, it may serve as the basis for future PK/PD modeling approaches. In addition, with more effort applied to developing in vitro and animal-to-human predictions, physiologic modeling may assume a higher position in the pharmacokinetic modeling hierarchy.

Controversy I: Patients or healthy volunteers for pharmacokinetic studies?

Many factors should be considered when choosing an appropriate population for a pharmacokinetic trial. Although there are some generalities that apply to most studies, each investigation must be judged separately, since the relevant considerations will vary depending on the particular study, the nature of the drug, and the population that will receive it for therapeutic benefit. Some of the most important information that is generated from pharmacokinetic studies concerns the pharmacokinetic variability among patients and the factors that can influence this variability under the conditions that the drug will be used. This information can best be obtained from a combination of baseline studies to define the variability within the patient population(s) and comparative studies to determine the impact of specific variables on the disposition of the drug and its pharmacokinetic variability. These data can provide valuable information to the clinician that can be used to individualize drug dosage and optimize therapy as well as to identify populations who may be at high risk of therapeutic failure or drug toxicity. It is our feeling that baseline studies in patients are necessary for understanding the pharmacokinetics of a drug, whereas the objectives of most comparative studies can be achieved using healthy volunteers. For most comparative studies, the data obtained from healthy volunteers will reflect what will occur in patients, especially if the variable of interest is drug absorption. This is particularly important when practical and ethical considerations preclude the use of patients. When considering studies in the elderly, one must decide whether the variable of interest may be influenced by age.(ABSTRACT TRUNCATED AT 250 WORDS)

An evolution of drug development and clinical pharmacology during the 20th century.

The current state of clinical pharmacology and drug development did not just happen. Clinical pharmacology and the drug development process were born, evolved, and have come to the fore during the past 100 years. The past century has been one of accelerating progress in science and medicine. The progress has not been a straight line but rather more like a sidewinder moving across the desert. Drug development has moved from small experiments with concoctions, extracts, and potions along with the manufacturing and promotion of the purported remedies to processes that exploited unknowing patients to a process that now requires concept generation, discovery, research, planning, and development with many checks and balances for the protection of human subjects. The factors that contributed to this progression from inappropriate use of potions, concoctions, and snake oil to the highly regulated drug development process of today is described in some detail.

Absorption kinetics of hydroflumethiazide.

The pharmacokinetics of hydroflumethiazide after oral administration of a 100-mg dose to 12 volunteers were evaluated to determine whether absorption of the drug is best described by zero-order or first-order absorption. Comparison of the two absorption models was based on three criteria: (1) correlation coefficients; (2) standard deviations of the parameter estimates; and (3) visual fits. Evaluation of the individual results in light of each criterion showed that the zero-order absorption model is the more appropriate one for describing the hydroflumethiazide data.